Modeling of thermodynamic properties of associated solutions with equilibrium constants defined on activities

Association constants (for example demerisation constants of carboxylic acids) defined in concentration units are strongly dependent on concentration and solvent. Depending on the solvent, these equilibrium constants vary two orders of magnitude at infinite solution. This is due to the differences in size and the physical interaction between monomer, associated and solvent molecules. This work presents a thermodynamic model to describe the properties (VLE, HE, Kx = xAixAi) of associated mixtures (carboxylic acids-inert solvent) where the equilibrium constants are based on activities and not on concentration units. The physical part of the activity coefficients is expressed by a group contribution method (distinguishing between monomers, chemical bonded and solvent molecules) based on the Bragg-Williams approximation. For the combinatorial part, the Flory-Huggins expression is used. In addition to being able to describe VLE-and HE-data as good as modified UNIFAC by Gmehling et al. (1993), the model yields values for Kx,∞ (Kx at infinite dilution) of the correct order.