ONSET OF HYDROPHOBIC ATTRACTION AT LOW SURFACTANT CONCENTRATIONS

We report here on the interaction between silica surfaces in a range o f CTA(+) (cetyltrimethylammonium cation) concentrations up to the poin t of zero charge (5 x 10(-5) Prl) in a background electrolyte (3 x 10( -3) M sodium acetate). In the absence of CTA(+) the static interaction is repulsive at all distances. The addition of CTAB up to 10(-5) M ha s no effect on the interaction. Between 2 x 10(-5) and 3 x 10(-5) M th e surface charge as deduced from interaction at large separations rema ins unchanged, but the interaction becomes attractive at distances les s than a Debye length. The strength of the attraction increases severa l times as surfactant concentration increases to 4 x 10(-5) M with sim ultaneous reduction in the electrostatic repulsion at larger distances . At 5 x 10(-5) M the interaction is attractive at all distances. The results confirm that CTA(+) adsorption is cooperative and nonlinear in concentration and is enhanced by surface proximity. Both effects lead to a long range hydrophobic force.