Calibration of pore volume in adsorption experiments and theoretical models

We propose a new definition of excess adsorption for use in molecular models of adsorption. This definition implies the calibration of theoretical models in a manner mimicking the experimental calibration procedure. The method of theoretical calibration is developed on the example of helium calibration. The notions of the He calibrated pore volume and the He calibrated pore size are introduced and the He calibrated theoretical excess isotherms are defined. The proposed method diminishes the discrepancies between the theoretical and the experimental excess adsorption isotherms and makes the theory and the experiment entirely consistent. The quantitative estimates are made by means of the nonlocal density functional theory (NLDFT) applied to the adsorption of He, N-2, and CH4 in micropores of active carbons at liquid nitrogen and ambient temperatures. It is shown that the experimental He calibration affects significantly the excess isotherms of vapors and supercritical fluids in microporous carbons. The nonmonotonous excess isotherms in molecular size pores are predicted. The theoretical predictions of He adsorption are in qualitative agreement with the experimental isotherm measured on the BPL active carbon at 77 K. The theoretical calibration is recommended for adjustment of any molecular model of adsorption phenomena, such as density functional theory, molecular dynamics, and grand canonical and other Monte Carlo simulations.