INVESTIGATION OF MICELLES, MICROEMULSIONS, AND PHOSPHOLIPID BILAYERS WITH THE PYRIDINIUM-N-PHENOLBETAINE ET(30), A POLARITY PROBE FOR AQUEOUS INTERFACES

The phenolbetaine et(30), a well-known internal probe of solvent polarity, is used to characterize the polarity of its environment in micelles, microemulsions, and phospholipid bilayers. The probe molecule, invariably solubilized in the aqueous interface, senses changes in polarity brought about by salt addition and variation of surfactant chain length and concentration, counterion, and temperature. These changes in polarity can be correlated with variations in the micellar aggregation number, signifying that the immediate probe surroundings become less polar when the micelle increases in size. With sodium dodecyl sulfate micelles a discontinuity in the variation of the et(30) value with the aggregation number is observed at 0.45 mole added NaCl, where the shape of the micelles has been reported to undergo a sphere-to-rod transition. The microemulsions behave as swollen micelles. The lowest polarity, comparable to that of 1-butanol, is observed for the phospholipid bilayers. 49 references.