Effect of molecular size on the W-shaped excess heat capacities: oxaalkane–alkane systems

Excess molar heat capacities, CEp, throughout the entire concentration range have been determined at 25 °C for the following oxaalkane–alkane systems: 2,5,8,11,14-pentaoxapentadecane (tetraglyme) with decane, nonane, octane and hexane; 2,5,8,11-tetraoxadodecane (triglyme) with hexadecane, pentadecane, tetradecane, dodecane, decane and hexane; 2,5,8-trioxanonane (diglyme) with 2,6,10,15,19,23-hexamethyltetracosane (squalane), 2,6,10,14-tetramethylpentadecane (pristane), hexadecane and decane; 2,5-dioxahexane (monoglyme) with squalane, pristane, decane and heptane; and 1,4-dioxacyclohexane (p-dioxane) with cyclohexane. For all these tures CEp has a W-shaped concentration dependence (two minima separated by a maximum). By increasing the molecular size of either component, the maximum in CEp is enhanced and displaced towards high concentrations of the smaller component. CEp behaviour correlates with degrees of non-randomness in the mixtures as quantified by the concentration–concentration correlation function Scc which is calculated using the Flory–Huggins theory with a group-interaction model and assuming interaction between molecular surfaces.