A Raman spectroscopic study of the Aerosol OT-water system

Abstract

A detailed Raman spectroscopic study of the bis-(di-2-ethylhexyl)sodium sulphosuccinate (Aerosol OT)-water system has been carried out at ambient temperature (20 ± 2° C). Marked changes were observed in the relative intensity of the band at 1460 cm −1 when the concentration of Aerosol OT was varied. Depolarisation ratio measurements on the viscous isotropic cubic phase indicate that this intense band at 1460 cm −1 is largely associated with the antisymmetric methylene rocking/deformation modes. Moreover, this was the only spectral region to vary with Aerosol OT concentration. A plot of I 1460/I 1734 versus Aerosol OT concentration indicated that the most striking changes in the intensity of the 1460 cm −1 band occurred in the regions where large changes or anomalies have been previously observed (a) in the observed X-ray spacings for one-dimensional swelling, (b) in the conductivity and (c) in the optical birefringence. The results of this Raman spectroscopic study therefore indicate the following. 1. (a) The methylene deformation region of the Raman spectrum at 1400–1500 cm −1 gives direct information on the chain packing arrangements of the hydrocarbon chains in the molecule in the liquid crystalline phase, as has previously been shown in the crystalline phase. Thus Raman spectroscopy is very useful for detecting changes in the hydrocarbon chain conformation within the lamellar (and possibly other) mesophase(s). 2. (b) The physical changes observed within the lamellar mesophase are associated with a chain reorganisation whereby the two branches of the hydrocarbon chains may separate in the concentration regions where the intensity ratio I 1460/I 1734 versus Aerosol OT content is a minimum. Thus, either chain reorientation occurs in several regions of the lamellar mesophase or the changes observed by the various physical techniques mentioned above may show possible phase transitions occurring within the lamellar region previously believed to be homogeneous.