Preferential ion adsorption in mixed electrolytes plays a crucial role in many practical applications, such as ion sensing and separation and in colloid science. Using all-atom molecular dynamics simulations of aqueous NaCl, CaCl2, and NaCl–CaCl2 solutions confined by charged amorphous silica, we show that Na+ ions can adsorb preferentially over Ca2+ ions, depending on the surface structure. We propose that this occurs when the local surface structure sterically hinders the first hydration shell of the Ca2+ ion. Introducing a protrusion metric as a function of protrusion of deprotonated silanols, ion-specificity is successfully predicted on isolated, vicinal, and geminal silanols alike, provided that no other deprotonated silanols are found nearby. Furthermore, we introduce a new strategy to analyze the results as a function of distance from the surface. This approach effectively removes surface roughness effects allowing for direct comparison with classical electric double layer theory and distinction of specifically adsorbed ions and electrostatically adsorbed ions.

Progress towards a phenomenological picture and theoretical understanding of glassy dynamics and vitrification near interfaces and under nanoconfinement

Note: This paper is part of the JCP Special Topic on Fluids in Nanopores.

Surface conductivity in the electrical double layer (EDL) is known to be affected by proton hopping and diffusion at solid-liquid interfaces. Yet, the role of surface protolysis and its kinetics on the thermodynamic and transport properties of the EDL are usually ignored as physical models consider static surfaces. Here, using a novel molecular dynamics method mimicking surface protolysis, we unveil the impact of such chemical events on the system's response. Protolysis is found to strongly affect the EDL and electrokinetic aspects with major changes in ζ potential and electro-osmotic flow.