Glasslike behavior in aqueous electrolyte solutions

Abstract

When salts are added to water, the viscosity generally increases suggesting the ions increase the strength of the water's hydrogen-bond network. However, infrared pump-probe measurements on electrolyte solutions have found that ions have no influence on the rotational dynamics of water molecules implying no enhancement or breakdown of the hydrogen-bond network. Here we report optical Kerr-effect and dielectric relaxation spectroscopic measurements, which have enabled us to separate the effects of rotational and transitional motions of the water molecules. These data show that electrolyte solutions behave like a supercooled liquid approaching a glass transition in which rotational and translational molecular motions are decoupled. It is now possible to understand previously conflicting viscosity data, nuclear magnetic resonance relaxation, and ultrafast infrared spectroscopy in a single unified picture.It is well known that when salts are added to water the viscosity typically increases, suggesting that the ions alter the hydrogen-bond network of the water, 1 which appears to be confirmed by, for example, neutron diffraction experiments. 2 However, recent ultrafast infrared pump-probe measurements on electrolyte solutions have found that ions do not influence the rotational dynamics of water molecules suggesting that there is no enhancement or breakdown of the hydrogen-bond network in liquid water. 3 As the effect of ions and ionic moieties on the structure of water is important for understanding protein stability, enzymatic reactions, and substrate binding, it is crucial to resolve this paradox. 4 Here we report ultrafast optical Kerr effect 5 (OKE) and dielectric relaxation 6 (DR) spectroscopy measurements, which show that salt solutions behave like a supercooled liquid approaching a glass transition, where rotational and translational molecular motions become decoupled. The rotational motions of bulk water molecules -observed as an -relaxation in DR -are essentially independent of concentration. The translational motions seen in OKE spectroscopy can be understood as a -relaxation 7 and their dynamics become increasingly inhomogeneous with increasing salt concentration. This insight reconciles previously conflicting viscosity data, 8 nuclear magnetic resonance relaxation, 9 and ultrafast infrared spectroscopy 3 data in a single unifying picture.When simple inorganic salts are added to water, the viscosity typically increases ( see FIG. 1). In the relatively low concentration range (up to ~0.5 M), this is described by the semi-empirical Jones-Dole equation, 1 , which expresses the shear viscosity in terms of the viscosity of pure water 0 , salt concentration c, and parameters A and B. For many salts additional terms have to be added to describe the rapid increase in viscosity at higher concentrations. The obvious conclusion to draw from this behavior is that ions alter the structure of water. Indeed, the empirical Jones-Dole B coefficient is often used to classify ions as either structure makers (kosmotropes...

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

“…In Figure 1, the τ 2 relaxation time measured by OKE has a strong correlation with the concentration, while the τ 1 relaxation time measured by DRS is almost invariant with concentration change. This observation has a plausible agreement with the experiments, 45 which allows us to further analyze the simulation results to understand the correlation between the ion hydration and solution dynamics, as well as its reflection on the OKE and DRS observables.…”

“…The relaxation of the total polarizability's TCF in Figure 4b is governed by the induced term, which contains information about the relative motions between the molecules including translation and rotation. The difference between the OKE and DRS measurements was considered to be the reflection of a glass-like behavior 45 suggested by the observation that the concentration dependence of τ 2 follows eq 19, which was designed to describe the glass transition. At a higher concentration such as 4.4 M, the ratio between the numbers of waters and magnesiums is about 11:1, with approximately seven water molecules in the hydration shell of the magnesium, presuming that the MgCl 2 is completely dissociated and evenly distributed in the solution.…”

“…11,45 This "complete dissociation" picture works nicely for the very dilute ionic solutions. When the concentration gets higher, however, this picture starts to fail.…”

“…We note here that in ref 45, τ 2 was calculated by fitting the OKE signal in the frequency domain using a single Cole−Cole function, and in our case, τ 2 was generated instead by fitting the time domain OKE signals with the time derivation of a stretched exponential function. The characteristic relaxation times from these two fitting procedures should reflect the same microscopic dynamics.…”

“…Due to their sensitivities to different aspects of the molecular mobilities, various spectroscopic techniques are often combined to achieve a more comprehensive understanding of the dynamics in certain ionic solutions. 11,13,39,41,45,52 As an example, DRS and OKE relaxation spectroscopic measurements were applied to the MgCl 2 aqueous solutions together. 45 The τ 2 relaxation time measured by OKE increases with concentration with a rapid acceleration, and the τ 1 relaxation time measured by DRS is almost invariant with the concentration.…”