Low-frequency dynamics of aqueous alkali chloride solutions as probed by terahertz spectroscopy

Revealing the Molecular Origin of Anisotropy around Chloride Ions in Bulk Water

A clear picture of the local solvation structure around halide anions in liquid water remains elusive. This discussion has been stimulated by pioneering simulation results that proposed a "hydrophobic cavity" around anions in the bulk, which is analogous to air at the air-water interface. However, there is also sound experimental and theoretical evidence that halide ions are rather symmetrically solvated in the bulk, leading to a different viewpoint. Using extensive ab initio molecular dynamics simulations of an aqueous Cl- solution, we indeed find an anisotropic arrangement of H-bonded versus interstitial water molecules. The latter are not H-bonded to the anions and thus do not couple much electronically to Cl-. The resulting purely electronic anisotropy of the local solvation environment correlates with that structural anisotropy, which however should not be understood as an empty cavity─as it would be at the air-water interface─but rather contains interstitial water molecules.

Terahertz Spectroscopy of Aqueous Solutions of Sodium Halides (NaX): Self- and Cross-Correlation Contributions of Ions and Hydration Shell Water for X- = F-, Cl-, Br-, and I. J Phys Chem B 2023;127:9323-9335. [PMID: 37871257 DOI: 10.1021/acs.jpcb.3c05228] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

We investigated the terahertz (THz) absorption spectra of aqueous sodium halide solutions through molecular dynamics simulations using polarizable models of both water and ions. Specifically, we have considered aqueous solutions (∼1 M) of NaF, NaCl, NaBr, and NaI and calculated the difference THz spectrum of these solutions by subtracting the corresponding pure water contribution from the total THz spectrum of an ionic solution. The difference absorption spectrum of a given solution is then dissected into contributions from ion and ion-water correlations and also modifications of water-water correlations in the presence of the ions. The different components are further decomposed into induced dipole and permanent charge/dipole components and also into self- and cross-correlation components. The ion-water cross-correlation components are subsequently decomposed into contributions coming from different solvation shells through radially resolved calculations of such ion-water cross-correlations. Through all of these dissections, we could investigate the origin of different parts of the difference THz spectra of the sodium halide solutions studied here. It is found that while features below or around 100 cm-1 and also around 200 cm-1 arise mainly from ion and ion-water motion, that at the librational region above 600 cm-1 primarily originates from changes in water librational motion influenced by the ions. The variations of intensities of different components are also linked to the size and charge density of the anions in the solutions.

Dielectric property measurement of human sweat using attenuated total reflection terahertz time domain spectroscopy

Sweat is one of the essential biofluids produced by the human body, and it contains various physiological biomarkers. These biomarkers can indicate human health conditions such as disease and illness. In particular, imbalances in the concentration of electrolytes can indicate the onset of disease. These same imbalances affect the dielectric properties of sweat. In this study, we used attenuated total reflection terahertz time domain spectroscopy to obtain the frequency-dependent dielectric properties of human sweat in a frequency range from 200 GHz to 2.5 THz. We have investigated the variation of dielectric properties of sweat collected from different regions of the human body, and we have observed that the real and imaginary part of dielectric permittivity decreases with the increase in frequency. A combination of left-hand Jonscher and Havriliak-Negami processes is used to model the results and reveal the presence of relaxation processes related to sodium and calcium ions concentrations. This information may help design novel biosensors to understand the human health condition and provide a hydration assessment.

Ion Hydration and Ion Pairing as Probed by THz Spectroscopy

Ion hydration is of pivotal importance for many fundamental processes. Various spectroscopic methods are used to study the retardation of the hydration bond dynamics in the vicinity of anions and cations. Here we introduce THz-FTIR spectroscopy as a powerful method to answer the open questions. We show through dissection of THz spectra that we can pinpoint characteristic absorption features that can be attributed to the rattling modes of strongly hydrating ions within their hydration cages as well as vibrationally induced charge fluctuations in the case of weakly hydrating ions. Further analysis yields information on anion-cation cooperativity, the size of the dynamic hydration shell, as well as the lifetimes of these collective ion-hydration water modes and their connecting thermal bath states. Our study provides evidence for a non-additive behavior, thus questioning the simplified Hofmeister model. THz spectroscopy enables ion pairing to be observed and quantified at a high salt concentration.

The effect of Hofmeister anions on water structure at protein surfaces

To understand the effects of specific ions on protein-water interactions and the thermodynamic stability of proteins in salt solutions, we use a molecular dynamics (MD) simulation to examine the water structure, orientational distribution, and dynamics near the surface of ubiquitin. In particular, we consider NaCl, NaBF₄, NaSCN, and NaClO₄ salt solutions containing ubiquitin, where the anions of the latter three salts are well-known chaotropic ions in the Hofmiester anion series. The number of hydrogen bonds (H-bonds) per water molecule is found to decrease significantly at the ubiquitin-water interface, indicating a significant disruption of the water H-bonding network. The distribution of the water H-bond numbers near the protein surface is modulated by dissolved ions, and the extent of the ion effect on the H-bonding network structure follows the order of the Hofmeister anion series, while there are no specific ion effects on water properties at distances larger than 5 Å from the protein surface. From detailed analyses of the surface area, volume, and root-mean-square deviation (RMSD) of ubiquitin, we show that changes in the properties of the protein could originate from the disruption of the water H-bond network induced by ions with a higher affinity for the protein surface instead of direct protein residue-ion interactions. An interesting observation made here is that the orientational distribution of water molecules at the protein-water interface is close to random, but there is a slight preference for interfacial water molecules with a straddle structure within 2.5 Å of the protein surface, where one of the two OH groups points away from the protein surface and the other points toward the surface. In addition, comparing the MD simulation results for ubiquitin solutions with dissolved NaSCN and KSCN, we show that Na⁺ affects the water H-bonding structure at the protein surface more than K⁺. It is clear that the H-bonding network structure of water more than one water layer away from the protein surface is not distinguishably different from that of neat water. We thus anticipate that the present work will provide insights into the scale of specific ion effects on the H-bonding structure and orientational distribution of water in the vicinity of protein surfaces in aqueous solutions.

Correlations in the Solute-Solvent Dynamics Reach Beyond the First Hydration Shell of Ions

While the real-space structure of solvation shells has been explored for decades, a dynamical perspective that directly relies on changes in the H-bond network became accessible more recently mainly via far-infrared (THz) spectroscopies. A remaining key question is how many hydration shells are affected by ion-induced network perturbations. We disclose that theoretical THz difference spectra of aqueous salt solutions can be deciphered in terms of only a handful of dipolar auto- and cross-correlations, including the second solvation shell. This emphasizes the importance of cross-correlations being often neglected in multicomponent models. Analogously, experimental THz responses of simple ions can be deciphered in a similar way. Dramatic intensity cancellations due to large positive and negative contributions are found to effectively shift intensity maxima. Thus, THz spectroscopy provides an unprecedented view on the details of hydration dynamics, which can be understood by a combination of experiment and theory.

Negligible cation effect on the vibrational relaxation dynamics of water molecules in NaClO4 and LiClO4 aqueous electrolyte solutions

Negligible cation effects on the vibrational relaxation dynamics of water molecules in NaClO₄ and LiClO₄ aqueous solutions.