Attractive forces between uncharged hydrophobic surfaces: direct measurements in aqueous solution.

Long, double-chained alkylammonium acetate surfactants are soluble in water and, under suitable conditions, adsorb onto sheets of muscovite mica, forming an electrically neutral, hydrophobic surface. Attractive forces measured between such surfaces are 10 to 100 times stronger than expected from van der Waals theory over distances D up to about 10 nanometers. The forces decay exponentially [with a force proportional to exp(-D/1.4)] instead of following the power-law behavior of continuum theory. The results of these and earlier experiments indicate that the strength of these attractive forces depends critically on the degree of hydrophobicity of the surface and is due to the long-range influence of the surface on the structure of water. In addition, for very hydrophobic surfaces, the cavitation effects on pulling the surfaces apart are described.