Long-range attraction between silanated silica materials studied by an electrolyte titration with atomic force microscopy

A parametric study of electrostatic and hydrophobic forces with changing electrolyte concentration shows characteristic features in dynamic cantilever snap-in from which the hydrophobic interaction can be assessed. A relatively simple and efficient experimental procedure for determining the strength and range of the hydrophobic effect is accomplished in the sphere-plate geometry between silanated glass and quartz by a systematic electrolyte titration of electrostatic forces. Snap-in distance acquired with atomic force microscopy (AFM) plotted against electrolyte concentration shows a minimum at some identifying intermediate salinity and a snap-in plateau at high concentrations. These prominent features, distinguishable from an adequate database represented by a single titration curve, facilitate deconvolution of the hydrophobic force from other interactions. Monovalent electrolyte concentration does not appear to affect the hydrophobic interaction directly between 10−4 and 10−1 M, but evidence of an independent and long-range electrostatic correlation force is observed.