S-Shaped composition dependence of excess thermodynamic quantities for cyclohexane mixtures with globular alkanes

Expansion coefficients α, isothermal compressibilities, thermal pressure coefficients γ and heat capacities have been measured at 25°C for the cyclohexane+trans-decalin system. An S-shaped composition dependence, positivelnegative for highllow cyclohexane compositions is found for Cp′E dVE/dT and the thermal expansion contribution to Cp′E namely ΔαγVT. The thermal motion contribution to Cp′E, namely ΔCv is close to zero. The positive excursion of these mixing quantities at high cyclohexane content is anomalous. Correspondingly, the mixing quantity-ΔγVT deviates strongly in this region from the predicted equality with HE. The literature and this work show that all these excess quantities behave similarly for cyclohexane mixed with cyclooctane, methylcyclohexane and some highly branched alkanes. The unusual composition dependence of the thermodynamic quantities is consistent with ‘order’ occurring when any large alkane molecule of globular shape is added to cyclohexane. This is speculatively associated with an interference by the globular alkane with the relatively free rotation of cyclohexane molecules.