(350f) 2D Phase Transitions of Fluid Adsorbed in Cylindrical Mesopores: Influence of Adsorbent-Adsorbate Interactions

Recently, molecular simulations of methane adsorption in mesoporous cylindrical silica identified a two-dimensional phase transition associated with non-trivial densification of the adsorbed liquid, a high-density local maximum in the isosteric heat of adsorption, and hexagonal rearrangement of annular layers of adsorbed fluid [1]. This observed rearrangement of adsorbed methane was proposed as a plausible explanation for the densification of methane adsorbed in MCM-41 and SBA-15 that was observed in small-angle neutron scattering (SANS) experiments [2]. The proposed phase transition was, however, unique to the methane-silica adsorption systems, not being observed in, for example, nitrogen-silica, argon-silica, or argon-carbon systems. To better understand this unique phase transition and why it occurs for the methane-silica systems in contrast to others, we perform additional Monte Carlo molecular simulations to identify key characteristics of the methane-silica system that give rise to its appearance. Specifically, we measure adsorption isotherms, heats of adsorption, and bond-orientational order parameters for the benchmark systems that do not exhibit such phase transitions, and then smoothly transform those benchmark systems into the methane-silica system to clearly identify the boundaries in adsorbent-adsorbate parameter space that trigger this type of phase transition. Lastly, we discuss engineering consequences of the observed phase transition and how it may impact other thermophysical properties of the adsorbed fluid.

[1] D.W. Siderius, W.P. Krekelberg, W-S. Chiang, V.K. Shen, and Y. Liu, Langmuir, 33:14252 (2017).
[2] W-S. Chiang et al., Langmuir, 32:8849 (2016).