Sodium Halide Solutions Research Articles

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.

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.

Microfluidic Investigation of the Ion-Specific Effect on Bubble Coalescence in Salt Solutions.

A microfluidic method was developed to study the ion-specific effect on bubble coalescence in salt solutions. Compared with other reported methods, microfluidics provides a more direct and accurate means of measuring bubble coalescence in salt solutions. We analyzed the coalescence time and approach velocity between bubbles and used simulation to investigate the pressure evolution during the coalescence process. The coalescence time of the three salt solutions decreased initially and then increased as the concentration of the salt solution was increased. The concentration with the shortest coalescence time is considered as the transition concentration (TC) and exhibits ion-specific. At the TC, the change in coalescence time indicates a shift in the effect of salt on bubble coalescence from facilitation to initial inhibition. Meanwhile, it can be seen that the sodium halide solutions significantly inhibit the bubble coalescence and the inhibition capability follows the order NaCl > NaBr > NaI. The results of the approach velocity show that the coalescence time decreases with increasing approach velocity, as well as the approach velocity was strongly influenced by concentration. The approach velocity undergoes a significant change at the TC. Furthermore, simulations of bubble coalescence in the microchannel indicate that the vertical pressure gradient at the center point of the bubble pairs increases as bubbles approach, driving liquid film drainage until bubble coalescence. The pressure at the center of the bubble pair reaches the maximum when the bubbles have first coalesced. It was further revealed that the concentration of the salt solution has a significant impact on the maximum pressure, as evidenced by the observed trend of decreasing pressure values with increasing concentrations.

Insight into the Ionizing Surface Potential Method and Aqueous Sodium Halide Surfaces.

Complementing the microscopic picture of the surface structure of electrolyte solutions set out by previous theoretical and experimental studies, the ionizing surface potential technique offers a unique approach to quantifying the impact of aqueous inorganic ions upon the interfacial electric field of the air-aqueous interface. In this Feature Article, we review the vulnerability of theoretical and empirically derived χwater values as a normative reference for aqueous ion surface potentials. Instead, we recognize and evaluate aqueous ion surface potentials relative to well-known ionic surfactants cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS). Additionally, we also explore factors that impact the magnitude of the measured surface potentials using the ionizing method, particularly in the type of reference electrode and ionizing gas environment. With potential measurements of sodium halide solutions, we show that iodide has a dominant effect on the air-aqueous electric field. Compared to chloride and bromide, iodide is directly observed with a net negatively charged surface electric field at all salt concentrations measured (0.2 to 3.0 mol/kg water). Also, above the 2 M region, bromide is observed with a net negatively charged surface. Although several scenarios contribute to this effect, it is most likely due to the surface enrichment of bromide and iodide. While the results of this study are pertinent to determining the specific interfacial reactivity of aqueous halides, these anions seldom transpire as single-halide systems in the natural environment. Therefore, we also provide an outlook on future research concerning surface potential methods and more complex aqueous electrolyte systems.

Ceftriaxone Degradation in the Presence of Sodium Halides Investigated by Electrochemical Methods Assisted by UV-Vis Spectrophotometry

The electrochemical stability of ceftriaxone (CFTX), belonging to the third generation of cephalosporin antibiotics, was studied by electrochemical measurements recorded on a platinum electrode (Pt) in aqueous solutions containing sodium halides. The electrochemical behavior of ceftriaxone was investigated by cyclic voltammetry (CV) and constant current density electrolysis assisted by UV-Vis spectrophotometry. Cyclic voltammetry highlighted that the addition of CFTX in sodium halide solutions leads to significant changes in the hysteresis characteristics due to specific interactions with active species from electrolytes, as well as with the platinum electrode surface. After CV, when an exterior electric stimulus in short time (40 s) was applied, the UV-Vis spectra illustrated that CFTX is stable in the presence of F− ions, it is electro(degraded/transformed) in the presence of Cl− and Br− ions and interacts instantly with I− species. Electrolysis at constant current density confirms the results obtained from cyclic voltammetry, showing that (i) in the presence of fluoride ions CFTX gradually decomposes, but not completely, in about 60 min, without identifying a reaction product; (ii) chloride and bromide ions determine the almost complete CFTX electro(degradation/transformation) in 10 and 5 min, respectively, with completion of the electro-transformation reaction after 60 and 30 min, respectively; (iii) instantaneous interactions between CFTX and the iodide ions occurred.

Examining the Hofmeister Series through Activation Energies: Water Diffusion in Aqueous Alkali-Halide Solutions.

The effect of ions on the properties of aqueous solutions is often categorized in terms of the Hofmeister series that ranks them from chaotropes ("structure-breakers"), which weaken the surrounding hydrogen-bond network to kosmotropes ("structure-makers"), which enhance it. Here, we investigate the Hofmeister series in ∼1 M sodium-halide solutions using molecular dynamics simulations to calculate the effect of the identity and proximity of the halide anion on both the water diffusion coefficient and its activation energy. A recently developed method for calculating the activation energy from a single-temperature simulation is used, which also permits a rigorous decomposition into contributions from different interactions and motions. The mechanisms of the salt effects on the water dynamics are explored by separately considering water molecules based on their location relative to the ions. The results show that water diffusion is accelerated moving down the halide group from F- to I-. The behavior of the diffusion activation energy, Ea, is more complex, indicating a significant role for entropic effects. However, water molecules in the first or second solvation shell of an ion exhibit a decrease in Ea moving down the halide series and Na+ exhibits a larger effect than any of the anions. The Ea for water molecules within the second solvation shell of an ion are modest, indicating a short-ranged nature of the ion influence.

Surface Affinity of Alkali and Halide Ions in Their Aqueous Solution: Insight from Intrinsic Density Analysis.

The surface tension of all aqueous alkali halide solutions is higher than that of pure water. According to the Gibbs adsorption equation, this indicates a net depletion of these ions in the interfacial region. However, simulations and experiments show that large, soft ions, such as I-, can accumulate at the liquid/vapor interface. The presence of a loose hydration shell is usually considered to be the reason for this behavior. In this work, we perform computer simulations to characterize the liquid-vapor interface of aqueous alkali chloride and sodium halide solutions systematically, considering all ions from Li+ to Cs+ and from F- to I-. Using computational methods for the removal of surface fluctuations, we analyze the structure of the interface at a dramatically enhanced resolution, showing that the positive excess originates in the very first molecular layer and that the next 3-4 layers account for the net negative excess. With the help of a fictitious system with charge-inverted ion pairs, we also show that it is not possible to rationalize the surface affinity of ions in solutions in terms of the properties of anions and cations separately. Moreover, the surface excess is generally dominated by the smaller of the two ions.

Understanding of chemical reactions induced by argon plasma in contact with sodium halide solutions: importance of surface properties for plasma–liquid interactions

Plasma in contact with liquid is widely studied in research involving water purification, nanoparticle synthesis, biomedical applications, and so on. However, plasma–liquid interactions, which are physical and chemical phenomena occurring at a plasma–liquid interface, have not yet been fully understood. To deepen our understanding of the interactions, in this study, we have investigated where specifically short-lived active species generated by plasma react with solutes at the plasma–liquid interface. We adopted chloride (Cl−), bromide (Br−) and iodide (I−) ions as indicators of the reaction region of the short-lived active species, because it is known that these halide ions are located closer to the gas–liquid interface in an order of I−, Br−, and Cl−. A DC argon plasma was irradiated to 2.1 mol l−1 sodium chloride (NaCl), bromide (NaBr), and iodide (NaI) solutions, and we found that chlorine was not detected after 600 s plasma irradiation while bromine and iodine were observed after 30 s plasma irradiation. To explain the different characteristics of halogen (chlorine, bromine, and iodine) generation, we postulated that, because of the surface activity and low kinetic energy of the short-lived active species generated by plasma, they could react with the solutes only at the topmost layer of the water surface, which is an atomic monolayer in liquid in contact with the gas phase. Based on our suggestion, we could say that the Cl− is unable to react with the active species into the chlorine because the ions were excluded from the topmost layer, and that, by contrast, the Br− and I− were able to react into the bromine and iodine, respectively, because some of these ions were located in the topmost layer. For understanding the plasma–liquid interactions more, a difference between chemical reactions induced by plasma and conventional electrolysis has been also discussed.

Surface of aqueous solutions of alkali halides: layer by layer analysis

ABSTRACTInterfacial layers of both vapour/liquid and crystal/liquid aqueous solutions of sodium and caesium halides have been analysed in detail using four different methods for the identification of interfacial molecules and layers, and both non-polarisable and polarisable models. It is found that each of the methods yields a somewhat different result with the best mutual agreement found between the ITIM (Identification of Truly Interfacial Molecules) and α-shape-based USTI (Universal Scheme for Triangulated Interfaces) methods. Concerning the water models, the commonly used JC [I. S. Joung, T. E. Cheatham, J. Phys. Chem. 112 (2008) 9020] model predicts the structural properties of the vapor/liquid interface different from both the non-polarisable MADRID and polarisable AH/BK3 models, and also from experiment. The same applies also to the structural properties of the solid–liquid interface; although the AH/BK3 and JC models yield a surplus of chlorine ions, a segregation of Na and Cl− ions in the interfacial layers predicted by the JC model is not observed in the polarisable model.

Heavy atom quenching of CdS quantum dots photoluminescence: Evidences for electron transfer mechanism

In this work, the quenching effect of sodium halide solutions on the fluorescence of mercaptoacetic acid-capped CdS quantum dots (MAA-CdS-QDs) has been explored. The dependence of MAA-CdS-QDs fluorescence quenching on the sodium halide concentration was described by the Stern-Volmer equation indicating the dynamic mechanism of quenching. An external heavy atom effect on the fluorescence was observed based on the non-radiative decay of singlet excited state, increasing in the order of NaI>>NaBr>NaCl>NaF. The results of the cyclic voltammetry and HOMO-LUMO energy estimation were consistent with a photo-induced electron transfer between halide species and MAA-CdS-QDs.

Corrosion behavior of Pb-39Mg-10Al-1.5B alloy in sodium halide solutions

The electrochemical corrosion behavior of Pb-39Mg-10Al-1.5B alloy in 3.5 wt% NaX (X = F, Cl, Br and I) sodium halide solutions was investigated by polarizing curve test, electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN), aiming to assess the corrosion damage degree of the alloy in the cooling waters containing halide elements. The results of polarizing curves, EIS and EN indicate that the corrosion resistance of Pb-39Mg-10Al-1.5B alloy in these sodium halide solutions is in the order of NaF > NaI > NaBr > NaCl. The obtained pitting characteristic frequency fn and noise resistance Rn reveal that the corrosion type of the alloy in NaF and NaI solutions is localized corrosion and that in NaCl and NaBr solutions mainly is general corrosion. Magnesium is the most prone to corroded phase in Pb-39Mg-10Al-1.5B alloy. The corrosion of magnesium is influenced by the corrosion product layer that the compact corrosion product layer can prevent its corrosion while the porous one can accelerate its corrosion.

A Microfluidic Method for Investigating Ion-Specific Bubble Coalescence in Salt Solutions.

This paper reports the direct and precise measurement of bubble coalescence in salt solutions using microfluidics. We directly visualized the bubble coalescence process in a microchannel using high-speed imaging and evaluated the shortest coalescence time to determine the transition concentration of sodium halide solutions. We found the transition concentration is ion-specific, and the capacity of sodium halide salts to inhibit bubble coalescence follows the order of NaF > NaCl > NaBr > NaI. The microfluidic method overcomes the inherent uncertainties in conventional large-scale devices and methods.

Role of hydrogen bonding in solubility of poly(N-isopropylacrylamide) brushes in sodium halide solutions **Project supported by the National Natural Science Foundation of China (Grant Nos. 21264016, 11464047, and 21364016) and the Joint Funds of Xinjiang Natural Science Foundation, China (Grant No. 2015211C298).

By employing molecular theory, we systematically investigate the shift of solubility of poly(N-isopropylacrylamide) (PNIPAM) brushes in sodium halide solutions. After considering PNIPAM–water hydrogen bonds, water–anion hydrogen bonds, and PNIPAM–anion bonds and their explicit coupling to the PNIPAM conformations, we find that increasing temperature lowers the solubility of PNIPAM, and results in a collapse of the layer at high enough temperatures. The combination of the three types of bonds would yield a decrease in the solubility of PNIPAM following the Hofmeister series: NaCl>NaBr>NaI. PNIPAM–water hydrogen bonds are affected by water–anion hydrogen bonds and PNIPAM–anion bonds. The coupling of polymer conformations and the competition among the three types of bonds are essential for describing correctly a decrease in the solubility of PNIPAM brushes, which is determined by the free energy associated with the formation of the three types of bonds. Our results agree well with the experimental observations, and would be very important for understanding the shift of the lower critical solution temperature of PNIPAM brushes following the Hofmeister series.

Electrolyte Jet Machining of Titanium Alloys Using Novel Electrolyte Solutions

Electrolyte Jet Machining (EJM) is a non-contact machining process whereby material is removed through highly localised electrochemical dissolution. Throughout the development of this technique, difficulties have been reported in the processing of titanium alloys due to the formation of an electrochemically inert non-conductive passivating layer. In this study, various electrolytes are investigated for the purpose of establishing more stable machining and controlled removal of the passivating layer. Surface finish, material removal rate and pit formations using solutions of sodium halides (bromide, chloride and fluoride respectively) are compared with those obtained using the more commonly used sodium nitrate solution. The concentration of electrolyte is varied in order to appraise the efficacy of each solution. Machined features produced are subject to typical characterisation methods. The removal mechanism for overcoming the passivating layer and the development of more stable machining regimes are explored. Sodium chloride machining is shown to increase mass removal rates by over 100% at concentrations less than 2.5M compared with that achievable using the commonly used sodium nitrate electrolyte. Doping sodium chloride electrolytes with sodium fluoride decreases the overcut effect in machined pits by half compared with the pits formed in chloride, bromide, and nitrate electrolytes.

Corrosion Behavior of ZK60 Magnesium Alloy in Sodium Halide Solutions

The corrosion behavior of ZK60 magnesium alloy in 3.5% sodium halides NaX (X =F, Cl, Br and I) solutions was studied by means of static immersion methods, potentiodynamic polar- ization measurement and SEM observation. The results showed that, the open circuit potential (OCP) of ZK60 alloy in 3.5% sodium halides solutions increased first and then became flat to a steady state. The corrosion behavior of ZK60 alloy was related to the halogen ions in the solutions: i.e. it was passivated in 3.5%NaF solution, while it was corroded in 3.5%NaCl, 3.5%NaBr and 3.5%NaI solutions and its corrosion rate decreased with immersion time, which may be ascribed to the formed protective corrosion products with the increasing pH value of the solutions.

Cation Depth-Distribution at Alkali Halide Aqueous Solution Surfaces

Using synchrotron radiation Near-Ambient Pressure X-ray photoemission spectroscopy (NAP-XPS) in surface and bulk sensitive conditions, we specifically explored the depth distribution of Na+ ions near the vapor/liquid interface of saturated solutions of sodium halides (NaCl, NaBr, and NaI) kept at 278 K under a pressure of 8 mbar. By varying the photoelectron kinetic energy, and hence the probing depth, we show that the Na+ depth distribution near the surface is highly sensitive to the size and polarizability of the anion. The large polarizable anions, Br– and I– that segregate at the surface, tend to drag the Na+ ions closer to the liquid–water surface. However, this situation is not observed for the NaCl solution, in line with recent theoretical results.

Dissecting ion-specific dielectric spectra of sodium-halide solutions into solvation water and ionic contributions.

Using extensive equilibrium molecular dynamics simulations we determine the dielectric spectra of aqueous solutions of NaF, NaCl, NaBr, and NaI. The ion-specific and concentration-dependent shifts of the static dielectric constants and the dielectric relaxation times match experimental results very well, which serves as a validation of the classical and non-polarizable ionic force fields used. The purely ionic contribution to the dielectric response is negligible, but determines the conductivity of the salt solutions. The ion-water cross correlation contribution is negative and reduces the total dielectric response by about 5%-10% for 1 M solutions. The dominating water dielectric response is decomposed into different water solvation shells and ion-pair configurations, by this the spectral blue shift and the dielectric decrement of salt solutions with increasing salt concentration is demonstrated to be primarily caused by first-solvation shell water. With rising salt concentration the simulated spectra show more pronounced deviations from a single-Debye form and can be well described by a Cole-Cole fit, in quantitative agreement with experiments. Our spectral decomposition into ionic and different water solvation shell contributions does not render the individual contributions more Debye-like, this suggests the non-Debye-like character of the dielectric spectra of salt solutions not to be due to the superposition of different elementary relaxation processes with different relaxation times. Rather, the non-Debye-like character is likely to be an inherent spectral signature of solvation water around ions.

Electric field effects on alanine tripeptide in sodium halide solutions

The electric field effects on conformational properties of trialanine in different halide solutions were explored with long-scale molecular dynamics simulations. NaF, NaCl, NaBr and NaI solutions of low (0.2 M) and high (2 M) concentrations were exposed to a constant electric field of 1000 V/m. Generally, the electric field does not disturb trialanine's structure. Large structural changes appear only in the case of the supersaturated 2.0 M NaF solution containing NaF crystals. Although the electric field affects in a complex way, all the ions–water–peptide interactions, it predominantly affects the electroselectivity effect, which describes specific interactions such as the ion-pair formation.

On the Effects of Temperature, Pressure, and Dissolved Salts on the Hydrogen-Bond Network of Water

We study the structure of water through molecular dynamics, specifically the compression/expansion of the hydrogen-bond (H-bond) network, with temperature and pressure, and in salt solutions of alkali chlorides and sodium halides, and relate the observed local spatial perturbations with the tetrahedrality and the average number and lifetime of water H-bonds. The effect of transient H-bonds and transient broken H-bonds on H-bond lifetimes is further investigated, and results are compared with depolarized Rayleigh scattering lifetimes for neat water. A significant electrostriction is observed in the first hydration shell of Li(+) and F(-), while larger ions cause a small expansion of the H-bond network of water instead. However, both alkali cations and halide anions induce a minor contraction of the H-bond network in the second hydration shell. Further, water in the second hydration shell of Li(+), Na(+), and K(+) is less tetrahedral than neat water, resembling water at high pressures, while the H-bond network in the respective hydration shell of halide anions resembles water at low temperatures. Nevertheless, neither ion induced H-bond contraction nor expansion can be exactly mapped onto P or T effects on the local structure of water. Even though the average number and lifetime of H-bonds in the ionic hydration shells of most ions are not very different from those found in neat water, Li(+) and F(-) significantly increase the lifetime of water donor and acceptor H-bonds, respectively, in the first hydration shell. The non-monotonic increase of cation and anion mobility, with ionic size, observed experimentally, is interpreted in terms of the water local tetrahedrality around cations and anions.

Influence of anion species on electrochemical dealloying of single-phase Al2Au alloy in sodium halide solutions

The electrochemical dealloying of single-phase Al2Au alloy in 1 M sodium halide (NaF, NaCl, NaBr and NaI) aqueous solutions at 270, 290 and 340 K has been systematically investigated in this work. The effects of halide ions and temperature on the dealloying behavior have been studied with electrochemical methods like open-circuit potential measurements and dynamic polarization tests. Nanoporous gold (NPG) with a typical three-dimensional, bicontinuous, interpenetrating ligament-channel structure can be obtained from the single-phase Al2Au alloy by electrochemical dealloying in the NaCl, NaBr, and NaI solutions. In addition, the dealloyed NPG samples and the samples after coarsening at ambient conditions were characterized using field-emission scanning electron microscopy (FESEM). Morphological features like cracks and ligament sizes in NPG have been discussed. Furthermore, NPG samples that were almost free of cracks have been successfully prepared in the NaI solution at 340 K. Besides, based on the surface diffusion controlled coarsening mechanism, the surface diffusivities of gold adatoms along the alloy–electrolyte interfaces during dealloying have been evaluated in this work, as well as the activation enthalpy and activation entropy.

Specific ion effects in aqueous eletrolyte solutions confined within graphene sheets at the nanometric scale

The underlying mechanisms of specific ion effects on structure and dynamics of aqueous solutions have been long debated. On the other hand, the role of polarization at hydrophobic interfaces when aqueous electrolytes are present is of great importance, as it has been observed at the air-vapor interface. In this work, we have explored influence of ionic species on microscopical properties of aqueous sodium halide solutions constrained inside a double layer graphene channel, as a model for a realistic hydrophobic interface. Our systems have been simulated by molecular dynamics techniques, explicitly including polarization in water molecules and ions. Water and ionic density profiles showed the tendency of ionic species to occupy the whole space available, in good agreement with spectroscopic experimental data. The exception to this general behavior was fluoride, which preferred to stay away from interfaces. Two main regions were defined: interfaces and the central part of the slab, the bulklike region. Ionic hydration numbers at interfaces were lower than those at the bulklike area by about one to two units. We have also analyzed water-ion orientations and polarization distributions and obtained a marked dependence on ionic concentration. Residence time of anions suffered important fluctuations and tended to be largest at interfaces. Large variations of the static permittivity between interfacial and bulklike regions were observed. Ionic diffusion was found to be between 10(-5) and 10(-6) cm(2) s(-1) and showed to be mainly dependent on the concentration, whereas the type of anion considered and the polarizability had significantly less relevance. Conductivities were found to be dependent on ionic concentrations and the polarizabilities of anions, as well as on the spatial direction considered.