Water-separated ion pairs cause the slow dielectric mode of magnesium sulfate solutions

Shavkat I. Mamatkulov, Klaus F. Rinne, Richard Buchner, Roland R. Netz, Douwe Jan Bonthuis

Research output: Contribution to journalArticleResearchpeer-review

Abstract

We compare the dielectric spectra of aqueous MgSO4 and Na2SO4 solutions calculated from classical molecular dynamics simulations with experimental data, using an optimized thermodynamically consistent sulfate force field. Both the concentration-dependent shift of the static dielectric constant and the spectral shape match the experimental results very well for Na2SO4 solutions. For MgSO4 solutions, the simulations qualitatively reproduce the experimental observation of a slow mode, the origin of which we trace back to the ion-pair relaxation contribution via spectral decomposition. The radial distribution functions show that Mg2+ and SO42- ions form extensive water-separated - and thus strongly dipolar - ion pairs, the orientational relaxation of which provides a simple physical explanation for the prominent slow dielectric mode in MgSO4 solutions. Remarkably, the Mg2+-SO42- ion-pair relaxation extends all the way into the THz range, which we rationalize by the vibrational relaxation of tightly bound water-separated ion pairs. Thus, the relaxation of divalent ion pairs can give rise to widely separated orientational and vibrational spectroscopic features.

Original languageEnglish
Article number222812
JournalJournal of Chemical Physics
Volume148
Issue number22
DOIs
Publication statusPublished - 14 Jun 2018
Externally publishedYes

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magnesium sulfates
Magnesium Sulfate
Ions
Water
causes
water
ions
zwitterions
molecular relaxation
radial distribution
field theory (physics)
sulfates
simulation
Sulfates
distribution functions
Distribution functions
Molecular dynamics
permittivity
molecular dynamics
Permittivity

ASJC Scopus subject areas

  • Physics and Astronomy(all)
  • Physical and Theoretical Chemistry

Cite this

Water-separated ion pairs cause the slow dielectric mode of magnesium sulfate solutions. / Mamatkulov, Shavkat I.; Rinne, Klaus F.; Buchner, Richard; Netz, Roland R.; Bonthuis, Douwe Jan.

In: Journal of Chemical Physics, Vol. 148, No. 22, 222812, 14.06.2018.

Research output: Contribution to journalArticleResearchpeer-review

Mamatkulov, Shavkat I. ; Rinne, Klaus F. ; Buchner, Richard ; Netz, Roland R. ; Bonthuis, Douwe Jan. / Water-separated ion pairs cause the slow dielectric mode of magnesium sulfate solutions. In: Journal of Chemical Physics. 2018 ; Vol. 148, No. 22.
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