Charge/mass dynamic structure factors of water and applications to dielectric friction and electroacoustic conversion

Abstract

We determine time correlation functions and dynamic structure factors of the number and charge density of liquid water from molecular dynamics simulations. Using these correlation functions we consider dielectric friction and electro-acoustic coupling effects via linear response theory. From charge-charge correlations, the drag force on a moving point charge is derived and found to be maximal at a velocity of around 300 m/s. Strong deviations in the resulting friction coefficients from approximate theory employing a single Debye relaxation mode are found that are due to non-Debye-like resonances at high frequencies. From charge-mass cross-correlations the ultrasonic vibration potential is derived, which characterizes the conversion of acoustic waves into electric time-varying potentials. Along the dispersion relation for normal sound waves in water, the ultrasonic vibration potential is shown to strongly vary and to increase for larger wavelengths.

The hydrogen-bond network of water supports propagating optical phonon-like modes

“…between fluctuations in mass density and fluctuations in charge density. 56 The large spatial range and coherent propagation of these modes is surprising and implies the existence of an extended hydrogen bond network, in contrast to earlier ideas about the structure of water which emphasize dynamics as being confined within small clusters. 57 Simulations with larger simulation boxes are needed to fully quantify the extent of the longitudinal modes.…”

“…However, normal mode analysis of liquid water and clusters shows that the H-bond stretching modes have a wide distribution of frequencies, which overlaps with the librational modes, so some coupling between these modes is possible 54 55 . Recently, it was shown that there is coupling between the acoustic and optic modes in water—that is, between fluctuations in mass density and fluctuations in charge density 56 .…”

Local Molecular Field Theory for Nonequilibrium Systems

“…al. [39]. For small perturbations, the response of a medium to an electric field takes a linear form…”

“…where P is the polarization of the medium, D is the electric displacement field, and where the dynamic structure factor can be written have been studied for multiple water models in different contexts [39]. In addition to the charge-charge autocorrelation function, we will also consider the force-force autocorrelation function, which is related to the friction coefficient [62].…”

“…The oscillations at low frequencies, and for the long range forces, are attributed to the librational modes of the water molecules[39]. These librational modes represent the fastest time scales of the solvent dynamics, and are responsible for the rapid relaxation of the solvent to equilibrium.…”