Forces between hydrophobed mica surfaces immerssed in dodecylammonium chloride solution

Abstract

A surface forces apparatus was used for measuring the interactions between hydrophobed mica surfaces (coated with a monolayer of dimethyldioctadecylammonium ions) in water solutions of dodecylammonium chloride. The interactions were compared with theoretical fits using the numerically exact solution of the Poisson-Boltzmann equation. It was found that a long-range hydrophobic attraction is present, but the long-range tail of the hydrophobic force is strongly influenced by the adsorption of dodecylammonium ions on the hydrophobed mica surfaces. The major part of this long-range tail is already screened at 10 −5 mole/liter, where the adsorbed densities correspond to 1 molecule/60 nm 2 at large surface separations. The short-range part of the hydrophobic force is reduced by a factor of about 3 in the concentration range 1 x 10 −4 mole/liter. The adhesion between the surfaces decreases with the concentration up to 6 x 10 −4 mole/liter. No adhesive contact was reached at higher concentrations indicating a formation of condensed monolayers on the surfaces. This occurs below the critical micelle concentration of the surfactant.