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 wellcharacterized 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, 2butanol); and, (c) halogenated adsorptives adsorbing with an S-shaped uptake extending over a broad relative pressure (dichloromethane).

Fluctuation adsorption theory: quantifying adsorbate–adsorbate interaction and interfacial phase transition from an isotherm

“…3 Adsorbate structure, or adsorbate-adsorbate interaction, has been considered not only to play an important role in the surface properties and reactivity of nanoparticles 2,3 but it is also one of the key factors determining the type (functional shape) of an isotherm. [4][5][6][7][8] However, precisely how the balance between adsorbatesurface and adsorbate-adsorbate interactions gives rise to each type of adsorption is still an unresolved question. [4][5][6][7][8] Furthermore, the surface structure expected from the type of isotherm may be different from reality.…”

“…[4][5][6][7][8] However, precisely how the balance between adsorbatesurface and adsorbate-adsorbate interactions gives rise to each type of adsorption is still an unresolved question. [4][5][6][7][8] Furthermore, the surface structure expected from the type of isotherm may be different from reality. 9 Hence, a better link should be provided between an isotherm and adsorbate structure, which is the goal of this paper.…”

Sorption: A Statistical Thermodynamic Fluctuation Theory

“…Such difficulties can only be overcome by a universal approach to determining the sorption mechanism directly from an experimental isotherm. We have shown that sorbate–sorbate interaction, the key to understanding the functional shape (type) of an isotherm, ,− can be quantified directly from an isotherm. We have constructed a theory applicable universally to adsorption and absorption, solid and liquid sorbents, , and vapor and liquid sorbates, making it possible to analyze an isotherm from both sides of the deliquescence transition.…”

“…Understanding a sorption isotherm microscopically means explaining its functional shape (i.e., the IUPAC types) based on the underlying molecular interactions. Sorbate–sorbate interaction has been considered to play a key role in determining the shape of an isotherm. ,− Recently, we have shown, via rigorous statistical thermodynamics, that adsorbate–adsorbate interaction can be quantified directly from an isotherm’s derivative, which is the key to classifying functional shapes; the activity ( a 2 ) dependence of the adsorbed quantity, ⟨ N 2 e ⟩, is related rigorously to the adsorbate–adsorbate number correlation, as …”

Detection of Ring and Adatom Defects in Activated Disordered Carbon via Fluctuation Nanobeam Electron Diffraction

“…The unactivated char and a sample corresponding to 45% conversion were prepared for further TEM analysis (see Supplementary Section S1). This activated carbon has been investigated previously for its adsorption properties [28,29,30,31] and it has a measured pore size of 0.58 ± 0.02 nm [32].…”