The Behavior of Sorbitan Surfactants at the Water–Oil Interface: Straight-Chained Hydrocarbons from Pentane to Dodecane as an Oil Phase

Abstract

The interfacial tension of four sorbitan surfactants (Span 20, sorbitan monolaurate; 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 values were calculated: critical micelle concentration (cmc), the interfacial tension at the cmc (γcmc), surface pressure at the cmc (πcmc), area per molecule at the cmc (Acmc), standard free energy of micellization (ΔG°mic), and standard free energy of adsorption (ΔG°ad). The shorter chained Span 20 and unsaturated Span 80 had higher cmc values and Span 80 had a larger molecular area than the other surfactants. With the same oil phase, differences between πcmc values of the four sorbitan monoesters were small, but the γcmc was slightly lowered as the hydrophobicity of the surfactant was increased. ΔG°mic was less negative for Span 20 and the ΔG°ad value was slightly more negative for Span 80. The effect of the oil phase was obvious. Increasing the hydrocarbon chain length of the oil phase increased γcmc and cmc values while πcmc and Acmc were decreased. As the length of the hydrocarbon chain of the oil phase was increased, ΔG°mic and ΔG°ad became less negative, which means a less spontaneous reaction.