Concentrated phases of an ionic surfactant (Aerosol OT) in polar solvents

We have studied concentrated phases of Aerosol OT (AOT) in polar (water, ethylene glycol, formamide, N,N-dimethylformamide) and apolar (isooctane) solvents. We investigated two surfactant volume fractions (phi = 0.2 and phi = 0.6) with small angle x-ray scattering (SAXS), rheology and electrical conductivity experiments. AOT self-assembles differently depending on solvent type and concentration. SAXS experiments show that the AOT/water system displays a lamellar phase. In the other cases, only formamide displays a lamellar phase for phi = 0.6. The other solvents (and formamide at phi = 0.2) promote the self-assembly of AOT in other microstructures. The SAXS spectra display correlation peaks consistent with a disordered array of cylindrical aggregates. The microstructure of the AOT/isooctane system at phi = 0.2 is that of an arrangement of spherical aggregates. The results are explained in terms of surfactant packing models and solvent properties. For instance, the ability of ethylene glycol to form hydrogen bonds with AOT promotes the formation of cylindrical aggregates by increasing the area per surfactant polar head. As N,N-dimethylformamide is slightly miscible in hydrocarbons it increases the volume of the surfactant tail promoting reverse structures.