Methodology for the Development of Thermodynamic Models Describing Substances That Exhibit Complex Association Interactions

Abstract

This work describes a methodology for the development of a thermodynamic model describing the substances that show strong self- and cross-association interactions. The methodology is fundamentally based on the chemical theory of association interactions. The system used as a case study in this work is a binary mixture containing hydrogen fluoride and water (HF + H2O). Earlier studies have failed to provide a reasonable description of this binary mixture because of the complex association interactions between these compounds, which were not adequately modeled. In this work, the phase behavior of this mixture is understood by exploring these complex association interactions. Pure HF was modeled using 14 different association schemes that allow the formations of different physically meaningful oligomers with different distribution schemes (1−2, 1−6, 1−2−6, etc.), where the 1−2 scheme allows the formation of monomers and dimers and likewise. The parameters for these pure component schemes were obtained by correlating the phase coexistence properties of pure HF and were also used to predict several other pure component properties (ΔHvap, CP, Cv, Z, etc.) The dominance of these association patterns and their distribution were understood on the basis of their predictive ability. The pure component association schemes that were developed for HF and water were extended to the binary mixture. The phase coexistence properties were correlated using different association patterns for the pure components with and without considerations for the strong association between them. The significance of these self- and cross-association patterns are studied and understood on the basis of the correlative and the predictive ability of the association schemes. The effect of including the cross-associates that are most likely to be formed in this mixture, from a molecular level hybrid meta-density functional theory study, is also discussed. The methodology described in this work can be utilized to understand and predict the bulk-phase thermodynamic properties of substances that show complex association interactions at a molecular level.