On the phase transition of argon adsorption in an open end slit pore-Effects of temperature and pore size

The phase evolution of argon adsorbed in open ended graphitic slit pores at temperatures below the 2D-critical temperature of the first layer was simulated using the grand canonical Monte Carlo (GCMC) and meso-canonical ensembles (MCE). In the latter the adsorption system is connected to a finite reservoir and the combined system is a canonical ensemble. Hysteresis loops and sigmoid van der Waals loops were found for the GCMC and MCE simulations respectively, corresponding to the observed 2D-transitions, which comprised vapor-solid, vapor-liquid and liquid-solid changes of state depending on the temperature. The MCE isotherms in large open end pores exhibited a sequential adsorption, not previously noted in the literature, where the monolayer filling on one wall is followed by monolayer filling on the opposite wall, giving rise to a double van der Waals loop. When the spacing between the pore walls is decreased, this double-vdW loop evolves to form a fused single loop, and the transition shifts from being predominantly a surface adsorption to pore volume filling.