pVT Measurements and Related Studies on the Binary System nC16H34 -nC17H36 and on nC18H38 at High Pressures

The phase diagram of the binary system nC16H34 -nC17H36 has been established at ambient pressure using DSC and crystallographic measurements. At low temperatures below the rotator phase RI there exist two crystal forms Op (about x(C17) = 0.25) and Mdci (about x(C17) = 0.67) which are different from the crystal structures of the pure compounds (Tp for C16 and Oi for C17). Furthermore two compositions: (a) C16/C17 = 3:1 and (b) = 1:2, which correspond to the coexistence range of Op and Mdci, were chosen for high pressure DTA and pVT measurements, yielding the following findings: The specific volume of the rotator phase of C17 is distinctly lower than those of the binary systems at the same state point. Assuming the existence of a metastable rotator phase for C16, an excess volume of Δ VE/V ≈ 0.01 can be estimated. Due to the very enlarged coexistence range of RI, the mixtures reach their lower transition point at considerably lower temperatures (in isobaric measurements) or higher pressures (in isothermal measurements), where the specific volume is lower than that of C17 at its transition point. Furthermore, the volume and enthalpy changes of the Φord -RI transition is distinctly smaller for the binary systems than for pure C17. Thus the specific volumes of the phases Op and Mdci are appreciably larger than ν(spec.) of C17. Op and Mdci have practically the same specific volume in accordance with the crystallographic results. Enthalpy values are obtained with the aid of the Clausius-Clapeyron equation which agree well with enthalpies derived from the DSC measurements. Furthermore, pVT data have been established for the liquid and solid phases of nC18H38 in the neighbourhood of the melting curve, allowing to determine volume and enthalpy changes of melting as a function of pressure.