EFFECT OF DIVALENT ELECTROLYTE ON THE HYDROPHOBIC ATTRACTION

Measurements of the attraction between hydrophobic surfaces (chemically modified mica and mica coated with Langmuir-Blodgett films) across water, 0.01 M magnesium sulfate solution, and 0.1 M magnesium sulfate solution (equivalent to concentrations of free 2:2 electrolyte of up to 0.01 M) have been carried out. The magnitude of the hydrophobic attraction is reduced with increasing electrolyte concentration but remains much larger than any van der Waals force. The apparent exponential decay length of the attraction is 9-13 nm for the monolayer surfaces at all concentrations and 5.4 nm for the chemically hydrophobed surfaces at 0.01 M MgSO4. These results cannot be reconciled with recent theoretical models that ascribe an electrostatic origin to the hydrophobic attraction. © 1990 American Chemical Society.