Reverse micelles solvation dynamics

Dynamic light-scattering experiments or the analysis of some physicochemical properties have shown that finite amounts of formamide, A-methylformamide, AA-dimethyl-formamide, ethylene glycol, glycerol, acetonitrile, methanol, and 1,2 propanediol can be entrapped within the micellar core of AOT-reversed micelles [33-36], The encapsulation of formamide and A-methylformamide nanoclusters in AOT-reversed micelles involves a significant breakage of the H-bond network characterizing their structure in the pure state. Moreover, from solvation dynamics measurements it was deduced that the intramicellar formamide is nearly completely immobilized [34,35],... 

In the past few years, a range of solvation dynamics experiments have been demonstrated for reverse micellar systems. Reverse micelles form when a polar solvent is sequestered by surfactant molecules in a continuous nonpolar solvent. The interaction of the surfactant polar headgroups with the polar solvent can result in the formation of a well-defined solvent pool. Many different kinds of surfactants have been used to form reverse micelles. However, the structure and dynamics of reverse micelles created with Aerosol-OT (AOT) have been most frequently studied. AOT reverse micelles are monodisperse, spherical water droplets [32]. The micellar size is directly related to the water volume-to-surfactant surface area ratio defined as the molar ratio of water to AOT,... 

The observation of slow, confined water motion in AOT reverse micelles is also supported by measured dielectric relaxation of the water pool. Using terahertz time-domain spectroscopy, the dielectric properties of water in the reverse micelles have been investigated by Mittleman et al. [36]. They found that both the time scale and amplitude of the relaxation was smaller than those of bulk water. They attributed these results to the reduction of long-range collective motion due to the confinement of the water in the nanometer-sized micelles. These results suggested that free water motion in the reverse micelles are not equivalent to bulk solvation dynamics. 

Investigation of water motion in AOT reverse micelles determining the solvent correlation function, C i), was first reported by Sarkar et al. [29]. They obtained time-resolved fluorescence measurements of C480 in an AOT reverse micellar solution with time resolution of > 50 ps and observed solvent relaxation rates with time constants ranging from 1.7 to 12 ns. They also attributed these dynamical changes to relaxation processes of water molecules in various environments of the water pool. In a similar study investigating the deuterium isotope effect on solvent motion in AOT reverse micelles. Das et al. [37] reported that the solvation dynamics of D2O is 1.5 times slower than H2O motion. 

In addition, water motion has been investigated in reverse micelles formed with the nonionic surfactants Triton X-100 and Brij-30 by Pant and Levinger [41]. As in the AOT reverse micelles, the water motion is substantially reduced in the nonionic reverse micelles as compared to bulk water dynamics with three solvation components observed. These three relaxation times are attributed to bulklike water, bound water, and strongly bound water motion. Interestingly, the overall solvation dynamics of water inside Triton X-100 reverse micelles is slower than the dynamics inside the Brij-30 or AOT reverse micelles, while the water motion inside the Brij-30 reverse micelles is relatively faster than AOT reverse micelles. This work also investigated the solvation dynamics of liquid tri(ethylene glycol) monoethyl ether (TGE) with different concentrations of water. Three relaxation time scales were also observed with subpicosecond, picosecond, and subnanosecond time constants. These time components were attributed to the damped solvent motion, seg-... 

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Extensive studies in reverse micelles revealed a similar water distribution [127-130], which is consistent with the distinct water model proposed by Finer [150]. For example, when the molar ratio (wo) of water to the surfactant is 6.8 in lecithin reverse micelles with a corresponding diameter of 37 A, three solvation time scales of 0.57 (13%), 14 (25%), and 320 ps (62%) were observed using coumarin 343 as the molecular probe. At w0 = 4.8 with a 30-A water core diameter, only a single solvation dynamic was observed at 217 ps, which indicates that all water molecules are well ordered inside the aqueous pool. The lecithin in these reverse micelles have charged headgroups, which have much stronger interactions with water than the neutral headgroups of monoolein in the... 

In this chapter, the recent progress in the understanding of the nature and dynamics of excess (solvated) electrons in molecular fluids composed of polar molecules with no electron affinity (EA), such as liquid water (hydrated electron, and aliphatic alcohols, is examined. Our group has recently reviewed the literature on solvated electron in liquefied ammonia and saturated hydrocarbons and we refer the reader to these publications for an introduction to the excess electron states in such liquids. We narrowed this review to bulk neat liquids and (to a much lesser degree) large water anion clusters in the gas phase that serve as useful reference systems for solvated electrons in the bulk. The excess electrons trapped by supramolecular structures (including single macrocycle molecules ), such as clusters of polar molecules and water pools of reverse micelles in nonpolar liquids and complexes of the electrons with cations in concentrated salt solutions, are examined elsewhere. 

To comprehend the solvation dynamics results, a thorough understanding of the microemulsion structure and location of the probe within the microemulsions is necessary. Zhu et al. [140,141] characterized the water/TX-lOO/cyclohexane microemulsions. According to them TX-lOO forms anonspherical reverse micelles in cycloheaxne, with cyclohexane penetrating the polar interior of the aggregates. Gao et al. [80] have showed that with the addition of [Bmim][BF4] in TX-lOO/... 

Sarkar, N., Das, K., Datta, A., Das, S., and Bhattacharyya, K. 1996. Solvation dynamics of Coumarin 480 in reverse micelles. Slow relaxation of water molecules. 

Hazra, P. and Sarkar, N. 2002. Solvation dynamics of Coumarin 490 in methanol and acetonitrile reverse micelles. Phys. Chem. Chem. Phys. 4, 1040-1045. 

Hazra, R, Chakrabarty, D., and Sarkar, N. 2002. Intramolecular charge transfer and solvation dynamics of Coumarin 152 in Aerosol-OT, water-solubilizing reverse micelles, and polar organic solvent solubilizing reverse micelles. Langmuir 18, 7872-7879. 

Shirota, H. and Segawa, H. 2004. Solvation dynamics of formamide and N,N-d.im-ethylformamide in Aerosol OT reverse micelles. Langmuir 20, 329-335. 

H. Shirota and H. Segawa, Langmuir, 20, 329 (2004). Solvation Dynamics of Formamide and N,N-Dimethylformamide in Aerosol OT Reverse Micelles. 

P. Setua, C. Ghatak, V. G. Rao, S. K. Das, andN. Sarkar,/. Phys. Chem. B, 116, 3704 (2012). Dynamics of Solvation and Rotational Relaxation of Coumarin 480 in Pure Aqueous-AOT Reverse Micelle and Reverse Micelle Containing Different-Sized Silver Nanoparticles inside Its Core A Comparative Study. 

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