Surface freezing of CTAC-hexadecane mixed adsorbed film at the isopropyl palmitate–water interface: a way to stabilize emulsions

In this study, we utilized the surface freezing transition of mixed adsorbed films of cetyltrimethylammonium chloride (CTAC) and hexadecane at the isopropyl palmitate (IPP)–aqueous interface to encapsulate IPP in a stable oil-in-water (OW) emulsion droplets. IPP is a widely used emollient oil in cosmetic creams; however, as often seen in oily ingredients in cosmetics, it is also surface-active and adsorbs at the oil–water interface. Therefore, under normal experimental conditions, surfactant emulsifier and cosmetic oil form a mixed adsorbed film at the OW emulsion surfaces. However, such films typically do not enhance emulsion stability and for that purpose we used a 1:9 mixture of hexadecane and IPP as oil phase together with CTAC as surfactant. The interfacial tension of the oil phase against CTAC aqueous solution as a function of CTAC concentration and temperature shows three distinctive interfacial phases: disordered mixed adsorbed film of CTAC and IPP (surface liquid) and two surface frozen monolayers in which CTAC or IPP expelled the other and gives rise to a highly ordered and stiff hydrocarbon chain layer with incorporated hexadecane molecules. The compositional transition of the surface frozen monolayer of IPP and that of CTAC occurred as the CTAC concentration increased in the aqueous phase. Above the transition concentration, an enhanced OW emulsion stability was observed, whereas IPP surface frozen monolayer forms only kinetically unstable emulsions. From these findings, we suggest the potential use of surface frozen films of CTAC to encapsulate surface-active cosmetic oils stably in the OW emulsions, which in addition have the property that the stability can be controlled by temperature. Graphical Abstract