VIBRATIONAL-SPECTRA OF METAL-SALTS OF BIS(2-ETHYLHEXYL)SULFOSUCCINATE(AOT)

The vibrational spectra of sodium bis(2-ethylhexyl)sulfosuccinate (NaA OT), and NaAOT exchanged with alkali-metal (Li+, K+, Rb+, Cs+), Ba2+ a nd Ce3+ and tetraphenylarsonium (TPA(+)) cations have been investigate d. The effect of the charge-to-radius ratio of the counterion on the s ymmetric and antisymmetric sulfonate stretching modes has been studied to obtain a more comprehensive assignment of these modes for NaAOT, T he symmetric sulfonate stretching mode shifted to lower wavenumber wit h increasing counterion radius, due to a decrease in the interaction b etween the cation and the SO3- group. A difference was observed betwee n the IR and Raman spectra for the wavenumber of the symmetric sulfona te stretching mode, which was attributed to correlation coupling. This difference also decreased with increasing counterion radius. In the l imiting case involving the large (TPA(+)) counterion, the symmetric su lfonate stretching mode appeared at the same wavenumber in the Raman a nd IR spectra. The spectrum of a model compound, bis(2-ethylhexyl)succ inate (i.e. NaAOT without the sulfonate moiety), was used to assign ba nds in the 1300-1100 cm(-1) region of the IR spectrum of NaAOT. This r egion consists of C-C and C-O stretching modes associated with the suc cinate backbone, and non-degenerate, antisymmetric stretching modes of the SO3- group. The correlation splitting yields four antisymmetric s ulfonate stretching modes. The splitting decreases with increasing cou nterion radius, due to a reduction in the interaction between the cati on and the SO3- group. The doubly degenerate, antisymmetric sulfonate stretching mode appeared as a single, broad band in the spectrum of (T PA)AOT. The carbonyl stretching mode of NaAOT appears as an asymmetric band between 1720 and 1740 cm(-1). This asymmetry was attributed to d ifferent conformations about the acyl C-C bond. Weak interactions betw een the Na+ counterion and the carbonyl group may also contribute to t he intensity of the lower-wavenumber band.