Heat capacity and heat of adsorption at orientational phase transition in nitrogen monolayer on graphite

Abstract

In this paper, we report a simulation study of the effect of the herringbone orientational phase transition on the isosteric heat of adsorption and the heat capacity of the nitrogen adsorption monolayer on graphite surface. We use the previously proposed regression equations of state for phases with the short- and long-ranged orientational ordering (Ustinov et al. in J Phys Chem C 122:2897–2908, 2018). The proposed approach allows one to give an exhaustive thermodynamic description of the isobaric phase transition without direct estimation of the chemical potential, using only the pVT data obtained in a canonical Monte Carlo simulation. The herringbone orientational phase transition is indicated by a peak in the temperature dependence of the heat capacity. Its height and shape is close to those observed experimentally. In addition, the heat spike is observed on the isosteric heat curves near the transition region. It decreases with increasing temperature and completely disappears at a temperature close to the melting point in the gas-crystal system. A combined analysis of the heat capacity and isosteric heat of adsorption demonstrates that the nitrogen monolayer on graphite surface has similar features inherent in a three-dimensional system and, therefore, the translational and rotational degrees of freedom normal to the surface are involved in the mechanism of the long-to-short-ranged orientational transition.