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

An atomic force microscope (AFM) was used to measure the short- and lo ng-range hydrophobic attractive forces between a silanated silica plat e and a glass sphere. Octadecyltrichlorosilane (ODTCS) and trimethylch lorosilane (TMCS) were used to render the surfaces hydrophobic with ad vancing contact angles (theta(a)) in the 88-115 degrees range. The for ces measured with surfaces coated with TMCS (theta(a) = 88 degrees) ar e comparable to those obtained previously using the surface force appa ratus (SFA). On the other hand, the hydrophobic forces measured with O DTCS-coated surfaces are much larger than those measured with mica sur faces coated with other long-chain surfactants such as dimethyldioctad ecylammonium bromide. The long-range hydrophobic force increases sharp ly 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 coverag es, their size and the distance between them remaining relatively cons tant with increasing theta(a). At the same time, the decay length of t he long-range hydrophobic force does not change significantly with the ta(a), while its strength increases sharply at theta(a) > 95 degrees. These findings suggest that the decay lengths of long-range hydrophobi c forces vary with the domain size and the distance between them, with their strength increasing with increasing packing density and hence t he ordering of the hydrocarbon chains in the domains. The AFM force me asurements conducted in the present work also show that the hydrophobi c force significantly increases in argon-saturated water, suggesting t hat the cavitation mechanism may play a role. Only short-range hydroph obic forces have been observed between hydrophilic silica and hydropho bic (silanated) glass. The adhesion forces measured by AFM show a stro ng dependence on theta(a), which can be explained by the Young-Dupre e quation with appropriate corrections.