Study of heat of adsorption across the capillary condensation in cylindrical pores

We have investigated the isosteric heat of argon adsorption in MCM-41 and carbon nanotubes using grand canonical Monte Carlo (GCMC) simulation, and find that the isosteric heat during the capillary condensation is practically constant irrespective of temperature, pore-curvature, and surface strength. We also studied the microscopic behavior during capillary condensation by analyzing the histogram of the number of particles, the local number fluctuation and the layer compressibility. The histograms exhibit increasingly broader peaks close to condensation which subdivide into two separate peaks, indicating the relative probability of two co-existing states. Local number fluctuation allows us to identify the boundary between the adsorbed phase and the gas phase inside the pore. The layer compressibility can be used to identify the states of the adsorbate in each layer within the adsorbed phase. Finally we propose a simple model for the heterogeneous surface of MCM-41 to simulate the effects of surface heterogeneity on the isosteric heat. This model is able to describe the experimental isosteric heat of argon adsorption at 87.3 K in MCM-41.

Isosteric heats of argon adsorption on a 5.1 nm MCM-41 cylindrical pore obtained from our new model simulation and that calculated from the experimental isotherm at 87.3 K.Figure optionsDownload as PowerPoint slideHighlights
► Heat of adsorption in cylindrical pores across the condensation and evaporation region.
► Constant heat in the multi-layer region is due to the balance between the decrease of the solid–fluid interaction and the increase in the fluid–fluid interaction.
► Constant heat across the transition and the multi-layer region is insensitive to pore size, surface strength and adsorption temperature.
► Development of the simple model of MCM-41 to describe correctly the isosteric heat versus loading.