Dynamics of water confined to reverse AOT micelles

Spehr, T.L.; Frick, B.; Zamponi, M.; Stuehn, B.

doi:10.1039/c1sm05204g

Journal Article

PreJuSER-15647

Dynamics of water confined to reverse AOT micelles

Spehr, T. L. ; Frick, B. ; Zamponi, M.FZJ* ; Stuehn, B.

2011
Royal Society of Chemistry (RSC) Cambridge

Soft matter 7, 5745 - 5755 (2011) [10.1039/c1sm05204g]

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Please use a persistent id in citations: http://hdl.handle.net/2128/7300 doi:10.1039/C1SM05204G

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 approximate to 12 angstrom and 7 angstrom. 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.

Keyword(s): Polymers, Soft Nano Particles and Proteins (1st) ; Key Technologies (1st) ; Soft Condensed Matter (2nd) ; J

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Note: We acknowledge the ILL for the allocated beam time and the co-financing of the PhD thesis of T. L. S. Financial support by the DFG (project number STU 191/4-1) is acknowledged. A portion of this research at ORNL's SNS was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

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