Forces between carboxylic acid surfaces in divalent electrolyte solutions

The behavior of self-assembled monolayers of thiohexadecanoic acid adsorbed onto gold interacting in asymmetric 2:1 electrolytes has been studied with direct force measurements. The effects of two divalent cations (Mg2+ and Ca2+) were studied at concentrations ranging from 1 mu M to 10 mM. As compared to interactions in the presence of Na+, the divalent ions adsorb strongly to the surfaces, with the effect of lowering the surface potential and decreasing the double-layer repulsion. At concentrations above 10 mu M, the Ca2+ ions were found to adsorb stronger than Mg2+. Ca2+ ions cause charge reversal at high concentrations, and the net interactions at 10 mM were attractive over the measurable range. (C) 2000 Academic Press.