Influence of the emollient on emulsions containing lamellar liquid crystals: from molecular organization towards applicative properties.

Abstract

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. 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. 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. 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.