Dynamics of structural transitions in amphiphilic systems monitored by scattering techniques

Abstract

Structural changes in amphiphilic systems can be triggered by mixing them with other surfactants or additives. In the described experiments such mixing was done rapidly by means of a stopped-flow device and followed by small-angle neutron scattering (SANS) or small-angle X-ray scattering (SAXS) as a structural detection method. In such experiments we followed the formation of unilamellar vesicles in a catanionic surfactant mixture and for an ionic surfactant solution upon the addition of a cosurfactant. For both cases slow formation of monodisperse unilamellar vesicles is observed that takes place in a way purely governed by diffusion, i.e. without the effect of external forces. The formation proceeds via disk-like intermediates in the first case and via rod-like intermediates in the second case. In another investigation the solubilisation of methyl heptanoate into a zwitterionic surfactant was studied, where the initially present emulsion droplets are slowly transformed into well-defined microemulsion droplets. However, this process does not follow the static phase diagram, in which a lamellar phase is present at intermediate concentrations, which is not observed as an intermediate in the solubilisation process that goes directly to the formation of microemulsion droplets. Finally the disintegration of micelles was followed as it occurs when mixing the micellar solution with a bad solvent for the micelles. Here it is observed that the disintegration process does not occur in a simple dissolution step but in a more complex way in which after passing through a minimum aggregation stage smaller micellar structures are reformed. In summary it can be stated that the stopped-flow method coupled to SANS/SAXS detection allows to obtain detailed information about the dynamics of structural transitions in amphiphilic systems with a time-resolution of down to 5–50 ms.