Dynamics of water confined to reverse AOT micelles

We use quasi-elastic neutron scattering (QENS) to study the dynamics of water confined inside reverse micelles. As a model system we use a water-in-oil droplet microemulsion based on the anionic surfactant AOT (sodium bis[2-ethylhexyl] sulfosuccinate), that forms spherical water droplets coated by a monolayer of AOT dispersed in the continuous oil matrix. Combining neutron time-of-flight (TOF) and backscattering (BS) spectroscopy, we access the dynamical behaviour of water over three decades in time from pico- to nanoseconds. We investigate the influence of reverse micelle size on the water dynamics by comparing two sample systems with bigger and smaller water core radii of about Rc [approximate] 12 A and 7 A. The temperature is varied over a range where both microemulsion systems are stable, from room temperature down to the region where the confined water is supercooled: 260 K [less-than-or-equal] T [less-than-or-equal] 300 K. Taking explicitly into account the previously measured diffusion of entire reverse micelles in the oil matrix we find the average mobility of the confined water to be considerably slowed with respect to bulk water. The translational diffusion decreases with decreasing reverse micelle size. Dependent on the reverse micelle size we can interpret our data by assuming two dynamically separated water fractions. We identify the faster one with bulk-like water in the middle of the core while the slower one seems to be surfactant bound water. We find that 4 molecules of water per AOT molecule are immobilized on the timescale of QENS, i.e. shorter than nanoseconds.