FT-IR investigation of the urea state in AOT reversed micelles

FT-IR spectra of the urea/sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/CCl4 system as a function of the urea-to-AOT molar ratio (R-urea) at a fixed AOT concentration ([AOT] = 0.1 mol . kg(-1)) have been recorded at 25 degreesC. The experimental data are consistent with the hypothesis that at low Rurea values urea is dispersed as monomers among the hydrophilic core of AOT reversed micelles, whereas at R-urea > 0.13 it is encapsulated as small molecular clusters. These clusters display some departures from the H-bonded structure of the pure solid urea attributable to their small size and to urea-AOT head group interactions. The stretching modes of AOT sulfonate groups are affected by the presence of urea, indicating specific urea-AOT head group interactions. On the other hand, analysis of the AOT CO stretching band provides evidence that these groups are not significantly involved in urea-AOT interactions. (C) 2001 Academic Press.