Reversal of Cation-Specific Effects at the Interface of Mica and Aqueous Solutions

Ion-specific effects are ubiquitous and have gained renaissance over the past few decades while remaining largely elusive. In this work, molecular dynamics simulations have been conducted to investigate the adsorption of different metal ions at the interface of mica and aqueous solutions, and cation-specific effects abide by the sequences of Na+ > K+ > Cs+ and Cs+ > K+ > Na+ for less and more charged surfaces, respectively. Mechanisms for cation-specific effects and reversal of Hofmeister series are then addressed on an atomic level. Hydration effect (i.e., interaction of metal ions with water) is the driving force for less charged surfaces, whereas interaction of metal ions with mica plays a larger role for more charged surfaces, which further result in a reversal of Hofmeister series. Clay minerals generally carry an abundance of negative charges, and the finding that Hofmeister series can be reversed with no change in the sign of surface charges provides new insights about related processes and ion-specific effects. These results have significant implications because of the ubiquity and significance of charged systems, especially in biology, chemistry, and colloid science.