SANS and rheology of elongated SDS–DDAO mixed micelles near the phase boundary

We examine the micellar phase of sodium dodecyl sulphate (SDS) and <i>N</i>,<i>N</i>-dimethyldodecylamine <i>N</i>-oxide (DDAO) in water, a synergistic anionic/amphoteric mixed surfactant system, in the vicinity of the phase boundary, employing small angle neutron scattering (SANS) and rheology. Specifically, we investigate the role of the SDS : DDAO mixing ratio at a fixed concentration at room temperature. While neat SDS and DDAO form near-spherical micelles with radius ≈20 Å, these elongate into prolates with ≈90 Å polar axis, at intermediate 60-70% mol DDAO ratios. Micellar charge remains largely invariant with a surfactant ratio up to ≤80% DDAO, decreasing thereafter towards uncharged, neat DDAO, except for a large increase in charge, and up to 4 orders of magnitude in solution viscosity (from ≈1 to in excess of 10⁴ mPa s), accompanied by scattering anisotropy, at those intermediate ratios and in 500 mM solutions. A strong correlation is found between solution viscosity and micellar dimensions (and structure factor peak) in the vicinity of the phase boundary.