VIBRATIONAL SPECTROSCOPIC STUDY OF THE STRUCTURE OF SODIUM BIS(2-ETHYLHEXYL)SULFOSUCCINATE REVERSE MICELLES AND WATER-IN-OIL MICROEMULSIONS

The structure of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles and water-in-oil microemulsions in isooctane and cyclohexane has been investigated using vibrational spectroscopy. The formation of reverse micelles resulted in an increase in the interaction between t he Na+ counterion and the -SO3- group, compared with the solid state. Hydration of reverse micelles in cyclohexane to form water-in-oil (w/o ) microemulsions led to a shift in the wavenumber of the symmetric sul fonate stretching mode from 1064 to 1050 cm(-1) in the Raman spectrum and from 1051 to 1045 cm(-1) in the infrared spectrum. The difference between the wavenumber of the band maxima in the infrared and Raman sp ectra, which decreased from 13 to 5 cm(-1) after hydration, is attribu ted to correlation coupling. The splitting of the antisymmetric sulfon ate stretching mode in the infrared spectrum, which produces two stron g bands at ca. 1212 and 1248 cm(-1) in the reverse micelles, is also r educed upon hydration. These results are interpreted in terms of the i ncrease in the separation between the Na+ counterion and the -SO3- gro up and an associated increase in the separation between the -SO3- grou ps of adjacent surfactant molecules at the micellar interface. The gre atest changes in the interaction between the sulfonate moiety and the sodium counterion occurred below w(0) = [H2O]/[AOT] = 3, with only sma ll band shifts in the spectra observed beyond w(0) = 6. No shifts were evident beyond ca. w(0) = 15. These results are consistent with a str ong hydrogen-bonding interaction involving three H2O molecules located between the oxygen atoms of the -SO3- group and a Na+ cation, and the requirement of at least six H2O molecules per AOT molecule to complet ely solvate the head-group. In contrast, hydration produced only minor changes in the relative infrared intensities of carbonyl stretching m odes at ca. 1735 and 1723 cm(-1). These observations reflect changes i n the relative populations of different conformations about the acyl C -C bond and an increase in the degree of rotation, which accompanies t he increase in the average area occupied by the head-group upon hydrat ion.