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