Direct Measurement of Hydrophobic Forces: A Study of Dissolved Gas, Approach Rate, and Neutron Irradiation

Theoretical interpretations of the, as yet, poorly understood long-range hydrophobic attraction are briefly reviewed. We report long-range hydrophobic attractive forces between silica surfaces made hydrophobic by adsorption of the cationic surfactants cetyltrimethylammonium bromide and cetylpyridmium chloride onto silica surfaces. The effects of dissolved gas, surface approach velocity, and neutron irradiation on the measured interaction have been studied in order to investigate possible non-electrostatic mechanisms for the long-range hydrophobic attraction. At large separations the presence of dissolved gas, neutron irradiation, and reduced approach velocities are, in each case, found to result in a stronger attraction between the hydrophobic surfaces. These results are consistent with mechanisms related to the metastability of the thin aqueous film separating the approaching hydrophobic surfaces.