USE OF ATOMIC-FORCE MICROSCOPE FOR THE MEASUREMENTS OF HYDROPHOBIC FORCES

An atomic force microscope (AFM) was used to measure the short- and long-range hydrophobic attractive forces between a silanated silica plate and a glass sphere. Octadecyltrichlorosilane (ODTCS) and trimethylchlorosilane (TMCS) were used to render the surfaces hydrophobic with advancing contact angles (theta(a)) in the 88-115 degrees range. The forces measured with surfaces coated with TMCS (theta(a) = 88 degrees) are comparable to those obtained previously using the surface force apparatus (SFA). On the other hand, the hydrophobic forces measured with ODTCS-coated surfaces are much larger than those measured with mica surfaces coated with other long-chain surfactants such as dimethyldioctadecylammonium bromide. The long-range hydrophobic force increases sharply at theta(a) > 95 degrees. The AFM images show that the surfactants adsorb on the silica surface forming domains (or molecular clusters). With ODTCS, elliptical domains begin to form at relatively low coverages, their size and the distance between them remaining relatively constant with increasing theta(a). At the same time, the decay length of the long-range hydrophobic force does not change significantly with theta(a), while its strength increases sharply at theta(a) > 95 degrees. These findings suggest that the decay lengths of long-range hydrophobic forces vary with the domain size and the distance between them, with their strength increasing with increasing packing density and hence the ordering of the hydrocarbon chains in the domains. The AFM force measurements conducted in the present work also show that the hydrophobic force significantly increases in argon-saturated water, suggesting that the cavitation mechanism may play a role. Only short-range hydrophobic forces have been observed between hydrophilic silica and hydrophobic (silanated) glass. The adhesion forces measured by AFM show a strong dependence on theta(a), which can be explained by the Young-Dupre equation with appropriate corrections.