Dynamical structure of water in aqueous electrolyte solutions by low‐frequency Raman scattering

Spectral Characteristics of Unsaturated and Supersaturated Inorganic Aerosols: Insights into Deliquescence Kinetics

The deliquescence phase transition of atmospheric aerosols is crucial for radiative forcing and atmospheric chemistry. However, the deliquescence kinetics of micrometer-sized aerosols, especially the formation and evolution of surface solution films, remain poorly understood. In this study, IR spectral characteristics were employed for the first time to quantify the solute concentration evolution in surface solution films. At a constant relative humidity (RH) of ∼65%, solution films on NaCl crystals exhibited a very low solute concentration (3.06 ± 0.18 mol/L), comparable to aqueous NaCl droplets above 90% RH. These films reached saturation at ∼74% RH, i.e., the deliquescence RH of NaCl, maintaining a nearly constant saturation level during deliquescence. In contrast, amorphous NaNO3 solids showed supersaturated solution films before deliquescence. Following deliquescence, the saturation level of solution phases increased due to faster solid dissolution rates than liquid water condensation. These findings address knowledge gaps in the complex nonequilibrium dissolution processes of crystalline or amorphous atmospheric aerosols.

Toward Metal Extraction from Regolith: Theoretical Investigation of the Solvation Structure and Dynamics of Metal Ions in Ionic Liquids

Lunar and Martian regoliths, containing feldspar, pyroxene, ilmenite, olivine, and aluminite minerals, are excellent sources of metals such as aluminum, sodium, magnesium, and iron. Ionic liquids (ILs), which are excellent solvents with extremely low vapor pressure and high electrochemical stability, can be potentially leveraged for extracting metals from regolith in an extra-terrestrial environment. A critical step in the solvation process, which determines the effectiveness of the IL solvent, is the formation of solvation shells around the metal cations. To determine the rigidity and stability of the solvation shells, which has a direct implication on the extraction of metals, we performed classical molecular dynamics simulations of dilute solutions comprising individual metal ions Na+, Mg2+, and Al3+ in two distinct ILs, [mppy][TFSI] and [mppy][HSO4]. Our results indicate that the compactness of the structure is directly related to the charge density of the metal cation and the relative size and symmetry of the IL anion. Potentials of the mean force of the metal cation with the solvating IL anion indicate the presence of energy minima with barriers that increase with the surface charge density of the cation. The increasing energy barrier leads to greater residence time of metal cations in the solvation shell, which was confirmed by evaluating corresponding autocorrelation functions. Overall, our calculations provide fundamental insights into key factors that influence the solvation of metals and can be useful in the screening of ILs for digestion of metal-containing minerals in lunar and Martian regoliths.

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.

Anion-water hydrogen bond vibration revealed by the terahertz Kerr effect

The microscopic mechanism for ionic influence on the hydrogen bond network of water has not been fully understood. Here we employ the terahertz Kerr effect (TKE) technique to map the intermolecular hydrogen bond dynamics in a series of aqueous halide solutions at the sub-picosecond scale. Compared with pure water, the significantly enhanced bipolar TKE response associated with polarization anisotropy in an ionic aqueous solution is successfully captured. We decompose the measured TKE response into different molecular motion modes and demonstrate that the obviously increasing positive polarity response is mainly due to the anion-water hydrogen bond vibration mode with the resonant THz electric field excitation. Our measurement results provide an experimental basis for further insight into the effects of ions on the structure and dynamics of a hydrogen bond in water.

Do water's electrons care about electrolytes?

Ions have a profound effect on the geometrical structure of liquid water and an aqueous environment is known to change the electronic structure of ions. Here we combine photoelectron spectroscopy measurements from liquid microjets with molecular dynamical and quantum chemical calculations to address the reverse question, to what extent do ions affect the electronic structure of liquid water? We study aqueous solutions of sodium iodide (NaI) over a wide concentration range, from nearly pure water to 8 M solutions, recording spectra in the 5 to 60 eV binding energy range to include all water valence and the solute Na+ 2p, I- 4d, and I- 5p orbital ionization peaks. We observe that the electron binding energies of the solute ions change only slightly as a function of electrolyte concentration, less than 150 ± 60 meV over an ∼8 M range. Furthermore, the photoelectron spectrum of liquid water is surprisingly mildly affected as we transform the sample from a dilute aqueous salt solution to a viscous, crystalline-like phase. The most noticeable spectral changes are a negative binding energy shift of the water 1b2 ionizing transition (up to -370 ± 60 meV) and a narrowing of the flat-top shape water 3a1 ionization feature (up to 450 ± 90 meV). A novel computationally efficient technique is introduced to calculate liquid-state photoemission spectra using small clusters from molecular dynamics (MD) simulations embedded in dielectric continuum. This theoretical treatment captured the characteristic positions and structures of the aqueous photoemission peaks, reproducing the experimentally observed narrowing of the water 3a1 feature and weak sensitivity of the water binding energies to electrolyte concentration. The calculations allowed us to attribute the small binding energy shifts to ion-induced disruptions of intermolecular electronic interactions. Furthermore, they demonstrate the importance of considering concentration-dependent screening lengths for a correct description of the electronic structure of solvated systems. Accounting for electronic screening, the calculations highlight the minimal effect of electrolyte concentration on the 1b1 binding energy reference, in accord with the experiments. This leads us to a key finding that the isolated, lowest-binding-energy, 1b1, photoemission feature of liquid water is a robust energetic reference for aqueous liquid microjet photoemission studies.

S-shaped velocity deformation induced by ionic hydration in aqueous salt solution flow

Ionic hydration shells are the most noticeable microscopic feature in an aqueous salt solution, and have attracted attention due to their possible contribution to its flow behavior. In this paper, we find by molecular dynamic simulations that an S-shaped velocity profile is induced by the ionic hydration shells in the nano channel flow. Our theoretical analysis implies a linear relationship between the energy density inside the first hydration shell of the ions and the deformation strength of the velocity profiles of aqueous salt solutions, where the deformation strength is quantified by the curvature length defined by the linear deviation extended from the velocity profile. Our simulation results confirm that such a linear relationship holds for chloride salt solutions with monatomic cations, e.g., K, Na, Ca, Mg, Al and the Na/Ca models by varying the valence number of Na and Ca in the salt solutions. Furthermore, the influence of the flow velocity and the channel width upon the velocity deformation strength are also investigated. Our results indicate that the calculated curvature length provides a numerical evaluation for nano flow behavior and would be helpful in nanofluidic device design.

Polarizability series of aqueous polyatomic anions revealed by femtosecond Kerr effect spectroscopy

Femtosecond OHD-RIKES measurements show that the hyperpolarizability series of aqueous polyatomic anions increases in the following sequence HPO₄²⁻ < HSO₄⁻ < CO₃²⁻ < AC⁻ < NO₃⁻ < SCN⁻.

A first principles theoretical study of vibrational spectral diffusion and hydrogen bond dynamics in aqueous ionic solutions: D2O in hydration shells of Cl(-) ions

A theoretical study of vibrational spectral diffusion and hydrogen bond dynamics in aqueous ionic solutions is presented from first principles without employing any empirical potential models. The present calculations are based on ab initio molecular dynamics for trajectory generation and wavelet analysis of the simulated trajectories for time dependent frequency calculations. Results are obtained for two different deuterated aqueous solutions: the first one is a relatively dilute solution of a single Cl(-) ion and the second one is a concentrated solution of NaCl ( approximately 3M) dissolved in liquid D(2)O. It is found that the frequencies of OD bonds in the anion hydration shell, i.e., those which are hydrogen bonded to the chloride ion, have a higher stretch frequency than those in the bulk water. Also, on average, the frequencies of hydration shell OD modes are found to increase with increase in the anion-water hydrogen bond distance. On the dynamical side, when the vibrational spectral diffusion is calculated exclusively for the hydration shell water molecules in the first solution, the dynamics reveals three time scales: a short-time relaxation ( approximately 200 fs) corresponding to the dynamics of intact ion-water hydrogen bonds, a slower relaxation ( approximately 3 ps) corresponding to the lifetimes of chloride ion-water hydrogen bonds, and another longer-time constant ( approximately 20 ps) corresponding to the escape dynamics of water from the anion hydration shell. Existence of such three time scales for hydration shell water molecules was also reported earlier for water containing a single iodide ion using classical molecular dynamics [B. Nigro et al., J. Phys. Chem. A 110, 11237 (2006)]. Hence, the present study confirms the basic results of this earlier work using a different methodology. However, when the vibrational spectral diffusion is calculated over all the OD modes, only two time scales of approximately 150 fs and approximately 2.7 ps are found without the slowest component of approximately 20 ps. This is likely because of the very small weight that the hydration shell water molecules carry to the overall spectral diffusion in the solution containing a single ion. For the concentrated solution also, the slowest component of approximately 20 ps is not found in the spectral diffusion of all water molecules because a distinct separation between the hydration shell and bulk water in terms of their stretch frequencies does not hold at this high concentration regime. The present first principles results are compared with those of the available experiments and classical simulations.

Temperature-dependent Time-resolved Fluorescence Study of Cinchonine Alkaloid Dication

Photo induced excited state dynamical processes of cinchonine alkaloid dication (C(++)) have been studied over a wide range of temperature using steady state and nanosecond time-resolved fluorescence spectroscopic techniques. The temperature-dependent fluorescence studies of C(++) clearly indicate the existence of two distinct emitting species having their own characteristic decay rates. The shorter-lived species shows a usual temperature dependence with increasing non-radiative deactivation at higher temperatures, while the longer-lived species show features resembling to the excited state solvent relaxation process with a large solvent relaxation time (tau(r) approximately 6 ns). The species emitting in the lower energy side, having longer decay time is found to be more sensitive towards chloride ion quenching and has a charge transfer character. Further, concentration quenching with decrease in tau(r) of long lived species shows the possibility of energy migration along with solvent relaxation in C(++).

Ion Association and Hydration in Aqueous Solutions of LiCl and Li2SO4 by Dielectric Spectroscopy

A systematic study of the dielectric relaxation spectra of aqueous solutions of LiCl and Li2SO4 has been made at solute concentrations of 0.05 < or = c/M < or = 1.0 and 2.0, respectively, and over a wide range of frequencies (0.2 < or = nu/GHz < or = 89) at 25 degrees C. The spectra were best described by a superposition of four Debye processes, consisting of the two well-known water relaxations at ca. 8 and 0.5 ps and two ion-pair contributions at ca. 200 and 20 ps, corresponding to the presence of double-solvent-separated (2SIP) and solvent-shared (SIP) ion pairs, respectively. Consistent with spectroscopic studies, no contact ion pairs were detected over the studied concentration range. The overall ion association constants K(o)(A) obtained were in good agreement with literature data for both salts. Detailed analysis of the solvent relaxations indicated that Li+ has a significant second solvation sheath although there were differences between the effective hydration numbers obtained from LiCl and Li2SO4, which might arise from competition for the solvent from the anions.

Confocal Raman Observation of the Efflorescence/Deliquescence Processes of Individual NaNO3 Particles on Quartz

Confocal Raman spectroscopy was used to study the structural changes of bulk NaNO3 solutions with molar water-to-solute ratios (WSRs) of 54.0-12.3 and NaNO3 droplets (10-100 microm) with WSRs of 9.5-1.0 on a quartz substrate. Upon reduction of the WSR, a blue shift of the symmetric stretching band (nu(1)(NO3-)) from approximately 1048 to approximately 1058 cm(-1) was observed in the confocal Raman spectra with high signal-to-noise ratios. Accordingly, the full width at half-height of the nu(1)(NO3-) band increased from approximately 8.4 cm-1 for the dilute solution (WSR = 54.0) to approximately 15.6 cm-1 for the extremely supersaturated droplet (WSR = 1.0), suggesting the formation of contact ion pairs with different structures. For the O-H stretching band, the ratio of weak hydrogen-bonding components to strong ones, i.e., I(3488)/I(3256), increased from approximately 1.2 at WSR = 54.0 to approximately 7.3 at WSR = 1.0, indicating that the strong hydrogen bonds were heavily destroyed between water molecules especially in the supersaturated droplets. In the humidifying process, two hygroscopic behaviors were observed depending on the morphology of solid NaNO3 particles. No surface water was detected for a solid NaNO3 particle with rhombohedral shape at relative humidities (RHs) below 86%. When the RH increased from 86% to 93%, it suddenly absorbed water and turned into a solution droplet. For a maple-leaf-shaped NaNO3 particle with a rough surface, however, a trace of residual water originally remained on the rough surface even at very low RH according to its Raman spectrum. Its initial water uptake from the ambient occurred at approximately 70% RH. The small amount of initially adsorbed water induced surface rearrangement of the maple-leaf-shaped particle. A further increase of RH made the particle gradually turn into a regular solid core swathed in a solution layer. Eventually, it completely deliquesced in the RH region of 86-93%, similar to the case of the NaNO3 particle with rhombohedral shape.

On the structure of aqueous LiCl solutions

The structure of highly concentrated aqueous lithium chloride solutions was investigated by the Reverse Monte Carlo method. Two total structure factors, obtained from neutron and x-ray diffraction experiments, were applied as input information. From the resulting particle configurations, partial pair correlation functions, coordination numbers and cosine distributions of bond angles have been determined. It was found that, in accordance with common-sense expectations, the hydrogen bonded network of water molecules is breaking up continuously as the concentration of the electrolyte increases. The hydration shell of the cations becomes more and more distorted as concentration grows whereas the hydration structure of the anions appears to be nearly invariant. Ion-pairing was not detected even at the highest salt concentration.

Possibility of breakdown of overdamped and narrowing limits in low-frequency Raman spectra: phenomenological band-shape analysis using the multiple-random-telegraph model

Depolarized low-frequency Raman spectra of liquid water and heavy water are investigated from 266 K to 356 K. The reduced Raman spectra below 250 cm(-1) are reproduced by a superposition of one relaxation mode and two damped harmonic oscillator modes. The multiple-random-telegraph (MRT) model, which takes into account inertia and memory effects, is applied to analyze the relaxation component. Two damped harmonic oscillators around 50 cm(-1) and 180 cm(-1) are known as a bendinglike mode and a stretchinglike mode, respectively. It is found that the intensity of the bendinglike mode in water (heavy water) gradually decreases with increasing temperature, and finally vanishes above about 296 K (306 K). The relaxation time of the MRT model is interpreted as representing the averaged lifetime of the vibrating unit. At high temperature, the relaxation time becomes short, that is to say, the vibrating unit is quickly destroyed before the 50 cm(-1) mode is oscillating sufficiently. In the present analysis, the strongly disrupted oscillation cannot be distinguished from the relaxation mode which includes the inertia and memory effects. It is found that the low-frequency Raman spectrum of liquid water at high temperature is a good example demonstrating an application of the MRT model.