Glasslike behavior in aqueous electrolyte solutions

Journal of the American Chemical Society, 1998

Rotational diffusion of organic ions in electrolyte solution is studied via optically heterodyned polarization spectroscopy and molecular dynamics (MD) simulations. Significant differences between the behavior of organic cation and anion species were observed in the experiments. While the rotational relaxation time of the anion normalized by the viscosity of the solution increases with the electrolyte concentration, the normalized relaxation time of the cation decreases with increasing electrolyte concentration. The experimental data are analyzed by using a continuum theory approach and MD simulations. It is demonstrated that one must include ion pairs to describe the dynamics of the anion, but the analysis of the relaxation of the cation does not require ion pairing. MD simulations show that the difference in the dynamics of the anion and cation in electrolyte solution is caused by the different ability of free anion, free cation, and ion paired species to associate with the solvent (DMSO).

The water molecule has the convenient property that its molecular polarizability tensor is nearly isotropic while its dipole moment is large. As a result, the low-frequency anisotropic Raman spectrum of liquid water is mostly collision induced and therefore reports primarily translational motions while the far-infrared (terahertz) and dielectric spectrum is dominated by rotational modes. Atomic and globular-molecular liquids have a zero dipole moment as well as an isotropic polarizability tensor. These spectrum-simplifying properties were exploited in a study of a number of liquids and solutions using ultrafast optical Kerr-effect (OKE) spectroscopy combined with dielectric relaxation spectroscopy (DRS), terahertz time-domain spectroscopy (THz-TDS), and terahertz field-induced secondharmonic generation (TFISH) spectroscopy. For room-temperature ionic liquids (RTILs), liquid water, aqueous salt solutions, noble gas liquids, and globular molecular liquids it was found that, in each case, surprising structure and/or inhomogeneity is observed, ranging from mesoscopic clustering in RTILs to stretched exponential dynamics in the noble gas liquids. For aqueous electrolyte solutions it is shown that the viscosity, normally described by the Jones-Dole expression, can be explained in terms of a jamming transition, a concept borrowed from soft condensed matter studies of glass transitions in colloidal suspensions.

The ultrafast rotational-diffusive dynamics of the peptide linkage model compounds N-methylacetamide (NMA), acetamide (Ac), and N,N-dimethylacetamide (DMA) have been studied as a function of temperature using optically heterodyne-detected optical Kerr effect (OHD-OKE) spectroscopy. Both NMA and Ac exhibit a non-Arrhenius temperature dependence of the rotational diffusive relaxation time. By contrast, the non-hydrogen-bonding DMA exhibits normal hydrodynamic behavior. The unusual dynamics of NMA and Ac are attributed to the decoupling of single-molecule rotational diffusive relaxation from the shear viscosity via a transition between stick and slip boundary conditions, which arises from local heterogeneity in the liquid due to the formation of hydrogen-bonded chains or clusters. This provides new insight into the structure and dynamics of an important peptide model compound and the first instance of such a phenomenon in a room-temperature liquid. The OHD-OKE responses of carboxylic acids acetic acid (AcOH) and dichloroacetic acid (DCA) are also reported. These, along with the terahertz Raman spectra, show no evidence of the effects observed in amide systems, but display trends consistent with the presence of an equilibrium between the linear and cyclic dimer structures at all temperatures and moderate-to-high mole fractions in aqueous solution. This equilibrium manifests itself as hydrodynamic behavior in the liquid phase.

The Journal of Chemical Physics, 2005

ChemPhysChem, 2020

We present an atomistic simulation scheme for the determination of the hydration number (h) of aqueous electrolyte solutions based on the calculation of the water dipole reorientation dynamics. In this methodology, the time evolution of an aqueous electrolyte solution generated from ab initio molecular dynamics simulations is used to compute the reorientation time of different water subpopulations. The value of h is determined by considering whether the reorientation time of the water subpopulations is retarded with respect to bulk-like behavior. The application of this computational protocol to magnesium chloride (MgCl 2) solutions at different concentrations (0.6-2.8 mol kg À 1) gives h values in excellent agreement with experimental hydration numbers obtained using GHz-to-THz dielectric relaxation spectroscopy. This methodology is attractive because it is based on a well-defined criterion for the definition of hydration number and provides a link with the molecular-level processes responsible for affecting bulk solution behavior. Analysis of the ab initio molecular dynamics trajectories using radial distribution functions, hydrogen bonding statistics, vibrational density of states, water-water hydrogen bonding lifetimes, and water dipole reorientation reveals that MgCl 2 has a considerable influence on the hydrogen bond network compared with bulk water. These effects have been assigned to the specific strong Mg-water interaction rather than the Cl-water interaction.

The Journal of Chemical Physics, 2000

Femtosecond mid-infrared pump-probe spectroscopy is used to study the orientational relaxation of HDO molecules dissolved in liquid D 2 O. In this technique, the excitation of the O-H stretch vibration is used as a label in order to follow the orientational motion of the HDO molecules. The decay of the anisotropy is nonexponential with a typical time scale of 1 ps and can be described with a model in which the reorientation time depends on frequency and in which the previously observed spectral diffusion is incorporated. From the frequency and temperature dependence of the anisotropy decay, the activation energy for reorientation can be derived. This activation energy is found to increase with increasing hydrogen bond strength.

Physical Chemistry Chemical Physics, 2001

A model is given where the complex permittivity of an electrolyte solution is calculated as a superposition of the contributions due to the translation of ions and the reorientation of water molecules, which occur in intermolecular potential wells during the lifetime of local order in liquids. Simple analytical expressions are found for the contributions of cations and anions to the linear-response spectral function. The one-dimensional rectangular potential well with perfectly elastic walls is considered. The contribution of water molecules to the orientational relaxation was calculated in terms of a hybrid model using the approach given recently in a book by Gaiduk (Dielectric Relaxation and Dynamics of Polar Molecules, World ScientiÐc, Singapore, 1999). A modiÐcation of this model is also suggested in which the walls of the potential well undergo rather slow vibration. If the angular frequency u is much less than the plasma frequency of an ion, then the theory u p yields a nearly constant real part p@ of the complex ionic conductivity p(u), while its imaginary part pA is very small. Large variations with u of both parts of p are predicted to occur at millimetre/submillimetre wavelengths, when u approaches Wideband (up to 1000 cm~1) theoretical spectra of the complex u p . permittivity and absorption coefficient are calculated for NaClÈwater and KClÈwater solutions. The theory predicts that an additional loss/absorption could arise in the far-infrared (FIR) spectral range, if the mean ionic lifetime is much longer than the lifetime q of the bulk water molecules. q ion

The Journal of Physical Chemistry A, 2003

Nuclear magnetic resonance chemical shifts are used to examine the perturbations in water structure that occur with concentration changes in aqueous KF, KCl, and LiOH solutions. Changes in the slope of ion chemical shifts as a function of solute concentration can be explained by changes in water structure. The equilibrium shift in water structure occurs as a result of changes in the hydrogen bond strength. The changes in hydrogen bond strength are a result of changes in electrolyte concentration and electron delocalization throughout the liquid. The location of the changes in slope with concentration is temperature dependent. A correlation of the changes in slope of chemical shifts to minima in specific heat capacity suggests the occurrence of a weak continuous transition in the solution structure at the critical concentration corresponding to the specific heat capacity minimum. By extrapolation the experiments reported here imply that there is a weak continuous transition associated with the heat capacity minimum for pure water. There must also be a structural relaxation time in the liquid associated with this transition. The results of these experiments provide confirmation for the model of aqueous solutions we recently proposed in which the solution is composed of regions of pure water and regions of liquid crystalline electrolyte hydrates. The subphase composed of structurally perturbed water is the part of the system that participates in the weak continuous phase transition that is evidenced by the NMR chemical shifts. In complete agreement with earlier Raman experiments it appears that the entire solution is a single electronic whole with exquisite electronic delocalization between the water and liquid crystalline subphases so that the ionic nuclei experience the electronic effects of the transition in the water subphase.

The Journal of Chemical Physics, 2002

A new theory is proposed to describe spectral effects of the coupling between molecular rotations and OH¯O motions in liquid water. The correlation function approach is employed together with a special type of development in which the coupling energy of these two motions is the expansion parameter. The isotropy of the liquid medium plays an essential role in this study. Based on this theory, a new infrared pump-probe experiment is described permitting a visualization of molecular rotations at subpicosecond time scales. Full curves relating the mean squared rotational angle and time, and not only the rotational relaxation time, are measured by this experiment. However, very short times where the incident pulses overlap must be avoided in this analysis. The lifetime of OH¯O bonds in water is rotation-limited.

Chemical Physics, 2011

The dynamics of hydrogen bond (H-bond) formation and dissociation depend intimately on the dynamics of water rotation. We have used polarization resolved ultrafast two-dimensional infrared (2DIR) spectroscopy to investigate the rotational dynamics of deuterated hydroxyl groups (OD) in a solution of 6M NaClO 4 dissolved in isotopically mixed water. Aqueous 6M NaClO 4 has two peaks in the OD stretching region, one associated with hydroxyl groups that donate a H-bond to another water molecule (OD W ) and one associated with hydroxyl groups that donate a H-bond to a perchlorate anion (OD P ). Two-dimensional IR spectroscopy temporally resolves the equilibrium inter conversion of these spectrally distinct H-bond configurations, while polarization-selective 2DIR allows us to access the orientational motions associated with this chemical exchange. We have developed a general jump-exchange kinetic theory to model angular jumps associated with chemical exchange events. We use this to model polarization-selective 2DIR spectra and pump-probe anisotropy measurements. We determine the H-bond exchange induced jump angle to be 49 ± 5 • and the H-bond exchange rate to be 6 ± 1 ps. Additionally, the separation of the 2DIR signal into contributions that have or have not undergone H-bond exchange allows us to directly determine the orientational dynamics of the OD W and the OD P configurations without contributions from the exchanged population. This proves to be important because the orientational relaxation dynamics of the populations that have undergone a H-bond exchange differ significantly from the populations that remain in one H-bond configuration. We have determined the slow orientational relaxation time constant to be 6.0 ± 1 ps for the OD W configuration and 8.3 ± 1 ps for the OD P configuration. We conclude from these measurements that the orientational dynamics of hydroxyl groups in distinct H-bond configurations do differ, but not significantly.