SIZE DEPENDENCE OF TRANSFER FREE-ENERGIES .1. A FLORY-HUGGINS APPROACH

It has been recently proposed that Flory-Huggins theory (FH), which fi nds widespread use in the polymer community, be applied to understand the partitioning of relatively short alkane chains between an organic phase and water. Use of this theory, which predicts that the molar vol umes of the different species play an important role in determining so lubility, results in a significant increase in the hydrophobic surface tension estimated from transfer experiments. However, the application of FH theory to the analysis of alkane solubility has been widely cri ticized. Here, we derive this theory accounting specifically for the p ressure of the lattice system, and show that it is appropriate for tra nsfers from a condensed polymer solvent phase to either a gas phase or a monomer solvent. The sweeping criticisms of the applicability of FH theory to partition experiments that have appeared in the recent lite rature are therefore not valid. A new result is that FH theory is vali d for treating partition data for solutes of arbitrary shape, as long as the solvent is chain-like. On the other hand, Monte Carlo simulatio ns using the chain increment method indicate that Flory-Huggins theory overestimates molecular size effects by 25% even in the case of ather mal lattice systems, indicating that the predictions of this theory sh ould be viewed as first-order estimates of true size effects. The phys ical origins of Flory-Huggins theory are discussed, and connections ar e made to Hildebrand's free volume theory and also to Sharp et al.'s i deal gas derivation of volume effects. The role of molar volume effect s in transfers of solutes from a gas phase to monomer solution are als o considered.