MODELING OF THERMODYNAMIC PROPERTIES OF ASSOCIATED SOLUTIONS WITH EQUILIBRIUM-CONSTANTS DEFINED ON ACTIVITIES

Association constants (for example dimerisation constants of carboxyli c acids) defined in concentration units are strongly dependent on conc entration and solvent. Depending on the solvent, these equilibrium con stants vary two orders of magnitude at infinite solution. This is due to the differences in size and the physical interaction between monome r, associated and solvent molecules. This work presents a thermodynami c model to describe the properties (VLE,H-E,K-x=x(Ai)/x(Ai)(i)) of ass ociated mixtures (carboxylic acids-inert solvent) where the equilibriu m constants are based on activities and not on concentration units. Th e physical part of the activity coefficients is expressed by a group c ontribution method (distinguishing between monomers, chemical bonded a nd solvent molecules) based on the Bragg-Williams approximation. For t he combinatorial part, the Flory-Huggins expression is used. In additi on to being able to describe VLE-and H-E-data as good as modified UNIF AC by Gmehling et al. (1993), the model yields values for K-x,K-infini ty (K-x at infinite dilution) of the correct order.