On the characterization of NaDEHP/n-heptane nonaqueous reverse micelles: the effect of the polar solvent.

The behavior of two polar solvents, ethylene glycol (EG) and dimethylformamide (DMF), entrapped in sodium bis-(2-ethylhexyl) phosphate (NaDEHP)/n-heptane reverse micelles (RMs) was investigated using dynamic light scattering (DLS), molecular probe absorption and FT-IR spectroscopy. DLS results reveal the formation of RMs containing EG and DMF as a polar component. To the best of our knowledge this is the first report where both polar solvents are entrapped by the NaDEHP surfactant to effectively create RMs. We use the solvatochromism behavior of the molecular probe, 1-methyl-8-oxyquinolinum betaine (QB), and FT-IR spectroscopy to investigate the physicochemical properties of the non-aqueous RMs. Our results demonstrate that the NaDEHP surfactant interacts through hydrogen bonds with EG at the EG/NaDEHP interface and this interaction is responsible for destroying the bulk structure of pure solvent EG when entrapped in NaDEHP RMs. On the other hand, when DMF is incorporated inside the RMs the bulk structure of DMF is destroyed upon encapsulation by the Na-DMF interaction at the DMF/NaDEHP interface. Our results are completely different than the one observed for DMF/n-heptane/AOT. Our results show how the physicochemical properties, such as micropolarity, microviscosity and hydrogen bond interaction, of nonaqueous NaDEHP/n-heptane RMs interfaces can be dramatically changed by simply using different non-aqueous polar solvents. Thus, these results can be very useful to employ these novel RMs as nanoreactors since the dimensions of the RMs are around 10 to 20 nm.

Comparison between two anionic reverse micelle interfaces: the role of water-surfactant interactions in interfacial properties.

The water/sodium bis(2-ethylhexyl) phosphate (NaDEHP) reverse micelle (RM) system is revisited by using, for the first time, molecular probes to investigate interface properties. The solvatochromic behavior of 1-methyl-8-oxyquinolinium betaine (QB) and 6-propionyl-2-(N,N-dimethyl)aminonaphthalene (PRODAN) in the water/NaDEHP/toluene system is studied, and the results are compared with those obtained in water/sodium 1,4-bis(2-ethylhexyl) sulfosuccinate (AOT)/toluene RM media. The results demonstrate that the micropolarity, microviscosity, interfacial water structure, molecular probe partition, and intramolecular electron-transfer processes are dramatically altered for NaDEHP RM interfaces in comparison to the AOT systems. Because of organic nonpolar solvent penetration into the interface, NaDEHP RM media offer an interface with lower micropolarity and microviscosity than AOT media. Also, the interfacial water in the NaDEHP system shows enhanced water-water hydrogen-bond interaction in comparison with bulk water. The AOT RM interface represents a unique environment for PRODAN to undergo dual emission.

Characterization of multifunctional reverse micelles' interfaces using hemicyanines as molecular probes. II: Effect of the surfactant.

In this work, we have investigated the behavior of the cationic hemicyanine trans-4-[4-(dimethylamino)-styryl]-N-methylpyridinium iodide (HC) in benzene/benzyl-n-hexadecyl dimethylammonium chloride (BHDC)/water reverse micelle media using absorption and emission spectroscopy in addition to the steady-state and time-resolved fluorescence emission techniques and compare the results to those obtained in benzene/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT)/water reverse micelle media (Moyano, F.; et al. J. Phys. Chem. B 2009, 113, 4284.) in order to gain more insight about reverse micelle interface properties. Our results show that HC spectroscopic behavior is completely different when dissolved in AOT or in BHDC reverse micelle media. While the dye experiences an intramolecular charge-transfer process upon excitation in the former media, in BHDC, this process is inhibited because of the cationic nature of the surfactant. Interestingly, we also show that the water properties are different for water molecules sequestrated inside of an anionic and cationic reverse micelle system. This come out because the water molecules entrapped inside of the BHDC reverse micelle media appear to be non-electron-donating because of its interaction with the cationic surfactant polar head group. On the other hand, the water molecules sequestrated inside of the AOT reverse micelle systems show its electron-donor ability enhanced in comparison with its water bulk structure. These results could also explain the lack of nucleophilicity shown by the water molecules entrapped in BHDC reverse micelle media reported in previous kinetic studies.

Characterization of multifunctional reverse micelles' interfaces using hemicyanines as molecular probes. I. Effect of the hemicyanines' structure.

In this work, we report the behavior of two different hemicyanines, trans-4-[4-(dimethylamino)styryl]-N-methylpyridinium iodide (HC) and 4-[4-(dihexadecylamino)styryl]-N-methylpyridinium iodide (DIA), in water/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT)/benzene reverse micelles media using absorption and emission spectroscopy in addition to the steady-state and time-resolved fluorescence emission techniques. Our results show that the AOT reverse micelles interface has the nontrivial deaggregation property, a result that may have potential application for the preparation of dye lasers, which require a noninteracting monomeric form of the dye. Also, we show that the water interacts with a different region of the AOT moiety depending on the external organic solvent used and, in addition, we also present a nice, simple, and noteworthy method that helps to examine the presence or the absence of organized media. In conclusion, our results show that HC and DIA are powerful dyes to characterize simultaneously different interfaces' properties as they can be used to sense, at the same time, fluidity and specific interactions at the interface. These results are important because those properties are the key for molecular recognition.