Molecular structure and orientational order effects in enthalpies and heat capacities of solute transfer into n-hexadecane. Part 2.—Cyclic and aromatic solutes

Abstract

Enthalpies ΔH(b→n) and heat capacities ΔC_p(b→n) have been obtained at 25 °C for the transfer of solute molecules into n-hexadecane (n) from the highly branched C₁₆ hydrocarbon 2,2,4,4,6,8,8-heptamethylnonane (b). Solutes (S) include the cycloalkane series, dimethylcyclo-hexane isomers, bicycloalkanes of differet configurations, alkenes, steroids, benzene and its methyl and chlorine derivatives, bicyclic aromatics and two nematogens. Values of ΔH(b→n) and ΔC_p(b→n) vary widely but fall on two similar correlation curves. They reflect a balance of two temperature-sensitive ordered contacts: one, between n molecules, is broken on transfer of the solute, while the second is formed between the S and n molecules. The extent to which the S–n contact is ordered varies with solute molecular shape and flexibility, leading to either a net gain or loss of order during transfer. The balance determines the sign of ΔH(b→n) and ΔC_p(b→n) and the position of the solute on the correlation curve.