Monte Carlo simulations of Wyoming sodium montmorillonite hydrates

Monte Carlo simulations have been used to predict the interlayer basal sepa rations of sodium-saturated Wyoming clays at constant stress (NPzzT ensembl e) and at constant chemical potential (mu VT ensemble). These simulations u se the Ewald summation technique to incorporate long-range Coulombic intera ctions in the calculation of the total potential energy and the pressure te nsor. A comparison is made between the use of one, two, and three sheets of clay. It is shown that, for small separations, at least two separate clay sheets must be used to avoid system-size effects. The stable interlamellar separations are determined by combining results from isostress-isothermal a nd grand canonical simulations. It is shown that, consistent with experimen ts, at the temperature and pressure studied here, the cations in the interl ayer are hydrated, except at the smallest basal separations. (C) 2001 Ameri can Institute of Physics.