Mutual Effects of Glycerol and Inorganic Salts on Their Hydration Abilities

It is a tough challenge to understand the mutual interactions among various components in aqueous solutions of inorganic mixed with organic solutes. The hydration number, n_h, and critical hydration number, n_cr, determined by the measurements of glass transition of the solutions, in conjunction with tracing the change in local water structure, can provide some insights into the complicated interplays in such a mixture. Here, the n_h and n_cr for aqueous solutions of glycerol, various chlorides, and mixtures of glycerol with a chloride are determined. The ratio of n_cr/n_h measures 4 for glycerol and 1.7 for all the chlorides, and for mixtures of glycerol with all of the chlorides except ZnCl₂, it falls within these two extremes. Glycerol content dependence of n_h and n_cr reveals a rich and interesting scenario of mutual effects therein, in particular, the glycerol's replacement and sharing of hydration water with salt. In the case of ZnCl₂, at most, one hydration water molecule is replaced by glycerol, and the excess glycerol molecules continuously reduce the number of glycerol molecules that share hydration water with ZnCl₂. Our results can help establish a pathway for the investigation of interactions among the organic and inorganic components in aqueous solutions, which is desirable for many applications.