Computational design of clay minerals: hydration of Mg-exchange cation located in ditrigonal cavity

The results of a quantum-chemical MNDO-PM3 investigation of the hydration of the Mg2+ cation located in a ditrigonal cavity of the basal surface of clays based on cluster approximations are presented. As many as 14 local minima related the position of Mg2+ in the ditrigonal cavity were found. Among them seven are located above the basal lattice plane, and seven are below this plane. The most favorable area of the ditrigonal cavity where an Mg cation could be located was determined. The hydration by one to four water molecules of Mg2+ placed in the most energetically preferable position was also investigated. We found that the calculated values of interaction energy are decreased during the stepwise hydration of Mg - kaolinite surface from 36.9 to 13.4 kcal mol(-1). Finally, calculations of dissociative adsorption proves that this process is slightly endothermic and the value of the proton transfer barrier depends on the number of hydrated water molecules. (C) 1998 Elsevier Science B.V.