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000840072 1001_ $$0P:(DE-HGF)0$$avan der Loop, Tibert H.$$b0
000840072 245__ $$aCommunication: Slow proton-charge diffusion in nanoconfined water
000840072 260__ $$aMelville, NY$$bAmerican Institute of Physics$$c2017
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000840072 520__ $$aWe investigate proton-charge mobility in nanoscopic water droplets with tuneable size. We find that the diffusion of confined proton charges causes a dielectric relaxation process with a maximum-loss frequency determined by the diffusion constant. In volumes less than ∼5 nm in diameter, proton-charge diffusion slows down significantly with decreasing size: for diameters <1nm, the diffusion constant is about 100 times smaller than in bulk water. The low mobility probably results from the more rigid hydrogen-bond network of nanoconfined water, since proton-charge mobility in water relies on collective hydrogen-bond rearrangements.The transport of protons through nanometer-sized volumes of liquid water occurs in systems ranging from porous minerals,1 fuel-cell membranes,2–4 metal-organic frameworks5–7 and zeolites,8 to the living cell. In contrast to proton diffusion in bulk water which has been studied extensively,9–13 comparatively little is known about proton transfer in such nanoscopic volumes. 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.14,15 It was however also found15 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 < 5nm. 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. Nano-confined proton mobility has also been investigated using quasi-elastic neutron scattering16–19 and nuclear magnetic resonance spectroscopy.20,21 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.22–24 Here, we probe the mobility of aqueous proton charges in nano-confinement directly by observing their response to an externally applied oscillating electric field. To investigate proton-charge transport in confinement, we prepare nanoscopic water volumes in self-assembling reverse micelles in cyclohexane (see supplementary material). We use a nonionic surfactant (Igepal CO-520) to avoid interfacial charge effects25–27 and size-dependent surfactant counter-ion concentrations. Igepal contains hydroxy and ether O atoms, which have pKb∼16and 18, so protons do not “stick” to the surfactant.Since reverse micelles can take up water molecules in their hydrophilic interior, the ratio w0 = [H2O]/[surfactant] can be used to tune the size of the enclosed water volumes.28,29 We use small-angle x-ray scattering to characterize the structure of the reverse micelles.30,31 The investigated reverse micelles have a spherical shape, a polydispersity parameter <0.2, and interact as hard spheres. We observe a linear size dependence on w0, with a proportionality factor of 0.42 ± 0.01 nm for the water pool diameter (Fig. 1(a)). To study ions in nanoconfined water, we use appropriate aqueous solutions in the preparation of the reverse micelles. Using 1M HCl or LiCl as the interior aqueous phase has no influence on the shape or size of the reverse micelles (Fig. 1(b)).
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000840072 7001_ $$0P:(DE-HGF)0$$aOttosson, Niklas$$b1
000840072 7001_ $$0P:(DE-HGF)0$$aVad, Thomas$$b2
000840072 7001_ $$0P:(DE-Juel1)130932$$aSager, Wiebke$$b3$$ufzj
000840072 7001_ $$0P:(DE-HGF)0$$aBakker, Huib J.$$b4
000840072 7001_ $$0P:(DE-HGF)0$$aWoutersen, Sander$$b5
000840072 773__ $$0PERI:(DE-600)1473050-9$$a10.1063/1.4979714$$gVol. 146, no. 13, p. 131101 -$$n13$$p131101 -$$tThe journal of chemical physics$$v146$$x1089-7690$$y2017
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