Structure and dynamics of reverse micelles containing supercooled water investigated by neutron scattering

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.