Freezing of argon in ordered and disordered porous carbon

Abstract

We report a molecular simulation study on the freezing of argon within two models of activated porous carbons. Model A is a regular slit-shaped nanopore, which represents an ordered graphitic porous carbon with a single pore width. Model B is a realistic sample of a disordered porous carbon obtained from reverse Monte Carlo. The morphological (pore shape) and topological (pore connectivity) disorders of model B represent in a realistic way the complex porous structure of materials obtained after carbonization and activation of pure saccharose. This study is aimed at estimating how the effect of disorder of the porous material affects freezing and melting of simple adsorbates. Freezing of argon in the slit pore model conforms to the classical behavior for an adsorbate confined in a strongly attractive pore; the in-pore freezing temperature is higher than that of the bulk fluid, and the shift in freezing temperature increases with decreasing pore size. It is found that the two-dimensional crystal layers of argon within the slit pores have a hexagonal structure (i.e., triangular symmetry). Freezing of argon within model B strongly departs from that observed for model A. No crystallization is observed for argon in the complex porosity of model B. Nevertheless the confined phase undergoes structural changes at a temperature $T=115\phantom{\rule{0.3em}{0ex}}\mathrm{K}$; this temperature is close to the freezing temperature found for the slit pore with width $H=1.1\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, which corresponds to the mean pore size in model B. For temperatures larger than $T=115\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the confined phase in model B exhibits a liquid-like behavior as revealed from pair correlation functions and bond-order parameters. On the other hand, the confined phase for $T<115\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ has more short-range order than the liquid phase but its overall behavior remains liquid-like. Our results indicate that the changes observed at $T\ensuremath{\sim}115\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ are due (1) to the appearance in the confined phase of a small amount of crystal atoms and (2) to the fact that the fraction of liquid-like atoms having at least seven nearest neighbors reaches a plateau value of 80%. The results provide a basis for the interpretation of experiments such as NMR and scattering experiments on freezing in disordered porous materials.