IMPROVED MODELS FOR THE PREDICTION OF ACTIVITY-COEFFICIENTS IN NEARLY ATHERMAL MIXTURES .1. EMPIRICAL MODIFICATIONS OF FREE-VOLUME MODELS

Mixtures containing exclusively normal, branched and cyclic alkanes, as well as saturated hydrocarbon polymers (e.g. poly(ethylene) and poly(isobutylene)), are known to exhibit almost athermal behavior. Several new activity coefficient models containing both combinatorial and free-volume contributions are proposed, tested and compared with classical models. It is shown that it is generally difficult to simultaneously predict the activity coefficients of both a short-chain solute in a long-chain solvent (gamma(1)) and a long-chain solute in a short-chain solvent (gamma(2)) with a single model. In addition, only few models can be applied to solutions with polymers. One of the proposed models (p-FV), however, can successfully predict both gamma(1) and gamma(2), and can also be extended to mixtures containing polymers.