On the Key Role of Polymeric Rheology Modifiers in Emulsion-Based Cosmetics

Abstract

Emulsions play a crucial part in the whole beauty and care market, especially in skin and hair care domains where, due to their extraordinary versatility, they represent most of the finite products. Being thermodynamically unstable, one key aspect of their formulation is the use of stabilizers that ensure a long lifetime under different conditions. In this framework a key role is related to rheology modifiers, which include all those raw ingredients added to achieve, among others, desirable inflow characteristics that would not be possible to gain in their absence. In this review, strong attention was dedicated to different polymers and formulation strategies to understand the key role of these ingredients, widely used in emulsion-based cosmetics formulations.

1. Introduction

Emulsions play a key role in the whole beauty and care industry, especially in skin and hair care segments, where, due to their extraordinary versatility and peculiarities, they represent most of the finite products put on the market [1,2,3]. Generally speaking, emulsions belong to the field of colloidal dispersions and are defined as a “thermodynamically unstable dispersion of two mutually insoluble liquids”, where one component is present as finely distributed droplets (usually spherical) in the bulk of the other liquid; the first is known as the distributed (or inner) phase, to distinguish it from the continuous (or outer) phase [4,5,6]. Depending on the nature and the mutual interaction of these two phases, together with the dimension of dispersed droplets, different types of emulsions exist on the market [7,8,9,10,11]. A fundamental aspect to remember when dealing with emulsions is their inherent thermodynamic instability. Being formed by mutually insoluble components, an emulsion is not able to last long if not properly stabilized by the addition of a third component, an emulsifier (more often a mixture of them), or by electrical or steric expedients [12,13,14]. Emulsions can be classified depending on the nature of inner and outer phase; this is the simplest and less-detailed way to sort them, but it enables us to rapidly understand the main features of the formulation under analysis [15,16,17,18]. As can be easily understood, their different nature results in quite opposite properties: O/W emulsions have creamy consistency, whereas, being based on oil, W/O ones usually exhibit a more greasy and oily texture.

In addition to simple ones, it is worth mentioning how complex emulsions exist too. These systems are characterized by vesicular structures where internal and external aqueous layers are separated by oil membranes (W/O/W), or vice versa (O/W/O) [19,20,21,22]. Due to their unique properties, multiple emulsions are currently applied in the pharmaceutical field for controlled drug delivery and release; nonetheless, several applications are available in the cosmetic industry, such as sustained release aerosol fragrances and prolonged skin moisturizers [23,24,25,26]. On the other hand, their large-scale diffusion is still limited as a result of huge manufacturing difficulties when dealing with industrial scale processes. In addition to what is presented above, emulsions can also be classified according to the dispersed particle dimension; in these terms, three types of systems can be recognized [27,28,29]:

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Macroemulsions, where dispersed droplet size ranges between 0.1 and 100 [μm], allowing light scattering and thus giving the typical white creamy appearance to the system [30];

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Microemulsions, characterized by droplets ranging from 10 to 100 nm and the presence of both emulsifiers and co-emulsifiers. Their peculiarity is to be thermodynamically stable (they form spontaneously) and transparent to the eye [31];

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Nanoemulsions, presenting droplets with a mean characteristic dimension between 20 and 200 nm and transparent or translucent appearance [32] (they are formed spontaneously like microemulsions).

From a formulation perspective, a key role in emulsions is played not only by emulsifiers, fundamental to decrease interfacial tension and stabilize the systems, but also by other ingredients. This review is focused on one of them that represents a key ingredient of emulsion-based cosmetics: rheology modifiers, commonly referred to as thickeners. Their nature can be inorganic or organic. Clays, fumed silica, or Al stearate belong to the first category while polysaccharide, cellulose, or acrylic polymers belong to the second one. They are incorporated in emulsion-based formulations to provide suspension control, reduce phase separation, and prevent syneresis. Moreover, they can also help to enhance the texture and feel of skin care products. Several aspects should also be considered, like its concentration or molecular weight in the formulation. Indeed, it was found that by increasing molecular weight, the rheology modifier diffuses slower at the interface with high stability, while smaller polymers can quickly adsorb at the interface with limited steric stabilization, resulting in phase separation. The combination of both low and high molecular weight rheology modifiers results in emulsions with low dispersity and controllable sizes for the extremely low concentration of rheology modifiers used [33]. Aspects related to their choice will be discussed together with the new trends and challenges behind their use in cosmetics.

2. Rheology Modifiers in Cosmetics

Rheology aims to study the deformation and flow characteristics of materials under the influence of external forces. In particular, the flow behavior of emulsions is a key property from the point of view of their applications and determines their ability to spread onto surfaces, sensory attributes, and long-term stability [34,35]. Emulsions belong to the family of non-Newtonian fluids, fluids for which the viscosity, in particular, depends on time or shearing and cannot be named “viscosity” but rather “apparent viscosity”. Non-Newtonian fluids are classified as shear-thinning, shear-thickening, thixotropic, and rheopectic fluids [36,37]. In shear-thinning fluids, the apparent viscosity decreases with increasing shear stress; in shear-thickening fluids, in contrast, it increases. Shear-thinning viscosity is seen when asymmetrical, rigid particles orient themselves in a flow current or when flexible, tangled polymers are deformed by a flow speed gradient. Shear-thickening behavior occurs occasionally; most frequently, it is observed in certain emulsions once the critical volume concentration of some components has been reached or exceeded.

In this case, the viscosity increases with increasing shear-rate and the product solidifies. Another category is represented by fluids with an initial yield stress that flow only above this value. The initial yield stress is interpreted as the point at which associated structures disintegrate. When the shear stress changes, the fluids all adopt the corresponding speed gradient almost instantaneously. However, for some fluids, a noticeable relaxation time is required [38,39]. If the apparent viscosity at constant shear rate or constant shear stress decreases over time, the fluid is called thixotropic; if the apparent viscosity increases, the term used is rheopexy. Another key concept is represented by viscoelasticity. The ideal elastic materials are deformed under stress but return to their original shape and give up the energy absorbed after the applied force ceases. In ideal viscous materials, the energy of the deformation is completely lost. Many materials are viscoelastic, that is, they exhibit both viscous and elastic behavior, which can be the cause of relaxation and retardation in technical processes and, depending on the application, may be useful or damaging. Rheological properties are plotted using rheograms, obtained using experimental apparatus [40,41]. A variety of different viscometers and measuring techniques are now available to measure rheological properties. An assortment of norms has been agreed upon in different fields to indicate the rheology of fluid formulations in connection with parameters relevant to the application. Viscosity measurements are only reliable if the flow measured is laminar, and so, viscometers are designed to take this requirement into account. These flow conditions are favored by high viscosity and low shear rate. In most measuring systems, the flow nevertheless becomes turbulent at high shear rates, a fact that should be considered. Different methods can be used:

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capillary viscometers flow through a cylindrical tube caused either by application of gas pressure or by hydrostatic pressure from the fluid column;

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rotation viscometers: the liquid is sheared between two surfaces on the stator–rotor principle using coaxial cylinders, cone and plate, or a disk and plate. Normally, the rotation rate is controlled, and the torsional is moment measured. However, there are also rheometers in which the applied force is controlled, which are used to determine initial yield stress;

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falling ball viscosimeter correlates the speed of motion of a ball through a fluid with its viscosity;

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bubble viscometer correlates the speed of motion of a bubble through a fluid with its viscosity;

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efflux viscometer measures the viscosity using the time taken for fluid to flow through a hole;

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forced oscillatory shearing: dynamic techniques that, using vibrations, permit the simultaneous investigation of both the dynamic viscous behavior and the elastic properties of fluid systems.

Examples of rheograms of different categories of non-Newtonian fluids are visible in Figure 1 [42].

Broadly speaking, the rheology modifiers category includes all those raw materials able to significantly participate in the rheology of the final product, being a monophasic or biphasic system such as a solution, suspension, or emulsion [43]. In other words, these substances are added to achieve desirable flow characteristics that would not be possible to obtain in their absence. From the technical point of view, polymers are used as thickening agents by the different mechanisms visible in Figure 2 [44,45]:

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chain entanglement (also referred to as physical cross-linking): a simple thickening mechanism in which polymer chains are dissolved into the solvent (usually water), providing soft entanglement that increases with increasing concentration of the polymer, since more chains are occupied in less space. As the concentration of the polymer continues to increase, it becomes more and more difficult to individually separate the entangled chains for the shear forces acting on the formulation [46] (Figure 2a);

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associative mechanism: characterized by the formation of association network structures leading to viscosity increase as a result of the bridge connections and winding offered by the polymeric agent [46]. This is possible due to the presence of chemically attached hydrophobic groups able to interact with hydrophobic associations similar to those typical of conventional surfactants [47] (Figure 2b):

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covalent cross-linking is realized when two polymeric chains attach to each other due to the interaction of a bifunctional monomer that forms a covalent bond linking them. The result is a tridimensional network of polymer chains throughout each particle. Cross-linking radically modifies the properties of the original polymer chains involved and represents one of the most important ways to achieve thickening at an industrial level (Figure 2c).

The first and the third effects can be obtained with both synthetic and natural polymers [48,49], while the second one is more typical for synthetic ones [50].

Rheological modifiers (Figure 3) play a key role in different areas, since the final flow behavior of the formulation is a crucial aspect in several fields like the pharmaceutical, food, and cosmetic industries. They can be classified depending on various criteria [43], but the most suitable for the purpose of the present paper is the one considering their chemical nature together with their origin as raw material. In this case, two different macro-classes of rheological modifiers can be considered:

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organic rheological modifiers are further sorted into three categories depending on the substance origin:

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natural modifiers are derived from plant, animal, or microbial origin and represented by large polymers chemically based on proteins or polysaccharides [51], like gums;

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naturally modified modifiers are natural polymers treated with specific chemical modifications to enhance their performances in the final product;

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synthetic modifiers are derived starting from oil-based polymers;

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inorganic rheological modifiers comprise only mineral-based substances like clays and pyrogenic (or fumed) silica [43].

As current market trends push for greener, sustainable, and non-oil-based raw materials, especially in the cosmetic field, the relevance and diffusion of natural-based ingredients is on the rise in the rheology modifiers field [43]. In fact, starting from the first decade of the 2000s, organic cosmetic products recorded a constant revenue increase, finally penetrating into conventional market segments. Moreover, natural-based cosmetics are still non-mature technologies, meaning that a multitude of opportunities for growth are still available in this developing field. On the other hand, the substitution of oil-based polymers with natural and natural-modified additives is only at the beginning in the cosmetic industry: in fact, even if this switch has already been accomplished in rinse-off products, leave-on formulations still rely on traditional polymers in most of the cases. The reason behind this delay must be sought in the unique features that oil-based polymers are able to guarantee when added as rheological modifiers: high viscosity, softness, smoothness, and white shiny appearance are only some of those peculiarities that consumers require but bio-based polymers struggle to recreate [52]. Moreover, it must be mentioned that natural polymers often show higher stock prices with respect to the oil-based equivalent and require addition at higher concentrations to obtain comparable performances, sometimes leading to unsustainable formulation costs [53,54]. This is particularly problematic considering that starting from the second half of 2022, raw material stock costs experienced an unceasing increase, leading to budget cuts and the need for rethinking formulation logics [55,56]. Based on the above, it is easy to conclude that the introduction of natural rheological modifiers in skin care products is still a challenge, especially when the idea is to completely substitute oil-based additives, linked with specific and pleasant features of the final product.

3. Synthetic Polymers

Synthetic rheological modifiers, the most common solution used by companies, can provide a smooth and creamy texture, ease of application, and optimal visual appearance while being fully compatible with the most widespread cosmetic ingredients in the skin-care field [53,57]. Oil-based polymers are divided into two major groups depending on the polymerization mechanism exploited during their production [58,59,60]: condensation polymers (produced according to step-growth polymerization) and additive polymers (obtained by chain growth mechanism).

Nowadays, the majority of the synthetic polymers sold as rheological modifiers for cosmetic formulations are additional polymers based on a few building block monomers, such as acrylic acid, acrylamide, and acrylic ester derivatives [58]. The reason behind this situation can be found in the availability on a global basis of these monomers, which leads to high-cost efficiency of the final product: in fact, the same monomers form the basis of other product lines such as dispersing agents, flocculant, and super-adsorbents [61,62]. Moreover, these products can be enhanced by varying the ionic charge and exist in several physical forms, increasing their ease of use. Among the most widespread synthetic polymers in cosmetics, it is worth mentioning polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), poly-ethylene glycol (PEG), and polyethylene oxide (PEO), usually blended with natural-based polymers [58]. The primary family of synthetic thickeners for cosmetic formulations is represented by acrylic polymers; given their key role in the production of skin-care products, these agents are further detailed in the following dedicated paragraph.

3.1. Acrylic Polymers

Acrylic polymers are considered one of the greatest innovation of modern industrial chemistry. Perfected just after World War II, they were first developed to manufacture more versatile and versatile paints, later known as acrylic paints [63,64]. Today, these polymers are extremely widespread in several industrial fields, also playing a pivotal role in the cosmetics industry. From the chemical point of view, acrylic polymers are synthetized via the chain growth mechanism (usually referred to as addition polymerization) starting from acrylic acid. An anionic monomer, in turn, is obtained from ethylene (and so, originally, from oil) [65,66]. The molecular weight commonly used in cosmetics is in the range of 100,000–450,000 g/mol. Even if pure acrylic polymers are possible, namely, polyacrylic acid or cross-linked polyacrylic acid compounds, most of the acrylic acid is applied to produce acrylic esters like methyl acrylate, ethyl acrylate, and butyl acrylate. These are then involved in polymerization processes, resulting in the formation of a wide variety of products. The different ester functionalities of the possible building blocks ensure high versatility, and so, the possibility to customize the properties of the final product depends on its specific use. Moreover, acrylamide and acrylic acid can be blended at any proportion, allowing the preparation of copolymers in which the ionic content can be chosen by the ratio of acrylamide to acrylic acid (the latter being anionic), as in the case of the sodium acrylate–acrylamide copolymer [67,68]. Nonetheless, acrylic polymers also account for polymeric chains obtained starting from the methyl derivative of acrylic acid, the so-called methacrylic acid; even in this case, the starting point for the production of the final polymer is often the methyl or ethyl ester obtained from the acid (namely, methyl methacrylate and ethyl methacrylate). The wide diffusion of these rheological additives in cosmetics is mainly attributable to their capability of ensuring high viscosity and pleasant visual appearance of the final product while offering a broad range of viscosities, up to extremely high values. Once added to the formulation, acrylic polymers act through swelling or chemical cross-linking depending on their specific nature, thus forming a polymeric tridimensional network able to heavily modify the rheology of the system while offering increased stability towards gravitational destabilization mechanisms. So, creating a gel network at a low concentration can increase the rheology of the final product.

3.1.1. Carbomer

Carbomer is the name used to identify a family of synthetic, crosslinked polymers obtained from acrylic acid and polyvinyl ethers, characterized by high molecular weight [69]. Generally, even if it is difficult to obtain an exact molecular weight due to the high presence of cross-links, the range is around 1 million g/mol.

They are usually sold in the form of white dried powders to be dispersed in water as thickeners and stabilizers for emulsion, but they also show optimal performance as gelling and suspending agents (in the case of insoluble solids) [70,71,72]. Due to their exceptional versatility, Carbomers are widely applied in the cosmetic industry: skin, hair, nails cosmetics, and make-up are frequently enhanced by these additives, as well as toothpaste and other personal care products [73,74,75]. Since several compounds belong to the Carbomer series, this label is usually associated with a number representing an indication about the specific components of the polymer and its mean molecular weight. Examples are Carbomer 910, Carbomer 934, Carbomer 940, and Carbomer 941. In particular, the current work started from emulsions containing Carbomer 940; its INCI name still being simply Carbomer, it can be univocally distinguished considering its CAS number (9003–01-04). Again, it is chemically described as 2-Propenoic acid, polymer with 2,2-bis(hydroxymethyl)propane-1,3-diol 2-propenyl ether) [76]. It is fundamental to mention how all Carbomers require a neutralization step to act as a thickening agent; this process leads to the dissociation of carboxylic acid pendant groups present on the polymeric chain, resulting in electrostatic repulsion, chain relaxation, and curing [77].

3.1.2. Sodium Polyacrylate

Sodium or potassium hydroxide are usually applied as a neutralizer to reach a final pH ranging between 5 and 12, depending on the specific purpose of the product. Univocally identified as CAS 9003–01-07, sodium polyacrylate is chemically classified as 2-Propenoic acid, homopolymer, sodium salt [78]. Due to its optimal thickening and suspending capabilities, sodium polyacrylate is highly diffused in the cosmetic industry; in particular, it is recommended for skin care emulsions to provide the glossy white appearance of the final product and a silky-soft after-feel on the skin. Since it shows good emulsifying properties, sodium polyacrylate can also be applied in emulsifier-free formulations for cosmetic purposes [78,79]. The molecular weight commonly used in cosmetics is in the range of 100,000–600,000 g/mol.

3.1.3. Considerations on Synthetic Polymers

As detailed above, cosmetic products are nowadays still largely enhanced by the presence of oil-based polymers. Since a vast portion of these products (namely, rinse-off cosmetics) are intended to be rinsed after the application, it is easy to understand how an enormous amount of polymers reach the household wastewater every day. Among these pollutants, microplastics represent a particularly noxious threat to land and maritime environment. For this reason, several institutions all around the world have been evaluating legal means to limit the use and consequent release of polymeric materials with persistency and damaging environmental effects [80,81]. In order to understand the complex panorama of microplastic pollution, let us first of all consider the definition and guidelines actually in force according to the European Chemical Agency (ECHA). Microplastics are defined as small solid plastic particles composed of polymeric chains and additives that are not biodegradable and are smaller than 5 mm. They can originate from the fragmentation of larger plastic during usage (secondary microplastics), like wear particles from tires, or they can be intentionally manufactured (primary microplastics), as in the case of microbeads in personal care products or industrial abrasives. Only particles of less than 5 mm in any dimension can be considered microplastics, even if some exceptions occur [82,83,84]. Due to their tiny dimension, microplastics entering a wastewater treatment as part of the influent water can easily overcome all the purification steps, eventually reaching surface water streams; in addition, that portion of microplastic particles adsorbed onto the solid surface in the sludges exiting the wastewater treatment plant is buried or spread in fields.

Once part of the soil, they are washed away by rain and sent back to rivers and the ocean. In other words, even if modern wastewater treatment facilities show incredible efficiency, the accumulation on surface water and soil of polymer-based particles coming from cosmetics cannot be avoided. As can be easily imagined, once these tiny particles reach oceans and their wildlife, their effect is often extremely harmful; using surface ocean circulations and marine fauna as vectors, microplastics can travel for hundreds of miles, finally reaching high latitudes or accumulation points when different currents meet (like the Great Pacific Garbage Patch, between Hawaii and California). It is important to notice that microplastic noxious effect on wildlife does not include only oceans: plastic debris washed up on shore can be ingested or further fragmented by the local terrestrial fauna, leading to additional circulation of the polymeric particles. Even if all microplastics are polymer-based, it is fundamental to mention that not all synthetic polymers must be considered microplastics, neither are they all persistent [85,86]. According to ECHA, water-soluble and liquid polymers are not included in the definition of microplastic since they do not fulfill the solid-state requirement, and the same holds for biodegradable polymers, which stay out of the definition because they are not persistent. On the other hand, most synthetic polymers applied in skin-care formulations are represented by non-biodegradable solid particles, which are able to swell once added to the aqueous environment. This is the case of polyacrylates, which remain at the solid state while increasing the viscosity of the emulsion due to cross-linking. From the normative point of view, starting from 2018, the ECHA started to promote the abandonment of all products answering the above-shown definition, with the aim of reducing microplastics accumulation in the environment. Europe leads the way in this field, with an ordinance banning microplastic from all rinse-off cosmetic products by 2020; although this normative only hit rinse-off formulations, it is widely believed that the ban will soon be extended to leave-on products, thus including skin-care formulations. In fact, even if leave-on products are not immediately removed from the skin, they will be sooner or later washed away by the consumer during their personal hygiene routine, leading to polymer accumulation in the household wastewater. In other words, leave-on formulations still contribute to microplastic pollution, even if to a minor extent with respect to rinse-off products, and will therefore likely be subject to restrictions in the near future.

Indeed, regulation (EU) 2023/2055 (https://eur-lex.europa.eu/eli/reg/2023/2055/oj/eng) (accessed on 17 February 2025) establishes the ban of synthetic polymer microparticles on their own or in mixtures in a concentration ≥ 0.01% by weight. The ban does not apply to the polymers that (i) result from a natural polymerization process; (ii) are degradable; (iii) present a solubility higher than 2 g/L; (iv) do not contain carbon atoms; (v) are permanently incorporated into a solid matrix; or (vi) present physical properties permanently modified during intended end use so that the polymer no longer falls within the scope of the above classification. This normative aspect, together with raising consumers’ environmental awareness all over the world, is pushing towards a paradigm shift in the cosmetic industry; the future of formulated products is represented by a new idea of cosmetics, based on the naturalness of the final product and low-impact raw materials coming from organic sources [51,87]. Indeed, even if the alternatives to synthetic polymers are possible, the shift to a natural one is the most promising. In detail, one possibility for synthetic polymers is represented by bio-based synthetic polymers that are derived from feedstocks by chemically processing the biomass to form small molecules for use as monomers [88]. Another one comprises the development of polymers with very high hydrophilicity due to chemical modification strategies [89]. Despite the very good results obtained, their cost is still very high, higher than natural polymers. This is due to the difficulty obtaining reliable and high amounts of monomers from natural sources [58].

4. Natural Polymers

Before the advent of synthetic oil-based polymers during the second half of the last century, natural polymers represented the better alternative to influence the rheological profile of a cosmetic formulation. Actually, the use of these natural compounds in personal care and beauty products can be traced back to the Egyptian, Roman, and Greek eras [51]. As already mentioned above, natural thickeners are intended as substances obtained from natural sources such as plants, seeds, seaweeds, and microorganism with the capability of acting on the viscosity and, in general, on the entire rheology of a product [90,91].

From the chemical point of view, natural thickeners applied in the cosmetic industry are represented by polysaccharides, macromolecules formed by the repetition of saccharide units linked together by glycosidic bonds for a minimum of ten monosaccharide molecules. For this reason, they are also known as hydrocolloids [92,93]. During the past years, they have been widely applied in cosmetic formulations because of their quick availability from natural sources and their exceptional multifunctionality; in fact, they can also serve as suspending agents, moisturizers, co-emulsifiers, emollients; and hair-conditioners [51].

Nonetheless, natural polymers show different limitations regarding the texture and visual appearance of the final product when applied as rheological additives in cosmetic emulsions. This aspect, linked with the cost-effectiveness and ease of production of oil-based substances, led to the heavy adoption of synthetic polymers. It must also be mentioned as organic synthetic polymers like polyacrylates are usually able to ensure a desired rheological behavior at a very low percentage, often lower than 0.8%; if the same result has to be obtained applying only natural-based polymers, the total concentration of these additives will easily overcome 1.5%, strongly impacting the cost balance of the formulation. In order to better understand the complex panorama of natural thickeners, it is possible to classify and analyze them according to their origin, as in

Table 1. Classification of the most widespread natural thickeners depending on their origin as raw materials.

Marine	Botanical	Microbial
Carrageenans	Guar gum	Xanthan gum
Agar-agar	Locust bean gum	Gellan gum
Alginates	Gum tragacanth	Pullulan
	Konjac glucomannan	Curdlan
	Tara gum	Dextran
	Cassia gum	Welan gum
	Gum Arabic	Rhamsan
	Pectin	Succinoglycan
	Starches

.

4.1. Marine Polysaccharides

Marine-based natural thickeners include all those polysaccharides directly extracted from seaweed. They are represented by three main families of compounds [94,95]:

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Carrageenans: a group of sulphated galactans extracted from different red seaweed (known as Rhodophyceae) species, mainly Eucheuma cottonii, Eucheuma spinosum, Chondrus crispus, and Gigartina. They are, in turn, split into three different types according to their ester sulphate content, which depends on the specific weed source: the lower the ester sulphate content, the higher the gelling inclination of the considered carrageenan. Fontes-Candia and coworkers prepared emulsions with sunflower oil using three different concentrations of carrageenan (from 0.5 to 2% w/v). The key role of carrageenan and salt was evaluated considering mechanical properties and rheology.

Increasing their content formed more stable emulsions [96]. Similar results were obtained from preparing sunscreen emulsions and toothpaste [97,98];

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Alginates: block copolymers composed of mannuronic (M) and guluronic (G) acid extracted from brown seaweed (Phaeophyceae) species like Macrocystis pyrifera, Laminaria hyperborea, and Ascophylum nodosum. The M to G ratio depends on the specific type of alginate and governs its final properties in terms of thickening and gelling performance. They are usually commercialized as water-soluble sodium salts, which require the addition of calcium to obtain the desired thickening effect.

In this direction, Russo and coworkers [99] studied the effect of Pluronic (synthetic polymer) and alginate in producing stable emulsions. Pluronic was used as amphiphilic molecules to reduce interfacial tension, while alginate was used as a rheology modifier to slow down the droplet coalescence. By optimizing both alginate and Pluronic concentrations, emulsions stable up to 90 days were obtained.

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Agar: a general term used to identify a complex mixture of polysaccharides extracted from the Gelidium and Gracilaria species of red seaweed. The major fractions are represented by agarose, a neutral polymer, and agaropectin, a charged sulphated polymer. When applied as a thickener, agar results in the formation of firm and brittle gels.

All these polysaccharides are able to create 3D networks that make them very interesting as rheological agents. Xiao and coworkers [100] studied the possibility of developing surfactant-free cosmetics using agarose (1.2–1.8% w/v) and carbomer 940. Based on rheological analysis, the surfactant-free cosmetic cream obtained showed shear-thinning behavior and strongly synergistic action. The main drawback is that there is need of further steps, like heating or adding other solutions, before obtaining the ability to work as thickeners, which is something that should be taken into high consideration. It is indeed fundamental that these steps can be compatible with the entire manufacturing process of the formulated products.

4.2. Botanical Polysaccharides

Being easily achievable and extremely widespread, botanical polysaccharides have been known to man for many centuries. Similar to marine-based polymers, they can be classified in different families [95]:

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Galactomannans (or botanical gums): composed of a C1 to C4 linked mannose backbone with single galactose substituents; they include guar gum, locust bean gum, tara gum, and cassia gum, which differ in the degree of galactose substitution going from one galactose per every two mannoses (guar gum) to one every four (locust bean gum). They can all be used as thickeners in cosmetic formulations. Indeed, their use was compared with xanthan gum by Niknam and coworkers [101]. The emulsions not treated with microwave or ultrasound present viscous-like behavior, while treated samples showed weak gel behavior. Rheological parameters (storage modulus, loss modulus, and apparent viscosity) indicated that galactomannan had higher impact on the rheological aspects of emulsions compared with xanthan gum. In addition, the synergistic interaction between the two biopolymers resulted in better rheological aspects. By treating the samples with ultrasound and microwaves, the emulsion stability values of the samples increased, connected with various parameters, especially viscosity.

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Pectins: extracted from different vegetal sources like apples and citrus fruits and composed of galacturonic acid residues with occasional rhamnose interruptions. They are usually classified depending on their degree of methyl esterification and not widely applied in the cosmetic field, whereas they are extremely diffused in the food industry. Four different low-methoxyl pectins (0.015–0.02 w/w) were prepared to be used as the dispersing phase in cosmetic emulsion gels [102]. The obtained formulated products were used, together with a common non-ionic surfactant (Tween 60), to prepare olive oil emulsion gels suitable to design new cosmetic products.

The storage and loss modulus obtained were found to vary with pectin content with a power-law behavior, and two different regions were observed, according to the biopolymer concentration range.

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Tara gum, namely, Caesalpinia Spinosa Gum, represents one of the most promising thickening alternatives to polyacrylates in cosmetic formulations. It is obtained by grinding the endosperm of Caesalpinia Spinosa (also known as Peruvian carob) seeds, a plant belonging to the Leguminosae family and native to the Peruvian and Bolivian Andes. It is completely odorless and white to ivory colored, even if the resulting gel is usually brownish and translucent.

Due to its long history as a thickening agent in the food industry, tara gum is nowadays internationally recognized as safe for other human applications, like cosmetic products [43]. As anticipated above, its chemical structure is characterized by (1–4)-β-D-mannopyranose linear chains, branched through (1–6) bonds with α-D-galactopyranose units in a 3:1 ratio [43]; the specific mannose to galactose ratio differentiates tara gum from the others botanical gums presented above and gives it those special features widely appreciated in different fields. Concerning its performance as a rheological modifier in cosmetic emulsions, tara gum shows linear Newtonian behavior at low concentrations (0.1 to 0.2%), while it acquires pseudoplastic features without thixotropy for higher concentrations (0.5 to 2%) [43]. The thickening power becomes important over 1.0% concentration, when a dramatic increase in the obtained viscosity can be appreciated, exceeding 50,000 cP when the concentration approaches 2.0%; moreover, good suspending power is highlighted starting from 0.1% concentration. Finally, tara gum is stable for a wide range of pH (3 to 12) and tolerates the addition of ethanol up to 10%; in other words, it appears as an optimal potential substitute for polyacrylates in cosmetic emulsion [43].

4.3. Microbial Polysaccharides

Several microbial polysaccharides have been produced in the last decades, but only a few of them have proven to be as efficient as promised when dealing with the application on an industrial manufacturing scale [103,104]. They are generally produced due to controlled and bioengineered fermentation processes of different microorganism; examples are xanthan gum (secreted by Xanthomonas campestris), succinoglycan (produced during fermentation by Agrobacterium tumefaciensis), and Welan and Rhamsan gums (both coming from Alcaligenes species). Due to the difficulty in obtaining high quantities of most of them, only xanthan gum found a stable and fructose application in the cosmetic field [105]. Xanthan gum is a high-molecular-weight polysaccharide obtained during the fermentation process of the microorganism Xanthomonas campestris, commonly found on the leaves of the Brassica vegetables, like cabbage [106]. It acts as a thickener and stabilizer with optimal performances, and its high solubility in cold water ensures ease of operation both on the lab and industrial scales. Xanthan gum shows an extremely complex chemical formula; its primary structure is a linear (1–4)linked-β-D-glucose backbone with a trisaccharide side chain one every other glucose at C3, containing a glucuronic acid residue linked (1–4) to a terminal mannose unit and (1–2) to a second mannose that connects to the backbone. Almost half of the terminal mannose are pyruvylated, and the non-terminal residue carries an acetyl group [107]. Moreover, a helical secondary structure can be recognized, which contributes to the high stabilization activity of Xanthan gum when added to an emulsion [108,109]. From the technical point of view, Xanthan gum solutions are highly pseudoplastic: viscosity is progressively reduced when shear stress is applied, but upon its removal, the initial value of viscosity is recovered almost instantaneously. This behavior comes from the capability of xanthan molecules, once in solution, to form intermolecular aggregates due to hydrogen bonding and polymeric entanglements. Such a phenomenology explains both the high thickening and stabilizing effect offered by xanthan gum in cosmetic formulations. In addition, the reduction of viscosity following an increasing shear is important to the pouring properties of suspensions and emulsions, as well as to the efficiency of this additive as a processing aid. Concerning the recommended dosage, xanthan gum is able to offer good thickening and stabilization properties already for 0.1 to 0.3% concentrations, giving lead to a solution with a gel-like appearance when increased to 1.0% concentration [110]. However, xanthan gum is not able to reproduce the smooth and silky texture typical of polyacrylate-containing skin care formulations. Emulsions enhanced by the presence of xanthan gum often appear jellylike and sticky, something extremely undesirable in the skin care segment.

For this reason, it is usually blended with botanical gums; this pattern shows synergistic interactions able to provide high viscosity and stability coupled with the pleasant texture and appearance offered by galactomannans [105].

5. Natural-Modified Polymers

Several natural polymers intended to be used as thickeners are subjected to chemical treatments aimed in modifying their chemical structure, with the final goal of magnifying the rheological activity of such polymers. This is the case of chemically modified polysaccharides, which include the following [111,112]:

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Modified cellulose products enhanced by chemical modifications to render the basic cellulose backbone soluble. This family of cellulose-based products offers a wide range of functions, from the thickening effect ensured by carboxymethyl cellulose (CMC) to the thermogelation in hydroxyethyl cellulose (HEC). Formulations based on CMC with bacterial cellulose are not only able to form high stable emulsions but also to reduce the amount of tensides needed due to their synergic effect through a Pickering effect and by structuring the continuous phase [113,114].

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Modified native starches consist of starches enhanced by chemical transformations to improve their heat and acid resistance and increase processability while reducing the tendency for retrogradation. This category includes hydroxyethyl and hydroxypropyl modified starch. The synthesized starch [115] was obtained using microwave irradiation with less energy, solvent, and time needed. The final viscosity increases as molar substitution increases up to 250 cP. Its thickener effect was investigated with very promising results;

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Modified alginates are obtained by esterification with propylene glycol to avoid precipitation at a pH lower than 4.0, a phenomenon typical of alginates. Self-supporting foams produced with functionalized alginate demonstrated high stability with proper mechanical properties and density with promising application in cosmetics [116];

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Modified guar gums are enhanced by carboxymethylation to improve alkali compatibility, hydroxyalkylation to improve solubility, or phosphatization to enable cross-linking. At a low concentration (10–30 mg/mL), the functionalized polymer exhibited a gel-like nature and visco-elastic properties with a storage modulus (G’) higher than loss one (G”) at all polymer concentrations investigated. The role of tenside and salt is fundamental for the proper rheological properties of the formulated product and showed promising use as a rheology modifier [117]. Due to their enormous versatility, such modified natural polymers are used in several industrial fields, including cosmetics, food, oil drilling, pesticides, textiles, and many others [87,118]; even if some of them show a partial overlap of their properties, each modified hydrocolloid tends to excel in a few specific areas. However, despite the interesting results, only a few authors applied bacterial cellulose to cosmetics due to the high costs and challenge to scale up its production.

5.1. Modified Cellulose Products

Being the major constituent of the majority of land plants, cellulose is the most abundant organic substance existing in nature; it represents the starting material for a wide range of modified natural polymers with several applications in the food and cosmetic industries [119,120]. Well-known examples are cellulose ethers like methyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, and sodium carboxymethyl cellulose, frequently referred to as Cellulose gum (or Na-CMC). All these valuable additives come from a specific chemical modification of the same raw material, cellulose. From the chemical point of view, cellulose shows a polymer chain of two repeating anhydroglucose units, namely, β-glucopyranose residues, linked through a 1,4 glycosidic bond [120]. The strong intermolecular hydrogen bonding typical of its structure make natural occurring cellulose highly crystalline and ordered, resulting in insolubility in water; this is the main reason behind cellulose modification to obtain industrially desirable water soluble rheology modifiers [51]. The abundance of modified cellulose products is made possible by the fact that each anhydroglucose unit contains three hydroxyl groups, which can be, in principle, substituted, giving lead to a various range of possible alternatives. The average amount of hydroxyl groups effectively substituted per unit is referred to as the degree of substitution (ds); note that desirable chemical and mechanical properties are usually achieved for a ds much lower than the theoretical maximum [121]. The degree of substitution, together with the polymerization degree and the type of substituents groups introduced in the polymer chain, defines the final set of properties of the modified cellulose product. Carboxymethyl cellulose is probably the most widespread cellulose derivative in the modern industry, being soluble both in hot and cold water and able to give clear and colorless solutions [122]. It is particularly appreciated as a thickener in food and cosmetic products. In this case, carboxymethyl groups are attached to each glucose unit, with a degree of substitution for industrial application usually ranging between 0.7 and 1.2 [51]. Considering such a ds, it is possible to produce an aqueous solution at 1.0% concentration of cellulose gum with a viscosity of 5000 cP at ambient temperature [123,124]. It is important to mention how the degree of substitution strongly influences the thickening capabilities of CMC; solutions of low-substituted cellulose gum show as thixotropic, whereas a higher ds leads to pseudoplastic behavior [125,126]. At a technical level, CMC provides a thickening effect upon entanglement of its high molecular weight chains, resulting in stable viscous solutions within the pH range of 4–7 and at low temperatures. Differently from cellulose gum, which shows anionic features, cellulose ethers are cationic semi-natural derivatives of cellulose. Due to their history of effective and safe use, they represent the main cellulosic alternative in the personal care and beauty market [51]. In general, cellulose ethers are able to dissolve in water at room temperature, due to the hydration of the cellulose backbone. This leads to the development of a sheath of tightly bound water molecules around each polymer chain, allowing them to expand and create the entanglement network needed to build up the desired viscosity [51].

As seen for CMC, high viscosities can be achieved at a low concentration due to the interactions between the high molecular, rigid cellulose macromolecules. It is fundamental to state how, once added to whatever emulsion, cellulose ethers are not able to act as suspending agents without the aid of other specific substances showing suspending properties, such as xanthan gum [51].

5.2. Modified Starches

Starch is one of the most widespread naturally occurring polysaccharides, stored in all green plants as an energy reservoir and commercially isolated from several crop plants such as corn, potato, rice, wheat, and tapioca [51,127].

Chemically speaking, natural starch consists of two basic polysaccharides, namely, amylopectin and amylose. Different starches differentiate one from the other depending on their amylopectin to amylose ratio, on which the final features of the polymer depend. Typical molecular weights of amylopectin and amylose are 500 million g/mol and 1 million g/mol, respectively [51]. Being characterized by strong molecular interactions based on hydrogen bonding, starch shows a reduced ease of dissolution in water compared with cellulose; for this reason, it is usually added to hot water to favor the solubilization process. Starches are popular and well-known thickening agents in the food and cosmetic industries, even if the tendency to form hazy and cloudy solutions limits their application in clear formulation manufacturing [51]. As mentioned, starches are frequently subjected to chemical modifications aiming to maintain their beneficial properties while reducing instability and retrogradation issues [128,129]. Each chemical manipulation is realized with a specific goal:

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Cross-linking, the most frequent chemical transformation applied on starch, consists of the replacement of the hydrogen bonding between starch chains with stronger and more permanent interactions, namely, covalent bonds. Such a modification avoids the swelling of the starch granule, preventing disintegration by chemical attack or imposed shear [130];

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Stabilization, usually applied in conjunction with cross-linking, is another fundamental chemical modification process applied on starch. Its aim is to prevent retrogradation by introducing some bulky groups able to create steric hindrance against chain re-alignment [131];

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Conversions, comprising all those transformations occurring by chain-cleavage reactions of starch, like oxidation, acidic hydrolysis, dextrinization, and enzymatic hydrolysis, are able to guarantee various effects on the final properties of starch [132];

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Lipophilic substitution is characterized by the addition of long hydrophobic chains to the starch macromolecules, resulting in higher lipophilicity and increased ability to stabilize interactions between oil and water. An example is given by starch octenylsuccinates, widely applied additives able to rapidly migrate towards an oil–water interface and stabilize it [133].

All these modifications were carried out in order to allow them to work as rheology modifiers following all the three possibilities visible in Figure 1.

6. Conclusions

Rheological modifiers play a crucial role in cosmetics, being responsible for the ability to spread onto surfaces (skin layer), their sensory attributes, and long-term stability. During the different production stages, different viscosities are needed. Indeed, during processing (stirring and pumping) low viscosity is essential (100–1000 mPa s) to reduce costs and facilitate the entire manufacturing procedure. Another key part of their product life is represented by the application onto the skin, and also in this step, low viscosity is required (50–100 mPa s) to allow easy spreadability. On the contrary, during transportation and storage, the viscosity of the emulsion should be higher (10,000–100,000 mPa s) to prevent aggregation and then phase separation. It is easy to state that shear thinning behavior is the most favorable one. The mechanisms behind the ability of rheology modifiers to lengthen the lifetime of emulsions are related to the formation of networks (physical or chemical) able to decrease the collisions between the particle of the disperse phase through the continuous phase. Many different synthetic and natural polymers are used in commercial products with corresponding advantages and disadvantages. In particular, synthetic polymers can induce high stability and high mechanical performance and are easily produced in large volumes at low costs, while natural polymers lack in terms of costs and rheological performances if not modified and scaled up.

As current request trends push for greener, sustainable, and non-oil-based raw ingredients, especially in the cosmetic field, the applicability of naturally grounded constituents in the rheology modifiers field is increasing. Indeed, despite the disadvantages already highlighted, the biodegradability associated with natural products makes them ideal candidates in cosmetics. So, the solution to the problems related to their production is pivotal for the market success of natural-based formulated products.