Specific Ion Effects on the Interaction of Hydrophobic and Hydrophilic Self-Assembled Monolayers

Interactions between mineral surfaces and organic molecules are fundamental to life processes. The presence of cations in natural environments can change the behavior of organic compounds and thus alter the mineral-organic interfaces. We investigated the influence of Na⁺, Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ on the interaction between two models, self-assembled monolayers, that were tailored to have hydrophobic -CH₃ or hydrophilic -COO(H) terminations. Atomic force microscopy in chemical force mapping mode, where the tips were functionalized with the same terminations, was used to measure adhesion forces between the tip and substrate surfaces, to gather fundamental information about the role of these cations in the behavior of organic compounds and the surfaces where they adsorb. Adhesion force between hydrophobic surfaces in 0.5 M NaCl solutions that contained 0.012 M divalent cations did not change, regardless of the ionic potential, that is, the charge per unit radius, of the cation. For systems where one or the other surface was functionalized with carboxylate, -COO(H), mostly in its deprotonated form, -COO^-, a reproducible change in the adhesion force was observed for each of the ions. The trend of increasing adhesion force followed the pattern: Na⁺ ≈ Mg²⁺ < Sr²⁺ < Ca²⁺ < Ba²⁺, suggesting that ionic potential, thus hydrated radius, controls the interaction. The presence of a -CH₃ surface in the asymmetric system leads to lower adhesion forces than in the hydrophilic system, whereas the ionic trend remains the same. Although specific ion effects are felt in both systems, the lower adhesion force in the asymmetric system, compared with the hydrophilic system, implies that the -CH₃ surface plays an important role.