Quasi-chemical approximation for polyatomic mixtures

TL;DR

This paper develops a generalized quasi-chemical approximation for the thermodynamics of polyatomic mixtures, providing a more accurate model for adsorption phenomena than previous methods, validated through comparisons with simulations.

Contribution

The paper introduces a new theoretical framework combining exact partition functions and generalized QCA for polyatomic mixtures in adsorption studies.

Findings

QCA outperforms BWA in modeling adsorption of polyatomic mixtures.

The framework extends to higher dimensions and multisite occupancy.

Good agreement with Monte Carlo simulation data.

Abstract

The statistical thermodynamics of binary mixtures of polyatomic species was developed on a generalization in the spirit of the lattice-gas model and the quasi-chemical approximation (QCA). The new theoretical framework is obtained by combining: (i) the exact analytical expression for the partition function of non-interacting mixtures of linear $k$-mers and $l$-mers (species occupying $k$ sites and $l$ sites, respectively) adsorbed in one dimension, and its extension to higher dimensions; and (ii) a generalization of the classical QCA for multicomponent adsorbates and multisite-occupancy adsorption. The process is analyzed through the partial adsorption isotherms corresponding to both species of the mixture. Comparisons with analytical data from Bragg-Williams approximation (BWA) and Monte Carlo simulations are performed in order to test the validity of the theoretical model. Even though a good fitting is obtained from BWA, it is found that QCA provides a more accurate description of the phenomenon of adsorption of interacting polyatomic mixtures.