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

Mixtures containing exclusively normal, branched and cyclic alkanes, a s 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 fre e-volume contributions are proposed, tested and compared with classica l models. It is shown that it is generally difficult to simultaneously predict the activity coefficients of both a short-chain solute in a l ong-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 c an be applied to solutions with polymers. One of the proposed models ( p-FV), however, can successfully predict both gamma(1) and gamma(2), a nd can also be extended to mixtures containing polymers.