Hydration and Mobility of Inter layer Ions of (Nax, Cay)-Montmorillonite: A Molecular Dynamics Study

In most of natural montmorillonites, Na+ and Ca2+ ions commonly coexist in the interlayer space as compensation ions. Molecular dynamics simulations have been performed to investigate the swelling properties, hydration behaviors, and mobility of the interlayer species of (Na-x, Ca-y)-montmorillonites with different water contents. Nine montmorillonites with different Na+/Ca2+ ratio were selected as model clay frameworks, and the content of interlayer water was set within a range from 0 to 486 mg(water)/g(clay). The results show that the montmorillonites with coexisting of Na+ and Ca2+ present slightly different swelling curves, hydration energies, and immersion energies from Na- or Ca-montmorillonite. The double-layered hydrates are the thermodynamically stable states for all montmorillonites in the regime of crystalline swelling. A total of 170 mgwater/gclay is found as the threshold water content for the complexing modes of interlayer Ca2+ and Na+ ions switching from inner-sphere complexes to outer-sphere ones. The self-confusion coefficient of interlayer species obviously reveals the confining effects of clay surfaces. In all montmorillonites, the mobility of Na+ is always much greater than that of Ca2+ due to their different hydration shells. According to the water residence time in typical Na+ and Ca2+ hydration complexes, Ca2+ hydration complexes is pronounced more stable than those of Na+, and in montmorillonites with high Ca2+/Na+ ratio, the inhibitory effects of Ca2+ hydration complexes on the mobility of Na+ is clearly revealed.