Effect of Oil Species on the Viscoelastic Behavior of a Surfactant Film Formed at the Oil/Water Interface

Clarifying the viscoelastic properties of oil/water interfacial films is important for evaluating the resistance of emulsions to coalescence. In recent years, strain-controlled rheometers with a bi-cone geometry have gained significant attention for measuring the viscoelasticity of liquid/liquid interfaces. In the present study, we sought to clarify the effect of oil species on the viscoelastic behavior of the oil/water interfacial film formed by a nonionic surfactant (Span 65) and correlate it with an emulsion's stability. A series of interfacial rheological measurements on saturated hydrocarbons with varying alkyl chain lengths as the oil phase showed that the elasticity of the oil/water interfacial film increased as the difference between the alkyl chain length of the oil phase and that of Span 65 increased. The stability of the water-in-oil emulsions prepared using each oil phase also improved with increasing alkyl chain length difference. These results demonstrated that viscoelastic parameters evaluated using this interfacial rheology are promising indicators for predicting the emulsion's stability. From the perspective of differences in the orientations of Span 65 and the oil phase at the interface, we also discussed the mechanism by which the viscoelastic behavior of the interfacial film differs depending on the alkyl chain length of the oil phase.