Second virial coefficients of n-alkanes

The second virial coefficient, B(T), of n-alkanes has been calculated. The molecular interactions were modelled as site-site interactions where the sites represent methyl or methylene groups (United-Atom approximation). Two cases have been studied: one with the interaction sites placed on the carbon nucleii (UA model) and a second case with the interaction sites placed approximately at the geometrical centres of the atoms in the methyl and methylene groups (AUA model). When a unique set of parameters is fitted to reproduce the experimental second virial coefficient of several n-alkanes, the AUA model gives the best agreement with the experiments for all the substances studied. Moreover, when a value of σ which gives good results in the liquid range is used, the values of the depth of the wells which fit the experimental data for B(T) are larger than the corresponding values for the liquid range, as is found for more simple molecules. Two different torsion potentials, providing different conformer populati...