Spreading behavior of cosmetic emulsions: Impact of the oil phase

Biotribology 16 (2018) 17–24

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Spreading behavior of cosmetic emulsions: Impact of the oil phase ⁎

T

Ecaterina Gore , Céline Picard, Géraldine Savary Normandie Univ, UNILEHAVRE, FR 3038 CNRS, URCOM, 25 rue Philippe Lebon, BP 1123, 76063, Le Havre cedex, France

A R T I C LE I N FO

A B S T R A C T

Keywords: Spreading Emollient Skin Oil phase Emulsion

Background: Emollients are an important ingredient in personal care products, they are currently prescribed in skin disorders like eczema, which affects up to 30% of children in developed countries. Methods: The aim of the study was to investigate the impact of two emollients (stearic acid and isohexadecane) and their mixtures in the spreadability and frictional effect on films obtained from oil-in-water emulsions. Rheological, textural, sensory and tribological analysis were performed on human skin and artificial substrates. Results: The emollients ratio influences the spreading behavior of emulsions: more isohexadecane in the oil phase easier to spread the product on the skin. Moreover, significant correlations were obtained for the spreading behavior obtained by textural measurements on artificial substrates and sensory analysis (Pearson coefficient = −0.871). The results obtained by frictiometer showed different developments over time after product application: the friction values increase with the stearic acid concentration in emulsion. Discussion: This study showed the importance to consider the emollient properties when one emollient is used in the emulsion, but especially their interactions, when several emollients are used, to better understand and anticipate their behavior. First, spreading was governed by the consistency of the emulsion, particularly impacted by the emollients ratio. But then, in a long-time spreading, when the emulsion broke down and residual film was formed, a particular interaction with skin influenced the spreadability. It appears that not only the physical state of the emollient but also its chemical nature, physical state, polarity, temperature might explain these phenomena.

1. Introduction Emollients are multifunctional ingredients supporting multiple formulation claims in cosmetology, dermocosmetology and dermatology [1–3]. For instance, emollient therapy is currently prescribed in managing eczema (or atopic dermatitis), since this skin disease can affect up to 30% of children in developed countries [4–6]. In skin care emulsions, emollients represent the major ingredient after water, being used at level between 3 and 20% (w/w) [7]. From a sensory perspective, emollients have a major impact on physicochemical properties of cosmetic emulsions such as consistency and spreadability, properties that are important to achieve adequate efficacy and user acceptance of the products [8]. Generally, during application on the skin, emollients decrease the friction coefficient of the emulsion due to their lubricant properties and modify its spreading performances [9]. Therefore, spreadability is one of the sensory characteristics commonly evaluated during emulsion application [10]. Spreading can be defined as the ability of a substance to cover a surface and depends on molecular weight, viscosity and chemical structure [7,11]. In general, constituents with low molecular weights

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and/or low viscosities have higher spreading properties. Thus, the choice of emollients is essential to control the efficacy of the product in terms of skin moisturizing, but also to achieve the satisfactory physical and chemical stability of the emulsion. In particular, the polarity of the emollients and the association of different ingredients affect the mechanism of interactions with the skin as well as the structural organization and organoleptic characteristics of the emulsion [12]. In this respect, some studies attempted to characterize different pure emollients for their spreadability properties using rheology, spreading value and contact angle measurements on synthetic substrates and sensory analysis [8,11,13–16], but very few studies focused on the impact of emollients on the spreading properties of complex emulsions and creams [17,18]. Moreover, to the best of our knowledge, no study has been devoted to the mixture of different emollients incorporated in complex emulsions. In this way, the first aim of our study was to observe the effect of two types of emollients (pure and their mixture), commonly used in the cosmetic field: a close to solid saturated fatty acid - the stearic acid (SA) and a mineral oil - the isohexadecane (IHD), on the spreading properties of cosmetic emulsions. These emollients were selected for their chemical structure and for their physical state,

Corresponding author. E-mail addresses: [email protected] (E. Gore), [email protected] (C. Picard), [email protected] (G. Savary).

https://doi.org/10.1016/j.biotri.2018.09.003 Received 22 May 2018; Received in revised form 18 September 2018; Accepted 24 September 2018 Available online 25 September 2018 2352-5738/ © 2018 Elsevier Ltd. All rights reserved.

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which may influence the consistency of the final product. Another target of this study was to establish how the consistency of the emulsions impacts the friction behavior on the skin. The skin is a critical interface between the human body and its surrounding environment which accomplishes multiple defensive and regulatory functions. In particular, the 10–40 μm outermost thick layer of skin, namely the stratum corneum, by its complex multicomponent organization and its compact architecture, acts as the primary interface during application of skin care products [19]. At this stage, it seems important to understand the interaction between the skin and the emulsion. Recent reports focused on skin tribology, since it plays an important role while using skin care products in our daily life, being closely related to the contact and friction behavior between human skin and product surfaces [20–23]. These studies tried to understand and characterize the tribological properties of the bared skin, but very few concerned the friction evolution during the application of skin care products on skin [24,25]. Since sensory and tribological measurements on human skin are expensive, time-consuming and restrictive, it becomes important to have alternative strategies to understand this feature. Thus, our third aim was to explore the skin-emulsion interaction on the spreadability of cosmetic emulsions, by exploring different methodologies. For this purpose, different methods were implemented in order to obtain a broader characterization of the spreading properties. Very few studies concerned the spreading of cosmetic emulsions [17,26] and, to our knowledge, no one reported a similar exhaustive work, including the use of a frictiometer and especially carried out on mixtures of emollients. Most of them only focused on correlations between sensory and rheological data [27,28] or tribological properties of naked skin [20,22]. The originality of this work was to fix the composition of the emulsion and to act on consistency only by varying the ratio of the two constituents of the oily phase. In the present investigation, we have assessed how the nature of the oil phase of five O/W emulsions may impact their spreadability on the skin. By nature is meant the physical state of emollients and their ratio in the oil phase. For this purpose, two different emollients (IHD and SA) were incorporated at different rates (from 0 to 10%) in cosmetic emulsions. The impact of emollients and their ratio on the friction behavior of the O/W emulsions formulated with these oil mixtures was studied by a combined in vitro instrumental/in vivo study approach (texture analysis and rheology/sensory analysis and friction tests). These complementary tools made possible to better understand how emollients influence the spreadability of emulsions onto the skin.

IHD (IMCD, France) and/or SA (Croda, France) with emulsifiers Steareth 2 and Steareth 21 (Croda, France) at 3 and 2%, respectively. The purified water (Qs) was heated at 75 °C. When all ingredients were at 75 °C, the oil phase and emulsifiers were added to the water under mechanical stirring (Turbo test, VMI, France) at 400 RPM for one minute. The emulsion was then homogenized at 11000 RPM during one minute using a T25 digital ultra-thorax (IKA, Germany) equipped with the rotor-stator turbine S25 N-25F. Each emulsion was continuously stirred using the Turbotest apparatus (VMI, France) until it cooled down to 40 °C. Then, 0.5% of cosmetic grade xanthan gum (Rhodicare® T, Rhodia, France) previously predispersed in 4% of butylene glycol (Across Organics, France) at room temperature under manual stirring, was added to the emulsion. At room temperature, water loss was compensated and 0.5% of preservatives Dekaben MEP® (Jan Dekker International, France) were incorporated and the emulsions were stirred for 15 additional minutes. The pH of the final emulsions was corrected by adding a sufficient amount of 1 M NaOH (CarloErba, Italy). The adjusted pH is indicated in Table 1. The emulsions were stored at 4 °C to ensure preservation before further analysis. Their physical stability (at ambient temperature, 4 °C and 40 °C) in time was checked by particle size and rheological measurements (results not shown). The microbiological safety was assessed prior to sensory and in vivo analysis of the skin. 2.2. Microstructural Characterization 2.2.1. Optical Microscopy The microstructure of the Amazons was analyzed using a photomicroscope equipped with a camera (DMLP/DC 300, Leica Microsystems, Germany) at 200 x magnification for qualitative evaluation. The images were captured by the LAS V4.9 software (Leica Microsystems, Germany) on thin layers of emulsions on a microscope slide with a cover plate. 2.2.2. Droplet Size Distribution Droplets size measurements were performed by means of a laser diffraction particle size analyzer SALD 7500 Nano (Shimadzu Co., Ltd., Japan), equipped with a violet semiconductor laser (405 nm) and a reverse Fourier optical system. The emulsions were dispersed in purified water in order to reach an absorption value of 0.200 ± 0.10. To ensure homogenous dispersion of emulsions, continuous stirring was applied during analysis in the batch cell (7 cm3). All measurements were made at ambient temperature on at least three separately prepared samples. Data was collected using WingSALD II-7500 software.

2. Materials and Methods 2.3. Instrumental Spreading Characterization 2.1. Emulsions Preparation 2.3.1. Rheology Continuous flow measurements were performed with a stress controlled rheometer (HR-1, TA Instruments, USA), using a cone-plate aluminum device (40 mm diameter, 1°59′38′′ cone angle, 47 μm gap) as described in previous work [18]. Here we only focused on shear-rate viscosities at 0.1 s−1 which can be representative of the consistency of

Five O/W emulsions (200 g) were prepared by varying the composition of the oil phase in order to analyze the effect of two emollients: IHD and SA incorporated at different ratios (Table 1). Emulsions consisted in 10% oil phase, 5% emulsifiers and 85% aqueous phase. The oil phase and emulsifiers were first prepared by mixing and heating (75 °C)

Table 1 Ratio of isohexadecane (IHD): stearic acid (SA) in the oil phase (in %), pH, median particle size (D50) from granulometric measurements (in μm) and values of viscosity (in Pa.s) obtained at different shear rates (in s−1) obtained from shear flow test for the five O/W emulsions. Emulsion

IHD:SA ratio

pH

I0SA10 I2.5SA7.5 I5SA5 I7.5SA2.5 I10SA0

0:10 2.5:7.5 5:5 7.5:2.5 10:0

5.73 5.72 5.73 5.74 5.71

D50 ± ± ± ± ±

3.93c 4.48b 5.62a 2.70d 1.23e

0.00 0.01 0.01 0.01 0.01

± ± ± ± ±

0.02 0.06 0.10 0.10 0.02

a-e Means in a column sharing common superscripts are similar as tested by Tukey's test (P > .05). Means of replicates ± SD. 18

η(0.1)

η(1000)

139.5b ± 2.2 167.5a ± 12.9 122.5c ± 2.6 102.2d ± 1.3 53.8e ± 0.9

0.06b 0.08a 0.06b 0.06b 0.06b

± ± ± ± ±

0.00 0.00 0.00 0.00 0.00

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Fig. 1. Bright field micrographs of the five emulsions at a magnification G x 200.

the emulsions at rest and shear-rate viscosities at 1000 s−1 which can be representative of a spreading process on skin [29,30]. For each product the assay was performed in triplicate.

were taken. A Frictiometer® FR700 equipped with a plain, smooth Teflon (PTFE) disk (16 mm of diameter), a Corneometer® CM825 and a skin-pHMeter® PH905 (Courage-Khazaka electronic GmbH MPA580, Germany) were used to measure the skin friction coefficient, the stratum corneum hydration and the pH, respectively, on the internal side of both forearms of the subjects. The measurements of the skin friction coefficient were made at 90 rpm, in order to reproduce the spreading speed of emulsions on skin, and during 300 s, with one measure taken per second, in order to observe the skin behavior at longer friction. The hydration and the pH were measured at least in triplicate for each forearm before applying the cosmetic emulsions. Afterwards, 40 μL of emulsion was applied manually (10 complete rotations at 90 rpm), by the same person for each subject, in a circle (4 cm of diameter) situated on the internal side of the both forearms, by avoiding the extremities for at least 5 cm. The Frictiometer® was applied immediately after, in order to measure the friction coefficient for each emulsion. The computer software readily returned the data in arbitrary units (A.U). Each emulsion was tested on both left and right forearm for each subject. Both studies, involving human subjects have been carried out in accordance with The Code of Ethics of the World Medical Association (Declaration of Helsinki) and has been approved in advance by the local institutional ethics committee at UNILEHAVRE (France). All subjects participated only after receiving detailed oral and written information and signing an informed consent agreement.

2.3.2. Texture Analysis Spreading of cosmetic emulsions were performed on a TA.XT Plus Texture Analyzer (Stable Microsystems, United Kingdom) in compression traction mode, equipped with the A/FR Friction rig module (ASTM-D 1894-90), as described previously [26]. The products (200 μL) were homogenously applied in 4 parallel lines and the assay was conducted in triplicate. 2.4. In Vivo Spreading Measurements 2.4.1. Sensory Analysis Our study involved 16 healthy Caucasian female expert assessors, from 21 to 26 years old, was selected and trained according to the guidelines in ISO 8586-1 [31]. To describe the spreading of emulsions, a sensory profile method was applied according to the recommendations of ISO 13299 [32]. The assessors were trained in two training sessions followed by three analysis sessions, in which samples were evaluated once. The attribute to evaluate ease of spreading was defined as the ease of moving cosmetic emulsions (50 μL) over 6 cm distance from the arm bend, as described by Savary et al. [26]. The assessors appraised the force necessary to apply the cosmetic emulsions onto the skin and indicated the corresponding scores on scales from 0 to 7 with 0.1 increments. Samples were labeled with different three-digit random numbers for each session and were randomly presented in 5 mL opaque vials. The spreadability was assessed on the internal side of the non-dominant forearm. Testing took place at the sensory facilities of UNILEHAVRE (France) at 21.1 ± 0.9 °C and 39.2 ± 1.6% relative humidity.

2.5. Statistical Analysis Statistical analysis of the collected data was performed on the XLSTAT® software package (Addinsoft, France). Single-way analysis of variance (ANOVA) were applied to data series in order to test the significance of instrumental and sensory parameters and two-way ANOVAs were computed on sensory data with samples and assessors as variables. When a significant (p < .05) difference was revealed between emulsions, groups of emulsions were formed using a Tukey multiple comparison test. Results are reported as mean ± standard deviation (SD). Pearson's correlation coefficients were calculated between rheological, textural and sensory data in order to determine

2.4.2. Friction Tests The same 16 assessors were implicated in the in vivo investigations. All subjects had no skin disorders at the study sites. No skin care products had been applied to the measured sites for at least 24 h prior to the measurement. All subjects remained inactive at 21.2 ± 0.8 °C, at a relative humidity of 41.7 ± 2.2% for 20 min before measurements 19

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Fig. 3. Mean value and standard deviation for ease of spreading (0 for difficult to spread and 7 for easy to spread) obtained from sensory analysis. The test was made by 16 assessors in triplicate. Data sharing common superscripts are not significantly different at 5% (Tukey’s test). Fig. 2. Flow characterization of the emulsions. Each curve represents the mean value of three replicates. The standard deviations remain lower than 5% and are not shown for better readability. Solid symbols: viscosity (η, Pa.s); hollow symbols: shear stress (σ, Pa)

emulsions obtained only with the close to solid emollient (10% SA) and the 5:5 IHD:SA ratio. The most difficult to spread emulsion was the one obtained with 2.5:7.5 IHD:SA ratio. The two-way ANOVA revealed that assessors were not a significant source of variation (p = .094). As a consequence, the composition of the oil phase was the major factor affecting significantly the spreading of emulsions.

whether significant (p < .05) correlations between them exist or not. 3. Results 3.1. Physical Characterization of the Emulsions

3.3. Tribological Behavior

Analysis of micrographs (Fig. 1) indicated an homogenous and fine microstructure of all investigated samples composed of spherical small droplets. The droplet size measured on the microscopy captures was in accordance with the droplet size measurements obtained by laser diffraction (Table 1), with mean droplet diameters (D50) ranging from 1.23 μm for the emulsion containing only IHD (I10SA0) to 5.62 μm for the emulsion containing equal concentrations of emollients (I5SA5). The finest emulsions were obtained when IHD was > 75% in oil phase. Fig. 2 displays the flow behavior of the five emulsions: the shear rate dependences of the apparent viscosity and shear stress. The rheological curves obtained for all studied systems are characterized by a classical shear thinning behavior. On the one hand, at low shear rates all emulsions showed significantly (p < .05) different viscosities, with the I10SA0 standing out more than other emulsions, with the lowest viscosity (Table 1). The 2.5:7.5 IHD:SA ratio led to the formation of the most viscous emulsion, but generally, the tendency was respected: more solid emollient (SA) in the oil phase, the more viscous the emulsion is. Due to shear thinning behavior, the difference between our products is less important with the increasing shear rate, thus clearly illustrating the different emulsion behaviors according to the shear rate. At higher shear rates (1000 s−1) all emulsions obtained similar viscosity (Table 1). Only the emulsion prepared with 2.5:7.5 IHD:SA ratio is significantly (p < .05) different and remained higher than all other emulsions.

3.3.1. In Vivo Performances on Human Skin The spreading performances of the five emulsions were performed on healthy subjects and were expressed in the friction value of Frictiometer®. Due to a lack of studies using a frictiometer after applying a cosmetic product, it seemed interesting to study the spreading behavior in a long-time scale after applying the emulsions on the skin. Fig. 4 depicts the spreading average curves obtained in five minutes. Two different behaviors can be observed at the end, represented by the emulsions obtained with 100% of one emollient in the oil phase, SA or IHD, having the highest and the lowest values of friction, respectively. The I0SA10 emulsion was characterized by the sharp increase in friction coefficient and the highest values as well. An intermediate behavior can be observed with different mixtures of emollients. The increasing proportion of SA in the emulsion induces higher friction values. One can observe that after one minute of friction, the I0SA10 emulsion exceeds the friction value of the bare skin, standing out more than other emulsions, with the highest friction value and the highest variation as well between subjects, for the five minute friction procedure. This result shows a very difficult to spread, even a braking behavior after one minute spreading of the emulsion containing only semisolid-waxy oil phase. The same behavior is observed after two minutes of friction for I2.5SA7.5. Otherwise, other emulsions presented lubricant behavior on skin, during the five minute friction procedure, with values lower than for bare skin. In this study, we could observe that the repeatability was better on the same subject (data not shown) and the spread between different subjects were narrower for the first minute test than after (Fig. 4). These results could be influenced by differences in skin properties, in particular its hydration level. In addition to the friction measurements, the skin surface pH and hydration values were recorded just before applying the products to obtain the Blank measurements (Fig. 5). Thus, the average skin surface pH in the investigated area was 5.0 ± 0.4 pH units. The average value of the skin moisture level was 32.5 ± 4.9 A.U. When considering mean friction parameter for one minute, the average value of all collected data on the bare skin was 84.1 ± 3.3 A.U., significantly (p < .05) higher compared to the emulsions (Fig. 5C). It may be noted that during one minute of friction procedure all the emulsions present mean lubricant behavior on the

3.2. Sensory Characterization of Spreading In order to evaluate the impact of the emollients on the spreading properties, a sensory analysis was carried out on the five emulsions formulated as described above, by a trained panel. The assessors were instructed to describe the ease of spreading of the samples on a 7-point scale, where « 0 » indicated a very difficult to spread and « 7 » an easy to spread property. The results obtained (mean ± SD) showed repeatability and a consensus between assessors for the 3 organized sessions (Fig. 3). As expected, the emulsion containing only liquid emollient (10% IHD) was characterized as the most easy to spread, followed by the emulsions obtained with descending IHD:SA ratio. No-significant (p < .05) differences were observed by the assessors between the 20

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Fig. 4. Friction curves obtained after application of the products on the volar forearm area, in comparison with the bare area (Blank). Data represents the mean value obtained for 16 volunteers, all products being tested on the right and the left forearms once on each volunteer.

provoke its motion and then to move it on a distance of 6 cm at a constant rate of displacement, thus, corresponding to the same distance of spreading as in the sensory analysis protocol. Fig. 6 depicts the curves obtained during the spreading of emulsions between two artificial substrates imitating the surface of the skin (Fig. 6A) and the calculated areas under the curves (Fig. 6B) obtained in comparison with the Blank (obtained without any emulsion addition). Significant (p < .05) differences between emulsions can be observed according to the area values proportional to the difficulty of spreading. The most difficult to spread seems to be, once again, the emulsion obtained with the 2.5:7.5 IHD:SA ratio, followed by the 10:0 one. Then, no significant (p < .05) difference was observed neither between the 5:5 and 10:0 nor between the 7.5:2.5 and 10:0 IHD:SA ratios, being the emulsions the easiest to spread on the artificial substrates.

skin even I0SA10. Significant (p < .05) differences of spreading were observed for all the studied emulsions, thus demonstrating different spreading properties. The classification of the friction value of the emulsions is correlated with the IHD:SA ratio: less solid emollient in the formulation, easier to spread the emulsion. 3.3.2. Friction Behavior on Artificial Surface A full instrumental test, performed on the texture analyzer, was also carried out to physically evaluate the spreading of the five emulsions. The spreading properties were measured using artificial substrates like Poly(methyl methacrylate) (PMMA) and polypropylene surfaces. Due to the total surface energy close to the one of the skin (39 mN/m), these surfaces made it possible to have appropriate friction conditions to measure the spreading of cosmetic emulsions and are easily found everywhere. Furthermore, they have been already successfully used to characterize the in vitro spreading properties of emollients or creams [26,33]. It is worthwhile to note that these surfaces enabled to avoid the variability and heterogeneity of real human skin, already highlighted by a numerous number of tribological studies [20,21,34], and were of practical relevance even if they do not completely mimic either the chemical composition or the surface free energy of the stratum corneum [26]. Spreading was estimated as the work required for a weight first to

4. Discussion This work aimed to study the relation between (i) the composition of the oil phase, with different ratios of two emollients: a saturated fatty acid - the stearic acid (close to solid state) and a mineral oil - the isohexadecane (liquid state) and (ii) the spreading and frictional properties of five O/W emulsions on skin. Indeed, this characteristic is known to be very important in the efficacy and the user acceptance of cosmetic

Fig. 5. Skin surface pH (A), skin surface hydration (B) and friction values obtained for one minute friction procedure (C). The blank represents the bare areas of skin. Data sharing common superscripts are not significantly different at 5% (Tukey’s test). 21

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Fig. 6. Instrumental measurements of the spreading properties : curves (A) and areas under the curves (g.s) (B) obtained during the spreading of the emulsions over 6 cm. The blank (dotted line in part B) was obtained by following the same protocol without emulsion addition. Data sharing common superscripts are not significantly different at 5% (Tukey’s test).

fact that texture measurement applied here was developed to better reproduce the sensory spreading of emulsions: the distance of spreading in both experiments was identical, assessors were trained to evaluate the force while applying the sample onto the skin and physical references were freely available for the assessors at each session. As a conclusion, the consistency of the product at rest (0.1 s−1) has a direct impact on the sensory perceptions, consisting here in displacing at a constant speed the emulsion and thus, cohesiveness of the product appears to be a key factor in this phenomenon.

or topical products [3,26]. Thus, this study provides new data to better characterize and understand this major feature. Our results confirmed that there is a link between the nature of the oil phase and the spreading properties of cosmetic emulsions. 4.1. Correlation between Spreading and Consistency of Emulsions at Rest The effect of the oil phase nature on spreading was primarily investigated by shear-stress controlled rheometry, since the rheological properties of emulsions could have an impact on their spreading behavior. In fact, according to Barnes [29,30], the different corresponding shear rate ranges could be related to physical operations usually applied to emulsions, such as draining under gravity for lotions on skin between 0.01 and 10 s−1 corresponding to the consistency of emulsions at rest, creams spooning and pouring between 10 and 100 s−1, and creams spreading or rubbing around 1000 s−1. The data obtained in this study showed a significant correlation (Pearson coefficient = − 0.954) between spreadability obtained by sensory analysis and consistency (viscosity at 0.1 s−1) of emulsions at rest, the negative correlation coefficient observed being explained by the use of opposite scales. This could imply that the spreading properties perceived by the assessors would be mainly governed by the consistency of the products and not by the viscosity obtained at 1000 s−1. It corresponds to the first seconds of moving the emulsions on skin. As highlighted on the rheograms (Fig. 2), at 1000 s−1, emulsions experience loss of structure followed by breakdown showing a sharp decrease in viscosity. These results are confirmed by numerous references in the literature: less consistent the emulsion is, higher its spreadability [7,11,15,35,36]. The emollient with the lower molecular weight and viscosity, in occurrence IHD, which is liquid, procured easier spreading properties to the cosmetic emulsions when found predominant in the oil phase, with respect to the emollient with the higher molecular weight and viscosity, for instance, SA. It is interesting to note that texture measurements realized on artificial substrates seem to be significantly correlated with the spreading of emulsions as evaluated by sensory method (Pearson coefficient = − 0.871): more consistent was the emulsion, more it was difficult to spread and more important the spreading area under the curve was obtained. The negative correlation coefficient observed being explained by the use of opposite scales, as explained above. These results highlight the interest of the instrumental approach and the use of artificial substrates imitating the skin surface, in order to determine the sensory properties implying generally a long procedure of recruitment, of training and analysis with a certain number of assessors and the implementation of in vivo measurements. In this sense, it would be possible to replace time-consuming and expensive sensory analysis by instrumental measurements, requiring far less time, money, but also higher safety. This result was undoubtedly possible to obtain due to the

4.2. Impact of the Skin-Emulsion Interaction on Spreadability The in vivo measurements realized with Frictiometer® on human skin emphasize different behavior between products, as well. In this case, our study focused on the properties of the emulsions while applying them on skin (90 rpm, 300 s). The Frictiometer® measurements are particularly interesting since they evidence other characters and particularly the skin-product interaction. Typically, the friction is higher directly after the start of the measurement and decreases to a plateau value (Fig. 4). The plateau value is reached after approximately 10 s and its lengths depend on the emulsions: approx. 40 s, for I0SA10 till 150 s for the emulsion containing only liquid emollient (I10SA0), who present the longest plateau. This curve progression is in accordance with the literature [9,37]. Then, as the emulsions were absorbed into the skin surface and partially evaporated, the hydrating effects overcame the diminishing lubricating effect, a gradual increase in friction coefficient was observed till 300 s of test. This phenomenon could reflect an increase in the adhesiveness of the stratum corneum brought about by hydration of the surface cell layers. This surface phenomenon can also be explained by other changes in the physicochemical state of the stratum corneum. The water absorbed by the stratum corneum can increase the size and surface characteristics of its cells. This could bring about an increase in the contact area between the probe and the skin cells and lead to an increase in friction coefficient [9]. The skin friction coefficient indicates skin surface resistance against the movement of objects on it and it includes other parameters of the skin's biophysical properties. Our study highlights that the type of emollient influence the spreading properties in time. For example, the I0SA10 emulsion (0:10 IHD: SA ratio) presented the most difficult spreadability in time. In fact, this result could be related to the fact that during the spreading (circular motion) the water was rapidly (after 40 s, Fig. 4) absorbed into the skin with another part evaporated, thus, only the oil phase, which is semisolid at ambient temperature, remained on skin surface, forming white spots very difficult to spread. This phenomenon was more pronounced on subjects presenting lower stratum corneum hydration levels (25.8 to 26.9 A.U. of Corneometer®) and consequently lower friction values on bare skin areas. The results in the 22

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directly influenced by the nature and the ratio of emollients used and the skin-product interaction. It highlights how the physical properties and the emollients ratio found in the oil phase influences the spreadability of emulsions: in the short term, when moving the emulsion on the skin, with an impact of the consistency of the product, and in the longer term, when the emulsion breaks down and residual film is formed, with a specific interaction of emollients with the skin. This study opens new perspectives regarding strategies of formulation optimization of skincare products and how to select the mix of emollients. It would deserve further investigations to better understand this major feature. In particular, it would be interesting to study the effect of temperature on the spreading properties, since it could influence the physical state of the oil phase and the skin-product interactions as well. The study of the thermal properties is in perspective.

current study confirm that, for each subject, there is a significant positive correlation (Pearson coefficient = 0.858) between the hydration of the bare skin and the coefficient of friction, in accordance with previous studies [20,22,38–40]. It was expected that lower hydration levels would lead to lower coefficients of friction on bare skin [37,41]. In this study, when applying emulsions on subjects with lower stratum corneum hydration levels and therefore lower friction values on bare areas, they would present higher friction values when applying skin care products containing only semisolid oil phase, when compared to subjects with higher hydration level. Therefore, subjects with medium or higher stratum corneum hydration levels (33.0 to 42.7 A.U. of Corneometer®) would present higher friction values on bare skin, in accordance with the literature cited above, and lower friction values while applying skin care products. We can, thus, hypothesize that the emollients have also an impact on the formation and the properties of the residual film remaining after application on skin care products. According to Guest et al. [35], the sensory characteristics of skin care products during application depend on physicochemical properties of the film left on the skin and its interaction with the skin. These characteristics are time dependent and change during application due to several simultaneous processes: changes in rheological properties due to the shear exerted during application, evaporation of water and volatile ingredients, melting of solid ingredients, changes in the structure of emulsions, mixture of the product with sebum and sweat, and absorption into the stratum corneum of the skin [10].

Conflict of Interest This statement accompanies the article “Spreading behavior of cosmetic emulsions: impact of the oil phase” authored by Ecaterina GORE and co-authored by Celine PICARD and Geraldine SAVARY and submitted to Biotribology as an Article Type. Authors collectively affirm that this manuscript represents original work that has not been published and is not being considered for publication elsewhere. We also affirm that all authors listed contributed significantly to the project and manuscript. Furthermore we confirm that none of our authors have disclosures and we declare no Conflict of interest. Consultant arrangements: no. Stock/other equity ownership: no. Patent licensing arrangements: no. Grants/research support: no. Employment: no. Speakers' bureau: no. Expert witness: no. This article includes tissue and/or Animal experiments: no. If yes, please confirm that ethical approval has been sought and the approval number, if applicable: This article includes human subjects: yes. If yes, please confirm that ethical approval and informed consent from participating subjects have been obtained. Please state approval number, if applicable: We confirm that ethical approval and informed consent from participating subjects have been obtained.

4.3. Impact of the IHD:SA Ratio on Spreadability The present study was focused on two emollients, presenting both a different structure. Furthermore, their mixture was also considered, in an original way, by varying their proportions. As discussed before, the two emollients procure different behaviors when found at 100% in the oil phase: a less viscous and consistent emulsion with an easier to spread property for IHD and a more viscous and consistent emulsion with a more difficult to spread property for SA. Regarding the mixtures, we can notice that the results are not necessarily intermediate between both emulsions obtained with 100% of IHD and 100% SA. For instance, Fig. 3 highlights that the evolution between the two endpoints is not linear. In this case, it would not be possible to anticipate the spreading properties of a mixture, only by considering the behavior of each emollient of this mixture apart. Moreover, in Fig. 4 it seems that the friction values obtained for five minutes for the mixtures are closer to the one obtained with the 10:0 IHD:SA ratio. It would therefore appear that the presence of IHD in emulsion governs the behavior of the emulsions. The emulsion obtained with 5:5 IHD:SA ratio does not even represent the sum of the two contributions obtained by 50% of IHD and 50% of SA. Exactly the same situation is characteristic for the mixture containing the higher content of SA, in occurrence 2.5:7.5 IHD:SA ratio, which is dissimilar to the values obtained with the 0:10 IHD:SA ratio. These results show the importance to consider the emollient properties when single in emulsion, but it appears to be more important to consider the interactions between the emollients, their chemical nature, polarity, temperature effect to better understand and anticipate the mixtures behavior.

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5. Conclusion In conclusion, this paper focused on the study of the spreading behavior of five O/W emulsions on the skin. Emulsions were formulated by varying the ratio of two emollients- isohexadecane and stearic acid, in the oil phase, from 0 to 10%. A combined approach of in vivo and in vitro characterizations was carried out. The results of both sensory and textural analysis highlight that higher the isohexadecane in the oil phase, easier the spreading of the emulsion on the skin. Additionally, the friction behavior is mainly governed by the consistency of products, 23

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