PHASE-STRUCTURE OF POLY(OXYETHYLENE) SURFACTANTS IN WATER STUDIED BY FLUORESCENCE QUENCHING

Binary systems of water and nonionic surfactants of the dodecyl poly(oxyethylene) type, C12H25-(OCH2-CH2)mOH(C12Em), have been studied by time-resolved fluorescence quenching (TRFQ). The interest has been focused on the diffusion-influenced quenching in liquid crystalline phases of the C12E4, C12E5, C12E6, and C12E8 systems and the ability of the TRFQ method to discriminate between different quenching geometries. The relative diffusion coefficients of pyrene and 3,4-dimethylbenzophenone have been measured in rod-shaped and lamellar aggregates in aqueous solutions as well as in neat surfactants. A new method, based on fluorescence quenching, for determination of layer thicknesses in lamellar systems is introduced and the results obtained are shown to be in close agreement with those from low-angle X-ray diffraction. In addition, we propose a method for determination of cylinder radii in rodlike micelles; even this method seems to produce values very similar to X-ray data. One attractive feature of the TRFQ method is that it does not require any liquid-crystalline order in the sample; it works equally well for aggregates in isotropic L1 phases, in which the occurrence of spherical, cylindrical, and disk-shaped micelles is demonstrated. For both rodlike and lamellar aggregates a slight increase in thickness with temperature is detected, which can be related to a decrease in effective surfactant head-group area, caused by a change in polarity of the ethylene oxide groups. The results of fluorescence quenching measurements in cubic, phases show that the bicontinuous V1 phase is essentially two-dimensional on the time scale of fluorescence, whereas for the I1 phase further evidence is given for the idea that the building stones in the I1 phase are small, slightly elongated, micelles with an axial ratio of approximately 1.5:1 for the C12E8-water system. The quenching in the spongelike L3 phase proves to be more efficient than in the L(alpha) phase, and the possible reasons for this are also discussed.