Nanometer-Scale Correlations in Aqueous Salt Solutions

Abstract

The intermediate-range (1-4 nm) correlation of cations, anions, and water in aqueous alkaline earth salt solutions is measured using synchrotron X-ray diffraction. We differentiate from the entire solution structure factor, <i>S</i>_X(<i>Q</i>), a separate region at low <i>Q</i> (<1.5 Å^-1) containing local diffraction maxima (prepeaks) that indicate nanometer-scale oscillatory behavior. These features are quantitatively reproduced with classical molecular dynamics simulations. At high concentrations, the prepeaks emerge from correlations arising from the existence of small quasi close-packed lattice-like structures comprised of cation hydration spheres. We also analyze the concentration dependence of the prepeak and discuss the overall results in light of the rich literature dealing with intermediate-range correlations underlying universal phenomena in concentrated electrolytes.