Collective Ion Adsorption on Silica Surfaces Driven by Ion Pairs

Simulating molecular-level structures of an electrical double layer is crucial to understanding the ion adsorption mechanism on solid surfaces. Ion-pair-based Stern layer description has been recently utilized to reveal the interaction between monovalent and divalent metal ions and the silica surface. Recent molecular dynamics (MD) studies show that the ion-site interaction is similar in nature to cation-anion pairs in the solvent. As a result, especially in the case of multivalent ions, solvated pairs and ion-site pairs are correlated. We present MD simulations for the alkaline-earth chloride (MCl2) solution on an amorphous silica surface. Collective multidimensional potentials of mean force are calculated to determine the different chemical species in the Stern layer. The ion-ion and ion-site specificities are closely analyzed with Bjerrum-like association constants. We show that in the studied case, the Stern layer adsorption results from collective effects that form a series of contact ion pairs. Also, the cation-anion pair affinity influences the adsorption. A new vision of the adsorption mechanism is given, which is necessary and mandatory for similar oxide surfaces.