The Long-range Ordering of Ions in Aqueous Salt Solutions

The importance of understanding the long-range structure in aqueous salt solutions has recently come to the forefront when it was suggested that the ordering of water in the presence of ions possibly reaches length scales up to 10 nm. A much more important question regards the long-range ordering of the ions themselves since their arrangement critically affects processes such as nucleation/crystallization and the solution energetics related to activity coefficients and osmotic pressure. We use a combination of molecular dynamics and X-ray diffraction to correlate experimentally measured long-range structure to the precise spatial distribution of species in aqueous solutions of alkali and alkali earth halides. For the first time, we quantitatively differentiate and explain a separate region in the fluid structure factor containing diffraction-like features at low Q values (“pre-peaks”) that lead to the oscillatory behavior in the ion-ion and ion-water pair distribution functions. These features are shown to be due to a combination of correlated (e.g., cation-cation and cation-water) and anti-correlated (cation-anion and anion-water) atomic positions. In addition, we also explore the influence of salt concentration on the position and magnitude of the pre-peaks.