Fluorescence of prodan and laurdan in AOT/heptane/water microemulsions:: Partitioning of the probes and characterization of microenvironments

The partitioning of laurdan and prodan between the external solvent and the microaggregates in AOT/ heptane/water water-in-oil microemulsions has been measured. We have also evaluated the dependence of the fluorescence spectra (as determined by the generalized polarization, GP) with the water content of the microemulsion and the characteristics of the emission and excitation GP spectra in order to assess the properties of the average microenvironments sensed by the microaggregate-bound probes. Partition constants (K, molar concentration units) were evaluated from the change in fluorescence intensity (at a fixed probe concentration) with the AOT concentration (at fixed W = (water)/(AOT) values). For W values greater than or equal to 10, a value K = 1.0 +/- 0.2 M-1 was obatined, both for laurdan and prodan. This result indicates that neither the length of the probe hydrophobic tail nor the size of the water pool significantly modifies the affinity of the probe for the microaggregates. Even lower values of K were obtained at lower W values. GP values strongly decrease for both probes when W increases up to ca. 10, a result that indicates an increase in water relaxation rates when the water/surfactant ratio increases up to the total surfactant head hydration. On the other hand, very small changes in GP are observed when W changes between 10 and 20. All these results indicate that probes associated to the microaggregates are located at the interface and that the properties of this region remain almost constant after total surfactant head hydration. GP values and CP spectra of laurdan molecules associated to the fully hydrated microaggregate interface are similar to those reported in phospholipid vesicles in the liquid crystalline state. As in this system, and despite their localization at the interface, laurdan molecules do not constitute a homogeneous population.