Journal
FLUID PHASE EQUILIBRIA
Volume 202, Issue 2, Pages 325-335Publisher
ELSEVIER SCIENCE BV
DOI: 10.1016/S0378-3812(02)00124-3
Keywords
polymer; activity coefficient; entropic-FV; hard-core volume; free-volume
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Elbro et al. [Macromolecules 23 (21) (1990) 4707] proposed a simple but successful entropic expression for polymer and other size-asymmetric solutions. The expression is similar to the Flory-Huggins model, but free-volume (FV) fractions are used instead of segment or volume fractions and much better results are obtained. The FV is simply defined as V-f = V - V-W,V- where V-W is the van der Waals (vdW) volume, essentially a measure of molecule's hard-core volume. In this work we adopt the statement by Bondi that due to the packing of molecules, a higher than V-W inaccessible volume represents more adequately the hard-core volume. Using experimental phase equilibrium data for athermal polymer (and other asymmetric) solutions, we show that the optimum V*/V-W ratio is equal to 1.2, which is very close to the value expected based on the packing density at 0 K for organic systems. Using the entropic expression by Elbro et al. [Macromolecules 23 (21) (1990) 4707] and V* = 1.2 V-W yields improved results of activities for athermal polymer solutions and alkane-alkane systems over an extensive database. The results are better compared to other existing entropic expressions, e.g. those of Oishi-Prausnitz (Unifac-FV) and of Zhong-Masuoka. (C) 2002 Elsevier Science B.V. All rights reserved.
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