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Emollients for cosmetic formulations: Towards relationships between physico-chemical properties and sensory perceptions
Colloids and Surfaces A 536 (2018) 156–164
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Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Emollients for cosmetic formulations: Towards relationships between physico-chemical properties and sensory perceptions
T
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Christina Chaoa, , Célina Génota, Corinne Rodriguezb, Harmonie Magniezb, Sandrine Lacourtc, Aurélie Fievezc, Christophe Lena, Isabelle Pezrona, Denis Luartd, Elisabeth van Heckea a
Sorbonne Universités, Université de Technologie de Compiègne, EA TIMR 4297 UTC/ESCOM, rue du Docteur Schweitzer, 60200, Compiègne, France Labosphère, R & D, 5 Rue de Sétubal, 60000, Beauvais, France c Oléon, R & D, Chemin Usine, 60280, Venette, France d ESCOM, EA TIMR 4297 UTC/ESCOM, 1 allée du réseau Jean Marie Buckmaster, 60200, Compiègne, France b
G RA P H I C A L AB S T R A C T
A R T I C L E I N F O
A B S T R A C T
Keywords: Emolliency Spreading Esters Hydrocarbons Silicone
Thirteen current commercial emollients from different chemical families (hydrocarbon, ester, ether, and silicone) were firstly characterized by eight physicochemical properties which were a-priori thought to be strongly related to sensory perception. Those include usual properties as well as more original ones such as volatility, estimated by the evaporated mass fraction during a fixed duration, as well as the polar and dispersive components of the surface tension. Principal Component Analysis and Hierarchical Agglomerative Clustering led to a partition of the emollients into groups that matched fairly well with the different chemical families. It also helped to highlight the main characteristics of each group. As expected, silicone had the lowest surface properties. Hydrocarbons showed the highest volatilities and the lowest viscosities. Esters mainly displayed the highest surface properties, and within this group, diesters showed higher polar components than monoesters. Sensory evaluation was performed at the same time by a professional panel. The sensory attribute “Spontaneous spreading” was found to be larger for silicone and hydrocarbons than for esters and ether. From our set of data, a suitable correlation was obtained between the sensory attribute “Spontaneous spreading” and the calculated spreading coefficient on Teflon® which combines surface tension with the cosine of contact angle.
1. Introduction Emollients are widely used in the cosmetic field for their numerous properties. In oil in water emulsions (O/W), their concentrations are between 5% and 30%, and can even be higher for water in oil emulsions (W/O) or anhydrous products. Thus, they represent the second major group of ingredients in a formulation after water. Actually, they refer
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Corresponding author. E-mail address: [email protected] (C. Chao).
http://dx.doi.org/10.1016/j.colsurfa.2017.07.025 Received 30 September 2016; Received in revised form 2 March 2017; Accepted 8 July 2017 Available online 13 July 2017 0927-7757/ © 2017 Elsevier B.V. All rights reserved.
more specifically to oils, and are associated with different sensory feels. As a definition, an emollient can be defined as a set of characteristics, felt or seen at a precise moment, that are directly linked to a sense of smoothness, elasticity and spreadability regarding skin feel, and to a sense of glossiness or matte degree for visual perception [1]. Emollients can be divided into four main groups depending on their chemical family: hydrocarbons, fatty alcohols, esters, and silicone
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increased the interfacial tension and viscosity but decreased permittivity. Furthermore, when the hydrocarbon chain was branched, the interfacial tension would be higher than for a linear chain. And finally, they showed that the ester function increased the permittivity of the compound. To sum up, these properties depend on the hydrocarbon chain length, the degree of unsaturation, the presence of branching, and the presence of multiple functions. Silicone derivatives emollients are synthesized from silicon. They are mainly divided into three groups: the linear, the cyclic, and the cross-linked silicones. The most common linear silicone derivative is polydimethylsiloxane (PDMS), which can be found with different degrees of polymerization, and thus different viscosities from 10 to 100 000 mPa s. Cyclic silicone derivatives usually refer to octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, or dodecamethylcyclohexasiloxane, respectively D4, D5, and D6 compounds. However, due to its proven toxicity on rats, the D4 compound has been banned from the cosmetic ingredients list [8]. Silicone derivatives can be found in almost all cosmetic products because of their outstanding properties. The strong silicon-oxygen bond makes them chemically inert, resistant to oxidation and humidity. Their low surface tension allows them to spread easily on most solid surfaces, forming a homogenous and protective film that let the skin breathe. In cosmetic formulations, linear silicone derivatives are used to reduce tackiness, and to leave a nongreasy and smooth skin feel. They can also be used as antifoaming agents. As for cyclic silicones, they are used to increase the volatility of a formulation thanks to their high vapor pressure [9]. As we can see, many emollients exist on the market and they are generally characterized by sensory analysis. However, sensory evaluation is time-consuming and requires an available and well-trained panel of assessors. On the contrary, physicochemical measurements are based on standard protocols with quasi-immediate and reproducible results. Thus, understanding the relationship between the physicochemical properties of an emollient and its sensory performances could greatly improve the work of formulators. Until now, a number of scientific papers deal with the correlations between physicochemical and sensory properties of formulations containing emollients. However, only a few authors have studied the correlation between the sensory properties of raw emollients and their physicochemical properties. The study of Zeidler [10] is probably the oldest one, and focused on spontaneous spreading on human skin. The results were compared to the chemical structure and physicochemical data, such as viscosity and surface tension, of the tested compounds. Around thirty so-called lipids within which hydrocarbons, esters, alcohols and triglycerides, were investigated. Although the author concluded that none of these data series correlated entirely with the spreading rates of all the lipids tested, trends were obtained within groups of compounds with similar structures: the spreading value decreased with an increase of viscosity or surface tension. Parente et al. [3] applied linear partial least squares to a set of physicochemical properties and sensory attributes of eight commonly used emollients. They found that glossiness, residue and oiliness are positively correlated to surface tension, and negatively correlated to spreadability. They also concluded that viscosity is positively correlated to the difficulty of spreading and stickiness, but negatively correlated to softness and slipperiness. M. Gorcea and D. Laura [7] compared four specific branched ester emollients. They concluded that esters with low molecular weights and low viscosities would feel less tacky and leave less residue on the skin. They also observed that esters with shorter chain length spread the most, gave the lightest and driest feel, and left the least residue and gloss. The aim of our study was to identify, among basic physicochemical properties characterizing thirteen current commercial emollients, which ones really affect the sensory perception of the emollients. For
derivatives. Hydrocarbon-based emollients can come from animal, vegetable or mineral sources, and can be synthetic. Natural hydrocarbons are very unstable because of their high degree of unsaturation, which makes them easily oxidized. Stable hydrocarbons are obtained after hydrogenation of the compound. The final saturated hydrocarbon shows many advantages such as chemical inertia, resistance to oxidation and to hydrolysis [2]. In terms of physicochemical properties, Parente et al. [3] compared several emollients, and showed that the mineral oil and squalane they used had higher superficial tensions and smaller surfaces of spreading at 0.5 and 1 min on a glass solid surface than other compounds. Hughes et al. [4] also studied several emollients within which four hydrocarbons with viscosities between 1 and 5 mPa.s. The four hydrocarbons showed the highest values of interfacial tensions with water, and the lowest contact angles at 1 s post contact on Vitro-Skin®. In term of sensory evaluation, hydrocarbon-based emollients are generally claimed to be able to decrease the oiliness of a formulation by leaving a pleasant and light skin feel. Fatty alcohols can also be from vegetable sources or synthetic. The hydroxyl group allows an increase in polarity, but also the formation of hydrogen bonds with water which increases the consistency of the formulation. Parente et al. [3], showed that octyldodecanol had intermediate properties in terms of surface tension, viscosity, and spreading at 0.5 and 1 min compared to the esters, hydrocarbons and silicones tested. In terms of sensory evaluations, it showed high values of glossiness, residue and oiliness. Usually used to stabilize emulsions, alcohols can also be used as emollients when added between 1% and 2% [5,6]. Ester-based emollients can either be synthesized from alcohols and fatty acids, or be natural. When synthesized, simple or complex esters can be obtained. For monoesters, fatty acids can either be vegetable-derived, animal-derived, or synthetic. When vegetable-derived, the acids chain lengths are even numbered, and mostly between C12 and C18. When animal-derived, chain length can either be even or uneven numbered (C15 or C17). The alcohol reactants, can also be natural, or synthesized from petrochemicals, but also come from fermentation in the case of shorter chains (methanol and ethanol). As for complex esters, one of the reactant must be a multifunctional alcohol or acid. Naturally occurring esters refer to triglycerides which can be found in plants or animals, they are characterized by their fatty acid composition. When vegetable triglycerides can directly be used as emollients, animal triglycerides are mainly used as a source of fatty acids. The six vegetable oils that are most used are palm oil, soybean oil, rapeseed oil, sunflower oil, palm kernel oil, and coconut oil. Palm oil is a major source of oleic (C18:1) and palmitic (C16:0) acids. Soybean, rapeseed and sunflower oils are main sources of linoleic (C18:2) and linolenic (C18:3) acids. Palm kernel and coconut oils are main sources of acids with medium chain lengths between C8 and C14. Note that triglycerides can also be synthesized from glycerol and fatty acids (e.g. caprylic/ capric triglycerides) [5]. Thus, a lot of different combinations are possible, which makes the ester-based emollients a very versatile group with a wide range of properties. For example, they can be used to modify the consistency of the formulation, or increase the spreadability of sunscreens [1]. M. Gorcea and D. Laura [7] studied four specific branched ester emollients: diisopropyl adipate, isodecyl neopentanoate, isocetyl stearate, and octyldodecyl stearate. They concluded that spreading values measured on Vitro-Skin® depend on molecular weight, viscosity, and chemical structure. They observed that the higher the viscosity, the lower the spreading values. They also concluded that more polar esters showed lower refractive indexes and surface tensions. In addition to a higher polarity, when the ester has a low molecular weight and viscosity, it showed a lower contact angle on a silicone substrate. Hughes et al. [4] showed that esters with a long hydrocarbon chain 157
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Measurements were performed in triplicate. The coefficient of variation was 2.2%. The second category of measurements concerns bulk properties such as viscosity and density. Viscosity was measured with a capillary viscometer, measuring device was an AVS400 (Schott Geräte, Germany), and capillary was with reference number 501 01 (SI Analytics GmbH, Germany). Calibration of the viscometer was done with distilled water. Emollients were all sampled twice, and three measurements were performed on each sample. The coefficient of variation was 1.3%. Density was measured with a density meter DMA45 (Anton Paar, Austria). Emollients were sampled thrice, and one measurement was performed per sample. The coefficient of variation was 0.01%. As an optical property, refractive index was measured with a refractometer. Emollients were sampled thrice, and one measurement was performed per sample. The coefficient of variation was 0.01%. Finally measurements of one thermodynamic property, which is the percentage weight evaporation after 2 h (%wt. evaporation) was performed. A Dynamic Vapor Sorption device (Surface Measurement Systems Ltd., England) was used. Measurements were performed in a temperature/humidity controlled incubator, and relative humidity was set to 5%. Emollients were sampled twice, and one measurement was performed on each sample. The coefficient of variation was 4.2%. All measurements were performed at 25 °C.
these purposes, emollients were characterized by eight physicochemical properties that were a-priori thought to be strongly related to sensory perception. Those include original properties like volatility estimated by the evaporated mass fraction during a fixed duration, as well as the polar and dispersive components of the surface tension. Sensory evaluation was performed at the same time by a professional panel. Statistics were used thus allowing correlations between properties and to some extent comparisons between chemical families. Finally, by referring to the physicochemical science, we aim at initiating the development of meaningful relationships between the physicochemical properties that could help predict the sensory attribute “spontaneous spreading”. 2. Materials and methods 2.1. Samples Thirteen commercial emollients were evaluated: three hydrocarbons (HC1, HC2, HC3), six esters (ES1, ES2, ES3, ES4, ES5, ES6), one ether (ET), one silicone (SI), and two mixtures of ester/hydrocarbon (MI1, MI2). The selected emollients have different chemical structures: hydrocarbons were all saturated (linear and branched), esters comprised three monoesters (with increasing degree of branching from ES1 to ES3) and three diesters, ES5 being shorter than the two others and ES4 having a higher degree of branching than ES6, and the silicone was cyclic.
2.3. Sensory analysis All samples were labeled with random 3-digit codes and evaluated at least three times by a professional panel of four expert assessors who come from the industrial partners of the project. The assessors were all women who had experience between 5–10 years in sensory analysis of cosmetic products, and still evaluate cosmetic ingredients and formulations on a daily basis. The two attributes “Spontaneous spreading” and “ease of spreading” were evaluated. Reference samples were used to identify the lower and higher intensity of each attribute scale. Each assessor was asked to examine a maximum of 5 samples per session, and assign each of them to a category on a scale. Experimental procedures for rating the samples according to each of the attributes are detailed in Table 1.
2.2. Physicochemical measurements The chosen physicochemical properties were those that could be related to an attribute from sensory evaluation, and could be divided into different categories. The first category deals with surface properties such as surface tension (γ), and contact angle (& z.Theta;). Surface tension was measured with the Wilhelmy plate method using a processor tensiometer K100 (Krüss, Germany). To ensure that measurements were performed at equilibrium, they were programmed to last 180 s, or to end when 5 consecutive identical values of surface tension were obtained. All emollients were sampled twice, and three measurements were performed on each sample. The coefficient of variation was 0.8%. Contact angle was measured with a Drop Shape Analysis system DSA10 Mk2 (Krüss, Germany). The sessile drop method was used on a Teflon® solid surface, with three repetitions for each emollient. Teflon® was chosen because it is a usual substrate for contact angle measurements, and it is also the only surface allowing a rapid characterization of the emollients with static contact angles. The coefficient of variation was 3%. Two secondary surface properties were deduced from surface tension: polar (γP) and dispersive (γD) components. When summing these two components, the total surface tension is obtained, which is expressed as equation 1 according to Fowkes [11]. More specifically, the polar component is a result of hydrogen and dipole-dipole interactions, when the dispersive component is a result of molecular interactions of the London forces [12]. These two components can also be expressed with the Owens-Wendt-Rabel-Kaelble (OWRK) equation (Eq. (2)) which gives the interfacial tension between two liquids, and in which γ1 and γ2 are the surface tensions of each liquid [13]. When combining these two equations, it is thus possible to calculate γP and γD of each emollient. γ = γP + γD
γ1,2 =γ1
+
γ2
− 2(
2.4. Data analysis The physicochemical properties were compiled in a table of 13 lines and 8 columns, with the compounds as lines, and the physicochemical properties as columns. When trying to visualize these data, a scatter plot is obtained in an eight-dimension space (8 properties). In order to simplify the plot, principal component analysis (PCA) was applied to the physicochemical properties. It consists in reducing the dimension of the dataset by combining the variables (physicochemical properties) into principal components, which would represent the axis of the new plot. PCA helps to assess the correlations between variables, but also to evaluate the influence of chemical structures on physicochemical properties. Hierarchical agglomerative clustering (HAC) was then used to see how the emollients could be clustered into groups according to their physicochemical properties. HAC is an iterative method which at first, consists in comparing pairs of individual emollients by calculating their squared Euclidean distances. The first pair with the shortest distance is gathered into a cluster, which is then compared as a unique individual to the (n − 2) emollients left to form a bigger cluster, etc. For this study, the method of the complete linkage (farthest neighbor) was used. Sensory data being categorical data, the frequency of occurrence of each grade was calculated for each attribute and each emollient. The results were plotted on charts called mosaic plots. PCA, HAC and mosaic plots were performed using the statistical software Statgraphics® Centurion XVII (Statpoint Technologies, inc., USA).
(1)
γ1dγ2 d
+
γ1pγ2 p )
(2)
Polar/dispersive components were measured with a Drop Shape Analysis system DSA10 Mk2 (Krüss, Germany). Measurements were performed with the emerging drop method with a curved syringe of diameter 1.493 mm, and with distilled water as a continuous phase, which surface tension, and polar/dispersive components are known. 158
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Table 1 Description of the attributes used for sensory evaluation. Stage of evaluation
Attributes
Description
Scale
During application
Spontaneous spreading Ease of spreading
Surface spreading of a drop after 30 s with no stress applied. Ease of spreading when spreading a drop of emollient in circle 4 times with 2 fingers.
3-point category scale: 1. Small 2. Medium 3. Large
3. Results and discussion
HAC could cluster the emollients into 4 main groups drawn on Fig. 2a and b. One group is made of SI which properties are completely different from the other emollients. Indeed, its surface tension, polar component of surface tension, contact angle, and refractive index are the lowest. The unique surface properties of cyclic silicones have already been pointed out especially in term of contact angle, whether on Vitro-skin® [4] or on a glass dish [3], but also in term of surface tension [3]. The presence of Silicon-Oxygen bonds and the cyclic form of the molecules are undoubtedly responsible for these properties. The second group is made of HC1, HC2, HC3, and MI2. Those emollients, with the cyclic silicone but also MI1, the other blend of hydrocarbon/ester, showed the highest evaporated mass fraction after 2 h (Fig. 1f). This result is in total agreement with the fact that hydrocarbons generally have lower boiling points and higher vapor pressures than chemicals with similar molecular size from any other chemical families. The volatility of hydrocarbons is a consequence of the weak van der Waals interactions between molecules. Besides volatility, the HC and MI2 group is located among emollients with the lowest values of viscosity (Fig. 1e) and density, which was expected considering their moderate molecular weight. Lastly, this group of mainly non polar molecules was, as expected, also characterized by low polar components of the surface tension (Fig. 1c). The next group of emollients clustered by HAC is made of MI1 and ES5 which essentially shows the highest values of polar component but also the lowest values of dispersive component of the surface tension (Fig. 1c and d). Both emollients in this group were obviously the most polar ones of the study. Both contained at least one ester, ie oxygen atoms able to form hydrogen bonds, which increases polarity compared to hydrocarbons. In addition, ES5 was the shortest chain diester and MI1, the mixture hydrocarbon/ester with the shortest chains also. Finally, the fourth group on Fig. 2 is made of ES1, ES2, ES3, ES4, ES6, and ET, which shows higher values of surface tension, viscosity, and contact angle than other emollients. This group can be divided into two smaller groups: one with ET, ES1, ES2, and ES3 which are the ether and the three monoesters, and one with ES4 and ES6 which are two diesters. The ether had properties very close to those of the three monoester (ES1, ES2 and ES3). So adding a C]O bond to an ether does not seem to affect the physicochemical properties of the molecule. The fact that the esters are monoesters (ES2, ES3) or diesters (ES4, ES6) does not seem to greatly influence the surface tension as well (Fig. 1a). However, the three monoesters (ES1, ES2, ES3) clearly showed lower polar components and slightly higher dispersive components than diesters (ES4, ES6) and, as expected, than the short-chain diester (ES5, from the previous group) (Fig. 1d). So adding ester groups in a molecule increases its polarity as already noticed by Hughes et al. [4] who determined the polarity of various esters by means of permittivity measurements. Hughes et al. also found differences between branched and straight chain esters, the branched ones having higher interfacial tension against water than the straight ones, meaning a lower polarity. In our study, we did not found any trend concerning the branching. Regarding the contact angles of this fourth group, as expected, high values are observed since the molecules are mainly polar and the measurements of contact angles have been made on Teflon®. Indeed, Teflon® has a very low surface energy of (18–19 mN/m), with a dispersive component of 17–18.5 mN/m [14]. Thus, this surface is mainly
3.1. Physicochemical properties Surface tensions, contact angles, polar and dispersive components of surface tensions, viscosities and percentages weight of evaporation of emollients are illustrated on Fig. 1. Fig. 1a shows that surface tensions of the emollients are between 23 and 30 mN/m, except for SI which shows a surface tension below 20 mN/m. On Fig. 1b, we observe higher contact angles for ET and all ES. The emollients with lowest contact angle are SI (around 10°), followed by HC3 and MI1. Fig. 1c shows that ES5 and MI1 have the highest polar component (respectively around 9 mN/m and 7 mN/m). The lowest values (< 1 mN/m) are obtained for SI and all HC. The lowest values of dispersive components are observed for ES5, SI and MI1 (Fig. 1d). On Fig. 1e, we observe higher viscosities for ES4 and ES6 (respectively 9 and 8 mPa s) and lower viscosities for all HC and MI1 (below 2 mPa s). Fig. 1f shows that all emollients have low percentages weight of evaporation except HC3 and MI2 (respectively 27%wt. and 23%wt.). PCA was performed on the eight physicochemical properties of the thirteen emollients. From the correlation matrix shown in Table 2, a high correlation between surface tension and contact angle on Teflon® is observed (0.877). Moreover, surface tension and contact angle appear to be well correlated with refractive index (respectively 0.806 and 0.838). These unforeseen correlations are probably specific to our data set. Contrary to the works of Hughes et al. [4], and Gorcea and Laura [7], we did not found any consistent correlation between viscosity and contact angle (0.519). This is probably due to the different surfaces that they used for the measurement of the contact angle, which are Vitro-Skin®, a synthetic surface mimicking human skin for Hughes et al. [4], and a thick silicone substrate adhered to a glass slide for Gorcea and Laura [7]. The high correlation found by those authors might also be explained by the fact that the emollients they studied were different (they covered a wider range of viscosities than in our study). Except for surface tension, contact angle and refractive index, all the other correlations are low, meaning that each of the physicochemical properties investigated brings a specific information. Regarding the characteristics of the principal components, the first three components accounted for 85.4% of the variance. As shown on Fig. 2a, principal component 1 (PC1) accounted for 45.5% of the variance, with a major positive contribution of contact angle, surface tension and refractive index. This indicate that the variability in the data set was mainly explained by these three physicochemical properties. PC2 accounted for 26.5% of the variance, with a main positive contribution of density and polar component of surface tension, and, to a lesser extent, a negative contribution of the dispersive component of the surface tension (Fig. 2a). In order to give a more precise view of the data, PC3, was combined to PC1 in another biplot shown in Fig. 2b. PC3 accounted for 13.4% of the variance, and mainly revealed the contribution of viscosity. The evaporated mass fraction after 2 h (% wt. evaporation) which was not colinear to any of the first three principal components, seemed to contribute only weakly to the principal components. However, the length of the vector representing it on Fig. 2a is similar to the length of the other properties meaning this property should not be neglected. 159
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Fig. 1. Physicochemical properties of emollients. (a) Surface tension. (b) Contact angle on Teflon®. (c) Polar component of the surface tension. (d) Dispersive component of the surface tension. (e) Viscosity. (f) Percentage weight of evaporation after 2 h.
Table 2 Correlation matrix of the physicochemical properties of the thirteen emollients.
Surface tension Contact angle Polar component Dispersive component Viscosity Density Refractive index %wt. evaporation
Surface tension
Contact angle
Polar component
Dispersive component
Viscosity
Density
Refractive index
0.877* 0.495 0.506 0.324 0.026 0.806* −0.428
0.293 0.583 0.519 0.271 0.838* −0.338
−0.499 0.218 0.348 0.142 −0.355
0.101 −0.322 0.662 −0.074
0.636 0.479 −0.326
−0.064 −0.331
−0.064
* Significant correlation coefficient (p-value < 5%).
160
%wt. evaporation
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Fig. 2. Principal Components Analysis of the physicochemical properties. (a) PCA with PC2. (b) PC1 with PC3.
Fig. 3. Mosaic plot of the grades obtained for the attribute “Spontaneous spreading”: (a) and “Ease of spreading”: (b).
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the results obtained by Parente et al. [3] on a glass dish. The evaluation of spontaneous spreading for ET, ES5, (shortest diester) and ES3 (branched monoester), was the most dispersed since the entire range of the scale was used (bottom of Fig. 3a). Since ES5 is the shortest chain ester, this last result disagrees with the work of Gorcea and Laura [7]. Indeed, by comparing four esters, they found that the lowest molecular weight ester was the one that spread the most. They also measured the spreading area of their esters on Vitro-skin®, and confirmed the high spreadability of the shortest chain ester. This disagreement might be explained by the fact that the emollients they studied showed a broader range of molecular weights than in our study. Regarding the “ease of spreading” (Fig. 3b), all the emollients were rated between “medium” and “large”. Despite the different chemical structures of the emollients, their ease of spreading were very similar. No apparent groups can be formed. There is only a slight progression from MI1, predominantly perceived as largely easy to spread, to HC1, predominantly judged as moderately easy to spread. In their studies, Parente et al. [3] also found that the sensory attribute “difficulty of spreading” was not discriminant except for the Dimethicone they used which was significantly perceived as more difficult to spread compared to the other studied emollients. They explained that this attribute is particularly complex to rate because of its dependence to the interaction between three materials: the forearm skin, on which the product is applied, the skin of the fingers that evaluates the product and the product itself.
Table 3 Correlation matrix of the physicochemical properties and the attributes “Spontaneous spreading” and “Ease of spreading” of the thirteen emollients.
Surface tension Contact angle Polar component Dispersive component Viscosity Density Refractive index %wt. evaporation Spontaneous spreading
Spontaneous spreading
Ease of spreading
−0.690* −0.865* −0.370 −0.321 −0.568* −0.542 −0.618 0.480
−0.134 −0.228 0.114 −0.247 0.080 0.034 0.078 0.008 0.074
* Significant correlation coefficient (p-value < 5%).
dispersive, which means the more polar the emollient is, the higher the contact angle will be. No distinction could be made between the various esters regarding the contact angle. In term of viscosity, the three monoesters (ES1, ES2, ES3), and the ether were significantly less viscous than the two diesters (ES4, ES6) (Fig. 1e). As expected, by increasing the number of ester groups, molecular interactions by hydrogen bonds are increased, leading to a higher resistance to flow and thus, to a higher viscosity of the diesters than monoesters.
3.2.1. Correlations between spontaneous spreading and physicochemical properties From the correlation matrix shown in Table 3, surface tension, contact angle on Teflon® and viscosity appear to be well correlated with the sensory attribute “Spontaneous spreading” (respectively −0.690, −0.965 and −0.568). Zeidler [10] found a more convincing correlation between spontaneous spreading and viscosity in the one hand, and with surface tension in the other hand. However, for his study, the number of emollients characterized was larger, the range of viscosities was much wider (4–480 mPa s), and the range of surface tensions was also slightly higher (27–35 mN/m). In contrast, no correlations can be observed between the second sensory attribute “Ease of spreading” and the other physicochemical properties. As it was explained in the previous section, this could be explained by the complexity of evaluation of this attribute. To confirm the correlations between “Spontaneous spreading” and the three physico-chemical properties, Fig. 4 shows the mean scores of spontaneous spreading versus viscosity (Fig. 4a), surface tension (Fig. 4b), and contact angle on Teflon® (Fig. 4c). Those figures indicate that the correlations are rather nonlinear. The knowledge of the spontaneous spreading process of a liquid droplet on a solid is of much interest in many practical situations. There are two main theories that have been established for describing the dynamics of spreading: the hydrodynamic theory and the molecular kinetic theory [15]. The hydrodynamic theory is based on NavierStokes equation and mainly involves viscosity, surface tension, contact angle, and the size of the droplet. The molecular kinetic theory has been developed for exploring scales that are smaller than those describable by the continuum theory. It combines surface tension with the droplet’s properties at a molecular scale such as the molecular displacement, its frequency, and the temperature [16]. In our study, the emollients were really low-viscous liquids which spread very fast on human skin, and spontaneous spreading was assessed 30 s after application. In these conditions, the evaluation was performed close to the equilibrium, thus we considered that spontaneous spreading should be viewed as a thermodynamic problem more than a kinetic one. In that sense, “Spontaneous spreading” is actually the representation of the wettability of a solid surface. It can be assessed with the spreading coefficient (S) which is the difference between the work of adhesion WA of the liquid onto the solid, and the work of
3.2. Sensory data Results for the attributes “Spontaneous spreading” and “Ease of spreading” are summarized as mosaic charts shown on Fig. 3a and b respectively. In these graphics, each row corresponds to an emollient. Each colored bar of the chart represents a category of the evaluation scale. Five categories are represented: the three initial ones (small/ medium/large), and intermediate grades. The width of a bar is proportional to the relative frequency of occurrence of one specific grade resulting from the twelve evaluations (4 assessors x 3 replicates). As an example, for the emollient HC3, 100% of the “spontaneous spreading” evaluations (Surface spreading of a drop after 30 s without any stress) were graded “large” while, for the emollient MI1, approximately 60% of the evaluations were graded “large”, 15% “medium” and 25% “medium to large”. The first observation from Fig. 3 is that, except for the surface of spontaneous spreading of HC3 which was unanimously perceived as large (top of Fig. 3a), each emollient was diversely rated according to the assessor and/or the replicate. The grades were generally divided into two main categories, but for some emollients, the entire range of the scale is used. This variability can be mainly attributed to the nature of the products which made the evaluations and the discrimination difficult. However, at the top of Fig. 3a, one can identify a group of five emollients which showed predominantly large surfaces of spontaneous spreading on skin 30 s after the deposition of a droplet. This group contains the three hydrocarbons (HC1, HC2 and HC3), the silicone (SI), and MI1. Regarding the silicone, this result is in agreement with numerous commercial papers in which silicones, and more specifically cyclic silicones, are claimed to spread easily on most solid surfaces, and in particular on skin. Parente et al. [3] also found that the spreadability on a glass dish of cyclomethicone was approximately twice as large as the ones of other tested emollients, which included mineral oil and squalane. In our work, the tested hydrocarbons and MI1 show a spreadability on skin that is similar to what can be observed with a cyclic silicone. Among the other emollients, MI2 and four of the esters (ES1, ES2, ES4 and ES6), at the bottom of Fig. 3a, showed predominantly medium surfaces of spontaneous spreading. The smaller surfaces of spontaneous spreading of the esters compared to the silicone, is in agreement with 162
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Fig. 4. Spontaneous spreading versus (a) viscosity, (b) surface tension, and (c) contact angle on Teflon®.
length diesters (ES4, ES6). Finally, the fourth group was made of the two most polar compounds: ES5 which is the shorter chain length diester, and another hydrocarbon/ester mixture, MI1. The physicochemical characterizations of the emollients were completed with sensory evaluations which lead to the following descriptions of each group. The outstanding properties of the cyclic silicone (SI) compared to the other emollients were confirmed. Indeed, its low surface tension and contact angle allows it to show a large surface of spontaneous spreading on skin. The three HC and MI2 are mainly characterized by high percentages weight of evaporation after 2 h and larger spontaneous spreading. Their viscosity and density were low. The group of esters (ES1, ES2, ES3, ES4, ES6) and ether (ET) shows the highest surface properties (surface tension and contact angle). This group also shows higher viscosities than other groups, except for ET, and is characterized by small surfaces of spontaneous spreading. Finally, the group with ES5 and MI1 logically shows the highest polar component of surface tension. However, ES5 shows a small surface of spontaneous spreading when MI1 shows one of the largest surface of spontaneous spreading. The sensory attribute “Spontaneous spreading” was suitably correlated with the spreading coefficient on Teflon®, a physicochemical quantity that combines surface tension to the cosine of contact angle. As one of the main claim of emollients is their spreadability regarding skin feel, this result provides a useful base for predicting emolliency from instrumental characteristics.
Fig. 5. Spontaneous spreading on skin as a function of the spreading coefficient on Teflon®.
cohesion WC within the liquid (Eq. (3)) [17].
S = WA − WC = (γS + γL − γSL ) − 2γL = γS (γSL − γL )
(3)
Where γS, γL and γSL are respectively the surface energy of the solid, the surface tension of the liquid, and the solid-liquid interfacial energy. When combining Eq. (3) with Young’s equation (Eq. (4)), the spreading coefficient is finally expressed as a function of surface properties: surface tension (γ) and contact angle (θ) in Eq. (5). γS = γSL + γL cos θ
(4)
S = γ x (cos θ− 1)
(5)
It is clear from Eq. (5) that S cannot be positive. When S equals to zero, the wetting is complete (θ = 0°). The more negative it is, the more partial the wetting is. From the measured surface tensions and contact angles on Teflon®, we calculated a spreading coefficient for each of the emollient. Then, we plotted the sensory attribute “Spontaneous spreading” as a function of the spreading coefficient (Fig. 5). Fig. 5 shows that the higher the spreading coefficient on Teflon® is, the larger the surface of spontaneous spreading on skin is. Which means that emollients with low surface tensions and low contact angles on Teflon® will have a larger surface of spontaneous spreading on skin. In spite of the sensory data variability, the correlation is quite good especially when S is approximately between −9 and −3. Below −9, the emollients moderately spread on skin. Those were the chemical families of the esters and the ether. Above −3, the emollients largely spread. The silicone, which has the highest spreading coefficient on Teflon®, shows a surface of spontaneous spreading similar to the ones of the hydrocarbons.
Acknowledgments This project is co-funded by the European Union. Europe is involved in the Picardy region (France) with the European Regional Development Fund. FEDER 2014-2020 (Synergy N° PI0001194). References [1] N. Loubat-Bouleuc, Les esters en cosmétologie: généralités et fonctionnalités, OCL − Ol. Corps Gras Lipides 11 (2004) 454–456. [2] G.N. Stamatas, Mineral oil in skin care: safety profile, in: A. Pappas (Ed.), Lipids Ski. Heal. Springer International Publishing, Cham, 2015, pp. 291–299. [3] M.E. Parente, Study of sensory properties of emollients used in physicochemical properties, J. Cosmet. Sci. 182 (2005) 175–182. [4] K.J. Hughes, V.F. Lvovich, D. Ph, J. Woo, B. Moran, A. Suares, Novel methods for emollient characterization, Cosmet. Toiletries 93 (2006) 19–24. [5] J.T. Alander, Chemical and physical properties of emollients, in: M. Lóden, H.I. Maibach (Eds.), Treat. Dry Ski. Syndr. 2012, pp. 399–417. [6] S. Cochran, M. Anthonavage, Fatty acids, fatty alcohols, synthetic esters and glycerin applications in the cosmetic industry, in: A. Pappas (Ed.), Lipids Ski. Heal. Springer International Publishing, Cham, 2015, pp. 311–319. [7] D. Laura, M. Gorcea, Evaluating the physiochemical properties of emollient esters for cosmetic use, Cosmet. Toiletries 125 (2010) 26–33 http://www. cosmeticsandtoiletries.com/testing/sensory/premium-Evaluating-thePhysiochemical-Properties-of-Emollient-Esters-for-Cosmetic-Use-209763151.html. [8] A. Colas, Silicones: preparation, properties and performances, dow corning, Life Sci. (2005) 14. [9] M.D. Berthiaume, Silicones in cosmetics, in: E.D. Goddard, J.V. Gruber (Eds.), Princ. Polym. Sci. Technol. Cosmet. Pers. Care, 1999, pp. 275–324. [10] V.U. Zeidler, Uber das Spreiten von Lipiden auf der Haut, Fett Wiss Technol. 87 (1985) 403–408. [11] F.M. Fowkes, Attractive forces at interfaces, Ind. Eng. Chem. 56 (1964) 40–52. [12] D.Y. Kwok, D. Li, A.W. Neumann, Fowkes’ surface tension component approach revisited, Colloids Surf. A Physicochem. Eng. Aspects 89 (1994) 181–191.
4. Conclusions From the analysis of the physicochemical properties of thirteen emollients, four main groups were statistically formed (PCA, HAC) which matched fairly well with the different chemical families investigated. The cyclic silicone (SI) formed its own group. The second group comprised the three hydrocarbons (HC1, HC2, HC3) and a hydrocarbon/ester mixture, MI2. The third group was made of the only ether (ET), the three monoesters (ES1, ES2, ES3), and the longest chain 163
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Physicochem. Eng. Aspects 193 (2001) 85–96. [16] M.J. Davis, S.H. Davis, Droplet spreading: theory and experiments, C. R. Phys. 14 (2013) 629–635. [17] J. Zhang, Work of adhesion and work of cohesion, in: Q.J. Wang, Y.-W. Chung (Eds.), Encycl. Tribol. 2013, pp. 4127–4132.
[13] D.K. Owens, Estimation of the surface free energy of polymers, J. Appl. Polym. Sci. 13 (1969) 1741–1747. [14] D.W. Van Krevelen, K. Te Nijenhuis, Interfacial energy properties BT − properties of polymers, Prop. Polym. fourth edition, Elsevier, 2009, pp. 229–244 (Chapter 8). [15] M. Von Bahr, F. Tiberg, V. Yaminsky, Spreading dynamics of liquids and surfactant solutions on partially wettable hydrophobic substrates, Colloids Surf. A
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Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Emollients for cosmetic formulations: Towards relationships between physico-chemical properties and sensory perceptions
T
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Christina Chaoa, , Célina Génota, Corinne Rodriguezb, Harmonie Magniezb, Sandrine Lacourtc, Aurélie Fievezc, Christophe Lena, Isabelle Pezrona, Denis Luartd, Elisabeth van Heckea a
Sorbonne Universités, Université de Technologie de Compiègne, EA TIMR 4297 UTC/ESCOM, rue du Docteur Schweitzer, 60200, Compiègne, France Labosphère, R & D, 5 Rue de Sétubal, 60000, Beauvais, France c Oléon, R & D, Chemin Usine, 60280, Venette, France d ESCOM, EA TIMR 4297 UTC/ESCOM, 1 allée du réseau Jean Marie Buckmaster, 60200, Compiègne, France b
G RA P H I C A L AB S T R A C T
A R T I C L E I N F O
A B S T R A C T
Keywords: Emolliency Spreading Esters Hydrocarbons Silicone
Thirteen current commercial emollients from different chemical families (hydrocarbon, ester, ether, and silicone) were firstly characterized by eight physicochemical properties which were a-priori thought to be strongly related to sensory perception. Those include usual properties as well as more original ones such as volatility, estimated by the evaporated mass fraction during a fixed duration, as well as the polar and dispersive components of the surface tension. Principal Component Analysis and Hierarchical Agglomerative Clustering led to a partition of the emollients into groups that matched fairly well with the different chemical families. It also helped to highlight the main characteristics of each group. As expected, silicone had the lowest surface properties. Hydrocarbons showed the highest volatilities and the lowest viscosities. Esters mainly displayed the highest surface properties, and within this group, diesters showed higher polar components than monoesters. Sensory evaluation was performed at the same time by a professional panel. The sensory attribute “Spontaneous spreading” was found to be larger for silicone and hydrocarbons than for esters and ether. From our set of data, a suitable correlation was obtained between the sensory attribute “Spontaneous spreading” and the calculated spreading coefficient on Teflon® which combines surface tension with the cosine of contact angle.
1. Introduction Emollients are widely used in the cosmetic field for their numerous properties. In oil in water emulsions (O/W), their concentrations are between 5% and 30%, and can even be higher for water in oil emulsions (W/O) or anhydrous products. Thus, they represent the second major group of ingredients in a formulation after water. Actually, they refer
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Corresponding author. E-mail address: [email protected] (C. Chao).
http://dx.doi.org/10.1016/j.colsurfa.2017.07.025 Received 30 September 2016; Received in revised form 2 March 2017; Accepted 8 July 2017 Available online 13 July 2017 0927-7757/ © 2017 Elsevier B.V. All rights reserved.
more specifically to oils, and are associated with different sensory feels. As a definition, an emollient can be defined as a set of characteristics, felt or seen at a precise moment, that are directly linked to a sense of smoothness, elasticity and spreadability regarding skin feel, and to a sense of glossiness or matte degree for visual perception [1]. Emollients can be divided into four main groups depending on their chemical family: hydrocarbons, fatty alcohols, esters, and silicone
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increased the interfacial tension and viscosity but decreased permittivity. Furthermore, when the hydrocarbon chain was branched, the interfacial tension would be higher than for a linear chain. And finally, they showed that the ester function increased the permittivity of the compound. To sum up, these properties depend on the hydrocarbon chain length, the degree of unsaturation, the presence of branching, and the presence of multiple functions. Silicone derivatives emollients are synthesized from silicon. They are mainly divided into three groups: the linear, the cyclic, and the cross-linked silicones. The most common linear silicone derivative is polydimethylsiloxane (PDMS), which can be found with different degrees of polymerization, and thus different viscosities from 10 to 100 000 mPa s. Cyclic silicone derivatives usually refer to octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, or dodecamethylcyclohexasiloxane, respectively D4, D5, and D6 compounds. However, due to its proven toxicity on rats, the D4 compound has been banned from the cosmetic ingredients list [8]. Silicone derivatives can be found in almost all cosmetic products because of their outstanding properties. The strong silicon-oxygen bond makes them chemically inert, resistant to oxidation and humidity. Their low surface tension allows them to spread easily on most solid surfaces, forming a homogenous and protective film that let the skin breathe. In cosmetic formulations, linear silicone derivatives are used to reduce tackiness, and to leave a nongreasy and smooth skin feel. They can also be used as antifoaming agents. As for cyclic silicones, they are used to increase the volatility of a formulation thanks to their high vapor pressure [9]. As we can see, many emollients exist on the market and they are generally characterized by sensory analysis. However, sensory evaluation is time-consuming and requires an available and well-trained panel of assessors. On the contrary, physicochemical measurements are based on standard protocols with quasi-immediate and reproducible results. Thus, understanding the relationship between the physicochemical properties of an emollient and its sensory performances could greatly improve the work of formulators. Until now, a number of scientific papers deal with the correlations between physicochemical and sensory properties of formulations containing emollients. However, only a few authors have studied the correlation between the sensory properties of raw emollients and their physicochemical properties. The study of Zeidler [10] is probably the oldest one, and focused on spontaneous spreading on human skin. The results were compared to the chemical structure and physicochemical data, such as viscosity and surface tension, of the tested compounds. Around thirty so-called lipids within which hydrocarbons, esters, alcohols and triglycerides, were investigated. Although the author concluded that none of these data series correlated entirely with the spreading rates of all the lipids tested, trends were obtained within groups of compounds with similar structures: the spreading value decreased with an increase of viscosity or surface tension. Parente et al. [3] applied linear partial least squares to a set of physicochemical properties and sensory attributes of eight commonly used emollients. They found that glossiness, residue and oiliness are positively correlated to surface tension, and negatively correlated to spreadability. They also concluded that viscosity is positively correlated to the difficulty of spreading and stickiness, but negatively correlated to softness and slipperiness. M. Gorcea and D. Laura [7] compared four specific branched ester emollients. They concluded that esters with low molecular weights and low viscosities would feel less tacky and leave less residue on the skin. They also observed that esters with shorter chain length spread the most, gave the lightest and driest feel, and left the least residue and gloss. The aim of our study was to identify, among basic physicochemical properties characterizing thirteen current commercial emollients, which ones really affect the sensory perception of the emollients. For
derivatives. Hydrocarbon-based emollients can come from animal, vegetable or mineral sources, and can be synthetic. Natural hydrocarbons are very unstable because of their high degree of unsaturation, which makes them easily oxidized. Stable hydrocarbons are obtained after hydrogenation of the compound. The final saturated hydrocarbon shows many advantages such as chemical inertia, resistance to oxidation and to hydrolysis [2]. In terms of physicochemical properties, Parente et al. [3] compared several emollients, and showed that the mineral oil and squalane they used had higher superficial tensions and smaller surfaces of spreading at 0.5 and 1 min on a glass solid surface than other compounds. Hughes et al. [4] also studied several emollients within which four hydrocarbons with viscosities between 1 and 5 mPa.s. The four hydrocarbons showed the highest values of interfacial tensions with water, and the lowest contact angles at 1 s post contact on Vitro-Skin®. In term of sensory evaluation, hydrocarbon-based emollients are generally claimed to be able to decrease the oiliness of a formulation by leaving a pleasant and light skin feel. Fatty alcohols can also be from vegetable sources or synthetic. The hydroxyl group allows an increase in polarity, but also the formation of hydrogen bonds with water which increases the consistency of the formulation. Parente et al. [3], showed that octyldodecanol had intermediate properties in terms of surface tension, viscosity, and spreading at 0.5 and 1 min compared to the esters, hydrocarbons and silicones tested. In terms of sensory evaluations, it showed high values of glossiness, residue and oiliness. Usually used to stabilize emulsions, alcohols can also be used as emollients when added between 1% and 2% [5,6]. Ester-based emollients can either be synthesized from alcohols and fatty acids, or be natural. When synthesized, simple or complex esters can be obtained. For monoesters, fatty acids can either be vegetable-derived, animal-derived, or synthetic. When vegetable-derived, the acids chain lengths are even numbered, and mostly between C12 and C18. When animal-derived, chain length can either be even or uneven numbered (C15 or C17). The alcohol reactants, can also be natural, or synthesized from petrochemicals, but also come from fermentation in the case of shorter chains (methanol and ethanol). As for complex esters, one of the reactant must be a multifunctional alcohol or acid. Naturally occurring esters refer to triglycerides which can be found in plants or animals, they are characterized by their fatty acid composition. When vegetable triglycerides can directly be used as emollients, animal triglycerides are mainly used as a source of fatty acids. The six vegetable oils that are most used are palm oil, soybean oil, rapeseed oil, sunflower oil, palm kernel oil, and coconut oil. Palm oil is a major source of oleic (C18:1) and palmitic (C16:0) acids. Soybean, rapeseed and sunflower oils are main sources of linoleic (C18:2) and linolenic (C18:3) acids. Palm kernel and coconut oils are main sources of acids with medium chain lengths between C8 and C14. Note that triglycerides can also be synthesized from glycerol and fatty acids (e.g. caprylic/ capric triglycerides) [5]. Thus, a lot of different combinations are possible, which makes the ester-based emollients a very versatile group with a wide range of properties. For example, they can be used to modify the consistency of the formulation, or increase the spreadability of sunscreens [1]. M. Gorcea and D. Laura [7] studied four specific branched ester emollients: diisopropyl adipate, isodecyl neopentanoate, isocetyl stearate, and octyldodecyl stearate. They concluded that spreading values measured on Vitro-Skin® depend on molecular weight, viscosity, and chemical structure. They observed that the higher the viscosity, the lower the spreading values. They also concluded that more polar esters showed lower refractive indexes and surface tensions. In addition to a higher polarity, when the ester has a low molecular weight and viscosity, it showed a lower contact angle on a silicone substrate. Hughes et al. [4] showed that esters with a long hydrocarbon chain 157
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Measurements were performed in triplicate. The coefficient of variation was 2.2%. The second category of measurements concerns bulk properties such as viscosity and density. Viscosity was measured with a capillary viscometer, measuring device was an AVS400 (Schott Geräte, Germany), and capillary was with reference number 501 01 (SI Analytics GmbH, Germany). Calibration of the viscometer was done with distilled water. Emollients were all sampled twice, and three measurements were performed on each sample. The coefficient of variation was 1.3%. Density was measured with a density meter DMA45 (Anton Paar, Austria). Emollients were sampled thrice, and one measurement was performed per sample. The coefficient of variation was 0.01%. As an optical property, refractive index was measured with a refractometer. Emollients were sampled thrice, and one measurement was performed per sample. The coefficient of variation was 0.01%. Finally measurements of one thermodynamic property, which is the percentage weight evaporation after 2 h (%wt. evaporation) was performed. A Dynamic Vapor Sorption device (Surface Measurement Systems Ltd., England) was used. Measurements were performed in a temperature/humidity controlled incubator, and relative humidity was set to 5%. Emollients were sampled twice, and one measurement was performed on each sample. The coefficient of variation was 4.2%. All measurements were performed at 25 °C.
these purposes, emollients were characterized by eight physicochemical properties that were a-priori thought to be strongly related to sensory perception. Those include original properties like volatility estimated by the evaporated mass fraction during a fixed duration, as well as the polar and dispersive components of the surface tension. Sensory evaluation was performed at the same time by a professional panel. Statistics were used thus allowing correlations between properties and to some extent comparisons between chemical families. Finally, by referring to the physicochemical science, we aim at initiating the development of meaningful relationships between the physicochemical properties that could help predict the sensory attribute “spontaneous spreading”. 2. Materials and methods 2.1. Samples Thirteen commercial emollients were evaluated: three hydrocarbons (HC1, HC2, HC3), six esters (ES1, ES2, ES3, ES4, ES5, ES6), one ether (ET), one silicone (SI), and two mixtures of ester/hydrocarbon (MI1, MI2). The selected emollients have different chemical structures: hydrocarbons were all saturated (linear and branched), esters comprised three monoesters (with increasing degree of branching from ES1 to ES3) and three diesters, ES5 being shorter than the two others and ES4 having a higher degree of branching than ES6, and the silicone was cyclic.
2.3. Sensory analysis All samples were labeled with random 3-digit codes and evaluated at least three times by a professional panel of four expert assessors who come from the industrial partners of the project. The assessors were all women who had experience between 5–10 years in sensory analysis of cosmetic products, and still evaluate cosmetic ingredients and formulations on a daily basis. The two attributes “Spontaneous spreading” and “ease of spreading” were evaluated. Reference samples were used to identify the lower and higher intensity of each attribute scale. Each assessor was asked to examine a maximum of 5 samples per session, and assign each of them to a category on a scale. Experimental procedures for rating the samples according to each of the attributes are detailed in Table 1.
2.2. Physicochemical measurements The chosen physicochemical properties were those that could be related to an attribute from sensory evaluation, and could be divided into different categories. The first category deals with surface properties such as surface tension (γ), and contact angle (& z.Theta;). Surface tension was measured with the Wilhelmy plate method using a processor tensiometer K100 (Krüss, Germany). To ensure that measurements were performed at equilibrium, they were programmed to last 180 s, or to end when 5 consecutive identical values of surface tension were obtained. All emollients were sampled twice, and three measurements were performed on each sample. The coefficient of variation was 0.8%. Contact angle was measured with a Drop Shape Analysis system DSA10 Mk2 (Krüss, Germany). The sessile drop method was used on a Teflon® solid surface, with three repetitions for each emollient. Teflon® was chosen because it is a usual substrate for contact angle measurements, and it is also the only surface allowing a rapid characterization of the emollients with static contact angles. The coefficient of variation was 3%. Two secondary surface properties were deduced from surface tension: polar (γP) and dispersive (γD) components. When summing these two components, the total surface tension is obtained, which is expressed as equation 1 according to Fowkes [11]. More specifically, the polar component is a result of hydrogen and dipole-dipole interactions, when the dispersive component is a result of molecular interactions of the London forces [12]. These two components can also be expressed with the Owens-Wendt-Rabel-Kaelble (OWRK) equation (Eq. (2)) which gives the interfacial tension between two liquids, and in which γ1 and γ2 are the surface tensions of each liquid [13]. When combining these two equations, it is thus possible to calculate γP and γD of each emollient. γ = γP + γD
γ1,2 =γ1
+
γ2
− 2(
2.4. Data analysis The physicochemical properties were compiled in a table of 13 lines and 8 columns, with the compounds as lines, and the physicochemical properties as columns. When trying to visualize these data, a scatter plot is obtained in an eight-dimension space (8 properties). In order to simplify the plot, principal component analysis (PCA) was applied to the physicochemical properties. It consists in reducing the dimension of the dataset by combining the variables (physicochemical properties) into principal components, which would represent the axis of the new plot. PCA helps to assess the correlations between variables, but also to evaluate the influence of chemical structures on physicochemical properties. Hierarchical agglomerative clustering (HAC) was then used to see how the emollients could be clustered into groups according to their physicochemical properties. HAC is an iterative method which at first, consists in comparing pairs of individual emollients by calculating their squared Euclidean distances. The first pair with the shortest distance is gathered into a cluster, which is then compared as a unique individual to the (n − 2) emollients left to form a bigger cluster, etc. For this study, the method of the complete linkage (farthest neighbor) was used. Sensory data being categorical data, the frequency of occurrence of each grade was calculated for each attribute and each emollient. The results were plotted on charts called mosaic plots. PCA, HAC and mosaic plots were performed using the statistical software Statgraphics® Centurion XVII (Statpoint Technologies, inc., USA).
(1)
γ1dγ2 d
+
γ1pγ2 p )
(2)
Polar/dispersive components were measured with a Drop Shape Analysis system DSA10 Mk2 (Krüss, Germany). Measurements were performed with the emerging drop method with a curved syringe of diameter 1.493 mm, and with distilled water as a continuous phase, which surface tension, and polar/dispersive components are known. 158
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Table 1 Description of the attributes used for sensory evaluation. Stage of evaluation
Attributes
Description
Scale
During application
Spontaneous spreading Ease of spreading
Surface spreading of a drop after 30 s with no stress applied. Ease of spreading when spreading a drop of emollient in circle 4 times with 2 fingers.
3-point category scale: 1. Small 2. Medium 3. Large
3. Results and discussion
HAC could cluster the emollients into 4 main groups drawn on Fig. 2a and b. One group is made of SI which properties are completely different from the other emollients. Indeed, its surface tension, polar component of surface tension, contact angle, and refractive index are the lowest. The unique surface properties of cyclic silicones have already been pointed out especially in term of contact angle, whether on Vitro-skin® [4] or on a glass dish [3], but also in term of surface tension [3]. The presence of Silicon-Oxygen bonds and the cyclic form of the molecules are undoubtedly responsible for these properties. The second group is made of HC1, HC2, HC3, and MI2. Those emollients, with the cyclic silicone but also MI1, the other blend of hydrocarbon/ester, showed the highest evaporated mass fraction after 2 h (Fig. 1f). This result is in total agreement with the fact that hydrocarbons generally have lower boiling points and higher vapor pressures than chemicals with similar molecular size from any other chemical families. The volatility of hydrocarbons is a consequence of the weak van der Waals interactions between molecules. Besides volatility, the HC and MI2 group is located among emollients with the lowest values of viscosity (Fig. 1e) and density, which was expected considering their moderate molecular weight. Lastly, this group of mainly non polar molecules was, as expected, also characterized by low polar components of the surface tension (Fig. 1c). The next group of emollients clustered by HAC is made of MI1 and ES5 which essentially shows the highest values of polar component but also the lowest values of dispersive component of the surface tension (Fig. 1c and d). Both emollients in this group were obviously the most polar ones of the study. Both contained at least one ester, ie oxygen atoms able to form hydrogen bonds, which increases polarity compared to hydrocarbons. In addition, ES5 was the shortest chain diester and MI1, the mixture hydrocarbon/ester with the shortest chains also. Finally, the fourth group on Fig. 2 is made of ES1, ES2, ES3, ES4, ES6, and ET, which shows higher values of surface tension, viscosity, and contact angle than other emollients. This group can be divided into two smaller groups: one with ET, ES1, ES2, and ES3 which are the ether and the three monoesters, and one with ES4 and ES6 which are two diesters. The ether had properties very close to those of the three monoester (ES1, ES2 and ES3). So adding a C]O bond to an ether does not seem to affect the physicochemical properties of the molecule. The fact that the esters are monoesters (ES2, ES3) or diesters (ES4, ES6) does not seem to greatly influence the surface tension as well (Fig. 1a). However, the three monoesters (ES1, ES2, ES3) clearly showed lower polar components and slightly higher dispersive components than diesters (ES4, ES6) and, as expected, than the short-chain diester (ES5, from the previous group) (Fig. 1d). So adding ester groups in a molecule increases its polarity as already noticed by Hughes et al. [4] who determined the polarity of various esters by means of permittivity measurements. Hughes et al. also found differences between branched and straight chain esters, the branched ones having higher interfacial tension against water than the straight ones, meaning a lower polarity. In our study, we did not found any trend concerning the branching. Regarding the contact angles of this fourth group, as expected, high values are observed since the molecules are mainly polar and the measurements of contact angles have been made on Teflon®. Indeed, Teflon® has a very low surface energy of (18–19 mN/m), with a dispersive component of 17–18.5 mN/m [14]. Thus, this surface is mainly
3.1. Physicochemical properties Surface tensions, contact angles, polar and dispersive components of surface tensions, viscosities and percentages weight of evaporation of emollients are illustrated on Fig. 1. Fig. 1a shows that surface tensions of the emollients are between 23 and 30 mN/m, except for SI which shows a surface tension below 20 mN/m. On Fig. 1b, we observe higher contact angles for ET and all ES. The emollients with lowest contact angle are SI (around 10°), followed by HC3 and MI1. Fig. 1c shows that ES5 and MI1 have the highest polar component (respectively around 9 mN/m and 7 mN/m). The lowest values (< 1 mN/m) are obtained for SI and all HC. The lowest values of dispersive components are observed for ES5, SI and MI1 (Fig. 1d). On Fig. 1e, we observe higher viscosities for ES4 and ES6 (respectively 9 and 8 mPa s) and lower viscosities for all HC and MI1 (below 2 mPa s). Fig. 1f shows that all emollients have low percentages weight of evaporation except HC3 and MI2 (respectively 27%wt. and 23%wt.). PCA was performed on the eight physicochemical properties of the thirteen emollients. From the correlation matrix shown in Table 2, a high correlation between surface tension and contact angle on Teflon® is observed (0.877). Moreover, surface tension and contact angle appear to be well correlated with refractive index (respectively 0.806 and 0.838). These unforeseen correlations are probably specific to our data set. Contrary to the works of Hughes et al. [4], and Gorcea and Laura [7], we did not found any consistent correlation between viscosity and contact angle (0.519). This is probably due to the different surfaces that they used for the measurement of the contact angle, which are Vitro-Skin®, a synthetic surface mimicking human skin for Hughes et al. [4], and a thick silicone substrate adhered to a glass slide for Gorcea and Laura [7]. The high correlation found by those authors might also be explained by the fact that the emollients they studied were different (they covered a wider range of viscosities than in our study). Except for surface tension, contact angle and refractive index, all the other correlations are low, meaning that each of the physicochemical properties investigated brings a specific information. Regarding the characteristics of the principal components, the first three components accounted for 85.4% of the variance. As shown on Fig. 2a, principal component 1 (PC1) accounted for 45.5% of the variance, with a major positive contribution of contact angle, surface tension and refractive index. This indicate that the variability in the data set was mainly explained by these three physicochemical properties. PC2 accounted for 26.5% of the variance, with a main positive contribution of density and polar component of surface tension, and, to a lesser extent, a negative contribution of the dispersive component of the surface tension (Fig. 2a). In order to give a more precise view of the data, PC3, was combined to PC1 in another biplot shown in Fig. 2b. PC3 accounted for 13.4% of the variance, and mainly revealed the contribution of viscosity. The evaporated mass fraction after 2 h (% wt. evaporation) which was not colinear to any of the first three principal components, seemed to contribute only weakly to the principal components. However, the length of the vector representing it on Fig. 2a is similar to the length of the other properties meaning this property should not be neglected. 159
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Fig. 1. Physicochemical properties of emollients. (a) Surface tension. (b) Contact angle on Teflon®. (c) Polar component of the surface tension. (d) Dispersive component of the surface tension. (e) Viscosity. (f) Percentage weight of evaporation after 2 h.
Table 2 Correlation matrix of the physicochemical properties of the thirteen emollients.
Surface tension Contact angle Polar component Dispersive component Viscosity Density Refractive index %wt. evaporation
Surface tension
Contact angle
Polar component
Dispersive component
Viscosity
Density
Refractive index
0.877* 0.495 0.506 0.324 0.026 0.806* −0.428
0.293 0.583 0.519 0.271 0.838* −0.338
−0.499 0.218 0.348 0.142 −0.355
0.101 −0.322 0.662 −0.074
0.636 0.479 −0.326
−0.064 −0.331
−0.064
* Significant correlation coefficient (p-value < 5%).
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Fig. 2. Principal Components Analysis of the physicochemical properties. (a) PCA with PC2. (b) PC1 with PC3.
Fig. 3. Mosaic plot of the grades obtained for the attribute “Spontaneous spreading”: (a) and “Ease of spreading”: (b).
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the results obtained by Parente et al. [3] on a glass dish. The evaluation of spontaneous spreading for ET, ES5, (shortest diester) and ES3 (branched monoester), was the most dispersed since the entire range of the scale was used (bottom of Fig. 3a). Since ES5 is the shortest chain ester, this last result disagrees with the work of Gorcea and Laura [7]. Indeed, by comparing four esters, they found that the lowest molecular weight ester was the one that spread the most. They also measured the spreading area of their esters on Vitro-skin®, and confirmed the high spreadability of the shortest chain ester. This disagreement might be explained by the fact that the emollients they studied showed a broader range of molecular weights than in our study. Regarding the “ease of spreading” (Fig. 3b), all the emollients were rated between “medium” and “large”. Despite the different chemical structures of the emollients, their ease of spreading were very similar. No apparent groups can be formed. There is only a slight progression from MI1, predominantly perceived as largely easy to spread, to HC1, predominantly judged as moderately easy to spread. In their studies, Parente et al. [3] also found that the sensory attribute “difficulty of spreading” was not discriminant except for the Dimethicone they used which was significantly perceived as more difficult to spread compared to the other studied emollients. They explained that this attribute is particularly complex to rate because of its dependence to the interaction between three materials: the forearm skin, on which the product is applied, the skin of the fingers that evaluates the product and the product itself.
Table 3 Correlation matrix of the physicochemical properties and the attributes “Spontaneous spreading” and “Ease of spreading” of the thirteen emollients.
Surface tension Contact angle Polar component Dispersive component Viscosity Density Refractive index %wt. evaporation Spontaneous spreading
Spontaneous spreading
Ease of spreading
−0.690* −0.865* −0.370 −0.321 −0.568* −0.542 −0.618 0.480
−0.134 −0.228 0.114 −0.247 0.080 0.034 0.078 0.008 0.074
* Significant correlation coefficient (p-value < 5%).
dispersive, which means the more polar the emollient is, the higher the contact angle will be. No distinction could be made between the various esters regarding the contact angle. In term of viscosity, the three monoesters (ES1, ES2, ES3), and the ether were significantly less viscous than the two diesters (ES4, ES6) (Fig. 1e). As expected, by increasing the number of ester groups, molecular interactions by hydrogen bonds are increased, leading to a higher resistance to flow and thus, to a higher viscosity of the diesters than monoesters.
3.2.1. Correlations between spontaneous spreading and physicochemical properties From the correlation matrix shown in Table 3, surface tension, contact angle on Teflon® and viscosity appear to be well correlated with the sensory attribute “Spontaneous spreading” (respectively −0.690, −0.965 and −0.568). Zeidler [10] found a more convincing correlation between spontaneous spreading and viscosity in the one hand, and with surface tension in the other hand. However, for his study, the number of emollients characterized was larger, the range of viscosities was much wider (4–480 mPa s), and the range of surface tensions was also slightly higher (27–35 mN/m). In contrast, no correlations can be observed between the second sensory attribute “Ease of spreading” and the other physicochemical properties. As it was explained in the previous section, this could be explained by the complexity of evaluation of this attribute. To confirm the correlations between “Spontaneous spreading” and the three physico-chemical properties, Fig. 4 shows the mean scores of spontaneous spreading versus viscosity (Fig. 4a), surface tension (Fig. 4b), and contact angle on Teflon® (Fig. 4c). Those figures indicate that the correlations are rather nonlinear. The knowledge of the spontaneous spreading process of a liquid droplet on a solid is of much interest in many practical situations. There are two main theories that have been established for describing the dynamics of spreading: the hydrodynamic theory and the molecular kinetic theory [15]. The hydrodynamic theory is based on NavierStokes equation and mainly involves viscosity, surface tension, contact angle, and the size of the droplet. The molecular kinetic theory has been developed for exploring scales that are smaller than those describable by the continuum theory. It combines surface tension with the droplet’s properties at a molecular scale such as the molecular displacement, its frequency, and the temperature [16]. In our study, the emollients were really low-viscous liquids which spread very fast on human skin, and spontaneous spreading was assessed 30 s after application. In these conditions, the evaluation was performed close to the equilibrium, thus we considered that spontaneous spreading should be viewed as a thermodynamic problem more than a kinetic one. In that sense, “Spontaneous spreading” is actually the representation of the wettability of a solid surface. It can be assessed with the spreading coefficient (S) which is the difference between the work of adhesion WA of the liquid onto the solid, and the work of
3.2. Sensory data Results for the attributes “Spontaneous spreading” and “Ease of spreading” are summarized as mosaic charts shown on Fig. 3a and b respectively. In these graphics, each row corresponds to an emollient. Each colored bar of the chart represents a category of the evaluation scale. Five categories are represented: the three initial ones (small/ medium/large), and intermediate grades. The width of a bar is proportional to the relative frequency of occurrence of one specific grade resulting from the twelve evaluations (4 assessors x 3 replicates). As an example, for the emollient HC3, 100% of the “spontaneous spreading” evaluations (Surface spreading of a drop after 30 s without any stress) were graded “large” while, for the emollient MI1, approximately 60% of the evaluations were graded “large”, 15% “medium” and 25% “medium to large”. The first observation from Fig. 3 is that, except for the surface of spontaneous spreading of HC3 which was unanimously perceived as large (top of Fig. 3a), each emollient was diversely rated according to the assessor and/or the replicate. The grades were generally divided into two main categories, but for some emollients, the entire range of the scale is used. This variability can be mainly attributed to the nature of the products which made the evaluations and the discrimination difficult. However, at the top of Fig. 3a, one can identify a group of five emollients which showed predominantly large surfaces of spontaneous spreading on skin 30 s after the deposition of a droplet. This group contains the three hydrocarbons (HC1, HC2 and HC3), the silicone (SI), and MI1. Regarding the silicone, this result is in agreement with numerous commercial papers in which silicones, and more specifically cyclic silicones, are claimed to spread easily on most solid surfaces, and in particular on skin. Parente et al. [3] also found that the spreadability on a glass dish of cyclomethicone was approximately twice as large as the ones of other tested emollients, which included mineral oil and squalane. In our work, the tested hydrocarbons and MI1 show a spreadability on skin that is similar to what can be observed with a cyclic silicone. Among the other emollients, MI2 and four of the esters (ES1, ES2, ES4 and ES6), at the bottom of Fig. 3a, showed predominantly medium surfaces of spontaneous spreading. The smaller surfaces of spontaneous spreading of the esters compared to the silicone, is in agreement with 162
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Fig. 4. Spontaneous spreading versus (a) viscosity, (b) surface tension, and (c) contact angle on Teflon®.
length diesters (ES4, ES6). Finally, the fourth group was made of the two most polar compounds: ES5 which is the shorter chain length diester, and another hydrocarbon/ester mixture, MI1. The physicochemical characterizations of the emollients were completed with sensory evaluations which lead to the following descriptions of each group. The outstanding properties of the cyclic silicone (SI) compared to the other emollients were confirmed. Indeed, its low surface tension and contact angle allows it to show a large surface of spontaneous spreading on skin. The three HC and MI2 are mainly characterized by high percentages weight of evaporation after 2 h and larger spontaneous spreading. Their viscosity and density were low. The group of esters (ES1, ES2, ES3, ES4, ES6) and ether (ET) shows the highest surface properties (surface tension and contact angle). This group also shows higher viscosities than other groups, except for ET, and is characterized by small surfaces of spontaneous spreading. Finally, the group with ES5 and MI1 logically shows the highest polar component of surface tension. However, ES5 shows a small surface of spontaneous spreading when MI1 shows one of the largest surface of spontaneous spreading. The sensory attribute “Spontaneous spreading” was suitably correlated with the spreading coefficient on Teflon®, a physicochemical quantity that combines surface tension to the cosine of contact angle. As one of the main claim of emollients is their spreadability regarding skin feel, this result provides a useful base for predicting emolliency from instrumental characteristics.
Fig. 5. Spontaneous spreading on skin as a function of the spreading coefficient on Teflon®.
cohesion WC within the liquid (Eq. (3)) [17].
S = WA − WC = (γS + γL − γSL ) − 2γL = γS (γSL − γL )
(3)
Where γS, γL and γSL are respectively the surface energy of the solid, the surface tension of the liquid, and the solid-liquid interfacial energy. When combining Eq. (3) with Young’s equation (Eq. (4)), the spreading coefficient is finally expressed as a function of surface properties: surface tension (γ) and contact angle (θ) in Eq. (5). γS = γSL + γL cos θ
(4)
S = γ x (cos θ− 1)
(5)
It is clear from Eq. (5) that S cannot be positive. When S equals to zero, the wetting is complete (θ = 0°). The more negative it is, the more partial the wetting is. From the measured surface tensions and contact angles on Teflon®, we calculated a spreading coefficient for each of the emollient. Then, we plotted the sensory attribute “Spontaneous spreading” as a function of the spreading coefficient (Fig. 5). Fig. 5 shows that the higher the spreading coefficient on Teflon® is, the larger the surface of spontaneous spreading on skin is. Which means that emollients with low surface tensions and low contact angles on Teflon® will have a larger surface of spontaneous spreading on skin. In spite of the sensory data variability, the correlation is quite good especially when S is approximately between −9 and −3. Below −9, the emollients moderately spread on skin. Those were the chemical families of the esters and the ether. Above −3, the emollients largely spread. The silicone, which has the highest spreading coefficient on Teflon®, shows a surface of spontaneous spreading similar to the ones of the hydrocarbons.
Acknowledgments This project is co-funded by the European Union. Europe is involved in the Picardy region (France) with the European Regional Development Fund. FEDER 2014-2020 (Synergy N° PI0001194). References [1] N. Loubat-Bouleuc, Les esters en cosmétologie: généralités et fonctionnalités, OCL − Ol. Corps Gras Lipides 11 (2004) 454–456. [2] G.N. Stamatas, Mineral oil in skin care: safety profile, in: A. Pappas (Ed.), Lipids Ski. Heal. Springer International Publishing, Cham, 2015, pp. 291–299. [3] M.E. Parente, Study of sensory properties of emollients used in physicochemical properties, J. Cosmet. Sci. 182 (2005) 175–182. [4] K.J. Hughes, V.F. Lvovich, D. Ph, J. Woo, B. Moran, A. Suares, Novel methods for emollient characterization, Cosmet. Toiletries 93 (2006) 19–24. [5] J.T. Alander, Chemical and physical properties of emollients, in: M. Lóden, H.I. Maibach (Eds.), Treat. Dry Ski. Syndr. 2012, pp. 399–417. [6] S. Cochran, M. Anthonavage, Fatty acids, fatty alcohols, synthetic esters and glycerin applications in the cosmetic industry, in: A. Pappas (Ed.), Lipids Ski. Heal. Springer International Publishing, Cham, 2015, pp. 311–319. [7] D. Laura, M. Gorcea, Evaluating the physiochemical properties of emollient esters for cosmetic use, Cosmet. Toiletries 125 (2010) 26–33 http://www. cosmeticsandtoiletries.com/testing/sensory/premium-Evaluating-thePhysiochemical-Properties-of-Emollient-Esters-for-Cosmetic-Use-209763151.html. [8] A. Colas, Silicones: preparation, properties and performances, dow corning, Life Sci. (2005) 14. [9] M.D. Berthiaume, Silicones in cosmetics, in: E.D. Goddard, J.V. Gruber (Eds.), Princ. Polym. Sci. Technol. Cosmet. Pers. Care, 1999, pp. 275–324. [10] V.U. Zeidler, Uber das Spreiten von Lipiden auf der Haut, Fett Wiss Technol. 87 (1985) 403–408. [11] F.M. Fowkes, Attractive forces at interfaces, Ind. Eng. Chem. 56 (1964) 40–52. [12] D.Y. Kwok, D. Li, A.W. Neumann, Fowkes’ surface tension component approach revisited, Colloids Surf. A Physicochem. Eng. Aspects 89 (1994) 181–191.
4. Conclusions From the analysis of the physicochemical properties of thirteen emollients, four main groups were statistically formed (PCA, HAC) which matched fairly well with the different chemical families investigated. The cyclic silicone (SI) formed its own group. The second group comprised the three hydrocarbons (HC1, HC2, HC3) and a hydrocarbon/ester mixture, MI2. The third group was made of the only ether (ET), the three monoesters (ES1, ES2, ES3), and the longest chain 163
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[13] D.K. Owens, Estimation of the surface free energy of polymers, J. Appl. Polym. Sci. 13 (1969) 1741–1747. [14] D.W. Van Krevelen, K. Te Nijenhuis, Interfacial energy properties BT − properties of polymers, Prop. Polym. fourth edition, Elsevier, 2009, pp. 229–244 (Chapter 8). [15] M. Von Bahr, F. Tiberg, V. Yaminsky, Spreading dynamics of liquids and surfactant solutions on partially wettable hydrophobic substrates, Colloids Surf. A
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