(279a) The Role of Non-DLVO Forces in Silica Interactions in NMP-Water Mixtures

The interaction forces between a silica sphere and a glass plate in NMP-water binary mixtures were measured using the AFM technique. The interactions between two hydrophilic surfaces in pure liquids qualitatively followed the prediction of the DLVO theory. The addition of NMP into water drastically altered the attractive and adhesive interactions compared to water alone. An unusually strong, long-range, multi-stepped attractive force was observed on approach of hydrophilic surfaces in the NMP content range of 30 - 50 vol %, where the pull-off force was also maximized. Together with thermodynamic considerations, analysis of force profiles and contact angles suggests attraction arises from bridging of surface adsorption layers composed of a macrocluster-like mutilayered structure driven by hydrogen bonding between the liquid components. For the asymmetric case where one of the surfaces is silanated, a strong and long-range attractive force was observed in pure water. This attraction was affected significantly by the topography of the hydrophobic surface and was attributed to capillary forces arising from nanobubble bridging. The pull-off force decreased with increasing NMP content, which was explained in terms of the wettability of the surface determined by solution contact angles. The longest-ranged attraction between the asymmetric surfaces was observed at 30 vol % NMP, in good agreement with the notion of the formation of surface macrocluster layer on the hydrophilic silica surface.