Phase transitions in phosphatidylcholine dispersion observed with an interference refractometer

An interferometer is used to measure the refractive index change accompanying the crystal-to-liquid-crystal phase transition in the dispersion of phosphatidylcholines. Two separate methods of obtaining the refractive index change are employed: the first method analyzes the intensity transmitted through a spatial filter and the second method utilizes a piezoeletric crystal-based electronic compensator. The results of the two methods agree well. The accuracy of the apparatus (6 X 10(-6)) permitted us to use a very dilute sample to detect the phase change. Only a fraction of a milligram of dry lecithin is needed to observe the change. The result confirms conclusively that the major reason for the turbidity change at the transition temperature is the alteration in the refractive index of the lipid membranes. The fractional change in the refractive index does not agree well with the fractional change in the density of lipid molecules in vesicles.