Influence of Interfacial Properties of Lipophilic Surfactants on Water-in-Oil Emulsion Stability

The purpose of this study is to investigate interfacial properties of lipophilic nonionic surfactants of the sorbitan fatty acid ester type (Spans 20, 80, 83, and 85) with the aim of selecting appropriate emulsifiers for water-in-oil (W/O) emulsions under different conditions. Interfacial tension and rheology were investigated at a planar mineral oil/double distilled water interface using a Wilhelmy plate method and an oscillatory ring surface rheometer, respectively. The effects of bulk concentration, temperature and the presence of salt in the aqueous phase on the interfacial properties of surfactant films were determined. The relationship between interfacial film properties and W/O emulsion stability in the presence of salt was assessed by visual observation of phase separation on storage. All the surfactants exhibited viscoelastic properties with the exception of Span 85. Interfacial association of inverse micelles and/or surfactant multilayer formation are probably responsible for the observed viscoelasticity. Addition of sodium chloride to the aqueous phase and increase in temperature influenced the viscoelastic properties. Emulsion stability to phase separation decreased in the presence of salt. The interfacial rheology data were predictive of emulsion stability for the systems studied. Copyright 1998 Academic Press. Copyright 1998Academic Press