Simulation of nitrogen adsorption–desorption isotherms. Hysteresis as an effect of pore connectivity

The sorption mechanisms in porous materials have been of long standing interest and debate. Concretely, the involved hysteresis during nitrogen adsorption–desorption processes and its interpretation has represented an important challenge for experimentalists and theoreticians. Moreover, a better understanding of the different observed hysteretic behaviors and the elapsed are as is still remaining. Such a scenario has motivated us to study the pore connectivity and pore size distribution effects upon the hysteresis in nitrogen adsorption–desorption isotherms of a porous solid. Our simulation has been carried out over the entire range of connectivity assuming three-dimensional pores randomly inscribed in a two-dimensional lattice with different occupation probabilities, related with pore connectivity, including the percolation threshold. The adsorption and desorption curves as a function of pressure have been simulated taking into account the monolayer–multilayer formation process and the capillary condensation–evaporation phenomena by using the Broekhoff de Boer and Kelvin equations, respectively. Results show that the employed methodology allows reproducing different types of hysteresis experimentally observed where the hysteretic behavior is strongly dependent upon both connectivity and the broadening of the pore size distribution. Specifically, the hysteresis loop area exhibits a maximum at the percolation threshold and then it decreases monotonically above the threshold.