Adhesive Force between Hydrophilic Surfaces in Alcohol-Water Solutions

The interaction and adhesive forces between a mica plate and SiO(2) surfaces in water-alcohol (ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol) mixtures were investigated on the molecular scale, using an atomic force microscope. The following results were obtained: (i) alcohols higher than 1-propanol adsorb standing on the hydrophilic surface vertically to form a structured monolayer, when the alcohol concentration is high enough; (ii) the adhesive force between surfaces depends on how closely two surfaces can be brought by breaking the adsorbed layers; (iii) the adhesive force between surfaces is maximized at w(w)/w(ws) approximately 0.25, independent of the kind of alcohol, where w(w) and w(ws) are the weight fractions of water and saturated water, respectively; and (iv) the adhesive force for a particle of rough surface is much smaller than the adhesive force predicted for a particle of smooth surface. It is hypothesized that this adhesive force much greater than the van der Waals attractive force originates from the water bridging between surfaces, and this hypothesis is confirmed by the predictions given by the Laplace equation and the Kelvin relation. Copyright 1999 Academic Press.