Potential of organogel‐based colloidal hydrogels for new cosmetic formulations

Abstract

Objective

The cosmetics industry is always seeking new formulations that offer pleasant and effective textures. This study aimed to explore new systems based on aqueous dispersions of organogel particles capable of forming colloidal hydrogels.

Methods

Aqueous dispersions of gelled oil particles were obtained by hot formation of oil‐in‐water emulsions followed by cooling. Depending on the type of oil and dispersing agent, different colloidal hydrogels were formed. Their microstructure, texture, and sensory properties were evaluated.

Results

By using 12‐hydroxystearic acid (HSA) as the organogelator, colloidal hydrogels were systematically obtained, regardless of the oil or surfactant used. However, microstructures and resulting textures could be modulated from a rheological or sensory point of view by adjusting ingredient type and proportion. Moreover, nonionic surfactants led to firmer textures than charged surfactants.

Conclusion

Stable colloidal hydrogels were developed using two emollient oils and three common surfactants. Texture analyses, as well as their rheological and sensory behaviour, demonstrated their potential interest in the development of new cosmetic formulations.

Colloidal hydrogels are formed by combining anionic, cationic, or nonionic surfactants with two oils of cosmetic interest: sweet almond oil and phytosqualane. These systems are obtained through hot emulsification followed by cooling, which leads to the aggregation of gel‐like particles. The texture and cohesion of the hydrogels depend on both the type of oil and the nature of the surfactant, resulting in gels with varying firmness and cohesiveness. This results in gels with different levels of spreadability. This approach offers a new pathway for designing stable, pleasant‐to‐use, and polymer‐free cosmetic gels.

INTRODUCTION

Colloidal gels represent a fascinating class of soft materials with a wide range of applications across various fields, such as food science, pharmaceuticals, personal care and construction materials [1, 2, 3, 4, 5, 6, 7, 8]. Depending on the functionalization of the particles, colloidal gels can also be used in advanced domains such as novel materials for biomedical applications and energy storage systems [9, 10, 11, 12, 13, 14]. Colloidal gelation involves attractive aggregation between colloidal particles, resulting in a three‐dimensional (3D) solid network embedded in a liquid phase, leading to the formation of a gel with unique mechanical and rheological properties [15, 16, 17, 18]. The gel network in such a system is obtained by physical interactions, so the application of a high mechanical stress can induce a reversible change in both the internal microstructure of the gel and its viscoelastic behaviour [19, 20, 21, 22]. Colloidal hydrogels are soft and deformable materials comprising a solid particle network interpenetrated by water‐filled pores. In materials science, such systems have found utility as templates for porous materials synthesis, scaffolds for tissue engineering, and matrices for drug delivery systems [23]. The tunability of gel properties allows control of pore size, mechanical strength, and drug release kinetics, making them highly adaptable for biomedical applications [8, 24]. In the field of cosmetics, colloidal hydrogels offer various possibilities due to their unique properties, such as water retention capacity, smooth textural profile, and stable structure. Overall, these systems offer versatility in cosmetic formulations, providing both functional and aesthetic benefits to various skincare and haircare products.

In a recent study, we described a new family of colloidal hydrogels based on a 3D network of connected gelled oil particles [25]. As an aqueous stable dispersion of gelled oil particles has already been described [26, 27, 28, 29, 30, 31, 32, 33], the development of colloidal hydrogels based on organogel particles had not been documented. The preparation process was based on hot emulsification (T > Tgel), due to the sol–gel phase transition of the organogel, in the presence of hydrolysed polyvinyl alcohol (PVA80) as a stabilizing agent, followed by cooling the system below its characteristic Tgel (T < Tgel). As organic gelators, we chose 12‐hydroxystearic acid (HSA), an efficient and well‐known gelator widely used for cosmetic formulations [3], and sweet almond oil or phytosqualane as oils. In this first study, we demonstrated that the formation of colloidal hydrogels based on gelled oil particles is strongly dependent on the nature of the oil and the presence or absence of a preservative. Surfactants as stabilizing agents are widely used in cosmetic formulations, so it was interesting to test the impact of such stabilizing agents on stability and the formation of colloidal hydrogels based on organogel particles.

MATERIALS AND METHODS

Chemicals/materials

Sweet almond oil was provided by Symrise (Holzminden, Germany) and phytosqualane by Sophim (Peyruis, France). Cosmetic‐grade HSA was purchased from Vertellus (Indianapolis, IN, USA) and tocopherol from BASF (Ludwigshafen, Germany). Polysorbate 80 (Tween 80®) was obtained from Sigma‐Aldrich (St. Louis, MO, USA), cetyltrimethylammonium chloride (solution at 25 wt% in water) from Cognis Care Chemicals (Monheim, Germany), and sodium cocoyl glutamate (SCG) from Ajinomoto (Tokyo, Japan). The preservative was made of a mixture of 3‐(4‐chlorophenoxy)‐1,2‐propanediol (99%; Thermo Scientific, Germany), 1,2‐octanediol (98%; Sigma‐Aldrich, Missouri, USA), and 2‐phenoxyethanol (Sigma‐Aldrich, Missouri, USA). Distilled water was used for the preparation of colloidal dispersions.

Preparation of organogels

Organogels were prepared by dissolving the organogelator, HSA (10 wt% of the oil phase), in the respective oil (phytosqualane or sweet almond oil). As sweet almond oil oxidizes easily, tocopherol (0.2 wt% of the oil phase) was also added to the mixture. The mixture was heated to 85°C and magnetically stirred (300 rpm) until complete dissolution and then cooled to room temperature to obtain the gel. The resulting organogel was yellow with sweet almond oil or white with phytosqualane when using HSA.

Preparation of the preservative

The preservative was prepared by mixing 15 wt% of 3‐(4‐chlorophenoxy)‐1,2‐propanediol, 15 wt% of 1,2‐octanediol (98%), and 70 wt% of 2‐phenoxyethanol. The mixture was magnetically stirred (2 h, 350 rpm) until a clear solution was obtained.

Preparation of colloidal hydrogels

Colloidal hydrogels were prepared by combining, in a beaker, the organogel (2 wt%–25 wt%) and the aqueous phase, consisting of a mix of preservative (1 wt% of the aqueous phase), surfactant (3 wt% of the aqueous phase), and distilled water. The mixture was placed in an oven (85°C, 1 h) until the organogel melted and then emulsified by sonication (Sonic & Materials Vibra‐cell VCX750, 10 min, amplitude 80%, pulse of 3 s, pause of 2 s). The resulting dispersion was cooled to room temperature and stored for 24 h before characterization.

Dynamic light scattering (DLS)

To analyse the particle size distribution, DLS was performed using a Malvern Zetasizer Nano‐S equipped with a He–Ne laser (633 nm) at a scattering angle of 170°C and at 25 ± 1°C. The refractive indices of phytosqualane and sweet almond oil are 1.457 and 1.469, respectively, as previously determined by B. Duret et al. [25]. The samples of the colloidal dispersions were diluted in 50 mL of distilled water, stirred manually, then for 10 s by vortex mixer, and finally homogenized for 10 min in an ultrasound bath. The diluted solution was then analysed, and the mean particle diameter (Z‐average) was obtained (calculated by the Malvern software using the CONTIN model and the average of nine runs).

Cryo‐scanning electron microscopy (cryo‐SEM)

Colloidal dispersions were observed using a Quanta 250 FEG cryo‐SEM. Prior to the observation, all the samples were frozen by a high‐pressure technique (high‐pressure freezing [HPF]) to maintain their natural, hydrated state. To achieve this, the samples were introduced into HPF specimen carriers, in which they were fixed and frozen by an ICE HPF Leica's machine at 2050 bar within 5 ms. The frozen samples were then transferred into cryovials under liquid nitrogen. Finally, the specimens were transferred and fractured at −140°C with the help of a scalpel blade in the PP3000T Quorum cryochamber (if not fractured naturally while separating the two carriers), sublimated at −95°C for 1 h, and metalized with platinum for 30 s at 10 mA before being introduced into the Quanta 250 FEG SEM chamber. The samples were observed at −140°C and at 5–10 kV.

Texture analysis

Analysis of the systems' texture properties was performed using a texture analyser TAXT Plus (Stable Micro Systems, Cardiff, UK).

Evaluation of the colloidal hydrogel texture at rest: Penetration test

A penetration test was conducted using a cylindrical probe (P/0.5R) with a diameter of 10 mm, which penetrated the colloidal hydrogel placed in 500 mL containers to a height of 20 mm. The probe penetrated to a depth of 15 mm at a specified rate of 1 mm/s, before returning to its initial position. The area of penetration was recorded [34].

Consistency at rest: Compression test

The compression test was performed using a cylindrical aluminium probe (P/35) with a diameter of 35 mm. One gram of product was carefully deposited onto the base of the texture analyser using a spatula to avoid shearing and preserve the structural integrity of the sample. The probe was then lowered to compress the product at a constant speed of 1 mm/s until a gap of 0.5 mm was reached between the probe and the base. After compression, the probe returned to its initial position at the same speed of 1 mm/s. The area of compression was recorded [34].

Application properties of colloidal hydrogels: Spreading test

Spreading measurements were performed on the different systems using a texture analyser following the method described by Gilbert et al. [34]. The test employed a friction module (A/FR probe). A volume of 200 μL of product was deposited on a poly(methyl methacrylate) (PMMA) plate with a surface roughness of 6 μm (Heliplate HD6), in the form of four parallel lines of 50 μL each. The plate was then mounted on a sled‐type support and drawn across a polypropylene surface at a constant speed of 3 mm/s over a distance of 120 mm.

The positive area under the force–distance curve (g.s) was subsequently calculated over the full 120‐mm travel distance. This area reflects the resistance to spreading, with higher values indicating greater difficulty in spreading the systems. For each formulation, a minimum of three replicates was performed at room temperature. A blank test (without product) was also conducted at the beginning of the analysis to simulate the behaviour of bare skin.

Stickiness properties of colloidal hydrogels: Adhesion test

Thirty‐five microlitres of the product are applied using 10 rotations on a 9.60‐cm² disk area defined on the Biody (formerly Bioskin, urethane elastomer; Beaulax, Japan). The measurement takes place 3 min after the product application. The texturometer is equipped with a spherical probe with a diameter of 6.4 mm and a 500‐g force sensor. The probe descends onto the surface until it reaches a force of 500 g, holds for 2 s, and then ascends at a speed of 0.4 mm/s. The negative area (g.s) under the curve is measured [35].

Data analysis

XLSTAT software (Version 2016.02.27444; Addinsoft, Paris, France) was used to perform the statistical analyses of the collected data. The ANOVA test (two‐way analysis of variance) was applied to the results to spot the significant differences between colloidal hydrogels (p < 0.05). Mean intensities were compared using the Fisher multiple comparison tests to distinguish different groups of products.

Predictive model

Predictive models of sensory texture attributes based on instrumental measurements were used to determine the sensory properties of the formulated gelled oil particles and colloidal hydrogels. The models developed by Gilbert et al. [34] (related to compression force and difficulty of spreading) and Eudier et al. [35] (related to stickiness after skin application) were applied to assess the respective sensory dimensions.

RESULTS AND DISCUSSION

Formulation and visual appearance of the systems

In this study, two emollient oils commonly used in cosmetic formulations, sweet almond oil and phytosqualane, were selected. These oils differ in their viscosity at 20°C, 60 mPa·s⁻¹ for sweet almond oil and 30 mPa·s¹ for phytosqualane, as well as in their polarity. These differences are closely related to their chemical compositions, namely triglycerides for sweet almond oil and hydrocarbons for phytosqualane. Consequently, their interfacial tension with water at 20°C also varies significantly, measured at 22 mN·m⁻¹ for sweet almond oil and 55 mN·m⁻¹ for phytosqualane. Sweet almond oil is therefore approximately twice as viscous and polar as phytosqualane. To investigate the influence of surfactant charge on the formation and texture of colloidal hydrogels, three surfactants with different ionic characteristics were selected: a nonionic, an anionic, and a cationic. Polysorbate 80 (Tween 80©), a polyethoxylated sorbitan ester, was used as the nonionic surfactant. SCG, a derivative of coconut oil and glutamic acid, served as the anionic surfactant, and cetyltrimethylammonium chloride (CTAC) was selected as the cationic surfactant. These surfactants are widely used in cosmetic formulations and are representative of their respective charge categories. Six different formulations were prepared, based on the following mass proportions: 3% surfactant, 25% oil, 10% HSA in the oil phase, and 62% water (containing 1% of preservative). Table 1 summarizes the visual appearance and consistency for each formulation.

TABLE 1.

Visual appearance and consistencies of the hydrogels depending on the oil and surfactant used (3% surfactant, 25% oil, 10% 12‐hydroxystearic acid (HSA) in oil and 62% water containing 1% preservative).

Regardless of the oil or surfactant used, colloidal hydrogels are observed (Table 1), exhibiting a firm consistency that varies in granularity depending on the formulation. The smoothest and most compact systems are observed with the anionic surfactant for both oils, as well as CTAC combined with phytosqualane. Although these differences are primarily visual, they may reflect variations in the firmness of the hydrogel and, consequently, of the interconnected 3D network formed by the gelled particles. A network composed of loosely connected particles would result in a weak structure and a more fluid gel (with a smooth appearance), as observed with the anionic surfactant. In contrast, more granular (less smooth) formulations may correspond to hydrogels with a more cohesive 3D network. The system combining phytosqualane and CTAC exhibits an intermediate consistency, which may reflect a colloidal hydrogel composed of moderately connected particles.

All formulations with both oils and the three surfactants were observed by electron microscopy. In Figure 1, the three systems with sweet almond oil and surfactants are presented. Regardless of the surfactant used, a connection between gelled particles can be observed in each sample. Such behaviour is characteristic of a colloidal hydrogel [25]. However, it is difficult to observe a difference at the sample level in terms of particles connection that could be correlated with the texture differences. Moreover, these images exhibit spherical polydisperse particles with mean diameters varying from 100 nm for SCG and CTAC to 400 nm for Tween 80‐stabilized particles. Such variation may be due to the better electrostatic stabilization of charged surfactants compared to Tween80, which is nonionic. We note that the same behaviour and particles sizes were obtained with phytosqualane oil systems.

FIGURE 1.

Cryo‐scanning electron microscopy (cryo‐SEM) micrographs of samples with sweet almond oil; 25 wt.% of the oily phase containing 10 wt.% of 12‐hydroxystearic acid (HSA) and stabilized with 3 wt.% of surfactant. (a) Sample stabilized with Tween 80. (b) Sample stabilized with sodium cocoyl glutamate. (c) Sample stabilized with cetyltrimethylammonium chloride (scale bar: 1 μm).

Influence of oil content and DLS analysis

The formation and cohesion of a colloidal hydrogel are strongly dependent on both the number of interconnected particles and their size [25]. For the systems formulated in this study, the oil phase content and the emulsification method are key parameters that directly affect the number and size of the gelled particles. Accordingly, for a given emulsification method and across the various surfactants tested, the influence of oil phase content on the colloidal hydrogel formation was investigated and the size of the resulting particles measured. DLS was employed to determine the mean nanoparticle diameter and size distribution. Before measurements, the colloidal hydrogel was diluted and sonicated to destroy the network. For each surfactant, an oil concentration range from 2% to 25% (w/w) was prepared to identify the lower limit required for colloidal hydrogel formation. This sol–gel transition was evaluated visually, based on the transition from a fluid to a more structured, solid‐like hydrogel state. For the anionic surfactant, the critical oil concentration for hydrogel formation was reached at 15% for phytosqualane and 20% for sweet almond oil. For the cationic surfactant, the threshold remained at 15%, regardless of the oil type. Whereas for the nonionic surfactant, it was significantly lower, at 5%, and also independent of the oil type. These preliminary observations indicate that colloidal hydrogel formation is more readily achieved with the nonionic surfactant than with the systems formulated with charged surfactants. In contrast, for charged surfactants, the threshold is substantially higher (15%) and can even reach 20% in the case of the anionic surfactant combined with sweet almond oil. The fact that all tested surfactants led to the formation of colloidal hydrogels suggests that none of them is sufficiently effective [25], with the worst being Tween 80©, the nonionic surfactant.

Analysis of Tables 2 and 3 reveals that, within the studied range of oil concentration, the average particle diameter of the colloidal hydrogel is significantly larger when using the nonionic surfactant. For this surfactant, the particles formed with sweet almond oil are slightly larger than those with phytosqualane, resulting from the higher viscosity of sweet almond oil. Moreover, for the nonionic surfactant, an increase in the average particle diameter at higher oil concentrations is observed, while for ionic surfactants it remains similar. For sweet almond oil, particle size increases from 253 to 386 μm and for phytosqualane oil, from 180 to 253 μm. This observation reflects a destabilization process produced by a coalescence phenomenon. With ionic surfactants (SCG and CTAC), due to their electrostatic repulsion properties, the particle coalescence was reduced and their average particle diameter remained stable, even at 25% of oil. As for the polydispersity index (PDI) with the nonionic surfactant, it remained low (0.1) at lower oil concentrations, but increased significantly at higher oil concentrations, reaching 0.2 at 20%, possibly due to greater shear resistance during the emulsification process. In contrast, the use of ionic surfactants yielded smaller and comparable‐sized particles for both oils with low PDI (0.1). However, a slight increase in PDI (0.2) was observed when ionic surfactants were used with sweet almond oil.

TABLE 2.

Mean particle diameter and polydispersity index of colloidal hydrogels formulated with varying sweet almond oil concentrations, depending on the surfactant type.

Sweet almond oil %	Tween 80© (nonionic)	SCG (anionic)	CTAC (cationic)
5	253 ± 25 nm (0.1 ± 0.05)	‐	‐
10	213 ± 15 nm (0.1 ± 0.04)	‐	‐
15	242 ± 17 nm (0.1 ± 0.03)	‐	175 ± 10 nm (0.2 ± 0.03)
20	332 ± 22 nm (0.2 ± 0.07)	131 ± 7 nm (0.2 ± 0.04)	137 ± 4 nm (0.2 ± 0.04)
25	386 ± 15 nm (0.2 ± 0.06)	162 ± 18 nm (0.1 ± 0.02)	141 ± 7 nm (0.2 ± 0.02)

Abbreviations: CTAC, cetyltrimethylammonium chloride; SCG, sodium cocoyl glutamate.

TABLE 3.

Mean particle diameter and polydispersity index of colloidal hydrogels formulated with varying phytosqualane oil concentrations, depending on the surfactant type.

Phytosqualane %	Tween 80© (nonionic)	SCG (anionic)	CTAC (cationic)
5	180 ± 16 nm (0.1 ± 0.04)	‐	‐
10	195 ± 7 nm (0.1 ± 0.06)	‐	‐
15	225 ± 15 nm (0.2 ± 0.09)	147 ± 8 nm (0.1 ± 0.02)	166 ± 35 nm (0.1 ± 0.03)
20	248 ± 45 nm (0.2 ± 0.10)	152 ± 26 nm (0.1 ± 0.03)	154 ± 15 nm (0.1 ± 0.02)
25	253 ± 9 nm (0.3 ± 0.08)	138 ± 6 nm (0.1 ± 0.02)	152 ± 19 nm (0.1 ± 0.02)

Abbreviations: CTAC, cetyltrimethylammonium chloride; SCG, sodium cocoyl glutamate.

These results are consistent with the observations made on the colloidal hydrogels, which clearly exhibit better structural outcomes when a nonionic surfactant is used, regardless of the oil type. As demonstrated in a previous study using PVA80 as stabilizing agent [25], the transition step from a hot emulsion to a dispersion of gelled oil particles upon cooling is a crucial step in the successful formation of a colloidal hydrogel. During this phase, the organogelator, HSA, self‐organizes within the oil droplets, initiating preferentially at the interface due to its amphiphilic‐like character. This transition significantly disrupts the droplet interface and promotes interconnections through gel bridging (coalescence), especially when the stabilizing agent is displaced from the interface and becomes less efficient. Consequently, a connected network of organogel particles (colloidal hydrogel) is formed, with a higher density of interconnections when the stabilizer is less efficient. In all cases, the presence of HSA is essential for colloidal hydrogel formation. The fact that all tested surfactants led to the formation of colloidal hydrogels suggests that none of them was sufficiently effective at maintaining interfacial stability during droplet gelation. Among these, Tween 80© was the least effective, being rapidly displaced from the interface even at very low oil concentrations. This displacement facilitates the development of a well‐connected 3D network, characteristic of colloidal hydrogels. In contrast, ionic surfactants generally resulted in less structured hydrogels, likely due to their enhanced ability to maintain interfacial stability because they are less sensitive to interfacial disruption during HSA gelation.

This charge‐related effect can be interpreted by considering that, during gelation, HSA introduces a slight negative charge to the surface of the gelled oil particles (zeta potential = −35 ± 6) which can be attributed to the carboxylic acid groups [31]. This surface charge introduces a stabilizing effect through electrostatic repulsion, which remains predominant with charged surfactants, but is greatly reduced when using a nonionic surfactant.

Instrumental texture analysis of colloidal hydrogels: Contribution of oil type, surfactants and their interaction on sensorial applications

To comprehensively characterize the texture of the formulated colloidal hydrogels and to better understand their behaviour during topical use, a series of instrumental texture analyses were conducted. These analyses were designed to mimic the sequential steps performed by consumers when applying a cosmetic product. Initially, a penetration test was performed to evaluate the firmness and structural integrity of the product at rest. This was followed by a compression test, simulating the withdrawal of the product from its container. Subsequently, a spreading test was carried out to reproduce the application and distribution of the formulation over the skin surface. Finally, an adhesion test was implemented to assess the properties of the residual film after application, with particular emphasis on stickiness. Together, these complementary tests provide a detailed instrumental approach to predict the sensory attributes and application behaviour of the colloidal hydrogel systems.

Figure 2 gathers the penetration areas collected for the six products during the penetration step of the probe. When considering the sweet almond oil‐based gelled particles, the systems exhibited significantly different penetration areas, with values ranging from approximately 450 to 6000 g·s. This variation reflects the impact of formulation parameters, particularly surfactant type, on the mechanical resistance of the gels. Among the tested formulations, the system containing Tween 80 showed the highest penetration area, indicating a firmer and more consistent texture. The formulation with CTAC presented an intermediate firmness, while the system based on SCG exhibited the lowest penetration area, suggesting a softer, less structured gel. Phytosqualane‐based systems exhibited significantly different penetration areas, like those with almond oil, with values ranging from approximately 1000 to 5000 g·s. The highest penetration area was observed in the formulation containing Tween 80, indicating the greatest firmness and consistency among the tested samples. This was followed by the system incorporating CTAC, while the formulation with SCG displayed the lowest area, corresponding to a softer and less structured texture. It is important to note that even though SCG‐based colloidal hydrogels had the lowest values compared to those with Tween 80 and CTAC ones, they still exhibited firmness and consistency. As proof, the penetration area, correlated with firmness and shape integrity (texture at rest), varied between 450 and 1000 g.s, values that are found in conventional emulsions stabilized with 1% carbomer‐type gelling polymers [36].

FIGURE 2.

Penetration area observed for each formulation.

When examining the influence of surfactants on the colloidal hydrogel texture, a clear trend emerges across all formulations: Tween 80 consistently led to firmer and more structured gels compared to CTAC, and SCG resulted in the least firm textures. This hierarchy of consistency Tween 80 > CTAC > SCG was maintained regardless of the oil phase used, underscoring the dominant role of the surfactant's molecular structure and charge in determining gel strength. Notably, when comparing the effect of oil phase polarity within the same surfactant type, differences in texture became apparent. For both Tween 80 and CTAC, formulations incorporating sweet almond oil exhibited higher penetration areas associated with ruptures in the penetration curve shapes, indicating the formation of a highly inhomogeneous and brittle network. Interestingly, this behaviour was also observed with Tween 80–phytosqualane‐based gelled particles (results not shown). However, sweet almond oil‐based colloidal hydrogels, stabilized by anionic surfactants, and phytosqualane‐based colloidal hydrogels, stabilized by anionic and cationic surfactants, exhibited a penetration curve similar to that of a classic emulsion, indicating a firmer structure. Finally, the trend was reversed with SCG: in this case, phytosqualane‐based systems displayed greater firmness than their sweet almond oil counterparts. This inversion of consistency suggests a specific interaction between the anionic surfactant and the oil phase that is dependent on their respective polarities. These observations highlight the complex interplay between the oil phase and the surfactant in modulating the internal organization of the colloidal hydrogel systems. As a result, the consistency of colloidal hydrogels is not solely a function of the surfactant's intrinsic properties, but also of how these properties are expressed in the presence of a given oil.

Figure 3 represents the predictive value for consistency, on a 0–9 scale. The firmness (compression force attribute) of the colloidal hydrogel systems was evaluated to better understand the role of the surfactant and oil phase in modulating texture under resting conditions and pick‐up. Sweet almond oil‐based formulations exhibited firmness values ranging from 6 to 9. Among these, the system containing Tween 80 demonstrated the highest firmness, followed by the cationic surfactant CTAC, while the formulation with the anionic surfactant SCG was significantly less firm. In parallel, phytosqualane‐based systems showed firmness values between 7 and 9, with overall narrower variations compared to the sweet almond oil systems. Once again, Tween 80 generated the firmest formulation, followed by CTAC and SCG. Interestingly, the firmness values for the CTAC‐ and SCG‐based systems in the phytosqualane series were nearly identical. When analysing the influence of surfactants across both oil phases, a clear trend emerged. For a given surfactant, the resulting texture remained relatively consistent regardless of whether sweet almond oil or phytosqualane was used. This suggests that the surfactant exerted a more dominant influence on the colloidal hydrogel consistency than the oil phase itself. Indeed, the firmness hierarchy Tween 80 > CTAC > SCG was preserved in both oil contexts. Once again, all colloidal hydrogels could be considered consistent and firm, with a sensory evaluation at least two points above the average (4.5).

FIGURE 3.

Predicted firmness score of the systems obtained from the compression test.

These results support the hypothesis that surfactant polarity and charge significantly impact the formation and stability of the gel network, whereas the oil phase, though it contributes to the overall formulation properties, does not serve as a major driver of textural modulation under static conditions.

The instrumental spreading test, conducted on colloidal hydrogels, enables the prediction of their spreading difficulty on the skin. Results are given on a scale from 0 to 9, with a score of 9 being the most difficult to spread (Figure 4a). This instrumental test, described in the Materials and Methods section, is highly correlated to in vivo difficulty of spreading, as already evidenced in previous works [34, 37]. This is achieved through the integration of the positive area under the force–distance curve, which is input into a predictive sensory model. The resulting spreading scores varied from 5 to 9 across the formulations, indicating notable differences in spreadability. All systems scored above the average score (4.5), suggesting that, overall, the formulations were relatively difficult to spread. Sweet almond oil‐based systems exhibited similar spreading resistance, with sensory scores ranging from 7.3 to 8.7. These values did not represent statistically or sensorially significant differences, implying comparable application behaviour. Trace analysis (Figure 4b) confirmed full product spreading for all three systems in this group. In contrast, phytosqualane‐based systems revealed more heterogeneous spreading profiles. The formulation containing the anionic surfactant SCG showed the highest spreading difficulty, with a score of 9. Meanwhile, the systems incorporating nonionic (Tween 80) and cationic (CTAC) surfactants displayed lower spreading resistance, with scores ranging from 5 to 6, indicating easier application. Trace analysis revealed continuous product spreading only in the SCG‐based formulation, with a trace like those in Figure 4b. For the other two systems, spreading was incomplete (Figure 4c): initial friction was observed during the first centimetres of movement, followed by free sliding of the probe without significant product transfer. During the instrumental spreading test, the product is first pipetted and deposited on the friction mobile before the mobile is placed onto the spreading substrate and the test is initiated. The product is therefore confined between the mobile surface and the spreading substrate and is driven by the mobile surface, which has a defined and constant weight. Both the pipetting step and the applied load induce shear, which may alter the initial consistency of the formulation and disrupt the particulate network within the different hydrogels. As spreading proceeds, the progressive reduction in the amount of product—and consequently in the film thickness beneath the mobile—leads to an increase in shear rate and in the force required to maintain the mobile motion. Therefore, we can notice that the sweet almond oil systems conserved their ranking, modulo the effect of shearing (CTAC more difficult to spread than Tween 80). On the contrary, considering phytosqualane‐based colloidal hydrogels, the effect of shear has more impact and SCG exhibits a better resistance compared to CTAC and, at last, Tween 80, probably in connection with the sensitivity of their respective networks to shear.

FIGURE 4.

(a) Predicted spreading difficulty score for the six systems. (b) Continuous trace observed over the full spreading path of sweet almond oil‐based systems (the red arrow indicates the spreading direction). (c) Observation of a non‐spreading trace in phytosqualane‐containing formulations (the red arrow indicates the spreading direction).

Nevertheless, this remains an instrumental test designed to discriminate between products by mimicking longitudinal spreading on non‐absorbent surfaces. As it is based on specific interactions between the two materials involved—PMMA and polypropylene—it may differ from the complex nature of real skin. As such, differences may be expected between this model and actual application conditions. A proper sensory evaluation would therefore be necessary to complete and accurately assess the in vivo spreading behaviour of these formulations. These results suggest that complementary in vivo evaluations could help clarify the respective contributions of shear stress and substrate physicochemical properties, as non‐biological surfaces do not fully reproduce the characteristics of human skin.

The adhesion test performed with the BIODY® material provides valuable insight into the perceived stickiness of systems after topical application. Specifically, the negative area measured during the probe detachment phase is a quantitative indicator of adhesive force between the product and the skin surface. This parameter is integrated into a predictive sensory model that translates mechanical data into stickiness scores on a standardized scale from 0 to 7. For the systems developed in this study, the predicted stickiness values ranged from 1.7 to 3.1 (Figure 5), which are considered low within the cosmetic sensory spectrum. These results suggest that all tested formulations possess a low degree of stickiness after application, a desirable result for this sensory attribute in terms of user comfort and product acceptance.

FIGURE 5.

Predicted stickiness after application rating of the residual film.

CONCLUSION

Colloidal hydrogels can be formulated using surfactants in combination with two oils exhibiting distinct physicochemical properties: sweet almond oil and phytosqualane. These colloidal hydrogels result from the aggregation of gelled particles during the cooling process and are associated with limited stabilization by the surfactant molecules. In fact, the greater the aggregation of these gelled particles, the more cohesive the system becomes. Results obtained with sweet almond oil demonstrate that the texture can be modulated by adjusting the system's cohesion, which is directly related to the stabilization of the gelled particles. Microstructural analysis of the hydrogel reveals that nonionic surfactants promote strong cohesion between gelled particles, whereas ionic surfactants result in weaker cohesion. These differences manifest macroscopically, as the colloidal hydrogels exhibit a range of textures depending on the type of oil used. The spreading of these hydrogels remains homogeneous and is favoured by the presence of ionic surfactants; such behaviour is very interesting for topical applications. However, in the case of phytosqualane, stronger cohesion between gelled particles results in increased firmness but also limits spreading.

This study provides valuable insights into how surfactants can be selected to modulate hydrogel texture based on the physicochemical properties of the oils, namely their viscosity and polarity as well as the nature of the surfactant. This approach enables the development of simplified dispersed and gelled systems, offering high consistency at rest and shear sensitivity, allowing for application on the skin comparable to typical commercial dispersed and gelled systems formulated with synthetic polymers such as Carbopol.

CONFLICT OF INTEREST STATEMENT

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available from the corresponding author upon reasonable request.