LONG-RANGE ATTRACTION BETWEEN A HYDROPHOBIC SURFACE AND A POLAR SURFACE IS STRONGER THAN THAT BETWEEN 2 HYDROPHOBIC SURFACES

Long-range attractive forces have been measured in the surface forces apparatus (SFA), for both a symmetrical situation with two mica surfaces made hydrophobic by surfactant adsorption and an unsymmetrical situation with one hydrophobic surface. The magnitude of the attractive force decreases as electrolyte is added to the aqueous medium and the 2:2 electrolyte MgSO4 has a stronger effect than a 1:1 electrolyte like NaCl. The force is also decreased by adding octanol in low concentrations (less than or similar to 10(-4) M). We find a qualitatively very similar behavior of the force in the symmetrical and the unsymmetrical situation although we find larger forces in the latter case. In conflict with a previous interpretation we conclude that the force in the two cases is due to the same molecular mechanism. We argue furthermore that the force has an electrostatic origin. We suggest that the electrical fields emanating from the hydrophobic surface are due to a lateral inhomogeneity on a nanometer to micrometer scale at the surface.