Specific Ion (Hofmeister) Effects on Adsorption, Desorption, and Diffusion at the Solid-Aqueous Interface

Certain salts in aqueous solution are known to significantly increase the affinity of molecules with hydrophobic moieties for a hydrophobic interface, but the mechanism of this effect is poorly understood. We used single-molecule total internal reflection fluorescence microscopy to directly observe the underlying dynamic interfacial phenomena for a fluorescent fatty acid probe molecule at a self-assembled monolayer surface. Both NaF and NaSCN increased the surface affinity of the probe from that observed in pure water, and consistent with expectations, the surface affinity was greater in the presence of NaF than that with NaSCN. Notably, the primary cause of the increase in surface affinity was specifically due to an increase in the absolute rate of adsorption from solution. In contrast, the surface residence time and the surface diffusion coefficient of the probe molecules did not depend significantly on the solution conditions.