Application of the rotational isomeric model to the calculation of binary interactions between flexible chains

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Abstract
Orientational and conformational averages of the Mayer function corresponding to binary interactions of flexible n-alkane molecules have been computed by means of a Monte Carlo sampling of pairs of conformations generated with randomly chosen rotational angles and relative orientations. The conformations are generated according to conditional probabilities and molecular parameters corresponding to the rotational isomeric state model. The intermolecular interactions between -CH2 or -CH3 groups are described through a Lennard-Jones potential, with a fixed value of the length parameter d and a value of the energy parameter ϵ fitted to reproduce experimental results or justified theoretical predictions for second virial coefficients over a wide temperature range. We find a good agreement with these data employing values of ϵ which has a modest variation with chain length (about 30 per cent with respect to the highest value within the range n = 4–16). The averaged results for the interactions are discussed and their dependence on temperature and chain length are analysed. Differences between these site-site results and the interactions described through fittings of the data to global molecular parameters are discussed.