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

Measurement of Enthalpies and Entropies of Activation as a Function of Pairwise Distance for the Pairwise Relative Diffusion of SrI2 in Water over Lengthscales from 6 Å to 40 Å

Quasi-elastic neutron scattering (QENS) has been used to provide insight into the motions within solutions for many decades, and coherent QENS (CQENS) determines the motions of one ion relative to...

Ion and water transport reasonably involves rotation and pseudorotation: measurement and modeling the temperature dependence of small-angle neutron scattering from aqueous SrI2

X-ray and neutron scattering have provided insight into the short range (<8 Å) structures of ionic solutions for over a century. For longer distances, single scattering bands have, however, been seen. For the non-hydrolyzing salt SrI2 in aqueous (D2O) solution, a structure sufficient to scatter slow neutrons has been seen to persist down to a concentration of 0.1 mol L-1 where the measured average spacing between scatterers is over 20 Å. Theoretical studies of such long distance solution structures are difficult, and these difficulties are discussed. The width of the distribution in distances between the scatterers (ions, ion pairs, etc.) remains less than 10 Å, which approximates the average size of the ions and their first hydration shell. Here, we measure the temperature dependence from 10 °C to 90 °C of the small angle neutron scattering (SANS) by a 0.5 molar SrI2 solution in D2O and find that this surprisingly narrow distribution of the distances remains constant within experimental uncertainty. This structure of the ions in the solution appears to endure because changes in interion distances along any single spatial dimension require displacements near the size of a water molecule. Together, the experimental measurements support a rotatory mechanism for simultaneous ion transport and water countertransport. Since rotation minimizes displacement of the solution framework, it is suggested that water transport alone also involves rotation of multimolecular structures, and that the interpretation of single-molecule water rotation is confounded by pseudorotation that results from paired picosecond proton exchanges. It is pointed out that NMR-determined millisecond to microsecond proton exchange times of chelated-metal-ion bound waters and the much faster chelate rotational correlation times around 10 picoseconds, both of which require making and breaking of hydrogen bonds, are difficult to impossible to reconcile.

Heparin's solution structure determined by small-angle neutron scattering

Heparin is a linear, anionic polysaccharide that is widely used as a clinical anticoagulant. Despite its discovery 100 years ago in 1916, the solution structure of heparin remains unknown. The solution shape of heparin has not previously been examined in water under a range of concentrations, and here is done so in D2 O solution using small-angle neutron scattering (SANS). Solutions of 10 kDa heparin-in the millimolar concentration range-were probed with SANS. Our results show that when sodium concentrations are equivalent to the polyelectrolyte's charge or up to a few hundred millimoles higher, the molecular structure of heparin is compact and the shape could be well modeled by a cylinder with a length three to four times its diameter. In the presence of molar concentrations of sodium, the molecule becomes extended to nearly its full length estimated from reported X-ray measurements on stretched fibers. This stretched form is not found in the presence of molar concentrations of potassium ions. In this high-potassium environment, the heparin molecules have the same shape as when its charges were mostly protonated at pD ≈ 0.5, that is, they are compact and approximately half the length of the extended molecules.

The relative diffusive transport rate of SrI2 in water changes over the nanometer length scale as measured by coherent quasielastic neutron scattering

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 non-hydrolyzing salt SrI2 in aqueous solution, a locally ordered lattice of ions 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 diffusive transport rate for the motion between pairs of ions relative to each other. Because CQENS measures the 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 diffusive transport rate Dp. In addition to this ion centered frame of reference, the diffusive transport rate can be measured as a function of the momentum transfer q, where q = (4π/λ)sin θ with a scattering angle of 2θ. Since q is related to the interion distance (d = 2π/q), for the experimental range 0.2 Å(-1)≤q≤ 1.0 Å(-1), Dp is, then, measured over interion distances from 40 Å to ≈6 Å. We find the measured diffusional transport rates increase with increasing distance between scatterers over the entire range covered and interpret this behavior to be caused by dynamic coupling among the ions. Within the model of Fickian diffusion, at the longer interionic distances Dp is greater than the Nernst-Hartley value for an infinitely dilute solution. For these nm-distance diffusional transport rates to conform with the lower, macroscopically measured diffusion coefficients, we propose that local, coordinated counter motion of at least pairs of ions is part of the transport process.