Nonideality in mixed monolayers of sorbitan oleates is enhanced by elevated ionic strength

Mixtures of sorbitan fatty acid eaters and the analogous polyoxyethylene ad ducts are often used to stabilize emulsions. However, the phase behavior of these mixtures is not clearly understood. In addition, the properties of f ilms of these surfactants depend on solvent conditions. Previous results fo r mixtures of polyoxyethylene sorbitan monoleate with sorbitan monooleate s orbitan sesquioleate, or sorbitan trioleate showed that these surfactants f orm nonideal mixtures at the air-water interface. In the work reported here , similar films have been investigated with high ionic strength subphases. Nonideality observed with monooleate- and sesquioleate-containing films is enhanced at elevated ionic strength. For mixed films containing sorbitan tr ioleate and polyoxyethylene sorbitan monoleate, condensation is observed fo r films of nearly equimolar composition. A model to explain this behavior i s based primarily on molecular shape complementarity.