Asymmetric Adsorbate and Substrate Interactions in Physisorbed Systems: NITROGEN(2) on Graphite and Dipolar Molecules on Ionic Substrates

Asymmetries in physisorbed systems give rise to interesting phases and phase transitions in two-dimensional (2D) monolayer and multilayer systems. The effects of asymmetric adsorbate and substrate interactions in monolayers of dipolar molecules on ionic substrates and N _2 on graphite are studied. In the case of dipolar molecules on ionic substrates, 2D dielectric phase transitions using a modified Blume -Emery-Griffiths (BEG) model are determined theoretically. A dipole adsorbed vertically above a metal ion lattice site, and pointing up (down), is assigned a spin S = +1 (S = -1). An empty latrice site is assigned a spin S = 0. Analytic solutions for both ferroelectrically and antiferroelectrically ordered systems are found. The model is applied to CO adsorbed on MgO and NaCl, and halogenated methanes on NaCl. Phase diagrams for CO on MgO and NaCl, and preliminary results for the phase diagram of CH_3F on NaCl, are presented. Multilayer phase transitions for N_2 on graphite are studied experimentally using synchrotron x-ray diffraction. The system is measured to undergo layering transitions, where the number of layers increases as the temperature of the system increases. A new multilayer phase diagram based on our results and the combined results published by other researchers is presented. The effects of capillary condensation on this multilayer system are quantified, and it is determined that its primary effect is to broaden the discrete layering transitions. The results for both studies are put into context with other adsorption systems with asymmetric interactions.