Does Saltwater Wobble?

Abstract

When ionic salts dissolve in water, the resultant solution tends to become more viscous than the pure liquid. This observation can be broadly understood based on a picture in which the web of hydrogen bonds holding the water molecules together rigidifies around solvated ions. However, the molecular details appear much more complicated, because time-resolved vibrational spectroscopy has suggested that most individual water molecules continue to rotate freely regardless of how much dissolved salt is present. Turton et al. explore this discrepancy using two related spectroscopic techniques to probe the molecular structure of aqueous salt solutions. The first, dielectric relaxation spectroscopy, is sensitive to individual molecules' orientations, and confirms the rotational freedom previously observed. The second technique, optical Kerr effect (OKE) spectroscopy, reflects polarizability and so is more sensitive to intermolecular changes that stem from translation. The OKE data reveal increasing translational restriction with rising salt concentration, as the ion solvation shells crowd against one another. The authors note that such a decoupling of rotation and translation is analogous to the jamming that occurs during transitions from a liquid to a glass upon supercooling. — JSY