Statistical Thermodynamics of Chain Molecular Fluids: Equation of State Parameters for PVT Scaling and Their Group Contributions
Extensive experimental studies of fluid hydrocarbons in the lubricating range of molar mass have been undertaken sometime ago by American Petroleum Institute Project 42, located in the Departments of Chemistry and Physics at Pennsylvania State University. In these studies systematic structural changes were introduced, so that the equation of state (e.o.s.) as well as the viscosities of linear paraffins, branched hydrocarbons, and various rings attached to n-alkanes tails are known. Hence this material became the basis for various semi-empirical or empirical structural correlations. We proceed here with the hole theory of Simha-Somcynsky (SS) which has proven quantitatively successful for low as well as high molar mass system and examine e.o.s. data. We demonstrate the success of the theory and obtain the characteristic volume (nu^*), energy ( varepsilon^*) and flexibility (c) parameters as functions of chain length for the different structures. For the short chains in question these represent averages over the terminal and internal units. By suitable generalization of the SS theory developed for physical mixtures we decompose these averages into the individual group contributions. The accuracy of the numerical procedures employed is tested by back computations. Sometime ago A. Bondi developed structural rules for the computation of Van der Waals excluded group volumes. Interesting correlations between these and the above nu ^* values, defined for a 6-12 potential, are obtained. In the same way we examine correlations between D. W. Van Krevelen's and P. J. Hoftyzer's cohesive group energies and varepsilon^* values.
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- Chemistry: Polymer; Physics: Molecular; Chemistry: Physical