Surface pressure, hysteresis, interfacial tension, and CMC of four sorbitan monoesters at water-air, water-hexane, and hexane-air interfaces

The purpose of this study was to investigate the interfacial properties of sorbitan monoesters (Span 20, 40, 60, and 80), The surface pressure was inv estigated at the water-air interface using a Langmuir-Blodgett apparatus. I nterfacial tensions at w-hexane-air and water-n-hexane interfaces were meas ured by a du Nouy tensiometer. The effects of different surface-active agen ts and their concentrations on the interfacial properties of surfactant fil ms were determined, With saturated sorbitan monoesters the lengthening of t he hydrocarbon chain increases the collapse pressure and molecular area at the water-air interface. Unsaturated Span 80 had a lower collapse pressure and a larger molecular area than its saturated counterpart Span 60, Under c ompression-expansion cycles, all sorbitan monoesters showed hysteresis effe cts. At the n-hexane-air interface there were no differences in the interfa cial tension between different sorbitan monoesters. At the water-II-hexane interface, differences in CMCs were small, but the surface excess of Span 8 0 was markedly smaller and the molecular area larger than the corresponding values of other sorbitan monoesters. (C) 2000 Academic Press.