Decoding gas-solid interaction effects on adsorption isotherm shape: II. Polar adsorptives.

Abstract A unique set of 6 polar adsorptives of relatively large dipole moment and of increasing kinetic diameter were used to probe pore volumes available and their mechanism of adsorption on a well-characterised microporous carbon. Multiple adsorption isotherm measurements were made and repeatable results with relatively small standard deviations in amount adsorbed at low relative pressures were obtained. Inconsistencies were observed between calculated Gurvitsch volumes. Sources of these were analysed and identified as contributions from one or more of: (a) molecular sieve effects; (b) molecular packing effects, and; (c) 2D molecular structure formation due to hydrogen bonding. These inconsistencies were further studied by comparison with pore volumes derived via the Dubinin-Radushkevich (DR) equation. Qualitative analyses of the micropore filling processes were proposed, and substantiated by complementary DR analyses. Although most of the isotherms showed Type I character, recasting the relative pressure axis in logarithmic format highlighted clear differences as contributions from fluid-fluid and fluid-solid interactions during pore filling. Overall, the adsorptives were classified into three groups: (a) polar adsorptives with primarily specific interactions adsorbing as a condensation process over a relatively narrow relative pressure range in a medium and late pressure range (iso -PrOH, MeOH, 2-methyl, 2-butanol, H 2 O); (b) polar adsorptives with potential for non-specific interactions adsorbing as a condensation process over a relatively narrow pressure range in a medium pressure range (pyridine, iso -PrOH, 2-methyl, 2-butanol); and, (c) halogenated adsorptives adsorbing with an S-shaped uptake extending over a broad relative pressure (dichloromethane). Graphical abstract Image 1 Highlights • Six polar molecules with increasing kinetic diameter used as adsorptives. • High-resolution gas/vapour adsorption isotherms including uncertainty in the data. • Pore volumes evaluated via Gurvitsch and DR methods – inconsistencies analysed. • Low pressure isotherms show pore filling dependence on adsorptive size, shape, and polarity. • Pore filling classified into low, intermediate, and high relative pressure ranges. [ABSTRACT FROM AUTHOR]