Ion-Specific Effects of Alkaline Earth Metal Ion Binding to an Anionic Carboxylate Monolayer

The ion-specific effects of alkaline earth metal ions binding to anionic carboxylate-terminated monolayers influence environmental chemistry, industrial processes, and nanotechnology. Experimental results obtained using interfacial techniques are often interpreted by using data from dilute aqueous solutions of carboxylate anions. However, atomic force microscopy (AFM) adhesion forces reported for alkaline earth metal ions binding at the aqueous interface of an anionic carboxylate-terminated monolayer contradict data reported for dilute solutions of aqueous carboxylate anions. To reconcile this data and provide molecular insight into the ion-specific trends observed in the AFM data, classical atomistic molecular dynamics (MD) simulations are used to model Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ ions at the aqueous interface of a deprotonated 11-mercaptoundecanoic acid (MUA) monolayer. We compare site-specific and collective ion binding approaches for calculating the strength of ion binding at the carboxylate interface. This comparison reveals that site-specific approaches (i.e., simulated adhesion force curves and potential of mean force binding free energies) qualitatively agree with dilute aqueous solution data, whereas a collective ion approach (i.e., binding free energies from the Langmuir isotherm) is necessary to reproduce experimental AFM results. Analysis of the validated MD simulations reveals that Ca²⁺ and Ba²⁺ ions more efficiently occupy the carboxylate interface, which leads to a larger portion of ions binding to the monolayer through direct contacts, forming contact ion pairs with multiple MUA ligands. These results rationalize the ion-specific results reported by AFM studies and may be used to inform future studies of interfacial processes involving alkaline earth metal ions and carboxylate-functionalized structures.