Effect of Ion Pairing on the Solution Dynamics Investigated by the Simulations of the Optical Kerr Effect and the Dielectric Relaxation Spectra

Abstract

ABSTRACT:The optical Kerr effect (OKE) and dielectric relaxation spectroscopy (DRS) signals of MgCl 2 aqueous solutions were modeled based on the same group of molecular dynamics simulation trajectories. Plausible agreement between the simulations and the experiments allows us to analyze the microscopic origin of the different concentration dependences of relaxation times probed by these two techniques. Our simulations suggest that a significant amount of cations and anions associate in the solutions due to the ion pairing effect. These ion assemblies as well as the free cations form suspending clusters together with the water molecules in their cation hydration shells. The dynamics of water molecules in these clusters is significantly hindered, while that of other water molecules remains largely unaffected. The relaxation times measured by OKE and DRS have different concentration dependences because DRS probes only the rotational dynamics of water molecules outside of the cluster while OKE contains the information of dynamics of all of the water molecules in the solutions. Such findings provide us a microscopic picture on how the ion hydration affects the water dynamics in certain ionic solutions in which ion pairing plays an important role.

Modeling the low frequency vibrational spectroscopy of ionic solutions

“…As OKE and DRS were considered to measure the translation and rotation, respectively, such phenomena were interpreted as the reflection of a glass‐like behavior with translational and rotational motions decoupled. Our theoretical modeling nicely reproduced the experimentally observed concentration dependence . Further analysis demonstrates that the deviation of relaxation times reflects the water dynamics inside and outside of the cation hydration shells instead of the translation rotation decoupling.…”

“…Our theoretical modeling nicely reproduced the experimentally observed concentration dependence. [44] Further analysis demonstrates that the deviation of relaxation times reflects the water dynamics inside and outside of the cation hydration shells instead of the translation rotation decoupling. A microscopic picture of how the ions affect the dynamics and its spectroscopic reflections in the MgCl 2 solutions was proposed in which the ion pairing effect plays an important role.…”

“…The results indicate that the contribution from the rotation in OKE signal is considerable and the relaxation time of rotation component exhibits non-negligible concentration dependence. [44] Such projection scheme originated from a series of treatments of Kivelson, Keyes, and Ladanyi to analyze the Raman signals. [60][61][62][63][64] In this scheme, P I is separated into a portion which has the same time-evolution as P M and a "collisioninduced" which does not.…”

Molecular mechanism of water reorientational slowing down in concentrated ionic solutions

“…There have been spectroscopic reports suggesting that ions can affect the collective water rotational dynamics (1,13,19,33). For instance, long time decay in the time domain optical Kerr effect signal of liquid water and aqueous solutions has been assigned to the slow collective water structure dynamics (α relaxation) (33) and is composed of the contributions from both the rotational and translational motions. Through a projection scheme, the rotational component can be separated out and studied in detail.…”

Low frequency 2D Raman-THz spectroscopy of ionic solution: A simulation study

“…The dipole-induced-dipole scheme with the fixed polarizability tensor was employed to generate the fluctuating induced polarization for spectrum modeling. The same scheme has been successfully employed to simulate the dielectric relaxation spectrum and optical Kerr effect spectrum 60,66,75 of the same ionic solution. In principle, polarizable ion-water models can be used to achieve a more consistent modeling of the spectra based on the MD simulation.…”