The behavior of sorbitan surfactants at the water-oil interface: Straight-chained hydrocarbons from pentane to dodecane as an oil phase

The interfacial tension of four sorbitan surfactants (Span 20, sorbitan mon olaurate; Span 40, sorbitan monopalmitate; Span 60, sorbitan monostearate; and Span 80, sorbitan monooleate) was determined at the water-oil interface . Seven straight-chained hydrocarbons from pentane to dodecane were used as an oil phase. From the interfacial tension measurements the following valu es were calculated: critical micelle concentration (cmc), the interfacial t ension at the cmc (gamma (cmc)), surface pressure at the cmc (pi (cmc)), ar ea per molecule at the cmc (A(cmc)), standard free energy of micellization (DeltaG degrees (mic)), and standard free energy of adsorption (DeltaG degr ees (ad)). The shorter chained Span 20 and unsaturated Span 80 had higher c mc values and Span 80 had a larger molecular area than the other surfactant s. With the same oil phase, differences between pi (cmc) values of the four sorbitan monoesters were small, but the gamma (cmc) was slightly lowered a s the hydrophobicity of the surfactant was increased. DeltaG degrees (mic) was less negative for Span 20 and the DeltaG degrees (ad) value was slightl y more negative for Span 80. The effect of the oil phase was obvious. Incre asing the hydrocarbon chain length of the oil phase increased gamma (cmc) a nd cmc values while pi (cmc) and Delta (cmc) were decreased. As the length of the hydrocarbon chain of the oil phase was increased, DeltaG degrees (mi c) and DeltaG degrees (ad) became less negative, which means a less spontan eous reaction. (C) 2001 Academic Press.