Vibrational spectra of metal salts of bis(2-ethylhexyl)sulfosuccinate (AOT)

Abstract

The vibrational spectra of sodium bis(2-ethylhexyl)sulfosuccinate (NaAOT), and NaAOT exchanged with alkali-metal (Li⁺, K⁺, Rb⁺, Cs), Ba²⁺ and Ce³⁺ and tetraphenylarsonium (TPA⁺) cations have been investigated. The effect of the charge-to-radius ratio of the counterion on the symmetric and antisymmetric sulfonate stretching modes has been studied to obtain a more comprehensive assignment of these modes for NaAOT. The symmetric sulfonate stretching mode shifted to lower wavenumber with increasing counterion radius, due to a decrease in the interaction between the cation and the SO₃^– group. A difference was observed between the IR and Raman spectra for the wavenumber of the symmetric sulfonate stretching mode, which was attributed to correlation coupling. This difference also decreased with increasing counterion radius. In the limiting case involving the large (TPA⁺) counterion, the symmetric sulfonate stretching mode appeared at the same wavenumber in the Raman and IR spectra. The spectrum of a model compound, bis(2-ethylhexyl)succinate (i.e. NaAOT without the sulfonate moiety), was used to assign bands in the 1300–1100 cm^–1 region of the IR spectrum of NaAOT. This region consists of C–C and C–O stretching modes associated with the succinate backbone, and non-degenerate, antisymmetric stretching modes of the SO₃^– group. The correlation splitting yields four antisymmetric sulfonate stretching modes. The splitting decreases with increasing counterion radius, due to a reduction in the interaction between the cation and the SO₃^– group. The doubly degenerate, antisymmetric sulfonate stretching mode appeared as a single, broad band in the spectrum of (TPA)AOT. The carbonyl stretching mode of NaAOT appears as an asymmetric band between 1720 and 1740 cm^–1. This asymmetry was attributed to different conformations about the acyl C–C bond. Weak interactions between the Na⁺ counterion and the carbonyl group may also contribute to the intensity of the lower-wavenumber band.