GENERALIZED THERMODYNAMIC PERTURBATION-THEORY FOR POLYATOMIC FLUID MIXTURES - I - FORMULATION AND RESULTS FOR CHEMICAL-POTENTIALS

For general mixtures of polyatomic molecules and their constituent ato ms, we first rigorously derive an exact statistical mechanical result relating the background pair correlation function y(1,2,...,m) to a ce rtain excess chemical potential difference involving its components, b eta Delta mu(e), extending and generalizing our previous results. Seco nd, using only thermodynamic methods, we develop a perturbation theory for the equation of state (EOS) which involves beta Delta mu(e); we t hen express this EOS in an alternative form involving y(1,2,...,m). Th e latter form coincides with results recently obtained by Zhou and Ste ll using a different approach and with the EOS of the Wertheim first-o rder perturbation theory (TPT1); our approach explicitly exposes the u nderlying thermodynamic approximations involved. Third, we show for th e case of tangent fused-hard sphere (FHS) systems, under the approxima tion that beta Delta mu(e) is independent of composition, that impleme ntation of the former form of the theory yields results analytically e quivalent to these obtained from the Boublik-Nezbeda (BN) EOS; and tha t the alternative implementation is only slightly less accurate, due t o a (numerically small) internal inconsistency in this EOS. This sheds light on the remarkable accuracy obtained for several previous implem entations of TPT1 for such systems. We present new computer simulation results for a particular ternary tangent FHS heteronuclear diatomic m ixture, which support the approximation that beta Delta mu(e) for mixt ures of such molecules is nearly composition independent. Finally, for several FHS mixture model systems, we test the Lewis-Randall rule and several other approximations for calculation of the mixture chemical potentials. The Lewis-Randall rule is generally superior for the indiv idual chemical potentials, and is competitive for beta Delta mu(e). (C ) 1998 American Institute of Physics.