Peculiar and normal dynamics of hydrogen-bonded liquids: a neutron scattering and simulation study of methanol

Methanol is a hydrogen-bond liquid of fundamental importance in pure and applied physics and chemistry, particularly in mixtures with water, and innumerable papers deal with its properties. Yet, the fundamental aspects of microscopic dynamics, even in pure methanol, have not been clearly elucidated so far. The study of the collective dynamics is hindered by the weakness of the acoustic excitations, which has led to the erroneous conclusion that sound modes propagate only in a surprisingly narrow range of small wave vector values. New, accurate, molecular dynamics simulations joint to neutron Brillouin scattering measurements reveal, however, a different situation, reported in this paper. We have determined energy and damping of the acoustic modes in the whole range between the hydrodynamic regime, where the fluid can be viewed as a continuum, and the regime where the excitation wavelength decreases down to the order of intermolecular distances. Methanol is shown, for the first time, to feature the "normal" viscoelastic behaviour characterizing the dynamics of a large variety of liquids, including the arrest of sound mode propagation in the vicinity of the peak of the static structure factor. In addition to this first result, however, the presence of a second excitation with lower frequency, smaller damping and negligible dispersion is displayed. The translational intermolecular dynamics emerging from this study classifies methanol as a fluid having properties similar to those of water, where the presence of a double excitation is a long standing result although its interpretation has remained controversial for long time. However, this study provides a further quite remarkable result, namely the fact that the low-frequency mode seems to be present in the single-molecule dynamics as well. This opens a wide space to further research work, in two directions: one is a better understanding of the effect of hydrogen bonds in the microscopic molecular motions; the other is the problem of the presence of inelastic components, i.e. of characteristic frequencies, in single-particle fluids dynamics.