MONTE-CARLO AND MOLECULAR-DYNAMICS SIMULATIONS OF ELECTRICAL DOUBLE-LAYER STRUCTURE IN POTASSIUM-MONTMORILLONITE HYDRATES

Monte Carlo and molecular dynamics simulations of interlayer molecular structure in the one-, two-, and (hypothetical) three-layer hydrates of K-montmorillonite were performed concurrently in order to elucidate counterion speciation and water structure in the electrical double la yer of this clay mineral. Calculated layer spacings, interlayer water potential energies, and counterion mobilities were in agreement with a vailable experimental data. In the one-layer hydrate, both outer-spher e and inner-sphere surface complexes of K+ were observed, the latter a lways near sites of tetrahedral charge substitution, with the counteri on species exchanging readily on the simulation time scale (up to 200 ps). In the two-and three-layer hydrates, the surface complexes persis ted, but an incipient diffuse layer of counterions also was observed, with all three types of surface species engaging in sluggish exchange. Water molecules in the one-layer hydrate resided at the interlayer mi dplane, whereas ill the two-layer hydrate they lay in two planes betwe en outer-and inner-sphere K+ surface complexes, as veil as at the midp lane. Hydrogen bonds in the one-layer hydrate were longer and more ben t than in bulk liquid water. For all three hydrates, water and cation interlayer mobilities remained below those observed in bulk solution, principally because of the restricted geometry and the retarding effec t of clay layer surface charge. Most of our results can be understood in terms of the weak solvation of the counterions by water molecules, which permits significant competition between K+ and water protons for negatively charged sites in the clay mineral.