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 with single-file adsorption. By considering the adsorbed phase as a one-dimensional fluid, phase equilibria thermodynamics can be used to derive a one-dimensional analogue of the monocomponent Gibbs adsorption isotherm. Equations such as Langmuir, Volmer, FowlereGuggenheim or Hillede Boer can thus be 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 state characterising the adsorbed phase and the adsorption isotherm equations allow great insight to be had simply by adsorption isotherm determination. In order to show the potential and limitations of this approach, 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 are relevant for adsorption of both species, being attractive for Xe and repulsive for CO2; Xe adsorption is mainly distributed while CO2 adsorption is mainly localised