Dilatometric studies of the subtransition in dipalmitoylphosphatidylcholine.

The phase transition between the newly discovered low-temperature subgel phase and the gel phase of dipalmitoylphosphatidylcholine has been studied by using dilatometry. Equilibrium measurements show that the subtransition upon heating is centered at 13.5 degrees C, has a dilatometric half-width of 0.6 degree C, and comprises a specific volume change of 0.009 mL/g (about one-fourth the size of the main transition). When the gel phase is cooled, the subtransition does not occur until below 5 degrees C. The rate of formation as a function of incubation temperature for 1 degree C less than TI less than 6 degrees C was determined; it is not well fit by quantitative theories based upon homogeneous nucleation. However, some form of nucleation is present since temperature-jump studies show that once the subgel phase has started to form, it continues to grow in the range 6 degrees C less than TJ less than 12.8 degrees C. Thus, the true transition temperature lies between 12.8 and 13.5 degrees C, but nucleation of the subgel phase is severely retarded above 6 degrees C, leading to the large hysteresis observed upon cooling.