Surface Migration of Fatty Acid to Improve Sliding Properties of Hypromellose-Based Coatings

Abstract

Hypromellose (HM) is a cellulose-derived polymer of pharmaceutical grade that forms easily from thin films and coatings. As few studies concern HM-formulated systems, this study focuses on the formulation of HM films by incorporating a fatty acid additive, making it possible to control surface properties such as wetting and slip behavior for pharmaceutical or medical applications. The results show that the addition of a very small amount (from 0.1 to 1% w/w) of fatty acid additive reduces HM film affinity for water and water vapor transmission rate, while film appearance and gloss are rather preserved. Surface properties were probed using wettability measurements, Tapping Mode AFM, ATR-FTIR spectrometry, and friction measurements. Tapping Mode AFM images show that the surface roughness reduces by up to 65%. Wettability results show that the surface energy decreases from 43 to 31 mJ.m⁻², whereas surface FTIR spectrometry measurements demonstrate that fatty acid molecules migrate on the surface of the formulated films, the driving force being the microphase separation between the polar HM macromolecules and the hydrophobic additive, leading to the formation of a weak boundary layer with poor cohesion. As a consequence, the surface coefficient of friction significantly reduces from 0.38 to 0.08, and fatty acid molecules thus act as a lubricant, improving the sliding properties of HM-based coatings.

1. Introduction

Hypromellose (HM) appears to be a successful polymer as a bio-alternative material for film-forming materials or coatings. HM is easily available on the market and is used in many biomedical, pharmaceutical, food, or building applications as film or coating. Coatings improve the quality of drug release and food products by preventing physical, chemical, and biological deterioration and limiting the migration of water vapor, oxygen, carbon dioxide, and other substances. Coatings can also tune wetting and sliding behavior as an indication to control drug release, provide improved swallowability in the case of oral drug tablets or caps, or lubricate medical devices like endoscopes. Proteins (gelatin, gluten, etc.), polysaccharides (starch, cellulose, chitosan, etc.), or lipids (waxes, fatty acids, etc.) are all film-forming biopolymers. They can be either hydrophilic or hydrophobic, and the only solvents that can be used are ethanol and water. Research over the past few years has concentrated on creating films and coatings made of environmentally friendly materials. Among these, cellulose ethers (carboxymethyl cellulose, methylcellulose, and hypromellose) have received special attention [1,2,3,4]. Because these materials are readily available, inexpensive, and biodegradable, using them has benefits. The film-forming biopolymers are frequently mixed with additives to change the films’ functionalities or physical characteristics [5,6,7,8]. But in order to reach the expected properties, with regard to moisture content, barrier, mechanical, and surface properties, cellulose ether biopolymer films need to be optimized. Adding additives to film or coating formulations, such as surfactants, plasticizers, and colorants, is the most effective way to enhance HM-based film or coating properties. In this work, the film-forming matrix will be hypromellose (HM) (also named hydroxypropylmethyl cellulose (HPMC)). A hydrophobic additive will be incorporated during the formulation process to change the characteristics of the films. Hydrophobic additives could be waxes, lipid compounds, or surfactants. In order to lower surface tension and alter the wetting properties by making the coating material more spreadable [9], surface-active agents, also known as surfactants, are added to coating solutions and films. Fatty acids appear interesting, as they reduce water affinity and moisture transfer [10,11,12]. Fatty acids are also common lubricants found in pharmaceutical formulations, primarily in oral formulations such as tablets and capsules. However, their addition to film coating formulations can occasionally have an impact [13] on the bulk properties of films, in addition to an impact on their surface properties. This work’s primary goals are to outline the HM film formulation process and assess the impact of a hydrophobic fatty acid additive, namely stearic acid (SA), on the formulated films’ surface characteristics.

2. Materials and Methods

2.1. Materials

Hypromellose (HM) was produced by Dow Chemical Company and purchased by Colorcon, France. This grade has a nominal viscosity of 3 mPa.s at 2% aqueous solution at 20 °C. The degree of substitution of methoxy groups (DS) and the mass molar substitution of hydroxypropoxy groups (MS) of HM used are 1.87 and 0.25, respectively (by weight percent, the amount of OCH₃ groups is 28.3% (w/w), and OCH₂(CHOH)CH₃ groups is 9.1%). This grade is a low-viscosity grade of HM that is well suited for making films and coatings. Indeed, it makes it possible to prepare an aqueous solution with about 20% w/v of solid polymer. The main benefit is the decreased drying time during the film preparation process, as solution viscosity rises with the amount of solid polymer in the solvent. HM was used as a film-forming matrix. Stearic acid (SA) (Sigma-Aldrich, Saint-Quentin-Fallavier, France) was used as additive for the film-forming process. SA molecular weight is 284.48 g.mol⁻¹ and melting point is in the range 69–72 °C. The purity of SA is greater than 99%. Sorption isotherm salts to control relative humidity during the film-forming process were potassium acetate CH₃COOK, magnesium chloride MgCl₂, potassium carbonate K₂CO₃, magnesium nitrate Mg(NO₃)₂, sodium chloride NaCl, and potassium chloride KCl (Fisher scientific, Illkirch, France). For water vapor transmission rate (WVTR) experiments, anhydrous calcium chloride CaCl₂ (Fisher scientific, Illkirch, France) was used.

2.2. Methods

2.2.1. Film Preparation Method

The general process for film formation is the “solvent process” or “casting process”, and it is based on the drying of the film-forming solution after deposition on a nonstick substrate. It is used to form edible films or coatings. The solvents used are restricted to water and ethanol. By making the HM solution (6% w/v), casting the mixture onto a glass plate, and letting it dry at room temperature, pure HM films were produced. To avoid lumping, a moderate agitation of HM powder in deionized water at 80–90 °C was performed to create an HM solution (6% w/v). A % w/v of a solution is calculated with the following formula: % w/v = g of solute/100 mL of solution. HM solutions thus contain 6 g HM/100 mL of solution. Vigorous agitation can cause severe foaming that can be challenging to remove. After cooling to 25 ± 2 °C, a transparent solution is obtained. The HM polymer was fully hydrated by keeping the solution at 5 °C for a full day. Solutions were deposited onto a glass plate in a homogeneous layer with a thickness of 1.6 mm to create, after drying, films that were 100 µm thick. Films were obtained by drying 48 h under natural convection. Before the experiments, the samples were conditioned at 25 ± 2 °C and 30 ± 5% RH after the dried films were peeled from the glass plate.

SA additive was added to HM solution to create HM-formulated films. The amount of SA was expressed as a weight percentage (% w/w) of HM powder in HM solution. Respectively, 5.4 mg, 27 mg, and 54 mg of SA were dissolved in 10 mL of absolute ethanol. Then, the 10 mL of SA solution was added to 90 mL of HM solution to obtain, respectively, a 0.1, 0.5, and 1% w/w HM-SA solution while being stirred by magnetic agitation. Three distinct solutions containing varying amounts of SA (0.1%, 0.5%, and 1% w/w) were made. After two hours of homogenization, all solutions were kept at 5 °C for twenty-four hours. Film-forming solutions were applied evenly in 1.6 mm thick layers onto a glass plate, and the layers were allowed to dry naturally, thus forming 100 µm thick dry films. Prior to experiments, all formulated films were stored at 25 ± 2 °C and 30 ± 5% relative humidity.

2.2.2. Determination of Moisture Sorption Isotherms

The HM and HM-SA films’ water adsorption tests were carried out utilizing a technique developed by Coma et al. [12]. Aluminum dishes were weighed to the closest 0.0001 g and contained roughly 0.2 g of film pieces for each sample. To achieve different water activities and maintain a constant relative humidity (RH), the dishes were stored for two weeks in sealed desiccators containing saturated salt solutions (KC₂H₃O₂, MgCl₂, K₂CO₃, Mg(NO₃)₂, NaCl, and KCl, respectively, for 22%, 32%, 44%, 53%, 75%, and 85% RH at 25 ± 2 °C). The experiments are conducted in triplicate, and the equilibrium moisture content was determined by comparing the mass increase in the film samples following equilibration to the dried mass (Equation (1)) that was attained following three hours of drying at 105 °C in an oven.

$$\mathrm{Moisture} \mathrm{content}={\displaystyle \frac{{\mathrm{W}}_{\left(\mathrm{RH}\right)}-{\mathrm{W}}_{(\mathrm{dried} \mathrm{film})}}{{\mathrm{W}}_{(\mathrm{dried} \mathrm{film})}}}\times 100$$

(1)

where W_(RH) is the weight of the HM film sample at a given RH and W_{(dried film)} is the weight after sample drying.

2.2.3. Film Water Vapor Transmission Rate (WVTR) Determination

The steady flow of water vapor per unit of time through a unit area under particular temperature and humidity conditions is represented by the water vapor transmission rate (WVTR). As a result, the latter is regarded as one of the most significant hygroscopic characteristics of films in biomedical and pharmaceutical industries. WVTR strongly depends upon the film thickness and the type of matrix and additive employed. WVTR is extensively studied for cellulose-based films, protein-based films, and composite films [14,15]. The WVTR of HM films and HM-SA-formulated films was measured using ASTM method [16]. Cups with a diameter of 5.5 cm and a depth of 4 cm containing 3 g of anhydrous CaCl₂ were covered by the film under investigation. Films were cut circularly with a diameter slightly larger than cup diameter and stuck on the cup edges by an adhesive. The area of vapor exchange is 24 cm². The cups were placed in an environment of controlled relative humidity 53 ± 5% RH (in a desiccator containing saturated Mg(NO₃)₂ solution) and a temperature of 25 ± 2 °C. The WVTR (g.m⁻².day⁻¹) was determined from the weight increase in the cup over time at the steady state of transfer using the following equation:

WVTR = (∆m × 24)/(∆t × S)

(2)

where ∆m (g) is the amount of water vapor passing through a film of area S (m²) during time ∆t (h). All gravimetric measurements were conducted in parallel and triplicated.

2.2.4. Contact Angle Measurements

The most common method of measuring the contact angle is to observe a sessile drop deposited onto the surface under investigation. Liquid drops (2–3 µL) were deposited onto the film surface, and a drop shape imaging device was used to image the drop, with the help of a Krüss G2 Goniometer, and determine the resulting contact angle. Three liquids were used for the contact angle measurements, water, diiodomethane, and α-bromonaphtalene. Measurements were performed with a relative humidity of 30 ± 5% RH and a room temperature of 22 ± 2 °C. In the same film, ten droplets were imaged at various locations. The mean of these ten values gives the contact angle. Contact angle measurements give access to the surface free energy of the HM- and HM-SA-formulated films. The surface tensions and experimental contact angles of probe liquids were used to calculate the surface free energy of the HM and HM-SA films. Fowkes’s theories [17] were expanded upon by Owens and Wendt [18] to include situations in which dispersion forces and polar forces are present. When Young’s equation was combined with Owens and Wendt’s approach, the following equation was obtained:

γ_l × (1 + cos θ) = 2 × (γ_s^D × γ_l^D)^1/2 + 2 × (γ_s^ND × γ_l^ND)^1/2

(3)

where θ is the contact angle of the probe liquid on the film solid surface, γ_l^D and γ_l^ND, respectively, are the dispersive and nondispersive (polar) components of the surface tension of the probe liquid, γ_s^D and γ_s^ND, respectively, are the dispersive and nondispersive (polar) components of the surface free energy of the solid (HM or HM-SA film), and γ_l is the surface tension of the probe liquid. Using a purely dispersive liquid to measure the contact angle, one can calculate the dispersive component of the surface energy of the film solid surface (γ_s^D). Using a polar liquid, contact angle measurements yield the nondispersive (polar) component (γ_s^ND). The total of the dispersive and nondispersive components of surface energy is known as surface-free energy.

2.2.5. FTIR-ATR Spectrometry

The presence of stearic acid on a film surface is investigated by ATR (Attenuated Total Reflectance) FTIR spectrometry in order to check the surface chemical signature and identify its composition. The ATR-FTIR wavenumbers range is 4000–700 cm⁻¹. ATR-FTIR spectrometry measures the amount of infrared light that a sample absorbs when in intimate contact with a high refractive index crystal. At the crystal–sample interface, the IR light interacts with the sample, absorbing a small amount of the light and reflecting the remainder back. An evanescent IR wave is generated at the interface and decreases exponentially in intensity within the sample. Since in ATR-FTIR spectrometry the penetration depth of the IR light is on the order of a few micrometers, it is especially useful for analyzing thin films, coatings, and material surface layers.

2.2.6. Tribology Measurements

Macroscopic tribological properties of HM- and HM-SA-formulated films were carried out using a conventional pin-on-disk tribometer [19]. HM and HM-SA films deposited on a glass plate came into contact with a glass ball (6 mm diameter) under a specified normal load. The glass plate was then rotated at a given speed, and the force opposed to the motion of the ball (tangential force F_T), which corresponds to the friction force, was recorded. The macroscopic friction coefficient is calculated automatically by dividing the measured friction force F_T by the specified applied normal load F_N, as indicated by Equation (4).

µ_macro = F_T/F_N

(4)

All experiments were conducted along a sliding distance of 100 mm at a sliding velocity of 5 mm.s⁻¹ and an acquisition rate of 100 Hz, under a normal load of 2 N, 5 N, and 10 N at 25 °C.

3. Results

3.1. Thickness and Appearance of HM-Formulated Films

A micrometer was used to measure the thickness of the films at various random locations. Every film has a 100 µm thickness. The standard deviation was 15% of the average thickness. Figure 1 shows images of pure HM and HM-SA films. Pure HM film is clear, transparent, uniform, and homogenous.

HM-SA films have a uniform appearance, but as the SA content increases, the gloss property increases, and the films turn white. Similar outcomes were obtained by Yang et al. [20] when palmitic acid was added to gelatin films. Lipid incorporation results in an increase in film opacity. Light scattering from lipid droplets that are dispersed throughout the film network following formation is most likely the cause of this phenomenon.

3.2. Water Affinity of HM and HM-Formulated Films

3.2.1. Moisture Sorption Isotherms of HM and HM-SA Films

In order to compare the behavior of the films based on the quantity of SA additive, the water affinity of HM and HM-SA films was investigated using sorption isotherms. The moisture sorption isotherm curves and experimental data for pure HM and HM-SA films containing SA are displayed in Figure 2. In general, any alteration to the film’s structure or composition may have an impact on its sorption isotherm. Nonpolar groups led to a decreased water affinity of films from the surrounding atmosphere.

Between 20% and 50% relative humidity, the equilibrium moisture content of all HM films increased gradually. It does, however, rise more quickly between 50% and 85% relative humidity. Increased water activities indicate that solubilization and swelling processes have resulted in a significant water gain in the films [21]. The sorption isotherm for the pure HM film resembled the one found by Chinnan et al. [22] for the films of pure hydroxypropylcellulose (HPC) and methylcellulose (MC). In contrast to pure HM film, HM-SA film with 1% (w/w HM) of stearic acid started to show a low water sorption isotherm. At lower acid contents (0.1% w/w HM), there was no discernible change. The hydrophobicity of fatty acid molecules causes films’ affinity for water to decrease. In our situation, the amount of fatty acid is limited because it significantly affects other physicochemical properties (mechanical, surface, and appearance properties) and renders films unusable for practical purposes. However, a number of studies have demonstrated that raising the SA/HM ratio increases the hydrophobic character of HM films [2] and reduces the sorption of water molecules [11,23].

3.2.2. WVTR Determination of HM and HM-SA Films

The influence of SA hydrophobic compound on water vapor barrier properties of HM- and HM-SA-formulated films was investigated by determining the WVTR. WVTR determination needs to have a steady flow of water vapor through the film per unit of time and per unit of area under controlled temperature and humidity conditions. During a transitory period of 4 days, both the contribution of moisture sorption isotherm and water vapor transmission process are observed, making WVTR determination not possible. Then, moisture sorption isotherm stops and water vapor transmission contribution only remains, making WVTR determination possible. Figure 3 shows the WVTR value of HM film and HM-SA-formulated film between day 4 and day 10.

Stearic acid incorporation in HM films results in a decrease in WVTR. A linear evolution of WVTR with time is observed for all films, but as the amount of SA increases in the formulated film, the value of the slope decreases. Water vapor transmission is taken as the slope of WVTR as a function of time in this linear region divided by the area of exchange. Slope values are reported in

Table 1. WVTR vs. time slope values of HM- and HM-SA-formulated films.

Film	Slope
HM	87 ± 1
HM–0.1% SA	86 ± 1
HM–0.5% SA	84 ± 1
HM–1% SA	80 ± 1

.

These findings concur with a number of studies on the characteristics of water barrier properties. According to studies by Tanaka et al. [24] and McHugh et al. [25], adding lipid compounds like fatty acids and beeswax to edible films made of proteins significantly reduces the permeability of water vapor. Péroval et al. [26], who investigated the impact of various lipids on the physicochemical properties of polysaccharide films, also discovered similar outcomes. According to Sebti et al. [2], the nonpolar nature of SA reduces the moisture affinity of films, so incorporating 15% (w/w HM) of SA allowed for a WVTR decrease of roughly 60%. Even though in the present study the amount of SA incorporation was much lower, the same tendency was observed.

3.3. Surface Roughness of HM and HM-SA Films

The surface roughness of HM and HM-SA films was carried out by tapping mode atomic force microscopy (TM-AFM). TM-AFM topographic images and section analysis profiles extracted from TM-AFM topographic images show variation in surface aspects, as reported in Figure 4.

The scan size of each topographic image is 1 µm × 1 µm, and the Z-range is 15 nm. Section analysis profiles allow the determination of average surface roughness, Ra, expressed in nm. Ra values are gathered in

Table 2. Average surface roughness values, Ra, of HM- and HM-SA-formulated films.

Film	Ra (nm)
HM	2.3 ± 0.2
HM–0.1% SA	1.8 ± 0.2
HM–0.5% SA	1.2 ± 0.3
HM–1% SA	0.8 ± 0.3

.

Ra value for pure HM film is equal to 2.3 nm and drops to 0.8 nm for films containing 1% stearic acid. The granular structure responsible for the high roughness value that distinguishes pure HM gradually vanishes as the amount of stearic acid rises. It is suggested that the migration of stearic acid molecules on the film surface is the cause of this effect [20]. When stearic acid content is high, homogenous layer formation is suspected. In this instance, a microphase separation between the hydrophobic SA additive and the HM polymer matrix is the cause of the potential migration of stearic acid at the surface. Characterizing the thermodynamic characteristics of the film surface will require contact angle measurements and surface free energy calculations in order to verify this hypothesis.

3.4. Water Contact Angle of HM and HM-SA Films

By measuring the water contact angle, the evolution of the surface hydrophobic character following the addition of SA in HM film was investigated. As previously stated, the angle formed by the substrate surface and the tangent line at the point where the liquid droplet and substrate come into contact is known as the contact angle. The contact angle values of the water droplets obtained for the HM and HM-SA films are displayed in

Table 3. Contact angles (θ°) values of water droplets on HM and HM-SA films.

Film	θ (°)
HM	69 ± 2
HM–0.1% SA	84 ± 2
HM–0.5% SA	91 ± 2
HM–1% SA	94 ± 2

.

It is well known that as surface hydrophobicity increases, so does the water contact angle. The results shown in Table 3 suggest that adding SA to HM films raises their water contact angle value, even for very low SA content (0.1% SA). A value of 94° is observed for HM film with 1% SA, indicating a highly hydrophobic surface. This tendency supports the hypothesis of phase separation of SA molecules and its accumulation at the film surface.

3.5. Variation in Surface Free Energy with SA Content

Wettability measurements of pure HM film yielded an initial surface free energy of 43 mJ.m⁻². Nevertheless, this energy is divided into two parts related to the hydrophilic and hydrophobic characteristics of the film surface, the nondispersive (10 mJ.m⁻²) and dispersive (33 mJ.m⁻²) components, respectively. The surface free energy obtained for HM-SA films as a function of stearic acid content is displayed in

Table 4. Surface free energy (γ_s), dispersive (γ_s^D), and nondispersive (γ_s^ND) components of HM and HM-SA films.

Film	γs (mJ.m−2)	γsD (mJ.m−2)	γsND (mJ.m−2)
HM	43 ± 1	33 ± 1	10 ± 1
SA	22 ± 1	22 ± 1	/
HM–0.1% SA	36 ± 1	32 ± 1	4 ± 1
HM–0.5% SA	34 ± 1	32 ± 1	2 ± 1
HM–1% SA	31 ± 1	30 ± 1	1 ± 1

.

The nondispersive component γ_s^ND makes up a significant part of the total surface free energy of HM pure film (10 mJ.m⁻²) compared with HM-SA films. The hydrophilic substituents (hydroxyl, methoxy, and hydroxypropoxy groups) of cellulosic-derived HM chains are responsible for this contribution. A significant drop in surface free energy is seen, even at low SA concentrations (0.1% w/w HM). When 1% SA (w/w HM) is added, the water contact angle rises from 69° to 94° (Table 3). Consequently, the surface free energy is reduced to 31 mJ.m⁻² by adding 1% SA (w/w HM). Considering that the surface energy of SA is equal to 22 mJ.m⁻², the nondispersive component is greatly impacted by the addition of SA, which indicates a decrease in the hydrophilicity of the film and the presence of nonpolar aliphatic chains at the top surface of the film. The initial outcome validates the hypothesis of phase separation and SA molecule migration at the formulated film surface. The variations in SA conformations and surface molecular orientation are most likely the cause of the surface energy differences between pure SA and HM-SA films. In fact, SAMs have been observed [27] for pure SA thin films but not for HM-SA films.

3.6. Quantification of the Migration of SA Molecules at the Film Surface

Finding the surface fraction of SA on the film surface is necessary to quantify the migration of stearic acid at the surface. The apparent contact angle θ* measured on a chemically heterogeneous solid surface consisting of two distinct materials is described by the Cassie–Baxter law [28,29]. The following Equation (5) can be used to express the Cassie–Baxter law:

cos θ*= f₁ × cos θ₁ + f₂ × cos θ₂

(5)

where θ₁ is the contact angle on pure HM film, and θ₂ is the contact angle on pure SA. The HM-SA film surface’s experimental contact angle is denoted by θ*. The corresponding surface fractions of HM and SA are represented by the factors f₁ and f₂, respectively, and f₁ = 1 − f₂. Equation (5) can be rewritten as follows:

cos θ*= cos θ₁ + f₂ × (cos θ₂ − cos θ₁)

(6)

One can compute the surface fraction of SA (f₂) on the film surface using θ* (Table 4), θ₁, and θ₂. The variation in f₂ obtained as a function of the SA concentration (% w/w HM) in HM-SA films is displayed in Figure 5. A value of SA surface fraction of 0.5 means that 50% of the HM-SA film surface is covered by SA molecules.

When SA concentration rises in HM films, the surface fraction of SA increases dramatically. Figure 5 shows that more than 50% of the HM-SA film surface can be covered by the 0.1% of SA added to the formulation and more than 80% of the HM-SA film surface can be covered by the 1% of SA added to the formulation. This finding emphasizes how phase separation causes hydrophobic additives (SA) to migrate on the surface, which lowers the surface free energy of HM-SA films.

3.7. IR Spectrometric Characterization of HM and HM-SA Films and Quantification of the Migration of SA Molecules at the HM-SA Film

Attenuated Total Reflectance (ATR) FTIR spectrometry is used to examine the film surface and detect the presence of SA molecules in order to determine the composition of the surface and verify its chemical signature. The ATR spectra of HM-SA films in the 3700–2700 cm⁻¹ range are displayed in Figure 6. The HM-SA spectra were normalized based on the peak at 1050 cm⁻¹, which corresponds to the stretching vibration ν (O-C-O) [30], in order to facilitate potential comparisons between them.

Due to symmetric and asymmetric CH₂ stretching modes, SA has two prominent peaks at 2847 cm⁻¹ and 2916 cm⁻¹, respectively. An accumulation of SA on the film’s surface is indicated by the increase in IR band intensity seen with SA content in HM films in the region of CH₂ stretching modes. The reasons for choosing the two IR peaks at 2847 cm⁻¹ and 2916 cm⁻¹ due to symmetric and asymmetric CH₂ stretching modes, respectively, are first because stretching modes are much more intense compared with the other bending, rocking, and twisting modes of the CH₂ functional groups, and second, because in the 3100–2700 cm⁻¹ wavenumber range, they contribute to strong IR absorption compared with HM, making them unambiguously highly visible on the IR spectrum. The composition of the first few micrometers of the film surface is revealed by ATR analysis. By monitoring the intensities of its designated peaks, the SA signature on the film surface can be found. The peak intensity at 2847 cm⁻¹ for a specific SA concentration in HM film can be written as follows:

I₁₊₂ = I₁ × f₁ + I₂ × f₂

(7)

where I₁ is the ATR peak intensity of pure HM film, I₂ is the ATR peak intensity of pure SA, and I₁₊₂ is the experimental ATR peak intensity of HM-SA film. The corresponding surface fractions of HM and SA are represented by the factors f₁ and f₂, respectively, and f₁ = 1 − f₂. Thus, it is possible to determine the surface fraction of SA by using Equation (7). The surface percentage of SA and the IR peak intensities for various HM-SA films are given in

Table 5. Intensity values of two representative IR absorption bands of SA molecules and a surface fraction of SA for HM-SA films.

Film	I (2916 cm−1)	I (2847 cm−1)	Surface Fraction of SA (%)
HM	0.081	0.050	0
SA	0.077	0.070	100
HM–0.1% SA	0.084	0.051	5
HM–0.5% SA	0.098	0.060	50
HM–1% SA	0.107	0.066	80

.

The ratio of the two peak intensities stays constant for all samples, but the IR absorption band intensities at 2847 cm⁻¹ and 2916 cm⁻¹ intensify with SA content. The presence of the nonpolar groups of SA molecules on the film surface is explained by the increase in peak intensities shown in Table 5. The SA surface fraction can be estimated using experimental intensities for HM-SA films (Table 5). The results show that the SA surface content is always significantly higher than the nominal bulk content. In fact, for a nominal content of 1% in HPMC-SA film, the surface fraction of SA is equal to 80%. This finding supports the hypothesis of SA molecule migration and is consistent with earlier findings regarding the surface free energy of HM-SA films, which demonstrated a clear increase in the nonpolar surface character and film surface hydrophobicity.

3.8. Effect of Stearic Acid on the Friction Properties of HM-SA Films

The macroscopic tribological properties of HM and HM-SA films were investigated through the friction of a chemically cleaned glass ball on the film surface with the help of a pin-on-disk tribometer. For each film, the friction coefficient is measured as a function of sliding time and sliding distance at a sliding velocity of 5 mm.s⁻¹. At the macroscale, there are no systematic variations in the friction coefficient with sliding distance. Generally speaking, values of friction coefficients remain rather constant, whatever the applied load in the range 2–10 N, for both HM and HM-SA films. The macroscopic friction coefficient, µ_macro, can be determined from the slope in the representation of the friction force (tangential force F_T) versus the applied normal load (normal force F_N).

First, for pure HM film, it is possible to observe a high dependence of friction force on normal force (i.e., applied load). The addition of a very low amount of stearic acid (0.1%) sharply alters the slope in the representation of F_T versus F_N and thus the friction coefficient. Adding more SA to the film has a limited effect on the lowering of the slope. On the basis of Figure 7, the macroscopic friction coefficient, µ_macro, is determined for HM and HM-SA films. Obtained values are gathered in

Table 6. Macroscopic friction coefficient, µ_macro, of HM and HM-SA films.

Film	µmacro
HM	0.38
HM–0.1% SA	0.12
HM–0.5% SA	0.10
HM–1% SA	0.08

.

The macroscopic friction coefficient for pure HM film is 0.38. This high friction coefficient found for pure HM film is explained by hydrophilic interactions between a polar glass ball and polar HM that is also composed of long, rigid polymer chains substituted by hydroxyl, methoxy, and hydroxypropoxy polar groups. Adhesion is therefore strong and inhibits easy sliding. On the other hand, the macroscopic friction coefficient of HM-SA films is significantly reduced when 0.1% (w/w HM) of SA is added. These findings are consistent with the hypothesis that a weak SA boundary layer forms [31], acting as an easy sliding agent at the top surface of formulated films. The macroscopic friction coefficient consequently drops as sliding begins under the application of a normal load.

4. Discussion

The addition of hydrophobic fatty acid SA molecules to HM films significantly alters the films’ surface characteristics. Wettability measurements logically supported the lowering of surface free energy caused by the addition of SA molecules. Section analysis shows that as the SA content increases, the average film roughness decreases. Surface migration of hydrophobic additive is hypothesized and supported by Cassie–Baxter law and FTIR-ATR spectrometry. Obviously, there is a strong correlation between each of these findings. At the macroscale, SA significantly affects friction properties. Indeed, SA molecule incorporation also lowers the macrofriction between a glass ball and the surface of the HM-SA-formulated films, allowing high sliding characteristics for HM-SA-formulated films. It is thought that hydrophilicity due to the polar hydroxyl, methoxy, and hydroxypropoxy groups on cellulosic chains of HM matrix, which causes phase separation and surface migration of hydrophobic SA molecules and thus the formation of a weak boundary layer, is the reason behind it. This weak boundary layer has low cohesion, leading to easy sliding properties during ball–film surface contact. Furthermore, the fatty acid’s low molecular weight promotes surface sliding and lowers friction. All these findings indicate that hydrophobic SA molecules act as a lubricant due to their surface migration driven by phase separation with a hydrophilic HM matrix.

5. Conclusions

In the present work, particular attention was focused on the surface properties of HM- and HM-SA-formulated films. Evidence for the chemical nature and concentration of additive control surfaces’ hydrophobic/hydrophilic character and friction properties was exposed. First, HM-SA films showed a decrease in transparency, suspected to be due to phase separation between the hydrophobic SA and HM matrix. A limited reduction in water uptake was observed with the addition of 1% SA, and the transfer of water vapor through the film was also reduced. Simultaneously the addition of SA decreases the surface roughness. The hypothesis of SA surface migration is proposed and confirmed by wettability measurements and ATR-FTIR analysis. Indeed, quantification of the migration of SA molecules at the film surface was performed by computing the surface fraction of the SA molecule. Moreover, it was demonstrated that the addition of SA in formulated films first sharply decreases the surface energy and second the friction coefficient of HM-SA films due to phase separation between the HM and SA. SA molecules play the role of a solid lubricant. The results clearly underline the strong dependence of HM-formulated film surface properties on additive nature and concentration and the interplay with additive–HM polymer matrix compatibility. The synergy of multitechnique and multiscale approaches allows a more comprehensive understanding of the relationships between structures and surface properties of HM-formulated films.