(a) Linear dependence of the radius r 0 of the reverse micelle (water plus the hydrophilic head group) on w 0. Using 1M HCl or LiCl as the interior aqueous phase has no influence on the shape or size of the reverse micelles (Fig. To study ions in nanoconfined water, we use appropriate aqueous solutions in the preparation of the reverse micelles. We observe a linear size dependence on w 0, with a proportionality factor of 0.42 ± 0.01 nm for the water pool diameter (Fig. The investigated reverse micelles have a spherical shape, a polydispersity parameter <0.2, and interact as hard spheres. Zemb, in Neutrons, X-Rays and Light Scattering Methods Applied to Soft Condensed Matter, edited by P.Lindner and T.Zemb ( Elsevier, North-Holland, Amsterdam, 2002), pp. H.Chen and R.Rajagopalan ( Springer, New York, 1990), pp. de Kruif, in Micellar Solutions and Microemulsions, Structure, Dynamics, and Statistical Thermodynamics, edited by S. We use small-angle x-ray scattering to characterize the structure of the reverse micelles. Zvelindovsky ( Springer, Dordrecht, 2007), pp. Sager, in Nanostructured Soft Matter, Experiment, Theory, Simulation and Perspectives, edited by A. Micelles, Membranes, Microemulsions, and Monolayers, edited by W. Since reverse micelles can take up water molecules in their hydrophilic interior, the ratio w 0 = / can be used to tune the size of the enclosed water volumes. Igepal contains hydroxy and ether O atoms, which have p K b ∼ 16 and 18, so protons do not “stick” to the surfactant. and size-dependent surfactant counter-ion concentrations. We use a nonionic surfactant (Igepal CO-520) to avoid interfacial charge effects 25–27 25. To investigate proton-charge transport in confinement, we prepare nanoscopic water volumes in self-assembling reverse micelles in cyclohexane (see supplementary material). Here, we probe the mobility of aqueous proton charges in nano-confinement directly by observing their response to an externally applied oscillating electric field. Both these techniques, however, probe the mobility of the proton mass rather than that of the proton charge, and these mobilities can be very different due to the contribution of the Grotthuss mechanism to the proton-charge mobility. and nuclear magnetic resonance spectroscopy. Nano-confined proton mobility has also been investigated using quasi-elastic neutron scattering 16–19 16. In addition, for small reverse micelles, the size of the photoacid probe molecule becomes comparable to the water volume, which sets an intrinsic limitation to this approach. that in reverse micelles with neutral surfactants, the photoacid molecules tend to attach to the surface (where no photo-induced deprotonation occurs), which complicates the results, while in nanoscopic reverse micelles, with ionic surfactants, the counter-ion concentration is prohibitively large: typically >10M for water-pool diameters d < 5 nm. Previous work has demonstrated that the kinetics of photo-induced deprotonation and subsequent geminate recombination of photoacids can change upon nanoscopic confinement in reverse micelles. comparatively little is known about proton transfer in such nanoscopic volumes. In contrast to proton diffusion in bulk water which has been studied extensively, 9–13 9. Sauer, Proton Transfer in Zeolites ( Wiley-VCH, 2007), pp. The transport of protons through nanometer-sized volumes of liquid water occurs in systems ranging from porous minerals, 1 1.
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