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The Relative Diffusive Transport Rate of Srl2 in Water Changes over the Nanometer Length Scale as Measured by Coherent Quasielastic Neutron Scattering

Published

Author(s)

Kenneth A. Rubinson, Antonio Faraone

Abstract

X-ray and neutron scattering have been used to provide insight into the structures of ionic solutions for over a century, but the probes have covered distances shorter than 8 Å. For the nonhydrolyzing salt SrI2 in aqueous solution, a locally ordered lattice of scatterers exists that scatters slow neutrons coherently down to at least 0.1 mol·L-1 concentration, where the measured average distance between scatterers is over 18 Å. To investigate the motions of these scatterers, coherent quasielastic neutron scattering (CQENS) data on D2O solutions with SrI2 at 1, 0.8, 0.6, and 0.4 mol L-1 concentrations was obtained to provide an experimental measure of the diffusion coefficient for the motion between pairs of ions relative to each other. Because CQENS measures the relative motion of one ion relative to another, the frame of reference is centered on an ion, which is unique among all diffusion measurement methods. We call the measured quantity the pairwise collective diffusion coefficient Dpc. In addition to this unique frame of reference, the diffusion coefficient can be measured as a function of interparticle distance, here Dpc versus distances from 40 Å to ~ 6 Å. (This distance range has the momentum transfer 0.2 Å- 1 ¿ q ¿ 1.0 Å-1 , where q = (4¿/¿)sin ¿ with a scattering angle of 2¿.) The measured diffusion coefficients increase with increasing distance between scatterers over the entire distance range, and Dpc is greater than the Nernst-Hartley value for an infinitely dilute solution at the longer distances. We interpret this to result from a dynamic coupling that causes an increase in diffusion rate with increasing distance between scattering pairs. For these nm-distance diffusion coefficients to conform with the lower, macroscopically measured ones, we propose that local, coordinated counter motion of at least pairs of ions is part of the process.
Citation
Physical Chemistry Chemical Physics
Volume
18

Keywords

Ionic solution structure, SANS, QENS, relative diffusion coefficient, ion transport

Citation

Rubinson, K. and Faraone, A. (2016), The Relative Diffusive Transport Rate of Srl<sub>2</sub> in Water Changes over the Nanometer Length Scale as Measured by Coherent Quasielastic Neutron Scattering, Physical Chemistry Chemical Physics, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=918537 (Accessed February 28, 2024)
Created May 14, 2016, Updated February 19, 2017