Hydrogen bonding in supercritical methanol studied by infrared spectroscopy

Abstract

We report the first Fourier transform infrared spectroscopic studies of the hydrogen-bonding interactions in supercritical methanol and methanol/methanol-d1 mixtures. At 523 K and below ∼0.3 mol/l, methanol chiefly existed in a monomeric form. However, at 523 K and between ∼0.3 and 2 mol/l, in the regions of O–H, C–O, and C–H stretching vibrations, there were clear indications of methanol monomer–dimer equilibrium. The effect of temperature on the equilibrium constant was also investigated. As a result, the partial molar volume difference and enthalpy of the dimerization were estimated and correlated with the behavior of the methanol isothermal compressibility. It was also concluded that at 523 K and density higher than ∼15 mol/l, the hydrogen-bonding equilibrium started to shift toward larger aggregates.