Time-resolved SANS for surfactant phase transitions

Surfactants spontaneously self-assemble in solution to form a variety of mi crostructures. Our understanding of the equilibrium properties of these agg regates, such as their shape and size, has made significant progress. Howev er, only limited information is available on the kinetics and pathways of p hase transitions and an the existence of nonequilibrium or metastable state s. Fortunately, this unsatisfactory situation can now be overcome owing to recent developments in small-angle neutron scattering (SANS) instrumentatio n which significantly improve the temporal and spatial resolution. These ne w, fascinating possibilities are illustrated by two examples. Firstly, time -resolved SANS reveals the structure of the intermediate in a spontaneous t ransition from micelles to vesicles in aqueous mixtures of lecithin and bil e salt. Secondly, a microemulsion containing spherical oil droplets is temp erature quenched into a two-phase situation where excess oil separates out. The nucleation and growth of the separating oil droplets is, again, follow ed by time-resolved SANS. (C) 2000 Elsevier Science B.V. All rights reserve d.