Bilayer phases in aqueous mixtures of dodecylpentaoxyethylene glycol monoether (C₁₂E₅) and sodium decyl sulfonate (C₁₀SO₃Na)

When small amounts of the anionic surfactant sodium decyl sulfonate (C₁₀SO₃Na) are mixed with the non-ionic surfactant dodecylpentaoxyethylene glycol monoether (C₁₂E₅) in water, the ionic surfactant has a profound influence on the equilibrium between lamellar (L_α) and L₃ phases. C₁₂E₅ in water forms a highly swollen lamellar phase at elevated temperatures that is in equilibrium with an L₃ phase at lower surfactant concentrations. The swollen lamellar phase is stabilized by steric interactions caused by bilayer undulations. The addition of anionic surfactant ends the swelling of the L_α phase at higher surfactant concentrations, and the L₃ phase is replaced by a new bilayer phase. In the mixed surfactant L_α phase, bilayer undulations are inhibited by long-range electrostatic repulsions, and the L₃ phase appears to be destabilized by changes in the preferred curvature of the bilayer as charge is added. To determine microstructure and aggregate interactions, small-angle neutron scattering (SANS) and static light scattering (SLS) experiments were performed as a function of temperature and added anionic surfactant. The influence of a steady increase in electrostatic interactions can be seen clearly in the SANS spectra from mixed micellar solutions. The swelling behaviour and estimates of the bilayer width can be extracted from combined SANS and SLS spectra for the lamellar phase and the new bilayer phase.