Viscosity of aqueous solutions and local microscopic structure.

The effect of solutes on the structure of water has been debated intensively over the past years. Typical scenarios label different ions as water structure "makers" or "breakers": this is a quite elusive definition, which has been first introduced in the description of the effect of solutes on the viscosity of water and, although criticized, is still used in the current literature. Here, using a combination of neutron diffraction and computer modeling, we present a possible relation between the viscosity B coefficient and a local structural property of the solution. In particular, B appears in the Jones-Dole relation and its sign is traditionally used to characterize a solute as "structure maker" or "breaker". We find that B is linearly correlated to the difference between the average solute-water distance and the water-water distance in the pure liquid, in the case of monovalent electrolyte solutions.