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@ARTICLE{vanderLoop:840072,
      author       = {van der Loop, Tibert H. and Ottosson, Niklas and Vad,
                      Thomas and Sager, Wiebke and Bakker, Huib J. and Woutersen,
                      Sander},
      title        = {{C}ommunication: {S}low proton-charge diffusion in
                      nanoconfined water},
      journal      = {The journal of chemical physics},
      volume       = {146},
      number       = {13},
      issn         = {1089-7690},
      address      = {Melville, NY},
      publisher    = {American Institute of Physics},
      reportid     = {FZJ-2017-07636},
      pages        = {131101 -},
      year         = {2017},
      abstract     = {We 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)).},
      cin          = {PGI-5},
      ddc          = {540},
      cid          = {I:(DE-Juel1)PGI-5-20110106},
      pnm          = {143 - Controlling Configuration-Based Phenomena (POF3-143)},
      pid          = {G:(DE-HGF)POF3-143},
      typ          = {PUB:(DE-HGF)16},
      pubmed       = {pmid:28390345},
      UT           = {WOS:000399073300003},
      doi          = {10.1063/1.4979714},
      url          = {https://juser.fz-juelich.de/record/840072},
}