HYDROPHOBIC INTERACTIONS BETWEEN DISSIMILAR SURFACES

An atomic force microscope (AFM) was used to measure surface forces be tween a glass sphere and a silica plate. When the measurements were co nducted between untreated surfaces, a ''short-range'' hydration force with decay lengths of 0.4 and 3.0 mm was observed. When the surfaces w ere hydrophobized with octadecyltrichlorosilane (OTS), on the other ha nd, long-range hydrophobic forces with decay lengths in the range of 2 -32 nm were observed. The force measurements were conducted between su rfaces having similar and dissimilar hydrophobicities so that the resu lts may be used for deriving an empirical combining rule. It was found that the power law force constants for asymmetric interactions are cl ose to the geometric means of those for symmetric interactions. Thus, hydrophobic force constants can be combined in the same manner as the Hamaker constants. A plot of the power law force constants versus wate r contact angles suggests that the hydrophobic force is uniquely deter mined by contact angle. These results will be useful in predicting hyd rophobic forces for asymmetric interactions and in estimating hydropho bic forces from contact angles. (C) 1997 Academic Press