FORMATION AND WETTING CHARACTERISTICS OF ADSORBED LAYERS OF UNSATURATED CARBOXYLIC-ACIDS AT A FLUORITE SURFACE

Molecular layers of unsaturated carboxylates on a fluorite surface wer e prepared by spontaneous adsorption from alkaline aqueous solutions o f 10-undecenoic and 10-undecynoic acids. The adsorption density and th e kinetics of adsorption were examined by Fourier transform infrared i nternal reflection spectroscopy (FT-IR/IRS). Both 10-undecenoic and 10 -undecynoic acids chemisorb on the fluorite surface and may form a mon olayer 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 wi th 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 th e sessile-drop technique. The effect of surface heterogeneity, as infe rred from the extent of carboxylate adsorption, on contact angle hyste resis for various drop sizes was examined. It was found that both cont act angle hysteresis and contact angle/drop size relationships depend on the deviation of the carboxylate layer from the well-organized mono layer state. Further, the contact angle data suggest that the formatio n of carboxylate monolayers is not a uniform process over the entire s urface of the fluorite crystal, but rather involves the nucleation of hydrophobic aggregates, patches composed of adsorbed carboxylates, who se size increases with time. (C) 1996 Academic Press, Inc.