IR spectra in the region of the OH stretching band of liquid-like and supercritical methanol (T
c = 239.4 °C,
Pc = 80.8 bar), ethanol (240.9 °C, 61.4 bar), 2-propanol (235.2 °C, 47.6 bar), and 1-butanol (289.9 °C, 44.1
bar) (data from ref , J. Chem. Eng. Data
1995, 40, 1025) have been recorded over a wide range of temperature
and pressure up to 450 °C and 1000 bar. The spectra were obtained using a high-pressure, high-temperature
IR cell and a specially developed measuring technique, in which the integrated intensity of the absorption
band of the C−H stretching vibrations is used as an internal standard. This analysis allows the fraction of
hydrogen-bonded and non-hydrogen-bonded hydroxyl groups in 2-propanol and 1-butanol to be estimated
over the whole range of experimental conditions as a function of both temperature and pressure. With this
approach, it is unnecessary to know the density of the sample or even the path length at which the spectra
were obtained. However, the approach is less reliable in the case of ethanol and cannot be applied to methanol
because the spectra of these substances are complicated by rotational fine structure. An unexpected result of
this study is the possibility that only one type of hydrogen-bonded species exists in ethanol, 2-propanol, and
1-butanol when the mole fraction of hydrogen-bonded molecules is less than about 0.6. When the probability
of hydrogen bonding is greater, evidence for a strong cooperative effect is seen.
The behavior of aqueous 1.1 M NaCl solution at a constant pressure of 1000 bar in the temperature range 25-500 degrees C has been studied with the use of IR absorption, Raman scattering, X-ray diffraction, and molecular dynamics (MD) simulations. The results are compared with the data for pure water under identical external conditions. The main purpose of the experimental and theoretical studies was to understand in what way an electrolyte dissolved in water influences the hydrogen bonding and structural features of water. As was found, the vibrational spectra show no essential difference between the properties of solution and pure water. However, the experimental pair correlation functions and the results of MD simulations present an evidence for very different nature of these substances. A characteristic feature of the structure of NaCl solution is a considerable contribution of strong O-H...Cl- bonds. As the temperature increases, the number of such bonds decreases partially due to a phenomenon of ion pairing, so that at high temperatures the properties of the solution become closer to the properties of water.
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