Kosmotrope and Chaotrope Salts Influence on Water Structural Relaxation Investigated by Coherent Quasielastic Neutron Scattering

Aqueous solutions of salts have been the focus of many experiments and simulations because of their relevance in physics, chemistry, and biology. Typically, anions and cations are classified according to the Hofmeister series for their properties of enhancing or weakening the hydrogen bond network of water. However, at the molecular level the distinction between kosmotrope (structure makers) and chaotrope (structure breakers) is less clear as both have a coordination shell of water molecules around them and can perturb the water structure beyond the first hydration shell. Using coherent quasielastic neutron scattering, we have investigated the structural dynamics of NaCl/D₂O and KCl/D₂O, NaCl and KCl being well studied kosmotrope and chaotrope, respectively. By probing the dynamics of these systems at the structure factor peak, we measured how different salts affect the structural relaxation of water. Both salts clearly reduce the de Gennes narrowing effect observed in water. Interestingly, as the salt concentration is increased, the de Gennes narrowing effect gets slightly more pronounced in NaCl/D₂O whereas it keeps getting weaker in KCl/D₂O. These results indicate that competing effects play a role in determining how salts affect water structural relaxation.