Molecular Simulation Study of Montmorillonite in Contact with Ethanol

Molecular simulations were performed to explore the adsorption and transport mechanism of ethanol and ions in Na- and Ca-montmorillonite clays. Our results show that the uptake of ethanol by montmorillonite increases with increasing relative pressure (RP)/basal d-spacing, consistent with experimental observations. The basal d-spacing of montmorillonite grows in the presence of ethanol to about 13.0 Å with a monolayer arrangement of ethanol (1L). Further uptake of ethanol allows the basal d-spacing to grow to about 16.5 Å with a bilayer arrangement of ethanol (2L). For a given solvation state, the amount of ethanol uptake is almost independent of RP and the type of the counterion. The stable basal d-spacings of the montmorillonite + ethanol system are larger than those of the montmorillonite + water system. Also, the swelling transitions are relatively shifted to higher RP values in the montmorillonite + ethanol system. This may be because the clay has a weaker affinity for the less polar ethanol molecules as compared with water. The mobility of ethanol and ions in the interlayers increases with increasing RP due to the associated swelling of montmorillonite. For a given solvation state, the mobility of these species is almost unaffected by changes in RP, as in the case of adsorption. The mobility of ethanol and ions in the 1L and 2L states is about 3 orders of magnitude lower than that in the bulk. The species mobility in the montmorillonite + ethanol system is generally much lower than those in the montmorillonite + water system. This may be attributed to the higher steric hindrance in the alcohol molecules. However, the mobility of the Ca²⁺ ion in the 1L state is almost similar in both the montmorillonite + ethanol and the montmorillonite + water systems. This is possibly due to the stronger Ca²⁺-clay interactions in the 1L state.