The hydration numbers of typical aprotic polar substances bearing dipole moments larger than 3 D, such as nitro compounds and nitriles, were precisely determined in aqueous solution using high frequency dielectric relaxation techniques up to a frequency of 50 GHz at 25 °C. The hydration number is one of the most quantitative parameters for determining the hydrophilicity or hydrophobicity of a compound. The hydration numbers of various nitriles, such as acetonitrile, propionitrile and n-butyronitrile bearing cyano groups, were determined to be ca. 0, irrespective of the species of molecule. Moreover, the hydration numbers of various nitro compounds, such as nitromethane, nitroethane and 1-nitropropane, were also evaluated to be ca. 0. These findings clearly reveal that neither cyano nor nitro functional groups form strong hydrogen bonds to water molecules. Consequently, neither nitro compounds nor nitriles are hydrophilic, despite their high polarities due to their large dipole moments. Rather, these compounds are "hydroneutral," with hydrophilicities intermediate between those of hydrophilic and hydrophobic molecules. The molecular motions of the examined highly polar molecules in aqueous solution were well described with single Debye-type rotational relaxation modes without strong interactions between the solute and water molecules, but with relatively strong interactions between the polar solute molecules due to the Kirkwood factor being less than unity. This small Kirkwood factor indicated that both nitro and cyano groups have a tendency to align in an anti-parallel intermolecular configuration due to their strong dipole-dipole interactions as a result of their dipole moments greater than 3 D.

Poly(2-ureidoethylmethacrylate) (PUEM n ) was synthesized via reversible addition-fragmentation chain transfer (RAFT) radical polymerization and following polymer reaction. We prepared two PUEM n samples with different degrees of polymerization (n 5 100 and 49). The polymers exhibited upper critical solution temperature (UCST) in phosphate-buffered saline (PBS) solution. The phase separation temperature (T p ) in PBS can be controlled ranging from 17 to 55 8C by changing molecular weight of the polymer, polymer concentration, and adding NaCl concentration. The polymers in PBS formed coacervate drops by liquid-liquid phase separations below T p . Results of the dielectric relaxation measurement, the hydration number per monomeric unit was 5 above T p . Based on a fluorescence study, the polymer formed slightly hydrophobic environments below T p . The liquid-liquid phase separation was occurred presumably because of weak hydrophobic interactions and intermolecularly hydrogen bonding interactions between the pendant ureido groups.

Near-infrared (NIR) absorption measurements over a wavenumber range from 4000 to 12000 cm(-1) were used to investigate hydration behavior of poly(ethylene oxide)s bearing molar masses of 3 × 10(3) (PEO3) and 20 × 10(3) (PEO20) in aqueous solution at a dilute regime. NIR absorption spectra for solutions were perfectly decomposed into three components: bulk water, PEOs, and hydrated water to PEOs. While amounts of each component were proportional to the concentrations of PEOs (cEO), the wavenumber dependence of each spectrum was independent of cEO and also the molar masses of PEOs. A newly determined NIR absorption spectrum of hydrated water molecules to PEOs found in a wavenumber range from 5500 to 7500 cm(-1) was well described with the summation of two Gaussian-type vibration modes possessing the same magnitude, which were observed at different wavenumbers of 6480 and 6910 cm(-1), respectively. Each mode was assigned to O-H stretching first overtone of water molecules hydrated directly to ether oxygen atoms (6480 cm(-1)) and water oxygen atoms (6910 cm(-1)) belonging to the secondary hydration layer.