STUDIES OF MOLECULAR INTERACTION EFFECTS IN PHYSICAL ADSORPTION ON HETEROGENEOUS SOLID SURFACES (MONTE CARLO, MIXTURE)

Model studies were carried out on the inter-relationship between the vertical and lateral interactions present in physical adsorption. An expansion of the adsorption integral equation, (AIE), was performed in terms of the statistics of the energy distribution. This relates the isotherm to the shape of the energy distribution rather than to a particular distributions\u27s functional form. A new isotherm was derived from the expansion and fit to experimental data. It represents the first-order correction to the Langmuir isotherm for heterogeneity. Its properties permit the IAS theory to be stated in a simple form. An algorithm called FastIAS has been implemented on this basis and it performs 25 times faster than the conventional IAS method. Monte Carlo simulations of a lattice model of adsorption were performed, and the results were analyzed using the AIE to back-calculate an estimate of the energy distribution. The statistics of the calculated distribution (mean, standard deviation) varied linearly with the magnitude of the neglected lateral interactions. A complete set of thermodynamic information was generated for the model system. A lattice model of adsorbed mixtures was proposed and solved in the Bragg-Williams approximation, assuming random heterogeneity. The model includes the lateral interactions and the surface heterogeneity. It was verified by Monte Carlo calculations. The model was employed to make predictions of experimental mixture data. In most cases it agreed more closely with the data than IAS. Activity coefficients were calculated using the lattice model. Heterogeneity caused negative deviations whereas lateral interactions caused positive deviations from Raoult\u27s law. Contrary to intuition, the excess area remains finite at the limit of zero pressure. Application of the model to liquid adsorption yielded an interpretation of some data which had previously not been explained. This behaviour corresponds to double azeotropy. A high-precision apparatus for the measurement of adsorption isotherms was designed and constructed and an uncertainty analysis was undertaken. Data obtained using the apparatus were shown to be of high precision, and they yielded information about the heterogeneity of the adsorbents used