This work concerns a comparison of the hydration properties and self-association behavior in aqueous solution
of two biologically relevant simple molecules: the trimethyl-amine-N-oxide (TMAO) and the tert-butyl alcohol
(TBA). These molecules are geometrically very similar, having the same hydrophobic moiety and different
polar groups. Both molecules were used as a model to study hydrophobic behavior in water solution. In
particular, water perturbation induced by TBA and TMAO molecules was studied as a function of the solute
molar fraction X
2 (0 < X
2 < 0.05) by using the IR absorption bands due to the vibrational modes of water
in the 4000−2600 cm-1 frequency region. Furthermore, possible clustering effects in aqueous solution of the
TBA and TMAO hydrophobic groups were investigated by studying the behavior of the alkyl CH stretching
band in the 3200−2800 cm-1 frequency region as a function of X
2. The OH stretching absorption data show,
in agreement with molecular dynamics simulation results and other suggestion found in the literature, that
the interaction of the TBA and the TMAO with water are remarkably different. In fact, water molecules are
more coordinated by TMAO than by TBA. Significant differences are also evident in the CH stretching data
for the two molecules. For TBA, the data can be interpreted in terms of a self-aggregation process of the
alcohol molecules occurring beyond a threshold value of the alcohol molar fraction (X
2
* = 0.025). This
phenomenon seems to be absent in the TMAO samples.
The infrared absorption spectra of aqueous methanol, ethanol, 1- and 2-propanol, t-butanol, and n-butoxyethanol in the region of asymmetric and symmetric C–H stretching modes (3200–2800 cm−1) have been measured at 25 °C as a function of alcohol concentration in the whole cosolvent mole fraction region. Measurements of adiabatic compressibility were also performed on the same water–alcohol solutions at 25 °C. The two sets of experimental data are compared and discussed in terms of a possible mechanism of molecular aggregation in the various regions of alcohol concentration.
Transmittance Fourier transform infrared (FT-IR) spectra of liquid water in the 4-80 degrees C temperature range are reported in the whole mid-infrared (MIR) region (4000-360 cm (-1)). The spectra were recorded by using a newly developed, home-made transmittance cell, working in light vacuum conditions (pressures of the order of 3-4 millibar). This permits the elimination of the aqueous vapor bands from the liquid spectra, particularly in the bending region, and the rapid collection of data without fluxing large amounts of nitrogen through the interferometer sample chamber. The temperature evolution of the OH stretching and HOH deformation bands is discussed in terms of Gaussian components analysis and a two-state model describing the equilibrium between different H-bond structures of liquid water. From this picture, structural and thermodynamic information about the hydrogen-bonding network of water is obtained.
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