A molecular dynamics study of swelling, structural and dynamic properties of 2:1 Cis-vacant Cs saturated smectite

Smectite, a family of phyllosilicates, exhibits discrete hydration/swelling states and high cation exchange capacity based on interlayer water and ion activities, rendering it a crucial material in engineering and environmental fields. Both cis-vacant (cv) and trans-vacant (tv) structures are observed in natural smectites. Previous molecular-level studies have predominantly assumed a tv configuration, potentially leading to incorrect predictions of smectite's thermodynamic and microscopic properties. Here, three representative cis-vacant smectites (i.e., Wyoming-type, Ari-type, and beidellite) with Cs interlayer cations were employed as a model framework to explore the thermodynamic and microscopic properties of smectite using classical molecular dynamic simulations. The swelling behaviors of smectite with different interlayer water content, Cs binding structures, distributions and mobility of interlayer water and Cs species, and cation exchange between Cs/Na saturated smectites were investigated. The results suggested that the monolayer hydrate is the most stable hydration state of Cs saturated smectites. The diffusion of interlayer water was relatively slower compared to the corresponding bulk case, while Cs cations can be effectively fixed at low water content. Furthermore, Cs cations showed higher mobility in the direction parallel to the basal plane of smectite rather than in the perpendicular direction. Cs cations form a stable inner-sphere complexation structure within the interlayer, primarily binding at the hexagonal sites (H-sites) and triangular sites (T-sites). In addition, the calculated cationic selectivity coefficients between Cs/Na ions were found to be significantly high (Log K_c = 1.69), indicating effective retention of Cs cations and comparable to the reported experiments. Moreover, all the results obtained from the cv smectite are further compared to those derived from the tv smectite. The comparison results showed that both smectites have highly similar thermodynamic and microscopic properties. This study provides an extended understanding to facilitate fundamental research of smectite in geochemistry and environmental material science.