Application of IR-spectroscopy in thermodynamic investigations of associating solutions

This paper presents results from a study on the use of IR-spectroscopic information in the development of thermodynamic models of H-bonding solutions. It is based on a previous comprehensive IR-spectroscopic study on H-bonding in solutions of alkanols and phenol in different solvents, which is used here to develop a physico-chemical model of the Gibbs energy of those solutions. The chemical part of the model, describing the association, is parameterized using the IR-data alone. In the physical part of the model, UNIQUAC is used with parameters determined from infinite dilution activity coefficients, i.e. from thermodynamic data. This physico-chemical model, the UNIQUAC association model, is compared to the conventional UNIQUAC model using only the same thermodynamic data to parameterize both models. The separation of the chemical and physical effects based on IR-spectroscopic data in the UNIQUAC association model leads to a clearly improved predictive power. Comparisons for vapor–liquid equilibria, activity coefficients and excess enthalpies are presented. The paper is thus a case study on the benefits that can be drawn from using IR-spectroscopic data in the development of thermodynamic models of H-bonding solutions. The procedure can be extended to other types of models like equations of state or molecular models.