We study water-in-oil microemulsions, in particular dispersions of water droplets coated with a monolayer of the anionic surfactant AOT in a continuous phase of n -decane. Upon addition of the amphiphilic triblock copolymer PEO(polyethylenoxide)-PI(polyisoprene)-PEO, a transient network is formed. At constant droplet size we vary the polymer concentration and there is clear evidence for an increasing crosslinking of the droplets from structural investigations with small-angle x-ray scattering. The dynamics of concentration fluctuations consisting of the translational diffusion of the droplets and the relaxation of the network are monitored with photon correlation spectroscopy. We mainly focus on the variation of the dynamic behavior as a function of the number of polymer molecules per droplet and the droplet volume fraction, which may be taken as a measure for the interdroplet distance. With increasing polymer content the dynamics of the system slows down and three different relaxation processes may be distinguished. We discuss the origin of the different relaxation modes. In particular, it turns out that the intermediate relaxation mode may be suppressed by index matching the oil matrix and the PI block and that it is effectively slowed down by an additional loading of the emulsion droplets with polyethylene glycol of increasing molecular weight.

We present a detailed neutron scattering study of the structure, shape fluctuations, and translational diffusion of microemulsion droplets at low temperatures. We investigate the ternary microemulsion D2O , AOT [bis(2-ethyl-hexyl) sulfosuccinate], and toluene-d8 (or heptane-d16) which forms spherical water droplets surrounded by a monolayer of AOT dispersed in oil around room temperature. At T=290 K , varying the molar ratio omega of water to AOT between 3 and 12, we find using small angle neutron scattering water core radii R_{c} between 7 and 18 A , respectively. We characterize the structure at low temperatures down to T=220 K . Upon cooling the droplet structure is maintained and R_{c} stays roughly constant down to temperatures where the confined water is deeply supercooled. At an omega -dependent temperature T_{s} we observe for all compositions a shrinking of the droplets, which depends on the initial droplet size: the smaller the initial radii, the lower the T_{s} is. At the lowest investigated temperature T=220 K we find an omega -independent remaining water core corresponding to a number of about 2 water molecules per AOT molecule. Neutron spin-echo spectroscopy is used to monitor shape fluctuations and translational diffusion for one microemulsion ( omega=8 , R_{w}=12 A ) from T=300 K down to temperatures below the corresponding shrinking temperature T_{s} . Thereby we determine the bending elasticity to be kappa=0.3k_{B}T over the whole investigated temperature range where the droplets are stable. From these results we cannot establish a link between surfactant membrane elasticity and low temperature structural instability of the droplets. Moreover, our results show that reverse AOT micelles are an excellent tool for the study of soft confined water over a broad range of confining sizes and temperatures down to the supercooled state.

No abstract

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 R c z 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 # T # 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.