Interactions of surfactant monolayers across hydrocarbon liquids

The forces between surfactant-coated monolayer surfaces across hydrocarbon liquids have been measured. The aim was to study the short-range and adhesion forces between various hydrocarbon surfaces across various hydrocarbon liquids, and the effects of polar additives such as water and alcohols. Single-chained, double-chained and fluid-chained surfactants were used, and the oils were either pure straight-chained alkanes, branched alkanes, or a polydisperse mixture. It was found that the force-laws are oscillatory between close-packed solid-crystalline monolayers. Otherwise, they are monotonically attractive down to surface separations of ca. 1 nm, below which they are monotonically repulsive. There are interesting correlations between the forces, the structure of the surface monolayers, and the properties of the intervening liquids. At separations <2 nm the measured force laws are not simply describable by current theories of van der Waals forces. A possible explanation is additional entropic orientational interactions of the solvent molecules and monolayer chains which enhance the attraction.