An IR Study of Hydrogen Bonding in Liquid and Supercritical Alcohols

Abstract

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.

Hydrogen bonding in liquid and supercritical 1-octanol and 2-octanol assessed by near and midinfrared spectroscopy

“…Therefore, fundamental studies mainly devoted to lower alcohols such as methanol and ethanol have been performed to elucidate the structure and dynamics of pure alcohols in the supercritical state. Molecular dynamics simulations, 2 NMR experiments, 3,4 neutron diffraction, 5 and vibrational spectroscopies ͑Raman 6 and infrared [7][8][9] ͒ came to the converging conclusion that hydrogen bonding still exists in the supercritical domain even at high temperatures. As a matter of fact, at about 330°C and at intermediate reduced densities r = / c =2 ͑where c is the critical density͒, the degree of hydrogen bonding was found to be about 30% that existing in liquid methanol and ethanol at room temperature.…”

“…10 This investigation has been performed in order to clarify some quantitative disagreement between the degree of hydrogen bonding obtained with molecular dynamics ͑MD͒ simulations results, 2 NMR experiments, 3 and using IR spectroscopy. [7][8][9] This point has been critically discussed and it came out that the difficulty relies in the analysis of IR and Raman spectra in terms of clusters which is particularly difficult for methanol and ethanol. 11 Besides, tert-butanol has been the subject of a number of recent studies 12 which suggested that the extent of hydrogen bonding is less dominant in the liquid phase than in lower alcohols ͑methanol, ethanol͒ because of steric and packing effects.…”

Supercritical Transesterification of Waste Vegetable Oil: Characteristic Comparison of Ethanol and Methanol as Solvents

“…The non-ester component and glycerol collectively is less than 5% for both SCM and SCE. The difference in the product obtained while using SCM and SCE is still ambiguous as shown by many researches [32,41,42]. Numerous studies [23,30] have shown the suitability of SCM in biodiesel production, while studies by Madras et al [29] and Poudel and Oh [43] have presented the suitability of SCE over SCM.…”

Synthesis of nanoparticulate yttrium aluminum garnet in supercritical water–ethanol mixtures

“…scH 2 O was injected from the top at 415 • C (or lower) while temperature of the metal solution introduced from the bottom of the reactor was kept below 30 • C. In most experiments reactor temperature (R) was 385 • C, but experiments at 355 and 260 • C were also conducted. At these temperatures, decomposition of EtOH does not take place [29]. Pressure was kept at 240 bar.…”