Liquid structure and the excess volumes of cyclohexane + normal- and branched-alkane mixtures
Abstract
Using a Sodev vibrating-cell densimeter, the excess volume (VE) has been measured at several temperatures between 10 and 55 °C for cyclohexane mixed with the normal alkane (n-Cn) series, n= 10, 12, 14, 16, 17 and 20, with the highly branched (br-Cn) 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-Cn, n= 5–10. The VE values for c-C6+n-Cn, 5 ⩽n⩽ 17, lie ca. 0.4 cm3 mol–1 higher than for br-Cn, 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 HE data. Values of dVE/dT at 25 °C are negative for br-Cn, becoming positive for higher n, and can be predicted. For the n-Cn series experimental dVE/dT values become increasingly negative as n increases, deviating strongly from the br-Cn values and from the Flory predictions. The n-Cn 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 dVE/dT. The quantity d2VE/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 dVE/dT.