state of the water molecules adjacent to the hydrophobic Fluctuating gaps, generated by thermal perturbations between surface (12); (2) the fluctuating electric field induced by a hydrophobic solid and water, because of their incompatibility, the lateral mobility of adsorbed ions (13); and (3) the elecare considered to be responsible for the long-range attractive hytric field generated by the ordered structures of the adsorbed drophobic force. Their formation is stimulated by the traces of air surfactant films ( 14). Such explanations of the hydrophobic dissolved in water. The pressure field induced by the fluctuations attraction are, however, in disagreement with its weak depenof one of the water surfaces acts upon the fluctuating gaps of the dence, observed experimentally (11, 15), on the electrolyte other surface generating an attractive force. Equations are obconcentration. The stronger influence of the ionic strength tained which predict a dependence of the force on the viscosity h of the liquid of the form h 02 . This dependence can explain the on the long-range hydrophobic forces observed in some exthreefold increase of the force for a change in temperature from periments (5, 16) is probably due to the hydrophilization of 20 to 40ЊC observed experimentally. This observation brings supthe interacting surfaces by the selective adsorption of ions, port to the suggestion that the hydrophobic force may have a which generates a charge on the surface and hence a repulhydrodynamic origin. ᭧ 1997 Academic Press sive force.