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CONTENTS 1. Introduction 4569 2. Background 4569 2.1. Aqueous Reverse Micelles 4569 2.2. Definition of Aqueous and Nonaqueous Reverse Micelles 4572 3. Characterization of Reverse Micelles 4573 3.1. Methods Demonstrating the Presence of Reverse Micelles 4573 3.2. Surfactant Packing at the Reverse Micelle Interface 4574 4. Reverse Micelles Encapsulating Nonaqueous Polar Organic Solvents 4578 4.1. Reverse Micelles Containing Glycerol 4578 4.2. Reverse Micelles Containing Other Polyols 4580 4.3. Reverse Micelles Containing Formamide 4582 4.4. Reverse Micelles Containing Other Amides: DMF and DMA 4584 4.5. Other Polar Solvents 4585 5. Requirements for Effective Reverse Micelle Formation 4586 5.1. Polar Solvent Properties 4586 5.2. Problem Systems: Methanol and Acetonitrile 4587 6. Room Temperature Ionic Liquids in Reverse Micelles 4589 7. Nonaqueous Reverse Micelles as Nanoreactors 4593 8. Future Directions and Cautions 4596 Author Information 4597 Corresponding Author 4597 Notes 4597 Biographies 4597 Acknowledgments 4598 References 4598

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The behavior of 6-propionyl-2-dimethylaminonaphthalene (PRODAN) was studied in homogeneous media and in large unilamellar vesicles (LUVs) of the phospholipid 1,2-di-oleoyl-sn-glycero-3-phosphatidylcholine (DOPC), using absorption, emission, depolarization, and time-resolved spectroscopies. In homogeneous media, the Kamlet and Taft solvatochromic comparison method quantified solute-solvent interactions from the absorption and emission PRODAN bands. These studies demonstrate that the absorption band is sensitive to the polarity-polarizability (pi) and the hydrogen bond donor ability (alpha) parameters of the media. PRODAN in the excited state is even more sensitive to these parameters and to the hydrogen bond acceptor ability (beta) of the media. The transition energy (expressed in kcal/mol) for both absorption and emission bands gives a linear correlation with the well-known polarity parameter E(T30). The results from the absorption and emission bands also reveal that PRODAN aggregates in water. The monomer has two fluorescence lifetimes, 2.27 and 0.65 ns, while the aggregate has a lifetime of 14.6 ns. Using steady-state anisotropy measurements, the calculated volumes of the aggregate and the monomer are 5590 and 222 mL mol(-1), respectively. In DOPC LUVs, PRODAN undergoes a partition process between the water bulk and the DOPC bilayer. We show that the partition constant (K(p)) value is large enough that only at [DOPC] below 0.15 mg/mL PRODAN in water can be detected. PRODAN dissolved in LUVs at [DOPC] > 1 mg/mL exists completely incorporated in its monomer form and senses two different microenvironments within the bilayer: a polar region in the interface near the water and a less polar and also less viscous environment, between the phospholipid tails. These environments were characterized by their fluorescence lifetimes (tau), showing that PRODAN in the polar microenvironment has a tau value of approximately 4 ns while in the less polar region gives a value of 1.2 ns. Moreover, this probe also senses the micropolarity of these two different regions of the bilayer and yields values similar to that of methanol and tetrahydrofuran.