Aqueous Alkali Chloride Solutions Research Articles

Range and sensitivity of 17O nuclear spin-lattice relaxation as a probe of aqueous electrolyte dynamics.

The study of electrolytic solutions is of relevance in many research fields, ranging from biophysics, materials, and colloid science to catalysis and electrochemistry. The dependence of solution dynamics on the nature of electrolytes and their concentrations has been the subject of many experimental and computational studies, yet it remains challenging to obtain a full understanding of the factors that govern solution behavior. Here, we provide additional insights into the behavior of aqueous solutions of alkali chlorides by combining 17O relaxation data with diffusion and viscosity data and contrast their behavior with 1H nuclear magnetic resonance relaxation data. The main findings are that 17O relaxation correlates well with viscosity data but not with diffusion data, while 1H relaxation correlates with neither. Certain ionic trends match known ion-specific series behavior, especially at high concentrations. Notably, we also examine the ranges of the interactions and conclude that the majority of the effects are tied to local water reorientation dynamics.

Hydrodynamic slip of alkali chloride solutions in uncharged graphene nanochannels.

Using non-equilibrium molecular dynamics simulations, we demonstrate the effect of concentration and alkali cation types (K+, Na+, and Li+) on the hydrodynamic slip of aqueous alkali chloride solutions in an uncharged graphene nanochannel. We modeled the graphene-electrolyte interactions using the potential of Williams et al. [J. Phys. Chem. Lett. 8, 703 (2017)], which uses optimized graphene-ion Lennard-Jones interaction parameters to effectively account for surface and solvent polarizability effects on the adsorption of ions in an aqueous solution to a graphene surface. In our study, the hydrodynamic slip exhibits a decreasing trend for alkali chloride solutions with increasing salt concentration. The NaCl solution shows the highest reduction in the slip length followed by KCl and LiCl solutions, and the reduction in the slip length is very much dependent on the salt type. We also compared the slip length with that calculated using a standard unoptimized interatomic potential obtained from the Lorentz-Berthelot mixing rule for the ion-carbon interactions, which is not adjusted to account for the surface and solvent polarizability at the graphene surface. In contrast to the optimized model, the slip length of alkali chloride solutions in the unoptimized model shows only a nominal change with salt concentration and is also independent of the nature of salts. Our study shows that adoption of the computationally inexpensive optimized potential of Williams et al. for the graphene-ion interactions has a significant influence on the calculation of slip lengths for electrolyte solutions in graphene-based nanofluidic devices.

Resolving the Equal Number Density Puzzle: Molecular Picture from Simulations of LiCl(aq) and NaCl(aq).

The change in number densities of aqueous solutions of alkali chlorides should be qualitatively predictable. Typically, as cations get larger, the number density of the solution decreases. However, aqueous solutions of lithium and sodium chloride exhibit at ambient conditions practically identical number densities at equal molalities despite different ionic sizes. Here, we provide an atomistic interpretation of this experimentally observed anomalous behavior using molecular dynamics simulations. The obtained results show that the rigidity of the Li+ first and second solvation shells and the associated compromised hydrogen bonding result in practically equal average water densities in the local hydration regions for Li+ and Na+ despite different sizes of the cations. In addition, in more distant regions from the cations, the water densities of these two solutions also coincide. These findings thus provide an atomistic interpretation for matching number densities of LiCl and NaCl solutions. In contrast, the number density differences between NaCl and KCl solutions as well as between LiCl and KCl solutions behave in a regular fashion with lower number densities of solutions observed for larger cations.

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.

Incipient charnockites from southern India: The role of brines

Southern India and Sri-Lanka are the places where “incipient charnockites”, i.e. the local transformation of amphibolite-facies gneisses into orthopyroxene-bearing, igneous looking charnockites, have been discovered in the early sixties. The fact that some incipient charnockites occur along a network of brittle fractures, together with CO2 remnants preserved in mineral inclusions, had called for the role of fluids during charnockite alteration. The present work presents new observations on fluid inclusions and microtextures of incipient charnockites from type localities in southern India. In addition to CO2-rich fluid inclusions in quartz and feldspar, all of the occurrences have disrupted remnants of concentrated aqueous alkali chloride solutions. CO2 inclusions are more abundant in paragneiss (Kerala) than in orthogneiss (Karnataka/Tamil Nadu). The finding of disrupted brine inclusions in the Kabbal charnockite is a key link between closely associated massive charnockites and Closepet Granite, both of which also share the brine remnants. All of the occurrences studied here have feldspar or feldspar-quartz microvein networks along grain boundaries of recrystallized quartz, feldspar and orthopyroxene. These metasomatic veins again indicate the action of alkali-exchanging fluids (i.e., saline solutions). Feldspar microveins, which have been found in most “massive” charnockites, along with the CO2-rich fluid inclusions, suggest a commonality of incipient charnockite and massive charnockite, both types differing in intensity of interaction with metasomatizing pore fluids.

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

Terahertz (far infrared) spectroscopy provides a useful tool for probing both ionic motions in solution and the effect of ionic solutes on the dynamics of the solvent. In this study, we calculate terahertz spectra of aqueous alkali chloride solutions using classical but novel (the water model includes three-body interactions, the ion parameterization is non-standard, and the dipole surface is polarizable) molecular dynamics simulations. The calculated spectra compare reasonably well to experimental spectra. Decomposition of the calculated spectra is used to gain a deeper understanding of the physical phenomena underlying the spectra and the connection to, for instance, the vibrational density of states for the ions. The decomposed results are also used to explain many of the cation-dependent trends observed in the experimental spectra.

Molecular simulation study on Hofmeister cations and the aqueous solubility of benzene.

We study the ion-specific salting-out process of benzene in aqueous alkali chloride solutions using Kirkwood-Buff (KB) theory of solutions and molecular dynamics simulations with different empirical force field models for the ions and benzene. Despite inaccuracies in the force fields, the simulations indicate that the decrease of the Setchenow salting-out coefficient for the series NaCl > KCl > RbCl > CsCl is determined by direct benzene-cation correlations, with the larger cations showing weak interactions with benzene. Although ion-specific aqueous solubilities of benzene may be affected by indirect ion-ion, ion-water, and water-water correlations, too, these correlations are found to be unimportant, with little to no effect on the Setchenow salting-out coefficients of the various salts. We further considered LiCl, which is experimentally known to be a weaker salting-out agent than NaCl and KCl and, therefore, ranks at an unusual position within the Hofmeister cation series. The simulations indicate that hydrated Li(+) ions can take part of the benzene hydration shell while the other cations are repelled by it. This causes weaker Li(+) exclusion around the solute and a resulting, weaker salting-out propensity of LiCl compared to that of the other salts. Removing benzene-water and benzene-salt electrostatic interactions in the simulations does not affect this mechanism, which may therefore also explain the smaller effect of LiCl, as compared to that of NaCl or KCl, on aqueous solvation and hydrophobic interaction of nonpolar molecules.

Microgel-like aggregates of isotactic and atactic poly(methacrylic acid) chains in aqueous alkali chloride solutions as evidenced by light scattering.

A comparative light-scattering study of isotactic and atactic poly(methacrylic acid), iPMA and aPMA, respectively, in aqueous solutions with added alkali chlorides, XCl (X = Li, Na, Cs), at 25 °C and XCl concentration of 0.1 mol L(-1), demonstrates that both PMA isomers are strongly associated at low degrees of neutralization, αN (= 0 for aPMA and 0.25 for iPMA), in the presence of all XCls. The shape parameter ρ and the scattering functions suggest that aggregates have the characteristics of microgel particles, with a dense core surrounded by a less dense shell. The extent of aggregation depends on the stereoregular structure of the polymer and on the type of the added cation. Li(+) and Na(+) ions support aggregation better than Cs(+) ions. Besides, iPMA chains are more strongly aggregated than aPMA chains and form particles with a denser core. A model of the aggregation process is suggested for iPMA. At high αN, a slow diffusive process (so-called extraordinary or anomalous mode in diffusion of polyelectrolytes), arising from electrostatic interactions between charged chains, is observed for both PMAs. Results suggest that under the same experimental conditions iPMA is effectively more charged than aPMA. The role of ions in the slow-mode phenomenon is less pronounced than in aggregation.

Raman spectroscopic study for the determination of Cl − concentration (molarity scale) in aqueous solutions: Application to fluid inclusions

From recent studies on the structure of aqueous alkali and alkaline chloride solutions, it can be deduced that the effects of monovalent and divalent cations are small, and water–chloride interactions are primarily responsible for the solutions. Therefore, Cl − molarity can be treated as an important parameter in describing these systems. In this work, the NaCl–H 2O, KCl–H 2O, CaCl 2–H 2O, MgCl 2–H 2O and NaCl–KCl–CaCl 2–H 2O systems are synthesized, and their Raman spectra are recorded. From the Raman OH stretching bands of the solutions, the isosbestic points can be discriminated. This means that there exists the equilibrium between various structural species, the Raman OH vibration can be applied to study the salinity of these solutions. In this study, the Raman OH vibration is fitted with two Gaussian sub-bands, the equilibrium constant can be determined, then the calibration equation of Cl − concentration (molarity scale) can be determined. After the effects of the birefringence of the host mineral on the Raman OH stretching bands are discussed, the method is applied to measure the salinity of natural fluid inclusions.

Kirkwood–Buff derived force field for alkali chlorides in simple point charge water

Solvated ions are a fundamental constituent of many biological systems. An important class consists of the alkali cations. In particular, potassium (K(+)) is the most abundant ion in the cytoplasm, whereas lithium (Li(+)), rubidium (Rb(+)), and cesium (Cs(+)) are of fundamental physicochemical and medical relevance. A powerful tool to understand ion specificity and cellular systems on a microscopic level is provided by molecular dynamics simulations. Previously, reliable force field parameters for Li(+), K(+), Rb(+), and Cs(+) in aqueous solution have not been available for the simple point charge (SPC) water model widely used in conjunction with the GROMOS force field. We used the Kirkwood-Buff theory to develop force fields for Li(+), K(+), Rb(+), and Cs(+) in SPC water to reproduce experimental data on respective aqueous alkali chloride solutions (LiCl, KCl, RbCl, CsCl). The force field developed reproduces many of the known properties of alkali metal chlorides solutions including densities and partial molar volumes. Our force field is shown to be superior to other common alkali chloride force fields in terms of reproducing the activity derivative, as a prerequisite for a realistic measure of ion-solute association underlying ion-specific phenomena (Hofmeister effects). For lithium and potassium, the ionic radii from cation-water oxygen pair correlation functions and hydration numbers are well reproduced. The force field developed will be useful for modeling physiological conditions and ion-specific phenomena for biomolecular systems.

The Viscosity of Aqueous Alkali-Chloride Solutions up to 623 K, 1,000 bar, and High Ionic Strength

An accurate viscosity (dynamic viscosity) model is developed for aqueous alkali-chloride solutions of the binary systems, LiCl–H2O, NaCl–H2O, and KCl–H2O, from 273 K to 623 K, and from 1 bar to 1,000 bar and up to high ionic strength. The valid ionic strengths for the LiCl–H2O, NaCl–H2O, and KCl–H2O systems are 0 to 16.7 mol · kg−1, 0 to 6 mol · kg−1, and 0 to 4.5 mol · kg−1, respectively. Comparison of the model with about 4,150 experimental data points concludes that the average absolute viscosity deviation from experimental data in the above range is within or about 1 % for the LiCl–H2O, NaCl–H2O, and KCl–H2O mixtures, indicating the model is of experimental accuracy. With a simple mixing rule, this model can be extrapolated to predict the viscosity of ternary aqueous alkali-chloride solutions, making it useful in reservoir fluid flow simulation. A computer code is developed for this model and can be obtained from the author: (maoshide@cugb.edu.cn).

Thermal diffusivity of eutectic of alkali chloride and ice in the freezing–thawing process by temperature wave analysis

Thermal diffusivity of eutectic formed in aqueous alkali chloride solutions (NaCl, KCl, RbCl and CsCl) was determined by temperature wave analysis (TWA) during the freezing–thawing process of aqueous alkali chloride solution. An obvious change of thermal diffusivity in freezing–thawing processes of eutectic was observed with the super-cooling phenomena for each alkali chloride solute. The rate of thermal diffusivity decrease during the eutectic melting in comparison to the total decrease from the solid to the liquid state was observed larger than that of the heat of fusion measured by DSC, especially in the dilute concentration. It was confirmed that thermal diffusivity was sensitively influenced by the highly structure of the coexistence of eutectic crystallization and ice.

Supercooling of Aqueous Solutions of Alkali Chlorides and Acetates

Abstract Homogeneous nucleation temperatures (TH’s) of aqueous alkali chloride and acetate solutions were measured as a function of salt concentration. There is a linear relation between TH and 1/r+ (r+: ionic radius of alkali ion) for aqueous alkali chloride solutions. In the supercooling of aqueous alkali acetate solutions, the ionization constant also plays an important role in their supercooling behavior. The TH results were discussed in relation to nucleation theory.

Surface charge density on spherical silica particles in aqueous alkali chloride solutions

Experimental data concerning the dependence of the surface charge density of AEROSIL 300 in aqueous alkali chloride solutions on pH are analyzed under application of two different models for a spherical electrical double layer. Data for the alkali ions Li + , Na + , K + , Rb + and Cs + are included at five electrolyte concentrations. In the first model a counterion binding in combination with a pure diffuse spherical double layer is considered ; here, a good agreement between the recalculated plots and the experimental data is found for low electrolyte concentrations (c < 0.05 mol/l) only. The second model with a charge free Stern layer and without counterion binding seems to be applicable for higher electrolyte concentrations also. Furthermore the surface charge density constants from the second model agree better with those published and determined with other models or experimental methods.

Surface charge density on spherical silica particles in aqueous alkali chloride solutions

Experimental data concerning the dependence of the surface charge density of AEROSIL 300 in aqueous alkali chloride solutions on pH are analyzed under application of two different models for a spherical electrical double layer. Data for the alkali ions Li+, Na+, K+, Rb+ and Cs+ are included at five electrolyte concentrations. In the first model a counterion binding in combination with a pure diffuse spherical double layer is considered; here, a good agreement between the recalculated plots and the experimental data is found for low electrolyte concentrations (c<0.05 mol/l) only. The second model with a charge free Stern layer and without counterion binding seems to be applicable for higher electrolyte concentrations also. Furthermore the surface charge density constants from the second model agree better with those published and determined with other models or experimental methods.

Apparent Molal Volumes of Urea and Its Derivatives in Aqueous Alkali Chloride Solutions

Abstract The apparent molal volumes (φv) at 25°C have been measured for the following urea derivatives in aqueous alkali chloride solutions (from density measurements) using a digital densimeter: urea, methylurea, ethylurea, 1,1-dimethylurea, 1,3-dimethylurea, 1,1-diethylurea, and 1,3-diethylurea. The limiting values (φv°) were obtained by a linear extrapolation using the least-squares method. A notable feature of the transfer volumes found in this study was that hydrophobic urea derivatives, such as dimethylureas and diethylureas, had negative values for transfer from water to lithium chloride solutions. For the remainder, however, all transfer volumes were positive.

Comparative studies on the conductances and energetics of transport of aqueous solutions of alkali chlorides through microporous membranes without and with control of pores using potassium salt precipitates

Comparative studies on the conductances of some common chloride solutions through microporous membranes, without and with control of pores by impregnating them with a precipitate of potassium zinc ferrocyanide, are reported. The microporous membranes are prepared from ion-exchange membranes of the “Neosepta” family treated with 6% H 2O 2; solution. The concentration range of the bathing chloride solutions is from 1 to 500 mol m −3. The temperature range studied is between 298 and 323 K at 5 K intervals. Both membranes have the tendency to show a smooth increase in conductance at all concentrations, tending towards a limiting value, and to maintain the order, K + > NH 4 + > Na + > Li + indicating that the proportionate hydrated sizes shown in aqueous solution are retained within the pores. The variations of conductance with temperature are utilised for the calculation of Arrhenius activation energies. These increase in the order, K + < NH 4 + < Na + < Li + parallel to their hydrated sizes, but with some variations in the case of membranes with precipitate.

Multicomponent electrolyte systems: Dependence of ionic mobilities on aqueous organic solvent structure

Ionic mobility data for multicomponent electrolyte systems at low concentrations are scarce due to experimental difficulties and are actually restricted to aqueous solutions of alkali chlorides. Some new results are presented which have been obtained by using a radiotracer method valid even if one of the ionic species is present at very low concentrations (tracer ion). The following electrolyte systems (two electrolytes with a common ion in a solvent) have been investigated at a 0.5N total ionic strength: NaNO3−AgNO3, KNO3−AgNO3, LiNO3−AgNO3 either in pure water or in water-rich (acetonitrile or dimethylsulfoxide) mixed solvents. Since ionic conductivity data processing by an extended law generalized to mixtures, such as that proposed by Quint and Viallard, has proved to be delicate to handle, our experimental results have been compared with the qualitative predictions of the classical Onsager-Fuoss limiting law. The main conclusion of this work is to give clear experimental evidence of the inability of any continuum theory to predict the ionic mobilities when solvent structural effect have to be taken into account. Consequently, the ionic behavior, particularly that of the Ag+ ion, has been interpreted in terms of preferential solvation and solvent microscopic structure. The trace mobility measurements reflect the maximum structural effect on the ionic transport properties.

The Volume, Adiabatic Compressibility, and Viscosity of Amino Acids in Aqueous Alkali-chloride Solutions

Abstract The apparent molal volumes (φV), apparent molal adiabatic compressibilities (φKS), and relative viscosities (nr) at 25 °C have been obtained for the following amino acids in aqueous alkali-chloride solutions from measurements of the density, the ultrasonic velocity, and the flow time: glycine, l-alanine, β-alanine, α-aminoisobutyric acid, l-serine, and l-threonine. Viscosity measurements were also carried out in water for reference. The alkali chlorides used were lithium, sodium, and potassium chlorides. The limiting values φVo, φKSo and the extended Jones-Dole viscosity coefficients, B and D, were calculated by a linear extrapolation using the least-squares method. All the φVo, φKSo, and B values obtained in the mixed solvents were larger than the corresponding ones in water, except in a few cases of the B coefficient. The results are discussed in terms of the dehydration effect of the electrolytes upon the amino acids.

Experimental Verification of the Space‐Charge Model for Electrokinetics in Charged Microporous Membranes

A space‐charge model for electrolyte transport in charged capillary pores was examined experimentally with aqueous solutions of alkali chlorides and in track‐etched mica membranes. The model combines the Gouy‐Chapman view of the double layer with the Nernst‐Planck and Navier‐Stokes transport equations. The pores of experimental membranes were uniform capillaries with a well‐characterized cross section. The pore sizes ranged from an order of magnitude smaller to an order of magnitude larger than the Debye screening lengths of solutions. Three independent quantities were measured: streaming potential, for which an applied pressure across the membrane is the driving force for transport; pore conductivity, for which an applied electrical potential is the driving force; and concentration potential, for which an electrolyte concentration difference is the driving force. Data follow the trends of model predictions but indicate that chloride ions affect the pore wall charge. For monovalent cations, the pore wall charge deduced from pore conductivity measurements yielded theoretical predictions for the streaming potential and for the concentration potential that agree quite well with the data. Such agreement was not obtained with Mg2+, probably because these divalent cations adsorbed onto the negative pore wall. We conclude that, in the absence of strong interaction between the charged pore wall and free ions in solution, the model is quantitatively accurate for pores larger than 30Å in radius and for aqueous electrolyte concentrations of 0.1M or lower.