Small-tingle neutron scattering (SANS) study of vesicles and lamellar sheets formed from mixtures of an anionic and a cationic surfactant

The various bilayer structures formed from aqueous mixtures of an anionic ( SDS) and a cationic surfactant (DTAB) with identical hydrocarbon C-12 chain s at 40 degrees C have been investigated using small-angle neutron scatteri ng (SANS) as well as static light scattering (SLS). The SANS data were anal yzed using a paracrystal lamellar model with respect to the layer distance distribution and the number of layers in a single cluster. Unilamellar or o ligolamellar vesicles form in the most diluted samples in the absence of ad ded salt where the number of layers in a single cluster is found to be 1-3. Beyond the compositions where micelles form (30:70 < [SDS]:[DTAB] < 70:30) , we observe a transition from vesicles to stacks of lamellar sheets upon i ncreasing the overall surfactant concentration, indicated by an abrupt incr ease of the number of layers in a single cluster from 1-3 to infinity. Comb ined SANS and SLS data for samples containing vesicles ([SDS] + [DTAB] = 0. 25 wt % in the absence of added salt) could be fitted with a model for unil amellar vesicles using a structure factor for sticky hard spheres, indicati ng that the vesicles attract each other and form clusters. However, at [SDS ] + [DTPB] = 0.125 wt %, the vesicles appeared to be too large for the size distribution to he determined from our SANS and SLS data. in 0.1 M NaBr, t he vesicles were clearly destabilized and either micelles or lamellar sheet s form at most compositions and concentrations where vesicles predominate i n the absence of added salt.