S-SHAPED COMPOSITION DEPENDENCE OF EXCESS THERMODYNAMIC QUANTITIES FOR CYCLOHEXANE MIXTURES WITH GLOBULAR ALKANES

Expansion coefficients alpha, isothermal compressibilities, thermal pressure coefficients gamma and heat capacities have been measured at 25 degrees C for the cyclohexane + trans-decalin system. An S-shaped composition dependence, positive/negative for high/low cyclohexane compositions is found for C-P(E), dV(E)/dT and the thermal expansion contribution to C-P(E), namely Delta alpha gamma VT. The thermal motion contribution to C-P(E), namely Delta C-V is close to zero. The positive excursion of these mixing quantities at high cyclohexane content is anomalous. Correspondingly, the mixing quantity -Delta gamma VT deviates strongly in this region from the predicted equality with H-E. 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.