Application of a Modified Generalized Flory Dimer Theory to Normal Alkanes

Abstract

The applicability of chain equations of state to real systems is discussed in this paper. For this purpose, the authors have compared four theories: the perturbed-hard-chain theory (PHCT) of Prausnitz and coworkers, the generalized Flory (GF) and generalized Flory dimer (GFD) theories of Hall and coworkers, and the statistical associating fluid theory (SAFT) of Radosz, Gubbins, and co-workers. In this comparison, the perturbation expansion in the attractive term was truncated after the first-order term for all theories. Comparison of these theories with Monte Carlo simulation data for hard chains and square-well chains showed that the GFD theory, which explicitly takes into account the effect of the formation of chains in both the repulsive and the attractive part of the equations, is in best agreement with the data. The GFD theory was further improved by using the simulations data directly to reevaluate the shape parameters c and q, which were found to be density dependent. The new simplified GFD theory gives a significantly better correlation of the properties of normal alkanes than the other four theories.