HYDROGEN-BONDING OF METHYL ALCOHOL-D IN SUPERCRITICAL CARBON-DIOXIDE AND SUPERCRITICAL ETHANE SOLUTIONS

Fourier transform infrared (FTIR) spectroscopy has been used to measure the degree of intermolecular hydrogen bonding between solute molecules of methyl alcohol-d in supercritical carbon dioxide, supercritical ethane, and liquid heptane. In these fluids, an equilibrium is established between the free non-hydrogen-bonded monomer and the various hydrogen-bonded species of which the tetrameric species is believed to predominate. The fluid pressure, temperature, and the alcohol concentration significantly affect the equilibrium distribution of the monomer and oligomeric species. Both supercritical and subcritical binary solutions containing up to 0.07 mole fraction of methyl alcohol-d were examined under conditions ranging from 30 to 400 bar and 40 to 80-degrees-C. The changes in the partial molar enthalpies and partial molar volumes of the alcohol upon hydrogen bonding are reported. These measurements, together with evidence from coupled rotational-vibrational bands, point to the formation of some type of weak complex between carbon dioxide and methyl alcohol. Such a complex is likely the result of the interaction of the large carbon dioxide quadrupole with the methyl alcohol dipole. The solvation phenomena presented in this paper will provide for more complete thermodynamic treatments of these systems.