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The human skin is a very complex living tissue, in a permanent evolution and self-renewing by constant lipids secretion. The characterization of this biological material is a major concern in dermo-cosmetic and pharmaceutics fields. Understanding the skin interaction with its environment, during application of skincare products, is consequently of genuine interest to better control the different phenomena occurring. In sensory language, the application of products on the skin is defined as the spreading behavior. Five O/W emulsions were formulated with different ratios of two emollients (isohexadecane and stearic acid). Complementary instrumental and sensory analysis of spreading behavior was carried out in vivo on human skin as well as in vitro on non-biological skin surface in order to investigate the impact of two emollients, and their mixtures in the spreadability and penetration of O/W emulsions. A first screening was made to link the physico-chemical properties (polarity, physical state and ratio) of emollients with the spreading behavior on human skin. Then, interesting parameters (the plateau value, its length and the increase of the friction value) from the tribological study on skin were considered to allow deepening the product/skin interactions after application of different emulsions and over time. In the last part of the study, an original method, using non-biological surfaces mimicking the human skin, was successfully tested with very good reproducibility of the spreading behavior. This original tool is of great interest to study the efficacy of new formulas on skin, but also for fundamental research and help performing standardized measurements as well as solving the logistic and safety problems of in vivo studies., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Objective: The sensory perception of cosmetic emulsions is complex as it is governed by an important number of parameters like the choice of raw materials, their interactions, the structural organisation of the system, etc. The aim of the present work was to go further in the interpretation of the emollient-surfactant interactions, towards the emulsions applicative properties. For this purpose, two systems containing liquid crystals of the lamellar type were formulated, differing only in the selected emollient., Methods: First, the liquid crystals types were checked using different tools like the optical microscopy under the bright and polarized light, the wide-angle X rays diffraction and, finally, thermogravimetric analysis. Next, two sensory attributes, namely compression force and difficulty of spreading, were evaluated by a sensory panel. In addition to that, complementary instrumental characterizations (flow tests, textural analysis and contact angle measurements) were performed in order to understand how the panel could discriminate the products., Results: The results showed that isohexadecane emollient induces the α-gel structures, while caprylic capric triglycerides favour the formation of the lamellar liquid crystals near to α-gel. For the compression force, the results point out that there is no direct interaction between the oil phase and the skin. For this attribute, depending on its chemical structure, emollient impacts the human perception only by changing the lamellar phase type. Concerning the difficulty of spreading, both the emulsion structure and the emollient properties should be considered. Immediate perception is impacted by the emulsions destruction, making the droplet roll one on each other. Then, once the droplets monolayer is disrupted, the emollient comes into direct contact with the skin. In this case, the perception is governed by the direct affinity of the emollient with the skin, nonpolar emollients being easier to spread if compared to polar ones., Conclusion: The sensory perception is guided not only by the choice of the raw materials but also by their interactions. It was shown that the chemical structure of the emollients affected the molecular organization of liquid crystals present in the emulsion and, consequently, directly or indirectly its sensory perception., (© 2018 Society of Cosmetic Scientists and the Société Française de Cosmétologie.)

The study focuses on the impact of structural and physicochemical properties of emollients on their spreadability. Fifty-three emollients, among which esters, silicones, vegetable and mineral oils, have been characterized. Their viscosity, surface tension, density and spreadability have been measured. Vitro-skin ® , an artificial skin substitute, was used as an artificial porous substrate to measure spreadability. Two different methods have been selected to characterize spreadability, namely contact angle and spreading value. Dynamic contact angle measurements showed that emollient spreadability is first governed by spontaneous spreading and that, in a second phase, absorption and migration into the porous substrate becomes the driver of the extension of the spreading area. Statistical analysis of physicochemical and spreading value data revealed that viscosity has a major impact on the spreading behavior of emollients whatever their chemical type. A special emphasis was placed on the ester family in which chemical diversity is very wide. The results highlighted a difference between "high viscosity esters" for which viscosity is the main factor impacting spreadability and "low viscosity esters" for which structural variations (mono/diester, saturated/unsaturated chain, linear/branched chain) have to be considered in addition to viscosity. Linear regressions were used to express spreading value as a function of viscosity for each of the four emollient families tested (esters, silicones, vegetable and mineral oils). These regressions allowed the development of reliable predictive models as a powerful tool for formulators to forecast spreadability of emollients., (Copyright © 2017 Elsevier B.V. All rights reserved.)

This study deals with the impact of emollients on the spreading properties of cosmetic products using a combined sensory-instrumental approach. To that purpose, three esters and one silicone were selected and incorporated separately into an oil phase. Different cosmetic o/w emulsions were then prepared with these different oil phases. Both of them were analyzed by instrumental techniques and in vivo sensory analyses. A significant effect of the emollient used was established in emulsions and in oil phases as well. Concerning emulsions, results reveal a clear correlation between in vivo spreading evaluation and friction coefficient parameters measured by texture analyzer, despite a fairly low correlation coefficient (Pearson coefficient=-0.78). Concerning oil phases, characterization of spreading was done by monitoring the contact angle relaxation of a drop of solution after deposition on a flat PMMA surface whereas sensory procedure was based on spontaneous spreading of oil phases onto the skin. Finally, good correlations between in vivo sensory analysis and instrumental measurements of both oils and emulsions were found, thus promising the possible development of predictive tools to evaluate spreadability., (Copyright © 2012 Elsevier B.V. All rights reserved.)