Interactions between a positively charged hydrophobic surface and a negatively charged bare mica surface

Direct measurements of interactions between one negatively charged mica surface and one positively charged modified mica surface immersed in electrolyte solutions are reported. The modified mica surface was obtained by Langmuir-Blodgett deposition of dimethyldioctadecylammonium ions which made the surface hydrophobic and gave it a small positive charge. The interactions between these dissimilar surfaces are dominated by attractive double-layer forces. However, in concentrated electrolyte solutions ( C > 10 −2 M) and/or at short surface separations ( D < 20 nm) the measured attraction is stronger than that expected from conventional DLVO theory. This is suggested to be due to either a breakdown of the Poisson-Boltzmann approximation or the presence of an additional force related to the attractive force previously observed between two hydrophobic surfaces in aqueous solutions.