M. Sandmann et A. Wurflinger, PVT MEASUREMENTS ON 4'-N-OCTYL-BIPHENYL-4-CARBONITRILE (8-CB) UP TO 300 MPA, Zeitschrift fur Naturforschung. A, A journal of physical sciences, 53(9), 1998, pp. 787-792
P, V-m, T data have been measured for the smectic, nematic and isotrop
ic phases of 4'-n-octyl-biphenyl-4-carbonitrile (8 CB) in the temperat
ure range 300-370 K and pressures up to 300 MPa. At atmospheric pressu
re all phase transitions appear to be of first order due to a disconti
nuity in the density. The volume change at the smectic A - nematic tra
nsition is only a tenth of the volume change at the clearing temperatu
re. At moderate pressures below 80 MPa the S-A-N transition could be d
etected as a discontinuity in the period of oscillation in measurement
s with a high-pressure vibrating tube densimeter. At higher pressures
the discontinuity seems to die away, possibly indicating a change from
first order to second order transition. From the volume changes and t
he slopes of the transition lines we calculate the enthalpy changes at
the phase transition. The p, V-m, T data enable us to calculate the v
olume part of the entropy and the molecular field parameter gamma = pa
rtial derivative ln T-NI/partial derivative ln V-NI.