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Small-Angle Neutron Scattering of Aqueous SrI2 Suggests a Mechanism for Ion Transport in Molecular Water

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Abstract

X-ray and neutron scattering have been used to provide insight into the structures of ionic solutions for over a century. Most of the structures discussed in the literature cover distances less than 8 Å. Outside that distance, single scattering bands have, however, been seen. For the non-hydrolyzing salt SrI2 in aqueous solution, a structure sufficient to scatter slow neutrons persists down to a concentration of at least 0.1 mol·L−1 at which the measured average distance between scatterers is over 18 Å. Over the concentration range of 1 to 0.1 mol·L−1, the full distribution of the distances between the scatterers remains within only about 10 Å, the size of an ion and its first hydration shell. This measurably correlated structure of the ions in the solution appears to hold because changes in hydration (and interior distances along any single spatial dimension) require displacements near the size of a water molecule. Together, these facts support a rotatory mechanism for simultaneous ion transport and water countertransport. A formula is presented for calculating the average, equally spaced interscatterer distance as a function of concentration. Using this relationship, the experimental results are interpreted as showing increasing ion association as the salt concentration is raised.

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Acknowledgments

Thanks are due for valuable discussions with John Barker, Craig Brown, Jack Douglas, Boualem Hammouda, Joseph Hubbard, Steve Kline, Susan Krueger, Yun Liu, and Dan Neumann. This work benefitted from SASView software, originally developed by the DANSE project under NSF award DMR-0520547.

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  1. NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD, 20899, USA

    Kenneth A. Rubinson

  2. Department of Biochemistry and Molecular Biology, Wright State University, Dayton, OH, 45435, USA

    Kenneth A. Rubinson

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  1. Kenneth A. Rubinson
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Correspondence to Kenneth A. Rubinson.

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Rubinson, K.A. Small-Angle Neutron Scattering of Aqueous SrI2 Suggests a Mechanism for Ion Transport in Molecular Water. J Solution Chem 43, 453–464 (2014). https://doi.org/10.1007/s10953-014-0148-5

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