Layering and orientational ordering of propane on graphite: An experimental and simulation study

Abstract

We report the results of an experimental and theoretical study of propane adsorption on highly oriented pyrolytic graphite. Simulations and experiments were carried out for temperatures from 90 to 110 K and pressures from ultrahigh vacuum up to about 0.5 mTorr. Both simulations and experiments show that propane adsorbs in a layer-by-layer fashion and exhibits continuous growth beyond the second layer at the higher temperatures studied. Experimental adsorption data were obtained by optical differential reflectance (ODR) and temperature programmed desorption (TPD). The ODR method is able to dynamically follow the adsorption and desorption process as a function of time over a wide pressures range, while TPD probes only the desorption events at ultrahigh vacuum conditions. The influence of the fluid–fluid potential on the adsorption isotherms has been investigated by comparing simulations using five different propane potentials. The pressure at which the second layer forms was found to vary by more than 1 order of magnitude among the potentials tested, whereas the isosteric heat of adsorption is less sensitive to the potential. We find that the propane potential that best describes the liquid phase energetics is in the best agreement with experimental adsorption isotherms and isosteric heats. The binding energy of propane in the monolayer measured from TPD experiments is in excellent agreement with that computed from simulations, both giving values of about 30 kJ mol−1. The isosteric heat of adsorption at incipient second layer formation is 23±2 kJ mol−1 as measured from ODR isotherms and 24±1 kJ mol−1 computed from simulations. The isosteric heat as a function of coverage computed from molecular simulations is roughly constant over the 1–2 and the 2–3 layering transitions at 91 K. We show that this unusual behavior is due to a rotational transition, whereby molecules in the first layer rotate from a parallel (all CHx groups in contact with the graphite plane) to a perpendicular (one CH3 group pointing up) orientation. This rotational transition has two effects: it allows more molecules to adsorb in the monolayer and increases the isosteric heat of adsorption in the second layer over that for adsorption onto an atomically smooth surface.