PVT measurements and related studies on the binary system nC(16)H(34)-nC(17)H(36) and on nC(18)H(38) at high pressures

The phase diagram of the binary system nC(16)H(34) - nC(17)H(36) has been e stablished at ambient pressure using DSC and crystallographic measurements. At low temperatures below the rotator phase RI there exist two crystal for ms Op (about x(C-17) = 0.25) and Mdci (about x(C-17) = 0.67) which are diff erent from the crystal structures of the pure compounds (Tp for C-16 and Oi for C-17). Furthermore two compositions: (a) C-16/C-17 = 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: T he specific volume of the rotator phase of C-17 is distinctly lower than th ose of the binary systems at the same state point. Assuming the existence o f a metastable rotator phase for C-16, an excess volume of DeltaV(E)/V appr oximate to 0.01 can be estimated. Due to the very enlarged coexistence rang e of RI, the mixtures reach their lower transition point at considerably lo wer temperatures (in isobaric measurements) or higher pressures (in isother mal measurements), where the specific volume is lower than that of C-17 at its transition point. Furthermore, the volume and enthalpy changes of the Phi (ord) - RI transiti on is distinctly smaller for the binary systems than for pure C-17. Thus th e specific volumes of the phases Op and Mdci are appreciably larger than v( spec.) of C-17. Op and Mdci have practically the same specific volume in ac cordance with the crystallographic results. Enthalpy values are obtained wi th the aid of the Clausius-Clapeyron equation which agree well with enthalp ies derived from the DSC measurements. Furthermore, pVT data have been esta blished for the liquid and solid phases of nC(18)H(38) in the neighbourhood of the melting curve, allowing to determine volume and enthalpy changes of melting as a function of pressure.