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 spectr a (as determined by the generalized polarization, GP) with the water conten t of the microemulsion and the characteristics of the emission and excitati on GP spectra in order to assess the properties of the average microenviron ments sensed by the microaggregate-bound probes. Partition constants (K, mo lar concentration units) were evaluated from the change in fluorescence int ensity (at a fixed probe concentration) with the AOT concentration (at fixe d W = (water)/(AOT) values). For W values greater than or equal to 10, a va lue K = 1.0 +/- 0.2 M-1 was obatined, both for laurdan and prodan. This res ult indicates that neither the length of the probe hydrophobic tail nor the size of the water pool significantly modifies the affinity of the probe fo r the microaggregates. Even lower values of K were obtained at lower W valu es. 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 be tween 10 and 20. All these results indicate that probes associated to the m icroaggregates are located at the interface and that the properties of this region remain almost constant after total surfactant head hydration. GP va lues and CP spectra of laurdan molecules associated to the fully hydrated m icroaggregate interface are similar to those reported in phospholipid vesic les in the liquid crystalline state. As in this system, and despite their l ocalization at the interface, laurdan molecules do not constitute a homogen eous population.