WAVE VECTOR-DEPENDENT DIELECTRIC-RELAXATION OF METHANOL-WATER MIXTURES

Using molecular dynamics (MD) simulation, we investigate the wave vect or (k)-dependent dielectric relaxation in room-temperature methanol-wa ter mixtures at three methanol mole fractions: x(m) = 0.2, 0.5, and 0. 8. The simulations were carried out on systems of 500 molecules using the TIP4P and Hl effective pair potentials for water and methanol, res pectively. We have calculated time correlation functions (TCFs) of the permanent dipole densities at k = 0 and of their transverse and longi tudinal components at the five smallest-magnitude k values attainable in the simulations. Frequency-domain results in the form of the dielec tric permittivity tensor, epsilon(k,omega), and the far-infrared (FIR) absorption coefficient are also reported. In order to assess the effe cts of intermolecular correlations on dielectric relaxation, we analyz e the dipole density TCFs in terms of contributions from intraspecies autocorrelations, interspecies cross-correlations, and, in the k = 0 c ase, single-molecule dipolar autocorrelations. We find that interspeci es cross-correlations strongly affect the intensity and relaxation rat e of the collective dipolar TCFs and that short-ranged structural inho mogeneities play an important role in these properties, especially in methanol-rich mixtures. The transverse and the k = 0 TCFs are dominate d by diffusive, but nonexponential, decay, which becomes more rapid as the mole fraction of water increases, while the longitudinal dipole d ensity TCFs relax mainly through inertial and librational mechanisms. As the water concentration increases, the low-frequency librational ba nds in the FIR absorption coefficient diminish, while the hydrogen-bon d librational band grows in width and intensity. We find that the tran sverse and longitudinal components of epsilon(k,omega) for the smalles t-magnitude k values are essentially identical for frequencies above t hose which characterize the rotational-diffusion regime.