How ion pair formation drives adsorption in the electrical double layer: Molecular dynamics of charged silica-water interfaces in the presence of divalent alkaline earth ions

International audienceThe study by numerical methods of ionic distributions in charged solid-liquid interfaces allows the interpretation of many concepts and phenomena such as the $\zeta$ potential or ion adsorption. Molecular dynamics (MD) can reveal detailed information about electric double layer (EDL), especially the Stern layer, by including electrostatic, van der Waals, and molecular forces. Here, we aim at analyzing the chemical species and the correlations between the ions and the surface including three-body e!ects, one particle belonging to the surface and two to the solution. Correlations are specified on the basis of interionic distance screening. An extensive description is provided from simulations of three alkaline earth metal chlorides (Mg$^{2+}$,Ca$^{2+}$, and Ba$^{2+}$) aqueous solutions at 0.6 mol/L$^{-1}$ in the centers of negatively charged silica nanochannels. The resulting McMillan-Mayer potentials of mean force (PMF) exhibit a decreasing affinity of deprotonated silanol along with the series Mg$^{2+}$ > Ca$^{2+}$ > Ba$^{2+}$, while the formation of bulk M$^{2+}$-Cl$^-$ pairs is in reverse order. A similar trend is obtained for the association constants and the residence times. Over 40% of surface-bound Ba$^{2+}$ ions are correlated with surface-bound Cl$^-$, while the other two cations do not show such trend. When the surface-bound and surface-correlated ions are taken apart, the remaining free ion distributions fit the Poisson-Boltzmann equation well, i.e., the Gouy-Chapman model. This work demonstrates the necessity to account for three-body associations on oxide surface at least for divalent ions