*Review* **Novel Gels: An Emerging Approach for Delivering of Therapeutic Molecules and Recent Trends**

**Trideva K. Sastri , Vishal N. Gupta \* , Souvik Chakraborty, Sharadha Madhusudhan, Hitesh Kumar , Pallavi Chand, Vikas Jain , Balamuralidhara Veeranna and Devegowda V. Gowda**

> Department of Pharmaceutics, JSS College of Pharmacy, JSS Academy of Higher Education & Research, Sri Shivarathreeshwara Nagar, Mysuru 570015, India; trideva.k@gmail.com (T.K.S.); souvik93pharmacist@gmail.com (S.C.); msharadha1996@gmail.com (S.M.); hitesh.sahu1921@gmail.com (H.K.); pallavichand1990@gmail.com (P.C.); vikasjain@jssuni.edu.in (V.J.); baligowda@jssuni.edu.in (B.V.); dvgowda@jssuni.edu.in (D.V.G.)

**\*** Correspondence: vkguptajss@gmail.com

**Abstract:** Gels are semisolid, homogeneous systems with continuous or discrete therapeutic molecules in a suitable lipophilic or hydrophilic three-dimensional network base. Innovative gel systems possess multipurpose applications in cosmetics, food, pharmaceuticals, biotechnology, and so forth. Formulating a gel-based delivery system is simple and the delivery system enables the release of loaded therapeutic molecules. Furthermore, it facilitates the delivery of molecules via various routes as these gel-based systems offer proximal surface contact between a loaded therapeutic molecule and an absorption site. In the past decade, researchers have potentially explored and established a significant understanding of gel-based delivery systems for drug delivery. Subsequently, they have enabled the prospects of developing novel gel-based systems that illicit drug release by specific biological or external stimuli, such as temperature, pH, enzymes, ultrasound, antigens, etc. These systems are considered smart gels for their broad applications. This review reflects the significant role of advanced gel-based delivery systems for various therapeutic benefits. This detailed discussion is focused on strategies for the formulation of different novel gel-based systems, as well as it highlights the current research trends of these systems and patented technologies.

**Keywords:** hydrogels; in situ gels; emulsion gels; microgels; nanogels; vesicular gels

#### **1. Introduction**

In recent years, novel drug delivery systems have proven very adept at delivering therapeutic molecules with site-specific and localized effects. Additionally, these systems facilitate drug release at desired rates and simultaneously lower the undesired effects [1]. Gels are three-dimensional, semi-solid systems consisting of polymeric matrices. These behave in the same way as solid systems; however, they consist of relatively higher liquid components than solid dispersions [2,3]. Gel systems comprise long, arbitrary chains, albeit with reversible links at precise points. These systems comprise minimum two components and are fundamentally coherent colloidal dispersion systems [4]. The system components, namely the dispersion medium and the dispersed constituent, are uniformly scattered throughout the system. Gels are usually transparent or translucent in appearance entailing higher amounts of solvent [5]. When a suitable solvent is employed, the gelling agents entangle to form a three-dimensional colloidal network that confines fluid movement by entrapment and achieves immobilization of solvent molecules [6]. The network governs the viscoelastic properties of the gel system by developing endurance against deformation. In other words, the thixotropic behavior is contributed by the matrix's structure [7]. Gels are prepared mainly by fusion technique or by employing gelling agents. Gel-based systems can be alienated into two categories, organogels and hydrogels, based on the physical state of the gelling agent dispersion [8]. Dispersible colloids and water-soluble

**Citation:** Sastri, T.K.; Gupta, V.N.; Chakraborty, S.; Madhusudhan, S.; Kumar, H.; Chand, P.; Jain, V.; Veeranna, B.; Gowda, D.V. Novel Gels: An Emerging Approach for Delivering of Therapeutic Molecules and Recent Trends. *Gels* **2022**, *8*, 316. https://doi.org/10.3390/ gels8050316

Academic Editor: Hiroyuki Takeno

Received: 30 April 2022 Accepted: 17 May 2022 Published: 19 May 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

components constitute hydrogels, while lipophilic oleaginous components are employed in organogels [9]. The systems are further classified into xerogels or aqueous gels based on the nature of the solvents. Xerogels are solid gels with a minimum solvent concentration obtained mainly by solvent evaporation, thereby attaining a gel network [10,11]. However, the gel state can be reinstated by incorporating an imbibing agent that swells the matrix. Novel gels are capable of controlled and sustained release of loaded therapeutic molecules. Figure 1 portrays common novel gel-based delivery systems [12]. Smart gels can be developed which respond to biological and external stimuli, such as temperature, pH, chemical, enzymes, electrical, light, antigens, etc. These systems are highly instrumental in lowering undesired effects and are biodegradable and biocompatible [13]. High drug loading can be achieved. Their size (nanogels) expedites high drug accumulation at the tissue level and enables stealth systems by evading phagocytic cells [14–16]. Their distinctive surface properties enable passive and active targeting. This review underlines the advances in gel-based delivery systems, their developments, and a current update in the delivery of therapeutic molecules. water-soluble components constitute hydrogels, while lipophilic oleaginous components are employed in organogels [9]. The systems are further classified into xerogels or aqueous gels based on the nature of the solvents. Xerogels are solid gels with a minimum solvent concentration obtained mainly by solvent evaporation, thereby attaining a gel network[10,11]. However, the gel state can be reinstated by incorporating an imbibing agent that swells the matrix. Novel gels are capable of controlled and sustained release of loaded therapeutic molecules. Figure 1 portrays common novel gel-based delivery systems [12]. Smart gels can be developed which respond to biological and external stimuli, such as temperature, pH, chemical, enzymes, electrical, light, antigens, etc. These systems are highly instrumental in lowering undesired effects and are biodegradable and biocompatible [13]. High drug loading can be achieved. Their size (nanogels) expedites high drug accumulation at the tissue level and enables stealth systems by evading phagocytic cells [14–16]. Their distinctive surface properties enable passive and active targeting. This review underlines the advances in gel-based delivery systems, their developments, and a current update in the delivery of therapeutic molecules.

based on the physical state of the gelling agent dispersion [8]. Dispersible colloids and

*Gels* **2022**, *8*, x FOR PEER REVIEW 2 of 21

**Figure 1.** Common novel gel-based delivery systems *(*Created with BioRender.com accessed on 28 April 2022). **Figure 1.** Common novel gel-based delivery systems (Created with BioRender.com, accessed on 28 April 2022).

#### **2. Advances in Novel Gel-Based Delivery Systems 2. Advances in Novel Gel-Based Delivery Systems**

Novel gel-based drug delivery systems are classified by the nature of their structural network and by their response to stimuli [15]. The former is either a chemically aligned gel network or a physically aligned gel network system. At the same time, the latter category entails responsive, intelligent gel systems that imbibe solvents and swell on exposure to stimuli, such as temperature, pH, chemical, enzymes, electrical, light, antigens, etc. [17,18]. Novel gel systems are evaluated for rigorous characterizations to understand their efficacy as delivery systems. The most commonly employed evaluation parameters comprise swelling capacity, size and morphology, rheological properties, surface charge, etc. [19]. Further, they are scrutinized for physical appearance for compliance, physical state, homogeneity, and phase separation to understand their stability, extrudability, and spreading coefficient is significant for topical gels, as well as bioadhesive strength is a vital Novel gel-based drug delivery systems are classified by the nature of their structural network and by their response to stimuli [15]. The former is either a chemically aligned gel network or a physically aligned gel network system. At the same time, the latter category entails responsive, intelligent gel systems that imbibe solvents and swell on exposure to stimuli, such as temperature, pH, chemical, enzymes, electrical, light, antigens, etc. [17,18]. Novel gel systems are evaluated for rigorous characterizations to understand their efficacy as delivery systems. The most commonly employed evaluation parameters comprise swelling capacity, size and morphology, rheological properties, surface charge, etc. [19]. Further, they are scrutinized for physical appearance for compliance, physical state, homogeneity, and phase separation to understand their stability, extrudability, and spreading coefficient is significant for topical gels, as well as bioadhesive strength is a vital element for mucoadhesive gels [20]. Drug content, permeability, and release play a substantial role in any drug delivery system. The International Council on Harmonisation

(ICH) (Geneva, Switzerland) dictates the stability guidelines. The gels are subjected to various stress conditions and later scrutinized for drug content, release, and entrapment efficiency to assess their compliance [21]. Figure 2 illustrates various potential delivery routes for novel gel-based delivery systems. (ICH) (Geneva, Switzerland) dictates the stability guidelines. The gels are subjected to various stress conditions and later scrutinized for drug content, release, and entrapment efficiency to assess their compliance [21]. Figure 2 illustrates various potential delivery routes for novel gel-based delivery systems.

element for mucoadhesive gels [20]. Drug content, permeability, and release play a substantial role in any drug delivery system. The International Council on Harmonisation

*Gels* **2022**, *8*, x FOR PEER REVIEW 3 of 21

**Figure 2.** Potential delivery routes for novel gel-based delivery system. (Created with BioRender.com accessed on 28 April 2022). **Figure 2.** Potential delivery routes for novel gel-based delivery system. (Created with BioRender.com, accessed on 28 April 2022).

#### *2.1. Intelligent Hydrogels 2.1. Intelligent Hydrogels*

A three-dimensional network of hydrophilic polymers composes of hydrogels that inherently imbibe water and maintain the system's integrity [22]. These systems are one of the most versatile delivery systems among novel gels. Researchers have comprehensively developed several intelligent hydrogels that precisely retort to numerous physical stimuli such as temperature, light, electric fields, pressure, sound, and magnetic fields in recent years. Furthermore, stimuli pertain to pH, ions, enzymes, etc. These systems are beneficial for formulating controlled delivery systems [23–25]. A three-dimensional network of hydrophilic polymers composes of hydrogels that inherently imbibe water and maintain the system's integrity [22]. These systems are one of the most versatile delivery systems among novel gels. Researchers have comprehensively developed several intelligent hydrogels that precisely retort to numerous physical stimuli such as temperature, light, electric fields, pressure, sound, and magnetic fields in recent years. Furthermore, stimuli pertain to pH, ions, enzymes, etc. These systems are beneficial for formulating controlled delivery systems [23–25].

Temperature-responsive hydrogels are triggered by a precisely established temperature range. These hydrogels are formulated with polymers that are capable of temperature-triggered phase transitions [26]. Regular polymers exhibit higher solubility with an increase in temperature; however, the polymers employed in temperature-responsive systems, such as poly (N, N-diethylacrylamide), poly (tertramethyleneether glycol), poly(Nisopropylacrylamide), and others, possess lower critical solution temperatures. Polymers with lower critical solution temperatures tend to shrink with the increase in temperature; these hydrogels are known as negative temperature responsive [22]. Poly(acrylic acid) and polyacrylamide polymers inherently imbibe at higher temperatures and shrink at lower temperatures; hence, hydrogels prepared from these polymers are positive temperature Temperature-responsive hydrogels are triggered by a precisely established temperature range. These hydrogels are formulated with polymers that are capable of temperaturetriggered phase transitions [26]. Regular polymers exhibit higher solubility with an increase in temperature; however, the polymers employed in temperature-responsive systems, such as poly (N, N-diethylacrylamide), poly (tertramethyleneether glycol), poly(Nisopropylacrylamide), and others, possess lower critical solution temperatures. Polymers with lower critical solution temperatures tend to shrink with the increase in temperature; these hydrogels are known as negative temperature responsive [22]. Poly(acrylic acid) and polyacrylamide polymers inherently imbibe at higher temperatures and shrink at lower temperatures; hence, hydrogels prepared from these polymers are positive temperature responsive. However, tetronics and pluronics are applied to formulate thermally reversible gels [27].

Electrical signal-responsive hydrogels endure swelling and contracting when subjected to an electric field. The demerit of some systems is that due to the charge orientation, one side swells while the other contracts, thus, comprising the stability [28]. Polyelectrolytes, such as poly(2-acrylamido-2-methylpropane sulphonic acid-co-n-butlymethacrylate), are usually employed to formulate these systems [29,30].

pH-responsive hydrogels tend to release or accept protons depending on the pH of the site. Researchers have studied these systems extensively and reported encouraging results. Poly(N,N′ -diethylaminoethylmethacrylate) is ionized at a low pH, unlike poly(acrylic acid), which ionizes at a higher pH. However, polycations tend to swell less at neutral pH [31,32].

In enzyme-responsive systems, a suitable enzyme is considered to trigger a release or deliver at a precisely targeted site where the enzyme is operational at a specific temperature or pH [33]. Enzyme-responsive hydrogels are usually prepared from cellulose and other suitable polymers that facilitate the macromolecular networks and function in a controlled environment [34]. The most explored enzymatic stimuli-responsive system consists of a triggerable agent (usually a polymer or a lipid) into which a therapeutic molecule is incorporated. Indeed, this active agent is sensitive to swelling or degradation when it reaches the target site. Some reported enzymes include protease- and glycosidase-based catalyzed enzymatic reactions [35].

Other intelligent systems include light-responsive hydrogels that are functional in ophthalmic delivery systems. These systems are responsive to light and other stimuli, including pressure, thrombin, antigen, and so forth [36–38].

#### *2.2. In Situ Gels*

Over the years, in situ gels have exhibited tremendous benefits in controlled drug delivery systems and emerged as a significant intelligent drug delivery technology [39]. These incredible systems remain in a liquid state at room temperature and achieve a sol-gel state when exposed to any biological environments, such as altered pH and temperature. In other words, in situ gelling is a spontaneous gelation process at a specific site postadministration [40].

This system accommodates numerous routes of administration, namely ocular, oral, intranasal, vaginal, rectal, depot system, etc. In situ systems have proven their benefits, including prolonged residence time at the site of application. As a result, there is a marked reduction in dosage regimen. Good thixotropic properties expedite the flexibility for formulation development. Rapid absorption and onset can be easily achieved. As a result, the therapeutic benefits can be achieved at lower doses with minimal side effects. Besides, they expedite systemic circulation, avoiding localized hepatic circulation, and targeting can be accomplished [41,42].

In situ gel systems principally work on stimuli such as temperature alteration (chitosan, poloxamer), ion activation (sodium alginate), pH changes (carbopol), solvent exchange, and environmental factors. The gels are ideally dependent on physical or chemical mechanisms. The physical mechanism constitutes of imbibing liquids, mainly water and diffusion, and absorbing water by gel polymers in site-specific locations. While diffusion entails the solvent penetration from the polymer solution to the neighboring tissues, the polymer solidifies. Temperature–responsive, pH-responsive, enzymatic cross-linking, and ionic cross-linking are effective mechanisms that govern the precipitation of solids in gel systems [43].

The pH-responsive systems include fewer polymers such as cellulose acetate phthalate, carbopol, pseudolatexes, polyethylene glycol, and polymethacrilicacid. The temperatureresponsive systems primarily form gels with temperature variations and they include polymers such as pluronics, chitosan, xyloglucans etc. [44,45]. Enzymatic cross-linking is governed by natural biological enzymes. The rate of gel formation is proportional to the enzyme concentration. Insulin delivery was studied with a smart stimuli-responsive delivery system and exhibited positive outcomes, e.g., in a study reported by Podual et.al., where the glucose oxidase enzyme was employed to facilitate the release [46–49]. In

ionic cross-linking, different ions dictate the phase transition of polymers. Gellan gum, carrageenan, and alginic acid are a few ion-responsive polymers. Several natural polymers are available in nature, such as carrageenan, which transform to a gel state on exposure to ions. Gellan gum is predominantly available as Gelrite (commercially available). It is an anionic polysaccharide that instinctively forms a gel in the presence of Mg++, Na<sup>+</sup> , K<sup>+</sup> , and Ca++. Electromagnetic radiation is applied to facilitate in situ gels in photo-polymerization techniques [50,51]. This is achieved by injecting reactive macromers or a solution with initiators and monomers into the desired tissue site. To initiate photo-polymerization, specific polymerizable functional groups and acrylate or similar macromers that undergo dissociation in the presence of a photo-initiator are subjected to radiation. In ultraviolet photo-polymerization, a ketone, such as 2,2 dimethoxy-2-phenyl acetophenone, is used as the initiator, while camphorquinone and ethyl eosin initiators are used in visible light systems [52–55].

#### *2.3. Emulsion Gels*

An emulgel is an amalgamation of gel technology and emulsions. This system offers controlled release, especially in topical formulations, as the therapeutic molecules are loaded in a dual delivery system emulsion and a gel core. The fusion of these dual delivery systems overcomes the demerits of these conventional systems, such as stability and drug loading [56]. Emulgels are prepared by incorporating gelling agents in the continuous (usually water) emulsion phase. Compared to other gels, these systems facilitate higher entrapment efficiency, desired thixotropic behavior, and better patient compliance. The formulation of emulgels is achieved by loading the drug-loaded emulsion into a pre-gel and applying a shear to achieve a homogenous gel system [57]. Gelling agents, emulsifiers, water, and penetration enhancers (primarily for topical formulations) are the fundamental components of emulgels. Polyethylene glycol, tweens, spans, and so forth are utilized as emulsifiers, while carbopols, including Hydroxypropyl methylcellulose (HPMC), are used as gelling agents. Menthol, oleic acid, etc., are mostly effective penetration enhancers. Emulgels are amphiphilic and enable the load of both hydrophilic and lipophilic moieties. Furthermore, the bioavailability of specific molecules was enhanced by lowering the globule size to a micron (µm). Microemulsions are isotropic, clearer, and stable systems. As a result, the increased effective surface facilitates a higher bioavailability [58]. These systems are proven better than emulgels due to the significant increase in the penetration of topical formulations and better compliance [59].

#### *2.4. Microgels*

Microgels are described as gels that have a size range in microns (µm). In contrast to typical gels, these systems have cross-linking structures in microns (µm) and are colloidal dispersions [60]. The molecular arrangements are different in these systems, in comparison to regular gels. Electric charge, polymer–water bonding, and cross-link density are a few factors that underlie the swelling capacity of these systems [61]. The internal forces impart steadiness to the microgels. Similar to microgels, stimuli-responsive methods yield novelty. The colloidal nature of microgels augments characteristics such as controlled delivery of therapeutic molecules, high responsiveness to stimuli, extrudability, mass transport, and high therapeutic efficacy with minimal adversity [62]. The microgels can be created by reducing the size of macrogels by applying a shear or by employing monomers or polymers. The basic underlying principles for formulating are emulsification, where a pregel is prepared in the oil phase, followed by polymerization, yielding microgels. In nucleation, the adjacent particles (in the solvent) initiate the nucleation process and deliver homogeneous microgels [63]. No external forces are required. In contrast, complexation, where complexes are formed in water, can be achieved by adding two mild polymers to water. Another peculiar method can be adopted for developing microgels by the mere addition of polyelectrolytes, which are oppositely charged and at diluted concentrations yield colloidal dispersion [64].

#### *2.5. Nanogels*

Nanogels are mostly hydrogel polymeric network dispersions with particles lying in the nanometer range (nm). Nanogels allow chemical modifications to facilitate ligand targeting and triggered release. This system is a blend of colloidal dispersions and cross-linking networks of polymers [65]. Nanogels are capable of intracellular delivery with enhanced cellular concentrations without significant cellular toxicity [66]. Other advantages include rapid swelling, sharp responsiveness to stimuli, high bioavailability, accommodating more therapeutic moieties, avoiding renal clearance, and remaining undetected by opsonins. Besides, nanogels have excellent stability and accommodate highly lipophilic moieties [67]. Distinctly, nanogels can be prepared by chemical cross-linking (emulsion polymerization), pulse radiolysis, and photopolymerization. In the first method, the cross-linking agent, monomers, surfactant, and water are added to an organic phase. Later, this organic phase is irradiated and purified. In pulse radiolysis, ionizing radiation is irradiated onto polymers in suitable solvents to promote internal structural reorientations of polymer radicles to yield nanogels. In the photopolymerization technique, monomers are exposed to UV radiation. Initiators, cross-linking agents, or surfactants are not required for this method; hence, pure nanogels are produced [68,69].

Similarly, other procedures include heterogenous controlled radical polymerization. Recently, developed techniques, including reversible addition–fragmentation chain transfer and atom transfer radical polymerization, have been instrumental in developing polymerconjugate systems. Chemical cross-linking is more commonly employed to formulate varied nanogels and is further classified into Michael addition reactions, carbodiimide coupling, and free radical polymerization [70]. Hydrophilic monomers are polymerized in the presence of varied cross linkers to yield synthetic nanogels. Self-assembly of polymers includes an aggregation of the hydrophilic polymers by electrostatic interactions, hydrophobic bonds, or hydrogen bonding in an aqueous medium. These systems accommodate large molecules; thus, they are effective in incorporating macromolecules, such as proteins and peptides [71].

#### *2.6. Vesicular Gels*

Vesicular gels are composed of carrier systems deliberate with amphiphilic molecules comprising lipids, surfactants, and co-polymers [72]. The carrier system consists of a hydrophilic core within an amphiphilic bilayer [73]. Vesicular drug delivery has been on the rise in recent studies for its versatility and broad application in drug delivery, cosmetics, etc. Vesicular gels have exhibited significant results in topical drug delivery. A hydrogel matrix can be incorporated with preformed vesicles in a simple technique [74,75].

#### 2.6.1. Liposomal Gels

Liposomes are a very effective and successful novel delivery system. One significant advantage of these systems is that they possess biosimilar structures with desirable properties [76]. Their amphiphilic properties enable the incorporation of both lipophilic and hydrophilic therapeutic molecules. These carriers are biodegradable, exhibit no toxicity, and support localization and site-specific release. They are capable of infiltrating several bio-obstacles otherwise difficult to any conventional systems [77,78]. However, often the topical applications are limited due to rheological constraints. Traditionally, liposomes can be formulated by the standard technique of lipid film hydration. Other techniques include the French pressure cell method and solvent injection methods. The addition of a gelling agent in the aqueous phase incorporates liposomes into the gel system. Carbopols, xanthan gum, poloxamers, gellan gum, polyvinyl alcohol, and others are explored as gelling agents [79].

#### 2.6.2. Niosomal Gels

Niosomes are similar to the liposomal system; however, these are prepared from non-ionic surfactants. This vesicular system resolves the instability of traditional liposomes due to phospholipids [80]. The bilayer, depending on the preparation technique, yields unilamellar or multilamellar niosomes. The system's integrity relies on the surfactant's chemical composition and on the hydrophilic–lipophilic balance (HLB) value. Primarily, proniosomes, emulsion niosomes, and aspasomes are different types of niosomes. Proniosomes are preliminary niosomes which are devoid of any aqueous phase [81]. These are reconstituted with a suitable buffer to yield niosomes. Proniosomes are advantageous over niosomes regarding dose dumping and stability. Niosomal gels comprise a therapeutic moiety, surfactant, and cholesterol that yield vesicles, which are later loaded into a gel base. However, broad applications of these systems are restricted to achieving controlled release, stealth systemic circulations, and enhanced stability [82,83].

#### 2.6.3. Transferosome Gels

Transferosomes are improved versions of liposomes. These systems overcome various shortcomings of other vesicular systems, including aggregation, dose dumping, and poor permeability [84]. Similar to liposomes, these systems also possess an aqueous core enclosed by a lipid layer. However, these bilayers are modified with edge activators, which enables flexibility. Usually span 80, tween 80, and sodium cholate are proven effective edge activators. Phospholipids, edge activators, alcohol, and hydrating media (buffers) constitute transferosomes [85]. Hydrocolloids are incorporated into buffers to yield transferosome gels. The delivery of proteins and peptides, such as insulin, has shown promising results when incorporated into the transferosome gel system [85–87].

## **3. Novel Gel-Based Delivery Approaches for Delivering Therapeutic**

## **Molecules—Recent Trends** *3.1. Hydrogel Systems*

Various nanocarrier systems have been utilized in drug delivery applications for different diseases. The number of cancer patients is increasing day by day worldwide. However, the therapy for cancer treatment is still adapted as a conventional method, i.e., surgery and radiotherapy followed by chemotherapy. There are several limitations of conventional chemotherapy, which produces long- or short-term side effects and adverse effects. Such chemotherapy delivery to cancer patients has low bioavailability due to poor solubility issues, resulting in adverse effects on the biological systems. Hence, researchers are finding a better way to deliver chemotherapeutics with enhanced bioavailability to resolve these problems safely. The hydrogel drug delivery system is one of the carrier systems that can deliver hydrophilic chemotherapeutics agents and hydrophobic drugs in a sustained and controlled manner. Generally, hydrogels are hydrated in nature and possess self-shrinking and self-swelling characteristics in different biological conditions [88]. Their 3D structural properties enable the efficient encapsulation of chemotherapeutic agents into their internal structure, which protects the drug from degradation either in storage or enzymatically during circulation in biological systems. The advantage of the hydrogel drug delivery system in cancer therapy is that nanogels can be modified according to the response to the cell or the tumor microenvironments [89].

Hydrogels can be functionalized by targeting ligands for enhanced, prolonged, and specific drug delivery, making it a safer carrier system. In general, hydrogels could achieve high drug delivery efficiency in cancer therapy with conformational changes and degradation under specific conditions, such as temperature, pH, redox, and ultrasound [90].

The second advantage is that hydrogel synthesis can be done as per the demand of the drug delivery to particular tumor microenvironments. Internal and external stimuli have been utilized to design hydrogels for the delivery of chemotherapeutic drugs to manage cancer. The thermo-sensitive stimuli hydrogels are the most common used hydrogels. These hydrogels are usually in a gel state at room temperature and are attributed to a low critical solution temperature. Once it is administered into the body, it will change its form into a solution due to the cellular temperature. Usually, poly(N-isopropylacrylamide) or elastin-like polypeptides have been used in thermo-sensitive hydrogels recently [91,92].

Moreover, thermo-sensitive hydrogel in situ sites avoid the accumulation of chemotherapeutic drugs in the liver or spleen, which overcomes the biosafety limitations of drugs [93]. The chemotherapeutic agent, 7-ethyl-10-hydroxycamptothecin or SN-38, has a lower solubility in drug delivery applications. Bai et al. developed a thermo-sensitive liposomal system, which showed a better antitumor effect and reduced systematic toxicities in in vivo models at the same dose of the pure drug [94]. Similarly, alginate-based thermo-sensitive gels enhanced cisplatin's in vivo antitumoral effects through an in situ injection. They spurred the tumor growth up to 95% compared to the control group, along with an increased prolonged survival rate of the animals [95]. Furthermore, in another study, the interaction of ligand–receptor with the hydrogel enhances transcorneal permeability and precorneal retention of the drug activity. Upon topical instillation, dexamethasone and Arggly-asp supramolecular hydrogel increased the transcorneal permeability in rabbits' eyes to treat ocular inflammation [96]. The development of a copolymer from a mono-functional polymer exhibits a good sol-gel transition phase, thereby enhancing water solubility to prolong the mucoadhesive system. The effect of silsesquioxane thermo-responsive hydrogel of FK506 improved drug solubility, biocompatibility, and prolonged retention time by enhancing drug efficacy in a murine dry eye model [97].

These hydrogels can be used for single-drug therapy and combinatorial therapy by coloading with other chemotherapeutics drugs [98,99]. For instance, Doxorubicin (DOX), IL-2, and IFN-g were delivered by a poly (g-ethyl-L glutamate)-poly (ethylene glycol)-poly(gethyl-L-glutamate) (PELG-PEG-PELG) hydrogel. The prepared hydrogel system showed long-term sustained drug release behavior for more than three weeks. The combination therapy enhanced the antitumor effect against B16F10 melanoma cells by inducing cell apoptosis and cell cycle arrest in the G2/S phase. The nanocarriers have not shown any systematic side effects in the xenograft mice model, suggesting an effective and promising approach to drug delivery in melanoma therapies [98].

#### *3.2. Thermosensitive Hydrogels*

Likewise, the thermo-sensitive poly(3-caprolactone)-10R5-PCL hydrogels, co-loaded with tannic acid and oxaplatin to manage colorectal cancer, restricted the CT26 colon cancer growth in a mice model. They improved the survival time of the animals (11). Moreover, the co-delivery of gemcitabine and cisplatin through the PDLLA-PEG-PDLLA hydrogels synergistically improved the anti-cancer efficacy against pancreatic cancer with sustained drug release. The dual drug-loaded hydrogels exhibited superior antitumor effects in the xenograft model than in single-drug therapies [100]. Similarly, when the thermo-sensitive stimuli hydrogel was utilized to co-deliver paclitaxel (PTX) and temozolomide (TMZ), it produced a synergistic effect against glioblastoma cells. The in vivo studies suggested that the combination therapy potentially reduced tumor growth and sustained drug release in mice brains for one month with no apparent side effects [101].

Chitosan-based thermo-sensitive hydrogels, including several polyols, have gained a critical identity that transforms to a hydrogel form upon contact with body temperature from a solution state. A study indicated that a hydrogel formulation that employs a green synthesis approach with chitosan, genepin, and poloxamer 407 has proved to sustain drug release. Brinzolamide-loaded nanostructured lipid carrier was entrapped into a hydrogel matrix using a hot-melt emulsification and sonication method that showed a sustained drug release for a longer duration (24 h) than marketed eye drops (8 h) in the management of glaucoma [102]. Moxifloxacin hydrochloride thermosensitive gel was formulated using chitosan-β-glycerophosphate to advance the ocular delivery. The drug release profile of the formulation was shown to be delivered in a sustained pattern and at a slower rate with a release of 53% in 1 h and 83.3% in 8 h due to the hydrogel's polymeric network, whereas a 75.6% release was identified from the drug solution. Hence, the formulated hydrogel was stated to be biodegradable, safe, and with a more significant drug loading to the administration site to manage bacterial infections [103]. The quaternized chitosan achieved a better swelling property as a therapeutic carrier. The thermo-sensitive transparent quaternized chitosan hydrogel was utilized to release timolol maleate in the management of glaucoma. Hemolysis and cytotoxicity profiles showed good biocompatibility. The in vitro release pattern of timolol maleate from the hydrogel showed a burst release initially and a linear release for one week, showing a sustained pattern. Hence, quaternized chitosan has a promising ability to sustain drug release in the anti-glaucoma model [104]. Similarly, chitosan has been an extensively used polysaccharide to prolong precorneal retention and corneal permeability, though it is poorly soluble in physiological solvents. Therefore, derivates of chitosan (glycol chitosan) have shown superior aqueous solubility in a broad pH range and are employed in ocular drug delivery systems, such as hydrogels, nanoparticles, and films. Hydrogel films for the topical ocular delivery of dexamethasone and levofloxacin were fabricated by utilizing many oxidation degrees of oxidized hyaluronic acid and glycol chitosan. The formulation showed potent activity in decreasing bacterial growth in different strains. Additionally, the formulation downregulated in vitro antiinflammatory activities. Overall, the formulated hydrogel film would serve as a treatment for endophthalmitis with minimal corneal irritation and biocompatibility [105]. In another study, a combination of carbon dots and thermo-sensitive hydrogels was evaluated for their in vitro cellular toxicity after effectively delivering diclofenac sodium to an eye. The in vitro release showed characteristic biphasic release of the drug. Cellular toxicity studies revealed that the formulation has a better cytocompatibility with CD44 targeting and serves as a novel way for ocular delivery of drugs [106].

#### *3.3. Light Stimuli Hydrogels*

Furthermore, light stimuli hydrogels enhanced drug release from hydrogels. The mechanism of photosensitive hydrogels is that they can undergo structural and conformational changes under radiation, ultraviolet, and visible light sources and achieve sol-gel transition [107,108]. Photo stimuli hydrogels have been utilized in the past few years. For example, Fourniols and his co-worker developed a polyethylene glycol dimethacrylatebased photo-polymerized hydrogel for the local and sustained delivery of TMZ in the management of glioblastoma [109]. Similarly, azobenzene, α-cyclodextrin-functionalized hyaluronic acid, and gold nano-bipyramids-mesoporous silica nanoparticles-conjugated polymer-based in situ injectable hydrogels loaded with DOX and stimulated under nearinfrared (NIR) radiation potentially improved the drug localization of drug into the nuclei of the tumor cells [110]. Another example of a NIR stimuli hydrogel was designed by Qui et al. in 2018. They synthesized DOX and black phosphorus-loaded agarose-based hydrogels. The drug release was enhanced after exposure to the light intensity of NIR, which therapeutically increased the anti-cancer efficacy for in vivo experiments [111].

#### *3.4. pH Stimuli Hydrogels*

Another most commonly used hydrogel for cancer therapy is the pH stimuli hydrogel. Due to the diverse microenvironments of tumor cells, the pH of the extracellular matrix of the tumor is in the range of 5.8–7.2 and the lysosomal or intracellular matrix pH is usually 5.5, which is acidic compared to normal cells (pH 7.4) [112,113]. Hence, both intracellular and extracellular acidic conditions stimulated the hydrogels for degradation and drug release at the tumor site. The pH-sensitivity could have been achieved by protonating the polymers' ionizable moiety or acid-cleavable bond break [114].

Usually, the pH stimuli hydrogels act as prodrugs, inactive in normal biological pH (7.4), but once they reach the tumor site, upon the release of the drug, they change to their active chemotherapeutic form, producing its effect. The FER-8 peptide hydrogel loaded with PTX exhibited high drug encapsulation and self-assembly of the peptide at pH 7.4. The hydrogel was stimulated by the acidic conditions of the tumor microenvironment. The PTX-loaded hydrogel significantly accumulated the drug in the tumor site and showed sustained and prolonged retention of PTX through an intratumoral injection. This hydrogel showed enhanced tumor inhibition [115]. The polyacrylic acid-based pH stimuli hydrogel for DOX delivery triggered the specific site of drug delivery with a less

acidic microenvironment. The hydrogels showed improved pharmacokinetics and drug accumulation in a mice xenograft model with significant tumor growth regression and lowered adverse and side effects [116]. Various hydrogels have received attention in the last few years due to their biocompatibility, biodegradability, and low toxicity. The hydrogelbased drug delivery of chemotherapeutic agents has been attended in recent years [27]. Most chemotherapeutic drugs are associated with lower solubility. Hence, efficient drug delivery and significant therapeutic effects cannot be achieved by a lower dose and the involvement of a higher dose in cancer management includes side effects. Therefore, the hydrogel system is the emerging nanocarrier for the delivery of chemotherapeutic agents. Some recent significant research related to novel gel-based systems is shown in Table 1.


**Table 1.** Recent published research for novel gel-based delivery systems.


#### **Table 1.** *Cont.*

#### **4. Descriptive Patents Established for Novel Gel-Based Delivery Systems**

Various publications illustrated the efficacy of novel gels over the last decades. The current section explains the established patents of novel gels. In patent 20210338211, aptamer and hydrogels cross-linked with DNAzyme are used for colorimetric identification of analytes in body fluids through an ocular device [150]. Patent 2021120395 tells that in situ hydrogels provide extended drug release by delivering to a tissue, usually agents with low water solubility [151]. Patent W/O/2021/113515 explains that hydrogels composed of Gelatin–hydroxyphenylpropionic acid (gelatin-HPA), hyaluronic acid–tyramine

(HA-Tyr), catalyzer, cross-linker, or other combinations can treat ocular disorders [152]. Patent 20210069496 describes polymeric formulations, including hydrogels formed by a UV cross-linking method. Here, the hydrogels act as a nasal stimulator that stimulates the lacrimal glands to mimic the production of tears electronically and to manage dry eye syndromes [153]. Patent WO/2021/038279 tells of the invention related to ion-exchange polymeric hydrogels for ocular treatment [154].

Patent 202121042889 talks about etoricoxib, which is formulated as a nanosponge hydrogel for the management of arthritis by the method of emulsion solvent diffusion using a polymeric organic solvent, ethylcellulose eudragit, and an aqueous phase. The formulated nanosponges were evaluated for differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR), polydispersity index (PDI), scanning electron microscopy (SEM), zeta potential, drug content, entrapment efficiency, viscosity, spreadability, in vitro diffusion, irritation test, and in vivo antiarthritic effect. The synthesized formulation proved to be effective as a novel way for managing arthritic pain [155]. In patent 202031000910, a cross-linked protein matrix hydrogel was prepared for topical application in skin regeneration and wound healing [156].

Conductive hydrogels with an adhesiveness method of preparation were invented and discussed in patent 112442194. The process involves dopamine modification of carbon nanotubes and grafting to saccharides, followed by acrylamide mixing and formation of hydrogels in the presence of an initiator and a cross-linking agent. Hydrogel structure, electrical conductivity, adhesiveness, and biocompatibility can be improved by dispersing the modified carbon nanotubes in an aqueous solution to form hydrogen bonds and crosslink with the supramolecules of the hydrogel. Hence, conductive hydrogels can be used in biomedical fields, as well as for human body monitoring and electronic skin, etc. [157].

Patent 20210023121 discloses thrombin-responsive hydrogels for prolonged heparin delivery for auto-anticoagulant regulation in a controlled feedback mechanism. The formulated microneedle, containing a patch, can activate the thrombin and release heparin to avoid blood coagulation. The insertion of a microneedle patch containing hydrogel regulates blood coagulation sustainably in response to thrombin without leakage [158].

Patent 20210393780 discloses the effectiveness of thermo-sensitive polymer–proteinbased hydrogels in the field of cancer therapeutics. The invention is enriched by photosensitizers, dyes, photothermal agents, and drugs. Hence, the invention proved to be less expensive, highly effective, and thermosensitive, resulting in a sustained drug release for targeted delivery [159]. Patent WO/2021/174021 describes a degradable hydrogel system for immunotherapy with an extended-release pattern of an anti-cancer drug linked with a hydrogel matrix synergistically for cancer treatment [160]. Patent 9758/CHENP/2012 explains self-assembling peptides and their use in hydrogels for the adhesion, proliferation, differentiation of neural stem cells, and their auto-healing properties. They are reported to be non-toxic in central nervous systems, as well as to avoid bleeding and have faster nervous regeneration [161]. Patent 20140286865 explores di-block co-polypeptide synthetic hydrogels in the central nervous system [162].

#### **5. Conclusions**

In this review, the authors have focused on recent trends in novel gel-based drug delivery systems and their applications. In recent times, these novel systems have exhibited proficient delivery of multiple therapeutic moieties and expressed desired properties and functions, such as selective targeting. The systems offer abundant benefits compared to conventional drug delivery approaches including controlled drug release, high drug loading, biocompatibility and biodegradability, and enriching patient compliance and comfort. The responsive gel technology is significant in formulating intelligent delivery systems; these systems respond to stimuli such as pH, temperature, enzymes, and so forth. Hence, these systems are site-specific and facilitate the controlled release of therapeutic molecules. Although, fundamentally, these systems have proven capabilities for effective drug delivery, there is a scope to explore new polymers to fabricate novel gels; therefore, the

currently employed components can be modified. Furthermore, recent studies revealed that employing plant extracts to develop novel delivery systems has enabled the development of various drug delivery systems with non-toxic procedures. The formulation of substances of natural origin has advantages in various magnitudes on the environment. Over the years, the evolution of green chemistry has provided more eco-friendly procedures resulting in minor harm to nature. Current findings suggest promising results for green synthesised delivery systems over conventional systems. Green technology does not require common harmful chemicals. Instead, this technology uses biological and biocompatible reagents. Besides, reports suggest that green technology delivery systems have better stability than traditional methods. Formulations developed with green technology employing plant extracts and biomaterials, such as proteins or peptides, yielded non-toxic and highly biocompatible systems; thus, they have resolved the most concerning issue with traditional delivery systems, i.e., toxicity. Green technology will play a significant role in formulating novel delivery systems. However, we require further understanding of the development of systems with green technology.

**Author Contributions:** Conceptualization, T.K.S., V.N.G.; Formal analysis, V.N.G.; Resources, T.K.S., S.C., S.M., H.K., P.C.; Data curation, T.K.S., S.C., S.M., H.K., P.C.; Writing original draft preparation, T.K.S., S.C., S.M., H.K., P.C.; Writing review and editing, V.J., B.V. and D.V.G.; Supervision, V.N.G., V.J., B.V. and D.V.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work received no external funding. The APC is supported by JSS Academy of Higher Education & Research, Mysuru.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The author(s) express deep sense of gratitude towards JSS College of Pharmacy, JSS Academy of Higher Education & Research (JSS AHER), Mysuru for their constant support and motivation.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


## *Article* **Interaction of TX-100 and Antidepressant Imipramine Hydrochloride Drug Mixture: Surface Tension, <sup>1</sup>H NMR, and FT-IR Investigation**

**Malik Abdul Rub 1,2, \*, Naved Azum 1,2 , Dileep Kumar 3,4, \* and Abdullah M. Asiri 1,2**

	- Ho Chi Minh City 700000, Vietnam

**Abstract:** Interfacial interaction amongst the antidepressant drug-imipramine hydrochloride (IMP) and pharmaceutical excipient (triton X-100 (TX-100-nonionic surfactant)) mixed system of five various ratios in dissimilar media (H2O/50 mmol·kg <sup>−</sup><sup>1</sup> NaCl/250 mmol·kg <sup>−</sup><sup>1</sup> urea) was investigated through the surface tension method. In addition, in the aqueous solution, the <sup>1</sup>H-NMR, as well as FT-IR studies of the studied pure and mixed system were also explored and deliberated thoroughly. In NaCl media, properties of pure/mixed interfacial surfaces enhanced as compared with the aqueous system, and consequently the synergism/attractive interaction among constituents (IMP and TX-100) grew, whereas in urea (U) media a reverse effect was detected. Surface excess concentration (*Γmax*), composition of surfactant at mixed monolayer (*X σ* 1 ), activity coefficient (*f* <sup>1</sup> <sup>σ</sup> (TX-100) and *<sup>f</sup>* <sup>2</sup> <sup>σ</sup> (IMP)), etc. were determined and discussed thoroughly. At mixed interfacial surfaces interaction, parameter (*β* <sup>σ</sup>) reveals the attractive/synergism among the components. The Gibbs energy of adsorption (∆*G* o ads ) value attained was negative throughout all employed media viewing the spontaneity of the adsorption process. The <sup>1</sup>H NMR spectroscopy was also employed to examine the molecular interaction of IMP and TX-100 in an aqueous system. FT-IR method as well illustrated the interaction amongst the component. The findings of the current study proposed that TX-100 surfactant could act as an efficient drug delivery vehicle for an antidepressant drug. Gels can be used as drug dosage forms due to recent improvements in the design of surfactant systems. Release mechanism of drugs from surfactant/polymer gels is dependent upon the microstructures of the gels and the state of the drugs within the system.

**Keywords:** amphiphilic drug; nonionic surfactant; surface property; thermodynamic; chemical shift; FT-IR

#### **1. Introduction**

Gels are used for various applications based on their drug-loading properties, rheological properties, and release mechanisms. Drugs can either be soluble in water with no interaction through any of the constituents, electrostatically/hydrophobically tied with polymer, or soluble within micelles and polymer/surfactant associates. The use of surfactant/polymer systems for gene therapy has a great deal of promise, and certain polymers can interact with the natural (nonionic) surfactant, which can be utilized to lock in bile salts for controlling cholesterol levels in the body. The interfacial/micellar characteristics of amphiphiles mixtures have been broadly studied due to their extensive applications, for instance, hydrate inhibitors, biologicals, foaming, in fabric moderating, pharmaceutics, improved oil recovery procedure, and so forth [1–3]. In aqueous/non-aqueous solvent, the

**Citation:** Rub, M.A.; Azum, N.; Kumar, D.; Asiri, A.M. Interaction of TX-100 and Antidepressant Imipramine Hydrochloride Drug Mixture: Surface Tension, <sup>1</sup>H NMR, and FT-IR Investigation. *Gels* **2022**, *8*, 159. https://doi.org/10.3390/ gels8030159

Academic Editor: Hiroyuki Takeno

Received: 28 January 2022 Accepted: 2 March 2022 Published: 4 March 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

surfactant monomers (comprising hydrophobic and hydrophilic parts into single molecules) were orientated into an associated form after surpassing a certain concentration into the solution (solvent) and formed the associate structure, called the micelle. The corresponding concentration is symbolized as the critical micelle concentration (*cmc*) [3–6]. Surfactant micelles revealed a considerable role in the solubilization of several hydrophobic materials including drugs [3,7]. Surfactant also acts as a drug carrier in combination with a specific additive, and therefore, extensive inspections of the influences of several additives (organic and inorganic) on the association performance of the drug are needed [3,7]. As compared with singular surfactant micelle formation, the mixed surfactants have substantial considerable properties in a variety of features [3]. Usually, a mixed surfactants system (ionic amphiphile with other ionic or nonionic amphiphiles) has smaller surface energy, higher solubilization capability, and smaller *cmc* along with higher surface activities as compared to the singular surfactants because of the attractive interaction/synergetic influence [3,8]. To diagnose osteoarthritis, Yin et al. [9] have made significant progress in eliminating major hurdles to using extracellular vesicles for delivery and as markers. Osteoarthritis therapeutics can be delivered effectively via extracellular vesicles because of their size, surface expression patterns, low immunogenicity, and low cytotoxicity.

Within various kinds of surfactants (cationic/anionic/nonionic), the non-ionic surfactant is valued as the best one for safe drug delivery, as they are physiologically more supportable than ionic surfactant [7]. TX-100 is one of the most applied surfactants in bio-chemical and chemical practices. The head groups of non-ionic surfactants consist of no electrical charge; therefore, they are generally soluble in water through H-bonding formation between the hydrophilic parts of the surfactant with water. Triton X-100 (TX-100) non-ionic surfactant has a huge industrial significance applied in the formulation of foams and found several applications in the pharmaceutical sciences for purpose of cleaning and as an ingredient in a few curative products [10,11]. TX-100 comprises a hydrophilic chain of 9 to 10 ethylene oxide units coupled with an aromatic ring, having a branched hydrocarbon chain. Different properties (interfacial, micellization, drugs solubilization ability, clouding property, etc.) of TX-100 in the occurrence of charge amphiphiles have been analyzed by means of experimental methods [3,12,13]. TX-100 varies from other conventional nonionic surfactants because their hydrophilic portion was found to be longer compared with the hydrophobic section of the monomer [14]. Herein, the interaction of TX-100 with antidepressant IMP was evaluated by means of different techniques. The mixed system of IMP+TX-100 reveals a compact packing at the surface as well as higher interfacial activity.

At a higher concentration, numerous amphiphilic drugs also formed a micellar structure in a similar manner to a conventional surfactant [15,16]. Pure amphiphilic drugs self-association studies, for any particular purpose are usually out-of-focus due to their high *cmc*, because of the use of a high amount of a drug, which might create numerous side effects [17]. Therefore, amphiphilic drugs are generally used in combination with additives such as surfactant, hydrotropes, bile salts, etc., as a drug carrier that generally forms mixed micelles [8,15]. As a mixture, the *cmc* value reduced more than 10 times. Hence, a very low quantity of drug is used along with a mixed micellar system to raise the absorption of numerous drugs [8].

Imipramine hydrochloride (IMP) is an amphiphilic tricyclic antidepressant drug that has two main parts, one is a large rigid tricyclic hydrophobic ring (tail) and the other one is a small alkyl amine part (head) and endures aggregation but higher concentration [15]. This drug color is white to off-white, odorless compound, and is employed to treat depression. The nature of IMP drug is protonated (cationic) at a lower pH range (below 7) and deprotonated at a high range (above 7) of pH (p*K*<sup>a</sup> = 9.5) [15]. Apart from their uses to heal depression, this drug also indicated some unwanted impact. Consequently, to lessen the unwanted impact of IMP, mixed micellization investigation of IMP with TX-100 (as a drug carrier) (Scheme 1) was conducted in different media by means of the several methods.

**Scheme 1.** Mixed micelle formation of the IMP+TX-100 mixed system.

*Γ β*<sup>σ</sup> <sup>σ</sup> σ ∆ୟୢ ୭ ∆ୣ୶ − − Previously, our group have examined solution (bulk) properties (mixed micellization behavior) of pure and mixed system of IMP and TX-100 in water, NaCl, and urea media [18] and the current study is an extension of our previous work [18]. Herein, the interfacial properties of IMP and TX-100 mixture were evaluated by tenstiometic method in different media, along with <sup>1</sup>H NMR and FT-IR spectroscopy, which were also employed to evaluate the interaction amongst IMP and TX-100 in an aqueous system. Combining IMP with TX-100, might enhance drug characteristics, such as their solubility along with stability in living atmospheres [8,19]. Previously, in an aqueous solution, Alam and Siddiq [20] examined the association and surface behavior of an IMP drug and TX-100 mixed system by differing the mole fraction of a drug by tensiometric method. Irrespective of surface tension and <sup>1</sup>H NMR methods, the FTIR study of the akin system in aqueous media was also investigated to crisscross the reliability of the interaction between IMP drug and TX-100. <sup>1</sup>H NMR of IMP+TX-100 mixture in five different ratios has been investigated to explain the mechanism of IMP and TX-100 interactions. Several theoretical models regarding the interfacial behavior are employed to illustrate the mixed monolayer formation of the drugsurfactant mixed system in three different media. Various parameters, such as surface excess concentration (*Γmax*), composition of constituent at mixed monolayer and the interaction parameter (*β* <sup>σ</sup>) at interface, activity coefficient of employed ingredients (*<sup>f</sup>* <sup>1</sup> <sup>σ</sup> (TX-100) and *f* 2 <sup>σ</sup> (IMP)) at the boundary, packing parameter, etc., at the mixed monolayer, have been assessed and discussed [3,21]. Different thermodynamic functions (Gibbs's energy of adsorption (∆*G* o ad), minimum free energy (*Gmin*), excess free energy at mixed monolayer (∆*G σ* ex)), and chemical shifts by <sup>1</sup>H NMR study have also been thoroughly evaluated and debated. According to the current study, the results have relevance to model drug delivery, but no direct evidence can be drawn for drug delivery. As a result of this study, drugs and their possible carriers are examined physiochemically using various theoretical models, which is vital since the surfactant may also be utilized as a drug carrier. In addition, the choice of 50 mmol·kg−<sup>1</sup> NaCl and 250 mmol·kg−<sup>1</sup> urea concentration was not based on any specific reason other than to examine the effects of salt and urea that are normally found in human being. To provide knowledge (thermodynamic and additional) for the widely used drug-surfactant combinations in the absence and presence of NaCl and urea in drug delivery, our primary goal had been to exhibit how the two ingredients interacted in the aqueous system as well as in salt and urea media. Further enhancement of drugsurfactant conjugate delivery systems is possible if salt/urea are present as their presence increases/decreases the spontaneity of the mixture.

#### **2. Results and Discussion**

#### *2.1. Characteristics at the Air-Interfacial Surfaces of Pure and Mixed System*

Amphiphiles are likely to settle at the air-interfacial surface as compared with the bulk solution. Gibbs's adsorption equation [22] is employed to assess a variety of surface parameters of drug–surfactant mixed system. All interfacial parameters were evaluated by using the surface tension plot given in our previous work [18]. The adsorbed quantity of molecules in each unit area of the surface is computed through the assistance of Gibbs adsorption equation [22]. The surface excess concentration (*Γmax*) along with minimum area per monomer (*Amin*) values in aqueous/non-aqueous media were determined utilizing the subsequent equations [3,22]:

$$T\_{\text{max}} = -\frac{1}{2.30 \text{\eth} nRT} \left(\frac{\partial \gamma}{\partial \log(\mathbb{C})}\right) (\text{mol} \cdot \text{m}^{-2}),\tag{1}$$

$$A\_{\rm min} = \frac{10^{20}}{N\_A \,\Gamma\_{\rm max}} \,\mathrm{(\,\text{\AA}^2\text{)}.\tag{2}$$

Here, the *<sup>γ</sup>*, *<sup>C</sup>*, *<sup>T</sup>*, *<sup>n</sup>*, *<sup>R</sup>*, and *<sup>N</sup><sup>A</sup>* is the surface tension (mN·m−<sup>1</sup> ), employed concentration of IMP, TX-100, or IMP+TX-100 mixtures, temperature, whole number of solute species obtained during adsorption, gas constant, and Avogadro number, respectively [3]. The *n* is considered 2 and 1 in the case of individual IMP and TX-100, respectively. However, in mixtures, *n* values were assessed using term: *n* = *n*1*X σ* <sup>1</sup> + *n*<sup>2</sup> 1 − *X σ* 1 [3], where *n<sup>1</sup>* = number of species in component 1 and *n<sup>2</sup>* = number of species in component 2 after ionization. *X σ* 1 = interfacial composition of component 1 at the mixed surface (Table 1). Throughout the study, the first component, or component 1, is used for TX-100 and the second component, or component 2, is used for IMP. The slope = *∂γ*/*∂log*(*C*) value is attained from the *γ* vs. *log*(*C*) plot of any fixed concertation in all cases.

**Table 1.** Different interfacial parameters for IMP+TX-100 mixture in several media at 298.15 K <sup>a</sup> .


<sup>a</sup> *<sup>A</sup>*<sup>1</sup> <sup>=</sup> *<sup>A</sup>min* of TX-100 and *<sup>A</sup>*<sup>2</sup> <sup>=</sup> *<sup>A</sup>min* of IMP. *<sup>A</sup>*<sup>1</sup> = 46.10 (in aqueous), 60.15 (in NaCl), 50.04 ´Å<sup>2</sup> (in urea). *A*<sup>2</sup> = 129.95 (in aqueous), 187.41 (in NaCl), 133.24 ´Å<sup>2</sup> (in urea).

In the ideal state, the minimum surface area per molecule (*A id*) was evaluated by means of Equation (3):

$$A^{id} = X\_1^\sigma A\_1 + (1 - X\_1^\sigma) A\_2. \tag{3}$$

Here, *A*<sup>1</sup> and *A*<sup>2</sup> = per monomer minimum head group area of surfactant and IMP correspondingly. The assessed *Γmax*, *Amin* and *A id* value of individual and mixed components (IMP, TX-100, and IMP+TX-100) in the existence of different media were revealed in Table 1.

Table 1 showed the value of *Γmax* and *Amin* of individual TX-100 in the aqueous system, which was found to be 36.02 mol m−<sup>2</sup> and 46.10 ´Å<sup>2</sup> respectively, revealing that their value is in the same range with the previously reported value [23]. The parameter *A*min value

showed the opposite trend with the *Γmax* value means, as each parameter was in reverse with each other. The *Γmax* value of singular IMP obtained lesser than the *Γmax* value of pure TX-100 means, and the *Amin* value showed the opposite behavior. This obtained behavior viewed that TX-100 molecules favored a compacted or strongly packed arrangement at the air-solvent interface as compared with IMP regardless of the media used, and therefore TX-100 showed more surface activity. The value of *Γmax* of the IMP+TX-100 mixed system was found above the *Γmax* value of singular IMP but was obtained below the *Γmax* value of TX-100, so we can observe that mixed system surface activity was found higher than pure IMP but less than pure TX-100. In an aqueous system, the *Γmax* value of the IMP+TX-100 mixed system was found to increase with an increase in *α*<sup>1</sup> of TX-100, observing that the mixed system surface activity increases with the increase of the composition of TX-100 in the solution mixture. However, in the presence of NaCl or U, the *Γmax* value of the IMP+TX-100 mixture has not viewed a specific trend, nor did *Amin*, since *Amin* is inversely proportional to *Γmax*.

The *Γmax* value in NaCl media of IMP+TX-100 mixtures was achieved higher than other employed media (H2O or U). The electrostatic repulsions between the ingredient's monomers decreased in NaCl media, observing that the efficiency of the molecules′ existence at the interfacial surface increased and high compactness of IMP+TX-100 mixtures existed. However, in U solvent, pure IMP, and TX-100, *Γmax* value found less but does not show any proper trend for mixed system.

The *A id* value of IMP+TX-100 mixtures were observed to be higher than experimental *Amin*, implying that the space taken by apiece monomers was found below as expected for their ideal behavior. For mixtures (IMP+TX-100), the *Amin* value was obtained below the value of *Amin* of pure IMP. This result indicates that the introduction of TX-100 in the solution of IMP causes decreases in the repulsive force between IMP monomer molecules, and hence the value of mixture *Amin* decreased. Figure 1 showed the *Γmax*/*Amin*/*A id* vs. α<sup>1</sup> plot for IMP+TX-100 mixture in diverse media (filled, open, and half-filled symbols represent *Γmax*, *Amin*, and *A id*, respectively), which shows the comparison of different surface parameters graphically.

*Γ* α *Γ* **Figure 1.** Plot of *Γmax*/*Amin*/*A id* against α<sup>1</sup> plot for IMP+TX-100 mixture in different media (filled, open, and half-filled symbols represent *Γmax*, *Amin*, and *A id*, respectively).

*γ γ* The parameter surface tension value at the *cmc*, is symbolized via *γcmc* and the obtained value is depicted in Table 1. The *γcmc* value of singular and mixed system (IMP+TX-100) in aqueous and NaCl media were taken from the graph of our group's earlier published

*π γ − γ*

−

*γ* γ *γ*

*α*

*γ*

γ π

π

*π*

−

work [18]. For individual TX-100 and IMP+TX-100 mixtures, the value of *γcmc* was found close to each other's means in the same range, irrespective of the solvent employed. However, their value for individual IMP was found quite higher. The surface parameters-surface pressure at the *cmc* (*πcmc*) and the adsorption efficiency, i.e., *pC*20, was also exploited for individual ingredients and the IMP+TX-100 mixtures in all media. At the *cmc,* the surface pressure (*πcmc*) parameter was explored by means of Equation (4) [3].

$$
\pi\_{cmc} = (\gamma\_0 - \gamma\_{cmc}).\tag{4}
$$

In Equation (4), *γ*<sup>0</sup> signified the pure solvent surface tension, and *γcmc* indicated the *γ* at the *cmc* of the single and mixed components. All assessed values of *γcmc* and *πcmc* are presented in Table 1. The obtained *πcmc* is lowest for IMP irrespective of the media utilized but was found to be close to each other for individual TX-100 and the IMP+TX-100 mixture [24].

Another parameter, called *pC*20, allowed the adsorption efficiency of the constituents at the interfacial surface. This parameter is demarcated as the negative logarithm of the concentration of monomer(s), as the individual solvent surface tension is lessened by 20 mN m−<sup>1</sup> (*C*20) [3]:

$$p\mathbf{C}\_{20} = -\log \mathbf{C}\_{20}.\tag{5}$$

The higher the *pC*<sup>20</sup> value, the larger the amphiphile efficiency for adsorption (higher surface activities) because a smaller amount (volume) of prepared solutions is needed to condense the solvent surface tension by 20 mN·m–1. The obtained value of *pC*<sup>20</sup> of IMP was considerably lower to a large extent, as compared to *pC*<sup>20</sup> achieved for individual TX-100 regardless of the solvent used, which again confirmed that the IMP drug was less surfaceactive as compared with TX-100 (Table 1). This obtained phenomena showed that TX-100 has better adsorption ability along with being more effective in surface tension reduction of the solvent [24]. The *pC*<sup>20</sup> value for IMP+TX-100 mixed systems was higher than individual IMP, observing that the mixed systems were more surface-active as compared with IMP, and their value increased with an enhancement in *α*<sup>1</sup> of TX-100, but blended systems *pC*<sup>20</sup> value was found near the *pC*<sup>20</sup> value TX-100 (Table 1). Pure species, as well as IMP+TX-100 mixtures *pC*<sup>20</sup> value enhanced in the existence of NaCl because of the better surface activity in NaCl media as compared with aqueous system and the reverse trend, which was detected in the existence of U (Table 1).

#### *2.2. Composition of Component and Interaction Parameters at the Air-Interfacial Surfaces*

Before the start of micellization, at the interfacial surface, a mixed monolayer formation took place through adsorption phenomena. Rosen′ s theory [25] was applied to assess the composition of the constituent at mixed monolayer as well as the interaction parameter (*β* <sup>σ</sup>) at the interface through subsequent equations.

$$\frac{\left(\mathbf{X}\_{1}^{\sigma}\right)^{2}\ln\left(\boldsymbol{\alpha}\_{1}\mathbf{C}/\mathbf{X}\_{1}^{\sigma}\mathbf{C}\_{1}\right)}{\left(1-\mathbf{X}\_{1}^{\sigma}\right)^{2}\ln\left[\left(1-\boldsymbol{\alpha}\_{1}\right)\mathbf{C}/\left(1-\mathbf{X}\_{1}^{\sigma}\right)\mathbf{C}\_{2}\right]} = 1,\tag{6}$$

$$\beta^{\sigma} = \frac{\ln\left(\alpha\_1 \mathbb{C} / X\_1^{\sigma} \mathbb{C}\_1\right)}{\left(1 - X\_1^{\sigma}\right)^2}. \tag{7}$$

In Equation (6), the *X σ* 1 = composition of the surfactant in the mixed monolayer (IMP+TX-100), and in Equations (6) and (7) *C*<sup>1</sup> = TX-100 concentration (first component), *C*<sup>2</sup> = IMP concentration (second component), and *C* = mixed monolayer concentration (IMP+TX-100) at different *α*1, which is used for lessening the surface tension of a solvent of any selected value for all cases (pure and mixture).

The assessed values of *X σ* 1 and *β <sup>σ</sup>* of all systems are itemized in Table 1. Herein, the *X σ* 1 (TX-100 composition at the mixed component surface) values were obtained amid 73% to 92% in all studied media, displaying that mainly TX-100 comprises the mixed

monolayer. By increasing the *α*<sup>1</sup> value, the *X σ* 1 value was exhibiting any regular behavior (i.e., increase or decrease), but overall, their value was found to be higher at higher *α*1. The *X σ* 1 value attained higher values in the NaCl system as compared with the aqueous solution at all *α*1, displaying that salt diminished the repulsive forces existing amid components. Accordingly, there was an affection for early micellization prompted via the progressively hydrophobic atmosphere.

The *β <sup>σ</sup>* values possess three possibilities: (1) *β <sup>σ</sup>* = 0 for an ideal monolayer, which means no interaction among the mixture ingredients, (2) *β <sup>σ</sup>* > zero for antagonistic interactions, whereas (3) *β <sup>σ</sup>* < zero signifies the supremacy of attractive or synergistic interactions amongst mixture ingredients.

The *β <sup>σ</sup>* values obtained was negative in all cases, revealing the existence of attractive interactions or synergism at the interfacial surface (Table 1) [26,27]. This occurred because of the closely packed formation of the mixed monolayer, owing to the clearly interactive forces amongst the ingredients at the interface in all utilized media. The decrease in electrostatic repulsion amongst molecules of both components applied its impact more at the planar interfacial surface, as compared in convex micelles [3]. The negative value of *β <sup>σ</sup>* was revealed interactions amongst the constituent allocated to ion-ion dipole as well as hydrophobic interactions irrespective of the employed media. Consequently, the merger of these forces overwhelmed all electrostatic repulsion amongst the ingredients. In NaCl or U media, the *β <sup>σ</sup>* value was not displaying a somewhat unique trend, but was found to be negative in the whole system (Table 1).

IMP+TX-100 mixtures display higher surface activity together with a much lesser *cmc* value as compared with individual IMP. The higher interaction amid the ingredients in the solution mixtures does not only serve as evidence for synergism in binary mixed system. Synergism in any mixed system at an interfacial surface occurs only if the subsequent circumstances are met [3]: (a) *β* <sup>σ</sup> value should be below 0, and (b) <sup>|</sup>*<sup>β</sup> σ* | value should be more than ln(*C*1/*C*2) value, otherwise attractive mixed monolayers will be found. By viewing these results, it is shown that for all systems only first the circumstance was satisfied (Table 1) However, the second circumstance was not fulfilled in almost all cases. Therefore, attractive interactions were observed irrespective of the type of media employed for the surface tension reduction efficiency.

Akin to mixed micelles, the value of the activity coefficient of the employed ingredients (*f* <sup>1</sup> <sup>σ</sup> (TX-100) and *<sup>f</sup>* <sup>2</sup> <sup>σ</sup> (IMP)) at the boundary was also evaluated via subsequent equations [28]:

$$f\_1^\sigma = \exp\left[\beta^\sigma (1 - X\_1^\sigma)^2\right],\tag{8}$$

$$f\_2^{\sigma} = \exp[\beta^{\sigma}(X\_1^{\sigma})^2]. \tag{9}$$

Table 1 shows that both *f* <sup>1</sup> <sup>σ</sup> (TX-100) and *<sup>f</sup>* <sup>2</sup> <sup>σ</sup> (IMP) values are obtained below one irrespective of the media employed [28]. Therefore, the system showed nonideal behavior as well as experienced attractive interactions amid the applied species at the boundary of the air-solvent. The results also showed that the *f* <sup>2</sup> <sup>σ</sup> was found to be lower as compared to *f* 1 <sup>σ</sup> (Table 1). This phenomenon showed that the involvement of IMP was much lower at the mixed monolayer than that of the TX-100. In NaCl or U media, no distinct behavior was detected.

#### *2.3. Thermodynamic Parameters*

Thermodynamic parameter, e.g., the Gibbs energy of adsorption (∆*G* o ad) of the existing systems (pure and mixed), was obtained from Equation (10) [29,30]:

$$
\Delta G\_{\rm ad}^{\rm O} = \Delta G\_{\rm m}^{\rm O} - \frac{\pi\_{\rm cm}}{\Gamma\_{\rm max}}.\tag{10}
$$

Table 2 showed the achieved ∆*G* o ad value of pure and mixed systems in different media. For the calculation of ∆*G* o ad of the current system, ∆*G* o *<sup>m</sup>* (Gibbs free energy) values were used from our previous article [18]. All ∆*G* o ads values were negative, which was symbolic of

the spontaneity of the adsorption process at the air-solvent interface and their magnitude were higher than those of the previously calculated ∆*G* o *<sup>m</sup>* value [18] of the corresponding system. The occurrence of ∆*G* o ad > ∆*G* o *<sup>m</sup>* hypothesized that adsorption phenomena were favored over the association process, meaning that after finishing the adsorption process, the micellization process starts, i.e., a slight effort is required to complete this phenomenon (energy supplied in micellization to bring the monomers from the surface to micellar state). The ∆*G* o ads value of the IMP+TX-100 mixture at all *α*<sup>1</sup> of the surfactant was more negative than the value associated with an individual component (IMP and TX-100) (Table 2). These obtained results showed that the adsorption phenomenon was additionally feasible in case of a mixed monolayer, as compared with the monolayer formed by a singular component. The ∆*G* o ads value did not view any specific trends in U or NaCl media in IMP+TX-100 mixtures. In the case of pure components, in NaCl/U media their negative value was found to increase/decrease, respectively.


**Table 2.** Various thermodynamic parameters along with packing parameter (*P*) for pure and IMP+TX-100 mixture in various media.

One more thermodynamic parameter, named minimum free energy (*Gmin*), which is attained at the outmost adsorption at equilibrium, is also used to determine the attractive interaction/synergism at the interfacial boundary via Equation (11) [31,32].

$$G\_{\rm min} = A\_{\rm min} \gamma\_{\rm cunc} N\_A. \tag{11}$$

The value of the evaluated *Gmin* value is given in Table 2. The value of *Gmin* is usually correlated by the shipping of a component from the bulk system toward the interfacial boundary. The smaller magnitude of the *Gmin* value detected in any studied case was characteristic of intensified stability of the air-solvent boundary [3]. The level through which the *Gmin* value of the system is decreased is directly proportional to the extent of synergism allied through the system. The obtained *Gmin* in our case was found to be lower in magnitude, showing the thermodynamic stable air-solvent boundary. The *Gmin* seemed to be guileless in respect of any increase or decrease in value in any proper way by the occurrence of U/NaCl (Table 2).

An additional parameter of mixed monolayer called excess free energy (∆*G σ* ex) of IMP+TX-100 was computed using Equation (12) [33–36].

$$
\Delta G\_{\rm ex}^{\sigma} = RT[X\_1^{\sigma} \ln f\_1^{\sigma} + (1 - X\_1^{\sigma}) \ln f\_2^{\sigma}].\tag{12}
$$

The obtained value of ∆*G σ* ex was found to be negative in each solvent, observing that mixed monolayer formation is more stable than compared with a monolayer of either singular constituent (Table 2). Usually, at higher *α*1, the ∆*G σ* ex value was found to be more negative, indicating that stability of the mixed monolayer was attained more at higher *α*1, however, the ∆*G σ* ex value is not exhibiting a specific trend with the change of solvent (Table 2). Figure 2 showed the variation of ∆*G* o ad/*Gmin*/∆*G σ* ex value with change in mole fraction (*α*1) of TX-100 in different media (filled, open, and half-filled symbols represent ∆*G* o ad, *Gmin*, and ∆*G σ* ex respectively) which depicted the comparison of different evaluated thermodynamic parameters graphically.

∆ୟୢ ୭ /Δ<sup>ఙ</sup> *α* ∆ୟୢ <sup>୭</sup> Δ<sup>ఙ</sup> **Figure 2.** Variation of ∆*G* o ad/*Gmin*/∆*G σ* ex value with change in mole fraction (*α*<sup>1</sup> ) of TX-100 in different media (filled, open, and half-filled symbols represent ∆*G* o ad, *Gmin*, and ∆*G σ* ex respectively).

#### *2.4. Packing Parameters*

The structural geometry can be supposed via the packing parameter (*P*), i.e., the shape of micelles/mixed micelles in aqueous and non-aqueous solution was assessed through the following equation [37]:

$$P = \frac{V\_0}{A\_{\text{min}} l\_c}.\tag{13}$$

 <sup>୫୧୬</sup> In Equation (13), *l<sup>c</sup>* and *V*<sup>0</sup> are the effective chain length and volume of micellar interior, respectively, of the hydrophobic part of the employed monomers. Here, *A*min value was used as achieved from the surface tension measurement. The *V*<sup>0</sup> and *l<sup>c</sup>* value were computed by employing Tanford's theory [38].

$$N\_0 = \left[27.4 + 26.9 \left(n\_c - 1\right)\right] \times 2 \left(\text{Å}^3\right),\tag{14}$$

$$l\_{\mathfrak{c}} = \left[1.54 + 1.26\left(n\_{\mathfrak{c}} - 1\right)\right] \left(\mathring{\mathcal{A}}\right). \tag{15}$$

 Here, *n*<sup>c</sup> represents the whole sum of C-atoms in the C-chain length. The entire sum of C-atoms is measured one beneath the real count of C-atoms for the calculation of *V*<sup>0</sup> and *l<sup>c</sup>* value, since the C-atom next to the head group is extremely solvated. Hence, the first corban is also considered as the head group portion [3]. Table 2 depicted the evaluated *P*

*≤*

(packing parameter) value of the entire system. Micelles can be found in several shapes, depending on the obtained *P* value. As stated in literature [3,39] spherical micelles were detected as *P* ≤ 0.333, cylinders or rods shapes micelles were noted for 0.333 < *P* < 0.5, vesicles and bilayers shapes micelles were found for 0.5 < *P* <1, whereas inverted micelles were reported for *P* > 1. In our case, a *P* value for IMP was obtained for 0.333 < *P* < 0.5 in the aqueous and U solvent, signifying that the micelles formed by IMP were cylinders or rods. In the NaCl solvent, the *P* value of IMP was found for 0.24, showing that IMP formed spherical micelles in the presence of NaCl (Table 2). For singular TX-100, the *P* was attained 0.5 < *P* < 1 irrespective of the employed solvent, representing that the micellar shape of TX-100 were vesicles (Table 2). For the IMP+TX-100 mixture of the different ratio in the presence of a different solvent, the *P* value was achieved 0.5 < *P* < 1, showing that a vesicle-shaped mixed micellar solution formed like pure TX-100, because mixed micelles consist of a maximum share of TX-100.

## *2.5. <sup>1</sup>H NMR Study*

<sup>1</sup>H NMR technique is one of the finest methods for confirming the structure and purity of compounds [40,41]. Currently, <sup>1</sup>H NMR is a very powerful method for examining an intermolecular interaction between both different compounds in their mixed micelles [42,43] and it gives us a great deal of information of interaction that is usually not available with other techniques. The present study also deals with the <sup>1</sup>H NMR study of the interaction among the drug IMP and TX-100 surfactant in different ratios in their mixed micellar solution of an aqueous system. The <sup>1</sup>H NMR signals of pure IMP, as well as TX-100, are clearly visible in D2O. The <sup>1</sup>H NMR spectra of singular drug IMP and TX-100 is shown in Figure 3 with labeled hydrogen atoms attached to various carbons and obtained chemical shift value exposed in Table 3. Related data of pure TX-100 <sup>1</sup>H NMR have also been given in previously published work [24,44]. The spectra of pure IMP clearly show distinct six proton signals, and their corresponding proton numbers are allotted in the structure given in Scheme 2. The pure TX-100 spectra clearly show eight proton signals and the corresponding proton numbers are allocated in Scheme 3 [24,44]. Protons attached to -N<sup>+</sup> (CH3)<sup>2</sup> signals (I1 protons) are highly deshielded, that is, they resonate at high δ values because of the occurrence of N-atom in the drug IMP head group. All the NMR signals in both compounds drug IMP and non-ionic surfactant TX-100 (I1-I6 and T1-T8), in their pure form, show an increase in chemical shift δ values, which shows that each proton signal was highly deshielded. The proton signal I4 resonates at low δ values. This can be clearly observed, from the change in chemical shift values of I1, I3, I2, and I5, that the proton signals that present nearby to the head group are highly deshielded because of the occurrence of an adjacent N atom, whereas the proton signal I4 is highly shielded. No doubt, due to the combined electrostatic and hydrophobic effects, the interaction is stronger. In both drug and surfactant, the aromatic protons I6, T7, and T8 resonate at high δ values, i.e., they shift downfield.


**Table 3.** <sup>1</sup>H NMR chemical shifts (δ, ppm) of pure IMP and TX-100 in aqueous system.

<sup>a</sup> References [24,44].

**Figure 3.** <sup>1</sup>H NMR (600 MHz) spectrum of singular compound IMP drug and TX-100 in D2O.

**δ**

**Scheme 2.** Molecular model of drug IMP.

T2

T3

T2

T1

T1

T1

T7

T8

O

T6

O

T<sup>5</sup> T<sup>4</sup>

OH

n (9–10)

**Scheme 3.** Molecular model of TX-100.

Substantiation of complex formation for the drug–surfactant mixtures was obtained by NMR spectroscopy [45]. The proton signals of both the drug and surfactant show a significant change upon mixing (IMP+TX-100), which can be clearly understood from the chemical shift values given in Table 4 and also from the spectra presented in Figure 4. Table 4 depicted the addition of TX-100 in pure IMP solution cause noteworthy displacement in chemical shift values, which clearly point towards molecular interaction between IMP and TX-100. Chemical shifts are used to describe signals in NMR spectroscopy and the location and number of chemical shifts is symbolic of the structure of a compound.

Upon addition of TX-100 to pure IMP, a slight increase in chemical shift values is seen, i.e., they show a downfield shift, but not much interaction is seen at lower mixing ratios i.e., 0.1 TX-100 and 0.3 TX-100. However, as the mole fraction of TX-100 reaches 0.5, a prominent enhancement in δ values is seen through a rise in mole fraction (0.5–0.9), and from these values, it can be concluded that the extent of downfield shift is caused by the addition of TX-100; this depends upon the *α*<sup>1</sup> of surfactant in the solution of a drug and surfactant mixture. This increase in a downfield shift can be ascribed to an interaction of rigid tricyclic ring of IMP and polyoxyethylene chain of TX-100 structure.


**Table 4.** <sup>1</sup>H NMR chemical shifts (δ, ppm) of IMP+TX-100 mixtures in aqueous system.

The changes in chemical shift values for the alkyl protons I1 to I5 upon addition of TX-100 are also given in Figure 4 and Table 4 and it is clear that upon mixing of both studied constituents, the resultant mixed micelles cause deshielding (a downfield shift) of all the hydrophobic tail protons of IMP. Upon mixing, the hydrophobic interactions, as well as electrostatic attractions, endorse spherically along with the compacted micelles, while steric repulsion sources the hindrance amongst the constituents, causing the exposure along with protons deshielding. Overall, the proton signals (I1–I6, T1–T8) for both drug IMP and surfactant TX-100 in mixed micelles resonating at high δ values show a downfield shift of protons. It is known that both electrostatic, as well as steric interactions, show the leading character during the mixed micelles formation. Therefore, through the rise in TX-100 mole fraction, all proton signals for drug-surfactant mixtures are highly deshielded, which point towards an increase in steric repulsion among the molecules, which leads to the formation of large micelles [45,46]. As compared to pure IMP, the length of peak I4 is increased in case of mixtures up to mole fractions 0.5, but as the mole fraction reaches 0.7 and 0.9, the length of the signal I4 is decreased, which shows that these mole fractions (0.7 and 0.9) of TX-100 are more effective as compared to IMP. Similar changes were recorded for other NMR signals, such as I1. It is clearly visible from the spectra as well, being stable, that the aromatic protons related to the tricyclic rigid ring in IMP as well as the protons related to the mono aromatic ring in TX-100 are highly deshielded and show high δ values. Therefore, a clear downfield shift is observed. In the case of mixtures, all peaks are showing a clear downfield shift for both compounds, I1–I6 as well as T1–T8, at different mixing ratios. The compactness of the micelles varies with the variation of mole fraction, which is clear from the chemical shift values [47,48]. This change in chemical shift values is attributed to the interplay of electrostatic and steric interactions.

*α* **Figure 4.** <sup>1</sup>H NMR (600 MHz) spectrum of IMP+TX-100 mixture having various *α*<sup>1</sup> of TX-100 in D2O.

**δ**

δ

#### *2.6. FT-IR Study*

The interaction impact can also be qualitatively followed via FT-IR spectra [49]. FT-IR spectroscopy was utilized to describe diverse functional groups and to examine the interaction amongst unlikely groups existing in the binary mixed system. Backgrounddeducted FT-IR spectra of a pure drug and IMP+TX-100 mixed system of equal ratio in an aqueous solution are depicted in Figure 5a,b. Amphiphilic compound head-groups along with hydrophobic portion frequencies give statistics on the structural change in the monomers of formed micelles [50,51]. The feasible interaction amongst IMP+TX-100 mixed system will possibly alter the C–H bending and stretching and C–N stretching frequency of the drug head group.

− **Figure 5.** FTIR spectra of IMP (**a**,**b**) in the absence and existence of TX-100 and FTIR spectra of TX-100 (**c**,**d**) in the absence and existence of IMP in the selected wavenumber regions (cm−<sup>1</sup> ).

To view the effect of TX-100 on the aliphatic C–N bond stretching band as well as C–H bond bending band in the IMP molecule of IMP+TX-100 mixture, a frequency range of 1195 to 1500 cm–1 was selected (Figure 5a). As shown from Scheme 2, the nature of the employed drug IMP is cationic, as it keeps a positively charged N atom allied with three alkyl groups. As depicted in Figure 5a, the singular IMP spectra showed C–N bond stretching at two different frequencies: one at 1212.61 and the second one at 1225.36 cm–1. However, in the occurrence of TX-100, the C–N stretching in IMP was shifted to a higher frequency. The first C–N bond stretching was shifted to 1244.47 from 1212.61 cm–1, and the second one was shifted to 1291.29 from 1225.36 cm–1. IMP showed C–H bending at three different frequencies (1446.10, 1472.01, and 1485.58 cm–1) (Figure 5a) and in the occurrence of TX-100, the frequency of C–H bending in IMP was significantly shifted to a higher frequency from their initial position (1457.12 cm–1, 1473.92 cm–1, and 1487.65 cm–1, correspondingly) because of the interaction of TX-100 with IMP. Shifting to a higher or lower frequency region is dependent on the environment of the interacting group of molecules. Through the addition of TX-100, the alteration in C–N stretching and C–H bending frequency in IMP showed the attractive interaction between constituents, owing to mixed micelles formation.

To investigate the C–H stretching in IMP, the frequency band region of 2800 to 2960 cm–1 was chosen to assess the effect of TX-100, and the achieved plotted graph is displayed in Figure 5b. As depicted in the graph, IMP showed a C–H bond stretching band at 2887.08 as well as 2932.08 cm–1 of the alkane methyl group. In presence of TX-100, the obtained C–H bond stretching band was shifted from 2887.08 to 2979.89 and 2932.08 to 2948.24 cm–1. In the presence of TX-100, the occurrence of this shifting in the C–H stretching frequency band in the IMP functional group, reveals an interaction amongst both employed ingredients (IMP and TX-100) [52].

Figure 5c,d depicted the FT-IR spectra of a singular TX-100 and TX-100+IMP mixture with an identical ratio. Figure 5c showed the spectra of singular TX-100 between 940 and 1470 cm–1 frequency that showed the C–O stretching at 947.11 and 1097.42 cm–1 , O–H bending at 1364.01 cm–1, and C–H bending band at 1455.63 cm–1. Upon addition of IMP in the solution of TX-100, both C–O bond stretching was shifted from their original position. The first one shifted from 947.11 to 948.12 cm–1, and the second one shifted from 1097.42 to 1091.14. In addition, in the presence of IMP, the shifting in the frequency band of O–H bending in TX-100 occurred from 1364.01 to 1364.49 cm–1 and the C–H bending band from 1455.63 to 1457.20 cm–1, signaling the interaction amongst the constituents. Figure 5d showed the spectra of TX-100 as well as the TX-100+IMP mixture in the range 2840–2930 cm–1. Pure TX-100 showed the medium and broad C-H stretching band (alkane) at 2868.83 cm–1. The C–H stretching band (alkane) attained at 2868.83 cm–1 in TX-100 was moved to a higher frequency (2879.98 cm–1) in the presence of IMP, signifying an interaction amongst TX-100+IMP mixture mixed micelles. The O–H stretching band was found in case of singular TX-100, but for the TX-100+IMP mixture, the O–H stretching band peak disappeared due to merging with the water peak (not shown graphically). Due to the interaction of the employed ingredients, the whole frequency band variation did not achieve much, but obtained to be reproducible. Overall, herein, the shifting in C–N stretching, O–H bending, along with C–H bending, and stretching frequency recommend the interaction between the employed ingredients [53–55].

#### **3. Conclusions**

Before a surfactant can be employed as an appropriate drug agent, a broad range analysis must be accomplished to examine the interaction of the surfactant through the proposed drug. Herein, <sup>1</sup>H NMR, FT-IR, and tensiometric studies were performed to explore the interaction of a TX-100 surfactant with the cationic drug IMP. Physiologically, the nonionic nature of surfactants is more suitable as compared with ionic ones (cationic/anionic) and, owing to their high surface activity, a nonionic surfactant, such as TX-10, is considered as an ideal nominee for drug delivery in comparison to other surfactants. The interfacial properties of IMP, TX-100 along with the IMP+TX-100 mixture of various ratios at the sur-

face were evaluated using a tensiometric method in different solvents (H2O/NaCl/urea). TX-100 decreases the surface thickness acquired by means of the water layer and enhances the hydrophobic film width of the studied systems. Interfacial composition (*X σ* 1 ) and the *β* <sup>σ</sup> values of the IMP+TX-100 system showed a much higher participation of TX-100 at the surface than IMP and attraction/synergism between the components at the surface, respectively. The obtained value of ∆*G* o ad specifies that the adsorption phenomena was a spontaneous process and the stability of the mixed monolayer. The *P* value of IMP+TX-100 was attained as 0.5 < *P* < 1, showing that the micellar solution was vesicles-shaped. The value of *Γmax* acquired more for the surfactant than IMP, confirming that the surfactant showed higher surface-activity as monomers of TX-100, favoring a compacted or strongly packed arrangement at the surface in all solvents. <sup>1</sup>H NMR study of solution mixed systems advocated that IMP and TX-100 interact with each other via hydrophobic interaction. FT-IR spectra showed that the frequency band of individual ingredients (IMP and TX-100) was shifted from the original position for the mixed system, proving the interaction amongst them. The conclusions of the current investigation contribute to the assessment of the implementation of the surfactant (as a capable drug delivery) with a drug mixed system and the supporting mechanisms, for a basic understanding required for the projected expansion of economical and efficient drug formulations.

#### **4. Materials and Methods**

#### *4.1. Materials*

Every material in the current study was used as received from their respective company. Drug IMP was obtained from Sigma (St. Louis, MO, USA) having purity ≥ 98.0%. Surfactant TX-100 was from Sigma (Taufkichen, Germany). Different additives such as NaCl was acquired from BDH (Poole, England), having a purity of 98.0%, and urea was obtained from Sigma (Taufkichen, Germany), with a purity of 98.0%. Deuterium oxide (D2O) was purchased from Sigma (St. Louis, MO, USA) with a purity of 99%, which was used as the solution preparation for the <sup>1</sup>H NMR study only. For the rest of the study, distilled water was used for the solution preparation. Using calculated quantities of NaCl and urea dissolved in distilled water, the prepared solutions of these additives were used as solvents. In the aqueous system and in the occurrence of fixed NaCl/urea concentrations, the stock solutions of both employed constituents (IMP and TX-100) of fixed concertation were made separately, clearly above their corresponding *cmc*. Combinations of both components (IMP (drug) and TX-100 (surfactant as drug carrier) were readied by mixing the prepared stock solutions of both constituents (IMP and TX-100) in diverse mass ratios, varying the mole fraction of component 1 (TX-100 surfactant) from 0.1 to 0.9. These prepared solutions of diverse mass ratios were employed in the experiments, assuming that the density of the component's dilute solution at the experimental temperature is roughly constant.

#### *4.2. Methods*

#### 4.2.1. Measurement of Surface Tension

For the surface tension (*γ*) measurement, an Attension tensiometer (Sigma 701, Darmstadt, Germany) working with the ring detachment process was applied for pure (IMP and TX-100) and mixed system (IMP+TX-100) in five ratios in aqueous/NaCl/U solvent. The *γ* of resultant system (IMP, TX-100, or IMP+TX-100) vs. *log* (*C* (conc.)) of pure IMP, TX-100, or IM+TX-100 were plotted, and each plot showed a break point that was termed *cmc* of the system [18]. Here, plots given for the IMP+TX-100 mixed system in different media in our previous work [18] were used for evaluation of different interfacial parameter evaluation. The error in *<sup>γ</sup>* and temperature was attained as <sup>±</sup> 0.2 mNm−<sup>1</sup> and <sup>±</sup> 0.2 K, respectively.

## 4.2.2. <sup>1</sup>H NMR Study

For the <sup>1</sup>H NMR study, D2O (as a solvent) was used rather than distilled water to prepare the solutions of the individual component (IMP, TX-100) and their mixtures (IMP+TX-100). The <sup>1</sup>H NMR spectra of IMP, the surfactant, and their mixture of various

mole fractions in the aqueous system, were noted using a Bruker ultrashield plus 600 spectrometer, Billerica, MA, USA (600 MHz proton resonance frequency). Approximately 1 mL of every studied system is placed in a 5 mm tube for spectra measurements and chemical shifts were noted on the *δ* (ppm) scale. The reproducibility of *δ* was within 0.01 ppm. An organosilicon compound-tetramethylsilane was employed as an internal standard, which is recognized for calibrating a chemical shift.

#### 4.2.3. FTIR Spectroscopy

In the aqueous system, the FTIR spectra (4000 to 400 cm–1 wavelength) of the singular components and IMP+TX-100 mixed system in an equal ratio were recorded by consuming a NICOLET iS50 FT-IR spectrometer possessing ATR accessory (Thermo Scientific, Madison, Waltham, MA, USA). Here, a particular part of the wavelength range is exposed in the graph for clarity purposes. From the entirely attained spectra of the chosen system, the water spectrum was consistently deducted. The concentration of IMP and TX-100 was maintained very well above their respective *cmc* value. Each spectrum was obtained at a resolution of 4.0 cm−<sup>1</sup> .

**Author Contributions:** Conceptualization, M.A.R., N.A. and D.K.; investigation, M.A.R. and N.A.; validation, M.A.R., N.A., D.K. and A.M.A.; formal analysis, M.A.R., N.A. and D.K.; mthodology, M.A.R. and N.A.; project administration, M.A.R.; visualization, M.A.R. and N.A.; supervision, M.A.R.; writing—original draft, M.A.R., N.A., D.K. and A.M.A.; writing—review and editing, M.A.R., N.A., D.K. and A.M.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors extend their appreciation to the Deputyship for Research and Innovation, Ministry of Education in Saudi Arabia for funding this research work through the project number IFPIP: 63-130-1442 and King Abdulaziz University, DSR, Jeddah, Saudi Arabia.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

