Grand ensemble Monte-Carlo studies of physical adsorption

Abstract

Singlet and pair distribution functions provide valuable insight into adsorbate structure and indicate that both solid-like and liquid-like properties can be found at the lower temperature (T* = 0·668), while at the higher temperature (T* = 1·002) liquid-like properties predominate. Singlet distribution functions confirm that a layer structure description of the adsorbate is appropriate, particularly at the lower temperature. The pair distribution functions indicate a transformation in the first-layer from liquid-like to solid-like at T* = 0·668 and a coverage of about 0·9. The simple BET theory and its modifications fit the isotherm fairly well, but layer filling and the isosteric heats are poorly reproduced. The assumptions of the slab theory are in contradiction to the filling process observed. Nevertheless good FHH plots are obtained for ϑ^(e) > ∼ 2 with indices of 1·71(2·44) at T* = 0·668(1·002), which demonstrate that surface heterogeneity is not necessary to explain indices below 3. Theories with lateral interactions are necessary to reproduce the form of the q_st curves and the step-like behaviour of the isotherms. Two such theories are discussed. Some aspects of long-range corrections for MC calculations in non-homogeneous systems are considered in an appendix.