Entropy of Mixing: Rigid vs. Flexible Molecules: Effect of Varying Solvent on Dissolution Temperature

We report a study of the dissolution temperature of n-hexacontane, as a function of concentration, with 52 different solvents, aimed at understanding the effect of molecular flexibility on the entropy-of-mixing. The entropy-of-mixing of rigid molecules is commonly expected to go as ln(mole fraction), while for flexible polymers it is expected to follow Flory-Huggins ln(volume fraction). By isolating the entropy-of-mixing, we have experimentally found that rigidity, through ring structures, causes deviations from the Flory-Huggins behavior; and we have proposed and derived a cross-over form for the entropy-of-mixing which varies between ln(volume fraction) and ln(mole fraction) according to molecular rigidity