Interfacial parameters for selected Spans and Tweens at the hydrocarbon-water interface

Sorbitan fatty acid esters (Spans) and the corresponding polyoxyethylene (POE) adducts (Tweens) have applications in the pharmaceutical, cosmetic, food and agrochemical industries. Interfacial parameters were determined for Span 80 (Sorbitan monooleate), Span 85 (sorbitan triooleate), Tween 80 (POE sorbitan monooleate), Tween 85 (POE sorbitan triooleate) and Tween 20 (POE sorbitan monolaurate), at the hydrocarbon oil-water (O/W) interface based on interfacial tension measurements. Kerosene, hexane and paraffin oil phases ave considered The apparent critical micelle concentration (CMC(a)) in a hydrocarbon (O/W) two-phase system was 1.2-7.6 x 10(-4) mol/dm(3). Based on the Gibbs adsorption isotherm, the surface excess concentration (Gamma) was 1.4-4.5 mu mol/m(2) for Spans and Tweens. The limiting area per molecule of surfactant at the O/W interface (A(m)) was 41-73 Angstrom(2) for the Spans and 37-116 Angstrom(2) for Tweens. It is concluded that Spans and Tweens tend to form expanded monolayer films at the planar O/W interface.