title: Influence of confined water on the photophysics of dissolved solutes in reverse micelles.
abstract: The photophysical parameters of two probes with largely different hydrophobic character, namely, coumarin 1 and coumarin 343, are investigated in sodium bis-(2-ethylhexyl)sulfosuccinate (AOT)/hexane/water reverse micelles at various water/AOT molar ratio w(0). Correlation of photophysical parameters such as fluorescence quantum yield, fluorescence lifetime, and emission maxima with w(0) indicate distinctly different trends below and above w(0) approximately 7 for both probes. The variation of the average rotational correlation times obtained from fluorescence anisotropy decays for both probes in reverse micelles further corroborate the above observation. Similar studies were also performed in nonaqueous reverse micelles with acetonitrile as polar solvent. Similar to aqueous reverse micelles, breaks in the photophysical parameters with increasing acetonitrile/AOT molar ratios w'(0) were also observed in these cases, although at a much lower w'(0) value of 3. The present results indicate that around w(0) approximately 7 for aqueous reverse micelles (and around w'(0) approximately 3 for nonaqueous reverse micelles) a distinct change occurs in the probe microenvironment, which is rationalized on the basis of the relative populations of interfacial and core water. We propose that until the ionic head groups and counterions are fully solvated by polar solvents, that is, up to w(0) approximately 7 (or w'(0) approximately 3), the interfacial water population dominates. Above these molar ratios coalescence of excess water molecules with each other to form truncated H-bonded water clusters leads to a sizable population of core water. This is further substantiated by changes in the IR absorption spectra for the O--D stretching mode of diluted D(2)O in reverse micelles with varying w(0). Critical comparison of the present results with relevant literature reports provide clear support for the proposals made on water structure in reverse micelles. The role of relative size of the probe and the reverse micelles for differences in polar solvent to AOT ratios (w(0)=7 and w'(0)=3) in the observed breaks in the two types of reverse micelles is also discussed.
authors.affiliation: Analytical Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India.
authors.forename: Ashis Kumar
authors.initials: AK
authors.lastname: Satpati
authors.forename: Manoj
authors.initials: M
authors.lastname: Kumbhakar
authors.forename: Sukhendu
authors.initials: S
authors.lastname: Nath
authors.forename: Haridas
authors.initials: H
authors.lastname: Pal
mesh_terms: D003374:Coumarins / Q000737:chemistry*; D041961:Deuterium Exchange Measurement; D005453:Fluorescence; D005454:Fluorescence Polarization; D057927:Hydrophobic and Hydrophilic Interactions; D008823:Micelles; D015335:Molecular Probes / Q000737:chemistry*; D012997:Solvents; D014867:Water / Q000737:chemistry*
chemical_list: D003374:Coumarins; D008823:Micelles; D015335:Molecular Probes; D012997:Solvents; D014867:Water; C030123:coumarin