Strong Slowing Down of Water Reorientation in Mixtures of Water and Tetramethylurea

Abstract

We use mid-infrared pump-probe spectroscopy to study the ultrafast dynamics of HDO molecules in mixtures of tetramethylurea (TMU) and water. The composition of the studied solutions ranges from pure water to an equimolar mixture of water and TMU. We find that the vibrational relaxation of the OD-stretching vibration of HDO proceeds via an intermediate level in which the molecule is more strongly hydrogen bonded than in the ground state. As the TMU concentration is increased, the lifetime of the excited state and of the intermediate increase from 1.8 to 5.2 ps and from 0.7 to 2.2 ps, respectively. The orientational relaxation data indicate that the solutions contain two types of water molecules: bulk-like molecules that have the same reorientation time constant as in the pure liquid (taurot = 2.5 ps) and molecules that are strongly immobilized (taurot > 10 ps). The immobilized water molecules turn out to be involved in the solvation of the methyl groups of the tetramethylurea molecule. The fraction of immobilized water molecules grows with increasing TMU concentration, reaching a limiting value of 60% at very high concentrations.

Hydration of Metal and (CH3)4N+ Ions in Solutions of Mixed Salts

“…Also in the presence of Me 4 NCl, ionic association does not occur. Therefore the radial distribution functions between Cl − and the water sites are identical in all simulated solutions.…”

“…The influence of the Me 4 NCl on the distribution of the oxygens around the sodium and magnesium cations is seen to be almost negligible. The first peaks of the radial distribution functions between the Na + and Mg 2+ ions and oxygen appear at 0.…”

“…The concentrations of both salts were 0.55 M. This means that in all simulations the periodic cube contained 400 water molecules, four molecules of (CH 3 ) 4 NCl, and four molecules of the inorganic salt. The lengths of the boxes were calculated from the experimental densities of the aqueous solutions of the mixed salts at 298 K.…”

“…The radial distribution functions for the metal cations and water-oxygen in solutions of Me 4 NCl are presented in Figure 1 and compared with the analogous functions in aqueous solutions. The influence of the Me 4 NCl on the distribution of the oxygens around the sodium and magnesium cations is seen to be almost negligible.…”

“…Though their 'hydrophobic' behaviour increases with the increasing length of the alkyl chain, the aqueous solution of (CH 3 ) 4 NCl can also serve as the model system since (CH 3 ) 4 N + is 'more apolar' then 'ionic' [13].…”