Definition and quantification of hydration water in aqueous solutions

Abstract

Water molecules in the very proximity to the solute differ a lot from those in the far and the bulk water in both structure and property, they are usually referred to as hydration water or bound water. There is no doubt about the effect of hydration water on the property and structure of solute in solution, in particular when biological macromolecules are of concern. However, by far, there are even significant controversies over the understanding of hydration water, including the accurate definition and quantification of hydration water, the quantitative evaluation of the difference in the properties between the hydration water and free water, and how the hydration water is involved in the various biological processes, etc. For resolving the aforementioned issues, it would be of essential importance to formulate a quantification scheme for the hydration water on a sound footing. In the present article, the principles of various spectrometric techniques for determining hydration water are briefly examined, and the main deficiency in quantification of hydration water for the individual techniques is analyzed. Those techniques based on the inflection point of the concentration dependence of some physical properties of the solution are also scrutinized. Finally, we present in detail a quantification scheme for hydration water based on the concentration dependence of glass transition temperature, which leads to quite a universal categorization of an aqueous solution into three distinct zones. Also the crystallization dynamics thus revealed might be helpful for understanding the water-involved processes in other circumstances.