Detection of lipid phase transitions by surface tensiometry.

A technique for the detection of lipid phase transitions is described, which involves measurement of the surface tension as a function of temperature. In the case of insoluble lipids, such as dipalmitoylphosphatidylcholine (DPPC) the lipid is spread as a multibilayer film on an aqueous substrate, while in the case of water-soluble lipids such as lysophosphatidylcholine (LPC) the surface tension of aqueous sols is measured. Surface tension at the interface, is monitored using a Wilhelmy plate while the temperature is continuously varied. Discontinuities or changes in slope in the surface tension-temperature (gamma-T) curve reflect phase transitions in the lipid. In the case of DPPC, the technique has been used to demonstrate the well-known gel-liquid crystalline thermal transition. This occurs at 36-38 degrees C in the multibilayer films; in bulk DPPC-water dispersions the transition is at 41 degrees. Cholesterol has the effect of lowering the thermal transition and broadening the temperature range. In films containing DPPC-cholesterol at a molar ratio of 2:1 or less, the transition is not present. These results are in agreement with a large number of previous studies of this system. In the case of LPC sols, a phase transition at about 70 degrees was detected when the concentration of SPC was close to the critical micelle concentration (CMC) at 70 degrees. This transition appears to reflect an increase in the equilibrium constant for micelle formation at this temperature. At higher concentrations of LPC a transition at 30 degrees, corresponding to a gel-liquid crystalline transition, was also detected. A complete description of gamma as a function of concentration and temperature in the range 10(-7) to 10(-3) g cm-3 and 20 degrees to 80 degrees has been obtained for LPC sols. The CMC varies from 6 X 10(-6) g cm-3 at 20 degrees to 10(-5) g cm-3 at 80 degrees.