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.

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Core Ideas Adsorption behaviors for different binary electrolytes at clay surfaces are presented. Major rather than minor adsorption modes are always more affected by co‐ions. Degrees of altering the adsorption numbers and strengths rely strongly on co‐ions. When alone and co‐existent, adsorption modes of metal ions remain consistent. Each binary system has peculiarities during adsorption as discussed in the text. Multiple electrolytes exist in almost all soil systems, and in this work, molecular dynamics was employed to tackle the adsorption of different binary electrolytes (PbCl2/CdCl2, PbCl2/NaCl, PbCl2/CsCl and KCl/CsCl) at montmorillonite–water interfaces, providing insights to the mechanisms of co‐adsorption and the impacts of co‐ions. In all cases, the adsorption modes of metal ions are not altered by co‐ions, while co‐ions exhibit the pronounced impacts on the adsorption numbers and strengths, and the main rather than minor adsorption modes are always more affected. In addition to these general principles, each type of binary systems has the peculiarities: Coexistence promotes Pb2+ adsorption while inhibits Cd2+ adsorption; Coexistence inhibits Pb2+ adsorption while promotes Cs+ adsorption; Coexistence promotes each other's adsorption of Pb2+ and Na+; Coexistence promotes Cs+ adsorption while inhibits K+ adsorption; All co‐ions improve Pb2+ stabilities, while coexistence of Pb2+ reduces Cd2+ stabilities and improves Cs+ and Na+ stabilities; Competition for inner‐sphere binding sites is not beneficial for the stabilities of both K+ and Cs+. Heavy and radioactive metal ions are a focus of this study and results are also helpful to manage their pollutions.

Adsorption of ions at the interface of clay minerals and aqueous solutions plays a critical role in a wide spectrum of colloidal, chemical, physical, and geological processes. Owing to the particular complexity of related systems and the femtosecond scale of related processes, the direct experimental observations often become a challenging task. As a contrast, computer simulations have proven to be a competent and powerful approach therein and already realized fruitful and significant contributions. In this chapter, we attempt to draw a relatively comprehensive picture of the interfacial adsorption of ions mainly within the context of computer simulations. As elaborated, quantum mechanics (QM) and molecular dynamics (MD), two popular simulation techniques currently used, have respective advantages, and with their collaborative efforts, we are striding toward the in-depth and systematic understanding of adsorption configuration, distribution, stability, reaction thermodynamics and mechanism, dynamics, diffusivity as well as electric double layer and other fundamental issues that are closely associated with the adsorption of ions at the interface of clay minerals and aqueous solutions. In addition, we demonstrate that investigation of the interfacial adsorption of ions greatly helps to unravel the origin and mechanism of ion-specific effects, whose importance has been explicitly suggested to be no less than Gregor Mendel's work to genetics.