A surface thermodynamics approach to modelling single-file adsorption in ultramicroporous materials

Abstract

A new thermodynamic approach is proposed to interpret adsorption equilibrium in ultramicropores withsingle-file adsorption. By considering the adsorbed phase as a one-dimensional fluid, phase equilibriathermodynamics can be used to derive a one-dimensional analogue of the monocomponent Gibbsadsorption isotherm. Equations such as Langmuir, Volmer, FowlereGuggenheim or Hillede Boer can thusbe used as representations of phenomenological models of the one-dimensional adsorption system,rather than just as mathematical correlations. The bidirectional relation between the equations of statecharacterising the adsorbed phase and the adsorption isotherm equations allow great insight to be hadsimply by adsorption isotherm determination. In order to show the potential and limitations of thisapproach, the adsorption isotherms of Xe and CO2 in four crystalline hydrophobic dipeptides of the VAclass are analysed with this approach. It was possible to assess that adsorbateeadsorbate interactions arerelevant for adsorption of both species, being attractive for Xe and repulsive for CO2; Xe adsorption ismainly distributed while CO2 adsorption is mainly localised