Molecular simulation study of hydrated Na-rectorite.

The swelling behavior of clay minerals is an important issue in industrial processes and environmental applications. Mixed-layer clay minerals containing a smectite fraction, such as rectorite, are neglected even though they could swell and exist in nature widely. The hydration of rectorite has not been well comprehended even though they are meaningful to mineralogy and industry. This study combines molecular dynamics (MD) and Monte Carlo (MC) simulations to disclose the swelling behavior of rectorite and compare with montmorillonite. From grand canonical Monte Carlo (GCMC) and MD simulations, we obtain swelling curves and swelling free-energy curves of rectorite with a relative humidity of 100%. With the comparisons of swelling free-energy minima, we find that the bilayer hydrate of Na-rectorite is more thermodynamically stable than the monolayer hydrate, which is similar to Na-montmorillonite. However, the interlayer sodium ions in rectorite show an asymmetrical distribution quite different from the symmetrical distribution in montmorillonite. Because of unequal layer charges between the smectite part and illite part of retorite, sodium ions prefer to distribute close to the illite part surface.