TEMPERATURE-DEPENDENCE OF EXCESS ENTHALPIES FOR SYSTEMS CONTAINING NORMAL HEXADECANE

Excess enthalpies, and heat capacities derived therefrom, have been obtained between 25 and 65 or 75°C at a constant concentration for cyclohexane and octamethylcyclotetrasiloxane mixed with normal hexadecane and with a highly branched C16 isomer, 2,2,4,4,6,8,8-heptamethylnonane, and also for cis-and trans-decalin mixed with n-C16. The hE values with n-C16 are positive and much larger than with the branched-C16. They decrease rapidly with T so that cpE is large and negative. These results imply the presence of orientational order in the n-C16, which is destroyed on mixing with the other component and which decreases with T. The hE for trans-decalin+n-C16 is much smaller than for cis-decalin+n-C16, and becomes negative with increase of T. This change of sign, which is unexplained by current theory, is interpreted as due to an interference of the flat, platelike trans-decalin molecule with the molecular motion of the n-C16 chain. © 1979 Plenum Publishing Corporation.