Hydration and aggregation in mono- and disaccharide aqueous solutions by gigahertz-to-terahertz light scattering and molecular dynamics simulations.

The relaxation properties of hydration water around fructose, glucose, sucrose, and trehalose molecules have been studied by means of extended frequency range depolarized light scattering and molecular dynamics simulations. Evidence is given of hydration dynamics retarded by a factor ξ = 5-6 for all the analyzed solutes. A dynamical hydration shell is defined based on the solute-induced slowing down of water mobility at picosecond time scales. The number of dynamically perturbed water molecules N(h) and its concentration dependence have been determined in glucose and trehalose aqueous solutions up to high solute weight fractions (ca. 45%). For highly dilute solutions, about 3.3 water molecules per sugar hydroxyl group are found to be part of the hydration shell of mono- and disaccharide. For increasing concentrations, a noticeable solute-dependent reduction of hydration number occurs, which has been attributed, in addition to simple statistical shells overlapping, to aggregation of solute molecules. A scaling law based on the number of hydroxyl groups collapses the N(h) concentration dependence of glucose and trehalose into a single master plot, suggesting hydration and aggregation properties independent of the size of the sugar. As a whole, the present results point to the concentration of hydroxyl groups as the parameter guiding both sugar-water and sugar-sugar interactions, without appreciable difference between mono- and disaccharides.