Characterization of mixed micelles of structural isomers of sodium butyl benzene sulfonate and sodium dodecyl sulfate by SANS, FTIR spectroscopy and NMR spectroscopy

The mixed systems of sodium dodecyl sulfate (SDS) and sodium salts of n-butyl benzene sulfonate (NBBS), iso-butyl benzene sulfonate (IBBS) and tert-butyl benzene sulfonate (TBBS), as hydrotropes, are studied by Small Angle Neutron Scattering (SANS), Fourier Transform Infrared (FTIR) spectroscopy and Nuclear Magnetic Resonance (NMR) spectroscopy for the effect of hydrotrope concentration, hydrotrope structure, and temperature on the SDS micelles. The mixed micelle formation of SDS with NBBS is confirmed with the contrast matching technique. NBBS when mixed with SDS replaces some of the SDS molecules from the micelle affecting its size. The hydrotrope structure shows a prominent effect on the dimensions of the mixed micelle. The behavior of NBBS, in the SDS-NBBS mixtures, is different from that of NaCl and is not the same as the salt effect. Higher temperatures result in the formation of smaller SDS-NBBS mixed micelles in the solutions. The FTIR spectroscopy results indicate enhanced gauche/trans conformer ratio of SDS in the mixed micelles unlike the reduction of the ratio in the mixed micelles of a cationic surfactant with NBBS. The up-field chemical shifts of the aromatic protons of the hydrotrope and the methylene protons of surfactant adjacent to ionic headgroup indicate that the hydrotrope resides near the surface of the SDS micelle.