Hysteresis loop and scanning curves of argon adsorption in closed-end wedge pores

Low temperature adsorption of CO2 in carbonaceous wedge pores: a Monte Carlo simulation study

A systematic study of carbon dioxide in wedge pores under subcritical conditions were conducted with Grand Canonical Monte Carlo simulation. The effects of various factors: temperature, pore geometry (i.e., pore width, length, tilt angle and corrugation), on the formation of stepwise isotherms, were particularly investigated. The occurrence of this step-wise adsorption behaviour is correlated to the alternate packing between the commensurate and incommensurate along the pore axis direction. The steps faded with temperature, due to the adsorbates are less structured and gradually receded mainly from the wide end where the weakest potential exerted to the fluids. The number of steps on desorption branch depends on the number of junctions and domains formed. The particles accumulating at the narrow end in open wedge pore play a role analogy to the closed end. Moreover, having corrugation on the interior solid surface also results different mechanisms illustrated in the stepwise behaviour due to the change in the actual pore size distribution and the interference to the force field inside the pore.

Unveiling the Molecular Origin of Vapor-Liquid Phase Transition of Bulk and Confined Fluids

At temperatures below the critical temperature, discontinuities in the isotherms are one critical issue in the design and construction of separation units, affecting the level of confidence for a prediction of vapor–liquid equilibriums and phase transitions. In this work, we study the molecular mechanisms of fluids that involve the vapor–liquid phase transition in bulk and confinement, utilizing grand canonical (GCE) and meso-canonical (MCE) ensembles of the Monte Carlo simulation. Different geometries of the mesopores, including slit, cylindrical, and spherical, were studied. During phase transitions, condensation/evaporation hysteretic isotherms can be detected by GCE simulation, whereas employing MCE simulation allows us to investigate van der Waals (vdW) loop with a vapor spinodal point, intermediate states, and a liquid spinodal point in the isotherms. Depending on the system, the size of the simulation box, and the MCE method, we are able to identify three distinct groups of vdW-type isotherms for the first time: (1) a smooth S-shaped loop, (2) a stepwise S-shaped loop, and (3) a stepwise S-shaped loop with just a vertical segment. The first isotherm type is noticed in the bulk and pores having small box sizes, in which vapor and liquid phases are close and not clearly identified. The second and the third types occurred in the bulk, cylindrical, and slit mesopores with sufficiently large spaces, where vapor and liquid phases are distinctly separated. Results from our studies provide an insight analysis into vapor–liquid phase transitions, elucidating the effect of the confinement of fluid behaviors in a visual manner.

Calculating adsorption isotherms using Lennard Jones particle density distributions

The computation of particle density distributions in pore channels is a fundamental post process practice in molecular adsorption simulations. The distributions, although not appropriate for direct experimental interrogation, when expressed in variable temperature, may be used to evaluate thermodynamic properties. As with molecular simulations, we can spotlight any frame or region of interest inside the computational cell, the distributions and subsequently the thermodynamic evaluations can be pore or site specific. This allows us to establish correlations for the adsorption capacity, on different pore partitions and surface textures and compare strengths of explicit interactions. We confirm this assumption using adsorption simulations of Lennard Jones particles such as argon in the dual cylindrical cavity of ZIF-69. We initiate the computations from the density distribution functions and reproduce full curves of adsorption isotherms which correlate well with the direct simulations. The computations are fast and implemented using linear van' t Hoff plots for the surface coverage at a sequence of pressures.