水溶液中结合水的定义与量化

水溶液中溶质的结合水具有不同于远离溶质的自由水的结构和性质.结合水的存在对水和溶质结构和动力学性质均具有显著甚至决定性的影响.然而,对结合水动力学和热力学性质的定量理解在诸多方面一直存在争议甚至严重分歧,其中重点包括如何定义和量化结合水,如何表征结合水和自由水的动力学差别,结合水如何参与生物大分子各种生物功能过程,以及溶质或界面影响结合水结构与性质的途径等.给出结合水定义的物理学依据和量化方法,是深入理解上述问题的第一步.本文简述了各种不同谱学方法定义结合水的基本原理及量化的困难,强调具有不同时间和空间响应尺度的测试方法所得结合水数不必完全可比.此外,系列水溶液物性随浓度升高会明显改变其浓度依赖关系,相应拐点浓度常被用于量化稀溶液中的溶质结合水数.我们近期研究的水溶液玻璃化转变温度-浓度关系,为结合水的定义、量化和水溶液的三区划分提供了物理依据,同时揭示了上述利用性质-浓度关系拐点浓度量化结合水方法的不足.

Definition and quantification of hydration water in aqueous solutions

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.