TOWARDS THE DEVELOPMENT OF A 2ND-ORDER APPROXIMATION IN ACTIVITY-COEFFICIENT MODELS BASED ON GROUP CONTRIBUTIONS

A simple modification of group contribution based models for estimatio n of liquid phase activity coefficients is proposed. The main feature of this modification is that contributions estimated from the present first-order groups in many instances are found insufficient since the first-order groups do not provide all the important molecular structur al information. Therefore, addition of second-order terms, which provi de the needed extra molecular structural information, is proposed. The scope of the new approach is demonstrated through the well-known UNIF AC model in terms of improved correlation/prediction capabilities, dis tinction between isomers and ability to overcome proximity effects.