Interpretation of the low frequency response of aqueous solutions: A molecular dynamics analysis

In the case of a mixture it is difficult to determine the nature and origin of the low-frequency response. The fundamental difficulty arises from the large number of different interaction mechanisms, and their corresponding cross terms, that contribute to the low-frequency response of the system. Molecular dynamics simulation is used to obtain qualitative information on the role of different contributions (translation, rotation, dipole-dipole-induced interactions) to the low-frequency response of aqueous solutions, as obtained by Raman spectroscopy and the time-resolved optical Kerr effect. On the basis of these results, three contributions to the low-frequency response of aqueous solutions are proposed. The first is associated with the vibrational motion of the solute or solvent molecule in a cage formed by the neighboring molecules. The second is associated with hydrogen bonding interactions between water molecules. The third contribution is associated with hydrogen bonds between solute and water and with the librational motion of urea molecules. This interpretation is applied to analyze the low-frequency response of the urea/water and acetone/water systems as a function of the concentration of the solute. The three criteria provided by the fitting procedure (the chi-square, the correlation matrix and the structure of the residue) are used to validate the three bands model proposed by the molecular dynamics simulation.