Formation and wetting characteristics of adsorbed layers of unsaturated carboxylic acids at a fluorite surface

Molecular layers of unsaturated carboxylates on a fluorite surface were prepared by spontaneous adsorption from alkaline aqueous solutions of 10-undecenoic and 10-undecynoic acids. The adsorption density and the kinetics of adsorption were examined by Fourier transform infrared internal reflection spectroscopy (FT-IR/IRS). Both 10-undecenoic and 10-undecynoic acids chemisorb on the fluorite surface and may form a monolayer from alkaline aqueous solutions. As much as 30 min is required to form such monolayers from 0.5-0.7 mM solutions at pH 9.5-9.6 under turbulent conditions. However, the formation of a perfect monolayer with a well-organized structure was difficult to control in this study. Advancing and receding contact angles were measured for water drops on submonolayers and monolayers of the unsaturated carboxylates using the sessile-drop technique. The effect of surface heterogeneity, as inferred from the extent of carboxylate adsorption, on contact angle hysteresis for various drop sizes was examined. It was found that both contact angle hysteresis and contact angle/drop size relationships depend on the deviation of the carboxylate layer from the well-organized monolayer state. Further, the contact angle data suggest that the formation of carboxylate monolayers is not a uniform process over the entire surface of the fluorite crystal, but rather involves the nucleation of hydrophobic aggregates, patches composed of adsorbed carboxylates, whose size increases with time. (C) 1996 Academic Press, Inc.