MOLECULAR MODELING OF THE ENTHALPIES OF ADSORPTION OF HYDROCARBONS ON SMECTITE CLAY

We have calculated the enthalpies of adsorption of 13 hydrocarbons on a smectite clay mineral using molecular modeling techniques. The molecules, C5-C-10 linear, branched, and cyclic hydrocarbons, were studied by computer simulation using classical mechanical energy minimization and molecular dynamics (MD) methods. We held the atoms in the clay surface rigid, while the hydrocarbons were allowed complete flexibility both internally and in relationship to the clay. The atomic coordinates for the clay were developed from a model proposed for an Ascan smectite. We modeled the nonbonded atomic interactions between the clay surface and the hydrocarbons with a simple Lennard-Jones (LJ) potential. The main purpose of our investigation was to determine the sensitivity of the results to the LJ potential parameters. We thus used two sets of values, those from the mm2 force field and those of Hopfinger. To evaluate the usefulness of these potential sets, we compared the calculated enthalpies of adsorption to experimental values from gas chromatography retention studies. The results show that both sets of LJ potentials can represent the trend in the enthalpies of adsorption but give substantially different absolute values. We also find that the calculated trends are quite different if the data are taken from MD simulations of ensembles of molecules, rather than energy minimizations of individual molecules. In particular, the MD calculated values are almost-equal-to 5.3 kcal/mol less negative due to the explicit inclusion of temperature. There is still much work needed in developing nonbonded potentials for these types of systems. However, the results demonstrate the great utility of molecular modeling techniques for studying adsorbate-clay interactions.