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

## TL;DR

This paper investigates how different alkaline earth metal ions interact with an anionic carboxylate surface, reconciling conflicting experimental and simulation data.

## Contribution

The study uses molecular dynamics simulations to explain ion-specific effects observed in AFM experiments at carboxylate interfaces.

## Key findings

- Site-specific approaches align with dilute solution data, while collective approaches match AFM results.
- Ca2+ and Ba2+ ions bind more efficiently to the carboxylate interface, forming contact ion pairs with multiple ligands.

## Abstract

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.