Microscopic characterization of xenon adsorption in wedge pores
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We explored the possibility of using xenon as a molecular probe to characterize graphitic non-uniform pores in activated carbon (AC), and studied its isotherms with a Grand Canonical Monte Carlo (GCMC) simulation. To model non-uniform pores, we used a wedge pore model, a better description of the pore space in AC, and studied the effects of the pore length, the wedge angle and the temperature on the behaviour of the adsorption and desorption isotherms. For temperatures below the triple point, the isotherm exhibits two distinct regions: in the region of low loadings it shows a step-wise behaviour as the adsorption progresses from the tip of the wedge, followed by the second region of gradual increase in the adsorbed density with pressure. A distinct feature is the possible existence of multiple hysteresis loop in the first region and the reversibility of the isotherm in the second region. The hysteresis is due to the commensurate packing of molecular layers across the pore to form domains along the axial direction of the pore and the number of layers is incremented by one as we move from one domain to the next. We have found that the hysteresis disappears when either the wedge angle is greater than a critical value or the temperature is greater than the so-called bifurcation temperature. The second region of reversibility is due to the formation of the condensate separated from the gas phase with a concave interface whose radius of curvature is found to satisfy the Kelvin equation.
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