Effects of stearyl alcohol crystallization on the mesoscopic mechanical properties of α-gel-based emulsions

Emulsions typically contain surfactants and co-surfactants to address their thermodynamic instability. Ternary mixtures of these amphiphilic molecules and water can form lamellar gel phases with a hexagonal crystalline subcell, so-called α -gels. In this work, we prepared α -gel-stabilized oil-in-water emulsions containing polyoxyethylene nonionic surfactants and stearyl alcohol. The lamellar structures were characterized by small- and wide-angle X-ray scattering (SAXS/WAXS). With increasing stearyl alcohol content, the emulsions exhibited a decrease in the lamellar d -spacing and multiple Bragg peaks. In addition, the retention of retinyl palmitate dissolved in the emulsions was evaluated by high-performance liquid chromatography after 1 month of storage at 50 ◦ C and 60% relative humidity. The residual content of retinyl palmitate also decreased at higher stearyl alcohol contents. Thus, excess stearyl alcohol was found to modify both the lateral and longitudinal packing of the multilamellar structures, resulting in reduced retention of the vitamin A ester. We further investigated the local mechanical properties of the emulsions using an in situ indentation-based mechanical approach with a rigid spherical punch. As the stearyl alcohol content increased, the emulsions exhibited stiffer indentation responses, consistent with their bulk rheology, while the adhesive term became less pronounced. These variations were summarized via the elasto-adhesion parameter, which tended to decrease logarithmically with the weight ratio of surfactants to stearyl alcohol. Our experimental results showed that the force pathways formed by the constituent α -gel structures evolved toward a more solid-like state due to the changes in the packing of the lamellar bilayers. These findings provide insight into the link between formulation guidelines and the mesorheological design of pharmaceutical and cosmetic products.