PVT MEASUREMENTS ON 4'-N-OCTYL-BIPHENYL-4-CARBONITRILE (8-CB) UP TO 300 MPA

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.