Liquid structure and the excess volumes of cyclohexane + normal- and branched-alkane mixtures

Abstract

Using a Sodev vibrating-cell densimeter, the excess volume (V^E) has been measured at several temperatures between 10 and 55 °C for cyclohexane mixed with the normal alkane (n-C_n) series, n= 10, 12, 14, 16, 17 and 20, with the highly branched (br-C_n) series 2,2-dimethylbutane, 2,2,4-trimethylpentane, 2,2,4,6,6-pentamethylheptane and 2,2,4,4,6,8,8-heptamethylnonane, and with other alkanes of intermediate degree of branching. Similar data are found in the literature for n-C_n, n= 5–10. The V^E values for c-C₆+n-C_n, 5 ⩽n⩽ 17, lie ca. 0.4 cm³ mol^–1 higher than for br-C_n, changing in both series from negative to positive values as n increases. These trends are predicted by the Flory theory with the X₁₂ parameter fitted to H^E data. Values of dV^E/dT at 25 °C are negative for br-C_n, becoming positive for higher n, and can be predicted. For the n-C_n series experimental dV^E/dT values become increasingly negative as n increases, deviating strongly from the br-C_n values and from the Flory predictions. The n-C_n behaviour is consistent with the presence of temperature-dependent short-range orientational order in long-chain n-alkanes, which increases dV/dT anomalously for the pure n-alkanes and which is destroyed on mixing with c-C₆, resulting in a negative order contribution to dV^E/dT. The quantity d²V^E/dT² is large and positive for c-C₆+n-C₁₆ at 25 °C, while the corresponding theoretical value is small and negative. The experimental value is consistent with the rapid decrease with T of the negative order contribution in dV^E/dT.