Single and collective dynamics in methanol and its aqueous solutions

Methanol is the simplest molecule containing a polar hydroxyl group and a non-polar methyl group, thus offering the possibility of studying the properties of small amphiphile systems in aqueous environments, as well as self-assembly phenomena. As for other alcohols it shows non-ideal behaviour when mixed with water and a large negative excess entropy that has been often attributed to an enhanced water ordering in the vicinity of hydrophobic groups. However the validity of such model has been questioned and alternative suggestions proposed. We have recently explored by means of quasielastic neutron scattering and MD simulations the possibility of observing anomalies in the dynamics of water and methanol in the mixtures that could be related with any of the available propositions. As part of this effort, the collective dynamics of pure methanol have also been measured using the BRISP spectrometer at the Institut Laue-Langevin (Grenoble, France). The experimental results have been analyzed in combination with the MD and show striking similarities with the collective dynamics of water. In particular a third high-frequency mode that in recent simulation results on supercooled water has been associated to four-coordinated molecules is also visible in methanol, suggesting that tetrahedrality is not an essential requisite for this mode to appear.