MOLECULAR MODELING OF THE ENTHALPIES OF ADSORPTION OF HYDROCARBONS ONSMECTITE CLAY

We have calculated the enthalpies of adsorption of 13 hydrocarbons on a smectite clay mineral using molecular modeling techniques. The molec ules, 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 sur face rigid, while the hydrocarbons were allowed complete flexibility b oth internally and in relationship to the clay. The atomic coordinates for the clay were developed from a model proposed for an Ascan smecti te. We modeled the nonbonded atomic interactions between the clay surf ace and the hydrocarbons with a simple Lennard-Jones (LJ) potential. T he main purpose of our investigation was to determine the sensitivity of the results to the LJ potential parameters. We thus used two sets o f values, those from the mm2 force field and those of Hopfinger. To ev aluate the usefulness of these potential sets, we compared the calcula ted enthalpies of adsorption to experimental values from gas chromatog raphy retention studies. The results show that both sets of LJ potenti als can represent the trend in the enthalpies of adsorption but give s ubstantially different absolute values. We also find that the calculat ed trends are quite different if the data are taken from MD simulation s of ensembles of molecules, rather than energy minimizations of indiv idual molecules. In particular, the MD calculated values are almost-eq ual-to 5.3 kcal/mol less negative due to the explicit inclusion of tem perature. There is still much work needed in developing nonbonded pote ntials for these types of systems. However, the results demonstrate th e great utility of molecular modeling techniques for studying adsorbat e-clay interactions.