Rheological properties of three component creams containing sorbitan monoesters as surfactants.

Creams from three components, surfactant, purified water and oil, were prepared. Comparable molar fractions of components were used in order to better understand the structural properties of the components used. The surfactants were sorbitan monoesters, sorbitan monolaurate, monopalmitate, monostearate and monooleate, which differed from each other in the length or structure of the hydrocarbon chain. The oils used were isopropylpalmitate and myristate, and they differed from each other in the length of the fatty acid chain. Rheological properties, droplet size distributions and types (either o/w or w/o) of the creams were studied. The rheological tests used were oscillation stress sweep test, creep recovery test and viscosity test. The modelling of the creep phase was based on the creep recovery test. Sorbitan monolaurate and monostearate formed w/o creams, sorbitan monopalmitate and monooleate o/w creams. It appeared that the double-bonded structure of the surfactant made the cream less elastic. Elasticity was increased due to lengthening of the alkyl chain of the surfactant and increased amount of surfactant. Also the lengthening of the fatty acid chain of the oil made the creams more elastic. The results of the rheological tests and droplet size distributions correlated well each other. According to the modelling of the creep phase, creams could be represented either with the Burger model or with the Maxwell model.