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 ≥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 v...