Phase transformations in lipids follow classical kinetics with small fractional dimensionalities

Abstract

The kinetics of the gel-to-crystal phase transformation have been studied dilatometrically for lipid dispersions of 1,2-palmitoyl-sn-glycerol-3-phosphatidylcholine (DPPC). A new crystallization procedure is introduced in which the sample is first briefly quenched to form a small population of nuclei followed by jumping to temperatures at which negligible nucleation takes place but at which growth of nuclei occurs. Kinetic data obtained this way fit the classical Kolmogorov-Avrami relation very well over all measured times, up to 95% crystallization, but the effective dimensionality of the growing domains is only n=1.0–1.3. For the same system, differential scanning calorimetry shows that the data for the crystal-to-gel phase transformation are well fit with n∼1.1 and the data for the gel-to-ripple phase transformation are well fit with n∼0.8. Among the different possible causes of the small value of n for the two transformations involving the gel and crystal phases, the most viable focuses upon circular domains growing within each bilayer and the finite-size effects present in particular geometry of lipid dispersions. The fact that the gel-to-ripple transformation has a pronounced one-dimensional character accounts for its n value being even smaller.