Excess heat capacities of carbon tetrachloride + alkane and benzene + alkane mixtures

Abstract

A Picker flow microcalorimeter has been used to obtain molar excess heat capacities (C^E_p) through the concentration range at 20, 40 and 55 °C for carbon tetrachloride and at 37.5 °C for benzene mixed with the normal-C_n alkanes, n= 8, 12 and 16, and the corresponding branched-C_n alkanes: 2,2,4-trimethylpentane, 2,2,4,6,6-pentamethylheptane and 2,2,4,4,6,8,8-heptamethylnonane. Values of C_p^E were also obtained for CCl₄+ n-C₈ and n-C₁₂ at 12 °C and for benzene + n-C₆ at 37.5 °C. Results are similar to those reported by S. N. Bhattacharyya and D. Patterson, J. Phys. Chem., 1979, 83, 2979, for cyclohexane + alkane systems. The C_p^E values, which are all negative, are small for the branched alkanes but are large for the normal alkanes and decrease in magnitude with increasing temperature. This is consistent with the presence of short-range orientational order in the n-alkanes, decreasing with increasing temperature, which is destroyed by the CCl₄ and benzene. Literature excess enthalpy values at 25 °C combined with integration of C^E_p give H^E for the CCl₄ systems as functions of T and, correspondingly, values of the X₁₂/s₁ parameter of the Prigogine–Flory theory. The order contributions to C^E_p and to X₁₂/s₁ for systems containing the quasi-spherical molecules CCl₄, benzene and cyclohexane + n-alkanes are single functions of alkane reduced volume. The order contributions apparently reflect only the order of the n-alkane which depends on the free-volume surrounding the alkane segments, i.e. packing density. A corresponding states treatment of n-alkane mixtures (A. Trejo Rodriguez and D. Patterson, J. Chem. Soc., Faraday Trans. 2, 1982, 78, 501) predicts the order contributions to C^E_p and X₁₂/s₁ for quasi-spherical molecules + n-alkanes.