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

Salts can either increase or decrease the viscosity when dissolved in water. This phenomenon has been traditionally interpreted within the classification of the solutes as kosmotropes (structure makers) and chaotropes (structure breakers), with reference to their hypothesized property of enhancing or weakening the hydrogen bond network. However, at the molecular level the distinction is less clear as both kosmotropes and chaotropes affect the structure of the surrounding water molecules. Using coherent quasielastic neutron scattering, we have investigated the dynamics of NaCl/D O and KCl/D O, NaCl being a kosmotrope and KCl being a 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. At the same time, by collecting data at the second peak in the deuterium intermolecular partial structure factor, the hydrogen bond dynamics was probed. Whereas the hydrogen bonding dynamics is largely unchanged by the presence of the salts, NaCl and KCl affect the structural relaxation differently: the former slows down the dynamics whereas the latter mostly leaves the relaxation unchanged.