W/O Emulsions Stabilized by Supramolecular Complexes Oriented at the Oil-Water Interface

Abstract

Polyethylene glycol (PEG)-modified silicone is widely used as a surfactant for water-in-oil (W/O) emulsions, but its emulsion stability at high temperatures is generally considered low. This is due to the fact that dehydrated PEG becomes hydrophobic and is removed from the oil-water interface. On the other hand, cyclodextrins (CDs) are water-soluble cyclic molecules with hydrophobic cavities and are generally known to recognize low-molecular-weight substances. We have found a remarkable emulsion-stabilizing effect of PEG-modified silicone with HLB 4.5 (PEG-MS) in combination with α-CD in a W/O emulsion system at high-temperature storage. In this study, we investigated the emulsion stabilization mechanism by rheological measurements, thermal analysis, and interfacial layer viscosity measurements using an amphiphilic spin probe. As a result, it was shown that PEG-MS and α-CD form a supramolecular complex (PEG-MS/α-CD complex) driven by hydrophobic interaction, which is considered to contribute to emulsion stabilization by being efficiently oriented at the oil-water interface. Furthermore, the PEG-MS/α-CD complex was indicated to form more stably at higher temperatures. This means that dehydration, an unstable factor in W/O emulsion systems using PEG-MS, can rather be utilized. This approach is expected to reduce the amount of surfactants and oil thickeners, and therefore, to create W/O formulations with both emulsion stability and good touch properties.