Formation and growth of anionic vesicles followed by small-angle neutron scattering

We present the first kinetic small-angle neutron-scattering experiments carried out on the instrument D22 (Institut Laue Langevin, Grenoble) with a stopped-flow apparatus (Bio-Logic company). D22 offers unique features for real-time experiments. The high flux and the large q range covered in only one instrumental configuration combined with the rapid electronics open up possibilities for few hundred millisecond resolution measurements. We have used these technical developments to study the formation and growth of spontaneous vesicles after addition of salts (NaCl, NaBr, KCl, and KBr) in a micellar solution of ACT in D2O from 500 ms to 5 h after mixing. The vesicle radii and the growth rate depend on the salt concentration and decrease with increasing ionic strength. The driving force of the transition is the screening of the electrostatic repulsion between adjacent surfactant headgroups that favors formation of a locally planar bilayer. Assuming that the aggregation is controlled by micelle diffusion, a simple kinetic approach predicts that the average radius increases with the power law R proportional to t(1/6), in close agreement with the experimental data.