Are All Polar Molecules Hydrophilic? Hydration Numbers of Ketones and Esters in Aqueous Solution

Abstract

Hydration numbers of typical polar compounds like ketones and esters in aqueous solution were precisely determined using high-frequency dielectric relaxation techniques up to a frequency of 50 GHz at 25 °C. Because the hydration number is one of the most quantitative parameters to demonstrate how much are molecules hydrophilic, it is a critical parameter to determine the hydrophilicity of compounds. Hydration numbers of some ketones bearing carbonyl groups were determined to be ca. 0 irrespective of the species of molecules. Moreover, hydration numbers of some esters were also evaluated to be ca. 0 as well as the ketones. These findings suggested that there is no hydrogen bond formation between the ester group and water molecules, nor is there the hydrogen bond formation between the carbonyl group and water molecules. Consequently, esters and ketones bearing typical polar groups are not classified into hydrophilic compounds, but into "hydroneutral" compounds positioned between hydrophilic and hydrophobic ones. Molecular motions of the examined polar molecules in aqueous solution were well described with single Debye-type rotational relaxation modes without strong interaction between solute and water molecules, and also between solute molecules because of the obtained Kirkwood factor close to unity. This independent rotational mode for the polar compounds results from the hydroneutral characteristics caused by the relationship n(H) = 0.