Studies on the phase behavior of perfluoropolyether carboxylic acids and their salts: Observation of a stunning temperature-dependent color effect for the birefringence in the pyridinium salt system

The phase behavior of a series of perfluoropolyether (PFPE) carboxylic acids and their salts (tetramethyl ammonium and pyridinium) was studied. The small solubility of the pure acids in water could be significantly enhanced by preparation of these salts that proved to be highly surface active. The most interesting result of our studies was a temperature-dependent color effect that appears for a multilamellar vesicle phase of a PFPE pyridinium salt between crossed polarizers. That effect is caused by a wavelength dependence of the birefringence in this system (dispersion) and the change in the dispersion with temperature is probably due to a change in the adsorption of the pyridinium ions to the surface of the surfactant aggregates.