title: Dissecting ion-specific dielectric spectra of sodium-halide solutions into solvation water and ionic contributions.
abstract: Using extensive equilibrium molecular dynamics simulations we determine the dielectric spectra of aqueous solutions of NaF, NaCl, NaBr, and NaI. The ion-specific and concentration-dependent shifts of the static dielectric constants and the dielectric relaxation times match experimental results very well, which serves as a validation of the classical and non-polarizable ionic force fields used. The purely ionic contribution to the dielectric response is negligible, but determines the conductivity of the salt solutions. The ion-water cross correlation contribution is negative and reduces the total dielectric response by about 5%-10% for 1 M solutions. The dominating water dielectric response is decomposed into different water solvation shells and ion-pair configurations, by this the spectral blue shift and the dielectric decrement of salt solutions with increasing salt concentration is demonstrated to be primarily caused by first-solvation shell water. With rising salt concentration the simulated spectra show more pronounced deviations from a single-Debye form and can be well described by a Cole-Cole fit, in quantitative agreement with experiments. Our spectral decomposition into ionic and different water solvation shell contributions does not render the individual contributions more Debye-like, this suggests the non-Debye-like character of the dielectric spectra of salt solutions not to be due to the superposition of different elementary relaxation processes with different relaxation times. Rather, the non-Debye-like character is likely to be an inherent spectral signature of solvation water around ions.
authors.affiliation: Fachbereich Physik, Freie Universität Berlin, 14195 Berlin, Germany.
authors.forename: Klaus F
authors.initials: KF
authors.lastname: Rinne
authors.affiliation: Physikalisches Institut, Universität Bayreuth, 95440 Bayreuth, Germany.
authors.forename: Stephan
authors.initials: S
authors.lastname: Gekle
authors.affiliation: Fachbereich Physik, Freie Universität Berlin, 14195 Berlin, Germany.
authors.forename: Roland R
authors.initials: RR
authors.lastname: Netz