Hydrophobic forces in the wetting films of water formed on xanthate-coated gold surfaces

Abstract

The kinetics of thinning of water films on hydrophobic gold substrates has been studied using the thin film pressure balance (TFPB) technique. The changes in the thickness of the wetting films have been monitored by recording the profiles of the dimpled films as a function of time using a high-speed video camera. It was found that the kinetics, measured at the barrier rim of a wetting film formed on a hydrophilic silica surface, could be predicted using the Reynolds lubrication approximation with the no-slip boundary condition. However, the wetting films formed on hydrophobized gold substrates thinned much faster, and the kinetics increased with increasing hydrophobicity. The data obtained with gold surfaces of different hydrophobicities have been fitted to the Reynolds approximation to determine the hydrophobic force constants (K₁₃₂) of a power law. K₁₃₂ increased with increasing contact angle and decreased with electrolyte (NaCl) concentration. It was also found that the K₁₃₂ values can be predicted from the hydrophobic force constants (K₁₃₁) for the interaction between hydrophobic surfaces and the same (K₂₃₂) for the foam films using the geometric mean combining rule that is frequently used to predict asymmetric molecular forces from symmetric ones.