Thermodynamic equations for phase transition of dimyristoylphosphatidylcholine (DMPC) are calculated numerically by using the Gibbs free energy per one mole of hydrocarbon chains that is given by Sugár as G/RT=F/RT+pV/RT+πA/2RT, where R is the gas constant, T is the absolute temperature, p the pressure, π the osmotic lateral pressure. The free energy F, the molar specific volume V and the molar specific area A are written as functions of the disorder parameter S. The compressibility B=-(1/V) (dV/dS) (dS/dp) is obtained as well as V against the temperature, the pressure and the lateral pressure. In DMPC we have the critical point where π_c=-0.01712N/m and T_c=273.15+22.06 K at the atmospheric pressure p=0. At the critical point B increases anomalously and V changes continuously from the disordered state as varying π. Contrarily as more increasing p, V changes more discontinuously and B behaves less anomalously at the first transition point. The results are compared with experiments for pressure dependence of V in DMPC liposome suspension.

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An automatic measuring system of ultrasonic velocity V and density ρ at high pressure was applied to a study of the phase transition of unilamellar liposomes consisted of dimyristoylphosphatidylcholine (DMPC) in distilled water. The compressibility B was obtained from the relation B=1/ρ V². With raising the temperarure at the atmosphere, the limiting density number [ρ]=lim (ρ-ρ₀)/ρ₀c decreased largely and the intrinsic compressibility [B]=lim (B-B₀)/B₀c had a break at 25°C. Such a transition was also observed at constant temperatures with varying the pressure. The temperature dependence of the transition pressure determined from the break pressure of [B] was ca. 50 atm/K, that agrees with the value of 49 atm/K referred in a paper of the quasielastic light scattering measurement. With the increase of the temperarure, the change in [ρ] associated with the transition slightly increased and the value of [B] at the break point decreased, which suggests that the second order transition could not be caused by the pressure increase. These experimental results agree semi-quantitatively with the calculated phase transition behavior reported in the preceding paper by using the phenomenological theory proposed by Sugár.

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