Raman and IR spectroscopic studies of the interaction between counterion and polar group in self-assembled systems of AOT-homologous “sodium dialkyl sulfosuccinates’'

Abstract

Headgroup–counterion interactions have been studied for a homologous series of sodium dialkyl sulfosuccinates (SDAS) with propyl, butyl, hexyl, octyl, decyl, undecyl and dodecyl chains as Aerosol-OT analogues. Raman scattering and IR absorption spectra were recorded and compared with those for dimethyl sulfosuccinate monohydrate, diethyl sulfosuccinate trihydrate and diheptyl sulfosuccinate dihydrate, whose crystal structures are known. The spectral features of the C2O and SO₃^- stretch modes directly reflect the interaction between the polar group and the Na⁺ ion and depend strongly upon the environment of hydration. The results may be summarized as follows. For the SDAS monohydrates in the solid state, there exists a strong interaction between the β C2O group and the Na⁺ ion, as a consequence of coordination of the β C2O to the Na⁺ ion, resulting in splitting of the C2O stretch modes. In particular, the common Raman (IR) bands observed at 1705–1707 (1706–1708) and 1730–1732 (1732–1733) cm^-1 may be assigned to the β C2O group coordinated to the Na⁺ counterion and the hydrated α C2O group, respectively. The extent of splitting of these bands is a measure of the strength of this C2O···Na⁺ interaction. Coordination of the β C2O to the Na⁺ ion also affects the C2O deformation modes of the O–C2O linkage. An increased hydration number and longer hydrocarbon chains induce a weak interaction between the C2O group and the Na⁺ ion. The SO₃^-···Na⁺ interaction reflects the SO₃^- stretch modes, depending upon the extent of hydration. Furthermore, for the SDAS samples in the organic and aqueous microphases, Raman (IR) bands characteristic of the C2O and SO₃^-1 groups have been used successfully to account for the interaction between the polar group and the Na⁺ ion.