Water structure enhancement in water-rich binary solvent mixtures

The excess partial molar volume of water in water-rich mixtures with co-solvents is a clear indication, if positive, that the bulky, low-density structure of the water is enhanced. This was demonstrated in the cases of aqueous methanol, ethanol, 1-propanol, 2-propamol, t-butanol, diethanolamine, triethanolamine, 1,2-diaminopropane, dimethoxyethane, dimethylformamide dimethylacetamide, 2-pyrrolidinone, N-methyl-2-pyrrolidinone, and dimethylsulfoxide. Some co-solvents that hydrogen-bond very strongly with water, such as ethylene glycol, glycerol, and formamide, do not enhance the water structure according to this criterion. The results are rationalized in terms of the sizes and hydrogen bonding properties of the co-solvent molecules.