Thermodynamics of mixtures of hexane and heptane isomers with normal and branched hexadecane

Molar excess mixing enthalpies h^E, Gibbs free energies g^E and hence entropies s^E have been obtained using calorimetry and the vapor sorption method at 25°C for hexane isomers+2,2,4,4,6,8,8-heptamethylnonane, a highly branched C₁₆. The h^E and g^E are negative while Ts^E are positive, but small. The values are explained by the Prigogine-Flory theory through negative free volume contributions to h^E and Ts^E, counterbalanced in the case of Ts^E by the positive combinatiorial Ts^E for mixing molecules of different size. No contribution is seen from the interaction between methyl and methylene groups. The excess quantities are also obtained for hexane and heptane isomers mixed with n-hexadecane. Values of h^E and Ts^E are now strongly positive, while those of g^E are only slightly less negative. The interpretation requires two recently advanced contributions in addition to those of the Prigogine-Flory theory: 1) a decrease of order when correlations of orientations between n-C₁₆ molecules in the pure liquid are replaced in the solution by weaker correlations whose strengths depend on the shapes of the lower alkane isomers. For lower alkane isomers of the same shape, but highly sterically hindered, h^E and Ts^E are small, manifesting, 2) a negative contribution, ascribed to a rotational ordering of n-C₁₆ segments on the sterically-hindered molecule. Enthalpy-entropy compensation is observed for these new contributions, arising from their rapid fall-off with increase of temperature.