Generalized Flory theory for hard alkane fluids

Abstract

Generalized Flory-dimer (GF-D) theory is extended to hard n-alkane fluids modelled as fused hard spheres constrained according to the rotational isomeric state approximation. Theoretical predictions for the second to the fifth virial coefficients for each conformer from n-butane to n-octane are compared with numerical results. Predictions for the second viral coefficient are in excellent agreement with recent simulation results; however, GF-D theory systematically overpredicts the third virial coefficient and underpredicts the fourth and the fifth virial coefficients. GF-D predictions for the compressibility factor for n-alkanes from n-butane to n-octane are compared with molecular dynamic simulation results for a closely related model. Despite the over- and underprediction of the individual virial coefficients, excellent agreement is observed between theory and simulation for the compressibility factors of n-butane and n-pentane using no adjustable parameters. However, in the cases of n-hexane, n-heptane and n-octane, GF-D theory slightly underpredicts the pressure at high volume fractions. Theoretical considerations suggest that probably the predictions of GF-D theory will deteriorate with increasing chain length.