Group definition in molecular solution theories by quantum mechanical methods: Application to 1-alkanol plus n-alkane mixtures

A group definition criterion based on Bader's theory of atoms in molecules was used to characterize functional groups in 1-alkanols. Application of th e criterion entails calculating the electronic charge density for the confo rmers of the molecules in the series HO(CH2)(n-1)CH3 (n = 1-5) using ab ini tio quantum mechanical methods at the HF/6-31G*//6-31G* level. The group de finition thus established was validated by analyzing the predictions of the group contribution molecular model of Nitta, Turek, Greenkorn, and Chao fo r 1-alkanol + n-alkane mixtures, which involved fitting the model parameter s to the values of various experimental properties for both the pure produc ts and their mixtures with special attention to the excess properties. The model predictions thus obtained were qualitatively and quantitatively bette r than those provided by previous applications. The model was found to accu rately predict second-order derivatives of the Gibbs free energy. Also, the behavior over wide temperature and pressure ranges was qualitatively repro duced.