Dilute Aqueous Electrolyte Solutions Research Articles

Changes in the Structure and Appearance of Elemental Peaks in Low-Frequency Dielcometric Absorption Spectra of Dilute Aqueous Solutions of Electrolytes

Changes in the Structure and Appearance of Elemental Peaks in Low-Frequency Dielcometric Absorption Spectra of Dilute Aqueous Solutions of Electrolytes

Evolution and Reversible Polarity of Multilayering at the Ionic Liquid/Water Interface.

Highly correlated positioning of ions underlies Coulomb interactions between ions and electrified interfaces within dense ionic fluids such as biological cells and ionic liquids. Recent work has shown that highly correlated ionic systems behave differently than dilute electrolyte solutions, and interest is focused upon characterizing the electrical and structural properties of the dense electrical double layers (EDLs) formed at internal interfaces. It has been a challenge for experiments to characterize the progressive development of the EDL on the nanoscale as the interfacial electric potential is varied over a range of positive and negative values. Here we address this challenge by measuring X-ray reflectivity from the interface between an ionic liquid (IL) and a dilute aqueous electrolyte solution over a range of interfacial potentials from -450 to 350 mV. The growth of alternately charged cation-rich and anion-rich layers was observed along with a polarity reversal of the layers as the potential changed sign. These data show that the structural development of an ionic multilayer-like EDL with increasing potential is similar to that suggested by phenomenological theories and MD simulations, although our data also reveal that the excess charge beyond the first ionic layer decays more rapidly than predicted.

Translocation and Binding in the Recognition of Short Oligonucleotides by a Biological Nanopore

Polymer interactions with pore-forming membrane proteins, as evidenced by the resulting block of ionic current, may be classified into two types: (1) the threading of an extended polymer chain of which only a small length contributes to the block and (2) a binding reaction, where the polymer enters the pore entirely and interacts as a particle. The former phenomenon typically occurs in dilute aqueous electrolyte solutions (e.g. KCl <1 M) and requires long and charged molecules and can allow sequencing (e.g. of DNA). In contrast, the binding reaction typically occurs with short, neutral polymer molecules, requires high salinity (e.g. KCl 3-4 M) and enables the high-resolution discrimination of polymer masses1. The aerolysin nanopore has recently been shown to strongly interact with short adenine oligonucleotides (A3-A10) and this interaction allows mass discrimination on the basis of the depth of block of ionic current induced by the binding of the analyte,2 suggesting a binding-type interaction. Using low-noise current recording, we have indentified strong dynamics of this interaction between DC and 50 kHz and indentified short visits to deeper blocked states preceding and following the principal, mass-dependent state (pre- and post-blocks). Statistical analysis indicates that the probability of post- but not pre-blocks decreases with (1) oligomer length and (2) transmembrane voltage. We interpret this finding in terms of a combined translocation and binding interaction, probably involving several binding sites for DNA in the pore. In line with this hypothesis, longer polynucleotides (A30) showed stable and long-lasting interactions with near-complete block of current.(1) Robertson et al. Proc. Natl. Acad. Sci. U S A 2007, 104, 8207-8211.(2) Cao et al. Nature Nanotechnology 2016, 1-7.

Energy transformation in water and oxygen-containing electrolytes

The conductivity of distilled water and dilute electrolyte solutions in the presence and absence of dissolved oxygen is studied. It is found that the circuit can be implemented with equal probability as a capacitive circuit with physically different current carriers and as an inductive circuit in water and dilute aqueous solutions of oxygen-containing electrolytes. The occurrence of an inductive resistance is caused by the presence of particles with intrinsic magnetic moment, i.e., reactive oxygen species. The frequency ranges of possible pulse energy transformation in an electrochemical system that contains oxygenated water are shown. In the absence of oxygen, this circuit is not implemented.

Adsorption of Ions to the Surface of Dilute Electrolyte Solutions: The Jones−Ray Effect Revisited

The controversial observation of a minimum in the surface tension of dilute aqueous electrolyte solutions by Jones and Ray in the 1930s is confirmed by new resonance-enhanced second harmonic generation (SHG) experiments demonstrating surface enhancement of simple inorganic anions in the same concentration range. New experiments show that the quadruply charged ferrocyanide, Fe(CN)(6)(4-), anion is not surface active at high concentrations, as expected, but at dilute concentrations, the anion is strongly attracted to the interface with a Gibbs free energy of adsorption of -6.8 kcal/mol. Using this value, the original Jones and Ray data are fit to a simple model of the surface tension with qualitative agreement, although better agreement is found for all 13 Jones and Ray salts with an even stronger surface adsorption.

Dielectric analysis of nanofiltration membrane in electrolyte solutions: influences of electrolyte concentration and species on membrane permeation

Dielectric properties of a nanofiltration membrane immersed in dilute aqueous electrolyte solutions were measured, and frequency dependence of capacitance and conductance of the systems was analyzed, based on the interfacial polarization theory, giving values of permittivity and conductivity of the membrane and the solutions. Permittivity, ε m , of the membrane slightly decreased whereas conductivity, κ m , of the membrane increased with increasing electrolyte concentration, as a result of entrance of ions into the membrane. The ratio of membrane/solution conductivity, κ m / κ w , also depended on the electrolyte concentration, showing that distribution of ions in the membrane and in solutions follow Donnan equilibrium, due to the presence of negative fixed charges in the membrane. New expressions were derived from Donnan equilibrium principle to explain this phenomenon, and negative fixed charge concentration c e of the membrane was obtained; thus the Donnan potential, Δ Φ Don, of the membrane in solutions at various concentrations could be calculated. The new expressions could be expected to be usable to analyze ion permeation property through membrane.

A conductometric study of dimethylsulfoxide effect on micellization of sodium dodecyl sulfate in dilute aqueous electrolyte solutions

Abstract Critical micelle concentrations (CMCs) of sodium dodecyl sulfate (SDS) have been determined in aqueous mixtures of dimethylsulfoxide (DMSO) between 25 and 45 °C and at very low NaBr concentration range (0.0025–0.03 mol dm−3). The ability of NaBr to lower the CMC in water is inhibited by DMSO below ∼0.014 mol dm−3 of NaBr. However, the contribution of DMSO in regard to its effect upon micellization process of SDS in aqueous electrolyte solutions have been discussed in terms of the observed behavior of thermodynamic properties, such as entropy (ΔSm°), free energy (ΔGm°) and enthalpy (ΔHm°). In particular, we discuss counterion binding as revealed by different experimental conditions. The data suggest mass action effect and salt-induced increase in micelle size with concomitant reduction in charge density of Stern-layer. These observations point to the counterion solvation effect of DMSO in addition to the loss of hydrophobic interactions due to strong intermolecular interactions.

On the thermodynamics and hydration in ternary aqueous solutions of electrolytes and saccharides

In dilute aqueous solutions of electrolytes and saccharides the hydration is the main cause of the thermodynamic coupling of the two solutes. This is shown for several systems for which the data are taken from the literature by applying an appropriate model theory.

Calculation of stoichiometric dissociation constants of monoprotic carboxylic acids in dilute aqueous sodium or potassium chloride solutions and p[ m(H +)] values for acetate and formate buffers at 25°C

Equations are given for calculation of the stoichiometric (molality scale) dissociation constants, K m, of weak acids in dilute aqueous electrolyte solutions at 298.15 K from the thermodynamic dissociation constant, K a, of the acid and the ionic strength, I m, of the solution. The equations for K m were based on the single-ion activity coefficient equations of the Hückel type. The equations were tested with the conductivity data for formic, acetic, propionic, n-butyric, lactic, chloroacetic, α-crotonic and cyanoacetic acids, and with data measured by Harned cells for formic, acetic, propionic, n-butyric and glycolic acids. These data were taken from the literature. According to these tests, K m can be obtained by the Hückel method within experimental error at least up to I m of about 0.1 mol kg −1. On the basis of the equations for K m, it is suggested p( m H) values {p( m H)=−lg[ m(H +)/(mol kg −1)] where m refers to the molality} for buffer solutions containing acetic or formic acid. A new calibration method is suggested for glass electrode cells, and this method is based on the p( m H) values instead of pH values (pH=−lg[ a(H +)] where a refers to the activity).

Conductivity of Dilute Aqueous Electrolyte Solutions at High Temperatures and Pressures Using a Flow Cell

A flow-through electrical conductance cell was assembled in order to measuremolar conductances of dilute aqueous electrolytes with a high degree of accuracyat high temperatures and pressures. The design of the cell is based on the conceptdeveloped at the University of Delaware and built in 1995, with modificationsthat will allow the cell to operate at much higher temperatures (to 600°C) andpressures (to 300 MPa). At present, the cell has been tested successfully bymeasuring aqueous (10−4-10−3 mol-kg−1) solutions of LiCl, NaCl, and KCl attemperatures 25–410°C and pressures 9.8–33 MPa. The results are in goodagreement with reported values, including those measured with the Delawareflow-through cell. These new results are also complementary to our previousresults, which were measured with a static high-pressure cell. Measurements attemperatures near the critical point of water (374°C, 22.1 MPa) require the useof lower solution concentrations that were unachievable in the past with thestatic cell.

The influence of dissolved gas on the interactions between surfaces of different hydrophobicity in aqueous media Part I. Measurement of interaction forces

An atomic force microscope (AFM) was used to directly measure the symmetric interaction forces between solid surfaces in dilute aqueous electrolyte solutions as the type and concentration of dissolved gas were varied. The gases studied were air, argon and carbon dioxide. The amount of dissolved gas was reduced by degassing the electrolyte solutions. The solid surfaces studied were silica, hydrophobised by either dehydroxylation or methylation. The approach of methylated surfaces in electrolyte solutions was typically characterised by a jump into contact from separations sufficiently large so as to not be attributed to van der Waals forces. The range of these attractive jumps was quite reproducible for a given pair of interacting surfaces. However the jump distance was found to vary dramatically (between 5 and 75 nm) for differing pairs of interacting surfaces, albeit prepared in an identical maner. For surfaces with large jump distances (>25 nm), degassing of the electrolyte solution caused a significant reduction in the jump distance. When smaller jump distances were measured, whether involved methylated or dehydroxylated surfaces, degassing had no significant effect on the jump distance. The effect of gas type on jump distance was examined for both types of surfaces. For dehydroxylated silica surfaces interacting in CO2 saturated electrolyte solutions, the jump distances were significantly greater than in the presence of air or argon. Overall this interaction behaviour may be explained by the presence of gas bubbles formed on hydrophobic solid surfaces. The gas bubbles are stabilised by a combination of hydrophobicity and chemical heterogeneity.

Importance of thermal diffusion in high temperature electrochemical cells

The significance of thermal diffusion in liquid systems with forced convection has been investigated over a wide range of temperature. Numerical calculations have been carried out for the case of dilute liquid solutions that flow into the entrance region of a tube or an annulus, and the theory has been developed for a first order chemical or electrochemical reaction that takes place on the electrode surface. As an example, dilute aqueous electrolyte solutions at sub-critical temperatures (up to 350°C) have been considered. As the Lewis number and the Soret coefficient increase, the impact of thermal diffusion on mass transport is predicted to become increasingly important at higher temperatures. Simple analytical relations for describing thermal diffusion effects in liquid solutions subject to forced convection have been derived, and good agreement between these analytical solutions and numerical results is shown. The use of an electrochemical cell with forced convection could be a promising experimental technique for determining Soret coefficients for electrolyte solutions and liquid mixtures at elevated temperature.

Simulation study of the role and structure of monatomic ions multiple hydration shells

The hydration structures of five monatomic cations and two monatomic anions are studied by Monte Carlo NpT simulations conducted on infinite dilute aqueous electrolyte solutions. The complete first and second hydration shells are defined by the successive minima of the radial distribution functions’ ion–water oxygen atoms. The first shell structure is determined essentially by the electrostatic charge of the ion and by the short-range ion–water molecule interactions so that it is always constituted by six molecules located at the vertices of regular octahedra unless if the distance between the charge of the ion and the opposite charge in the water molecule is large enough to allow the presence of more than six water molecules. This is the case of the K+ which presents eight molecules in its first shell. The pairs of hydrogen atoms are, in the first shell, preferentially perpendicular. The structure of the second shell is defined by the bulk tetrahedral structure induced by the water molecules so that they are similar for cations and anions. The resulting second shell densities are very close to the pure water density. Both complete hydration shells are maintained stable by means of a compromise between the strong attraction that the ion carry out on the water molecules and by the resulting repulsion between these molecules which are forced to get too close to one another.

Thermodynamics of phase equilibrium for systems containing gels

A gel is a three-dimensional cross-linked elastic polymer. When a gel is immersed into a liquid, it can swell, similar to a sponge. If the surrounding liquid is a solute-containing solution, a solute partitions between the gel and the surrounding liquid. This work derives the fundamental equations of phase equilibrium for such systems. These equations take into account the elastic properties of the gel. Criteria are given for the distribution of solutes between a gel and a coexisting liquid phase. Criteria are also presented for calculating three-phase equilibria where a swollen gel and a collapsed gel coexist with the surrounding liquid. Attention is given to a slightly ionized gel (polyelectrolyte) surrounded by a dilute aqueous electrolyte solution. The equations of equilibrium show that, when Donnan equilibria are taken into account, the pH inside the gel is not equal to that of the surrounding aqueous solution. The discussion presented here may be useful for design of gels used in medicine, pharmacy and biotechnology.

Description and estimation of adsorption from multicomponent aqueous solutions of dissociating organic substances on activated carbons

The general theoretical description of adsorption from multicomponent dilute aqueous solutions of weak organic electrolytes on solid surfaces is discussed. In the approach, the effect of solutionpH and ionic strength, and adsorbent energetic heterogeneity is taken into account. The presented model allows to estimate adsorption values in multicomponent systems by using the parameters characterizing single-solute adsorption data. The theoretical discussion is illustrated using new experimental adsorption data for two-component systems consisting of benzoic and salicylic acids over the wide range ofpH and equilibrium concentrations. The results of adsorption prediction are in good agreement with experiment.

Increasing ion pairing and aggregation in supercooled and glassy dilute aqueous electrolyte solution as seen by FTIR spectroscopy of alkali metal thiocyanates

FTIR spectra of aqueous solutions of lithium (1 M), sodium (0.04, 0.2, and 1 M), and potassium (1 M) thiocyanate in their glassy states at 83 K, and of 1 M sodium thiocyanate in supercooled solution from 298 to 230 K, are reported from 1950 to 2200 cm -1 . In the CN stretching band region of the glassy solution derivative spectra, up to six discrete bands are observed. The band at ≃2050 cm -1 is assigned to mainly «free» SCN - . Bands at 2071, 2068, and 2067 cm - are assigned contact ion pairs Li + NCS - , Na + NCS - , and K + NCS -

Nuclear spin relaxation in a hexagonal lyotropic liquid crystal

The hexagonal (E) phase in the sodium dodecyl sulphate (SDS)/decanol/water system is investigated by 2H and 23Na nuclear magnetic resonance (NMR) of the selectively deuterated SDS and the sodium counterion. Using macroscopically oriented E phase samples, prepared from the magnetically aligned nematic (NC) phase, we measure the orientation-dependent relaxation rates R1Z and R1Q as well as the line shape of both nuclei. The orientation dependence of the lab-frame spectral densities, determined from the relaxation rates, allow us to separate contributions from different types of molecular motion. In particular, we find a dominant contribution from molecular diffusion around the cylindrical aggregate. From this contribution we determine the lateral diffusion coefficient of SDS to (1.4±0.2)×10−10 m2 s−1 at 25 °C (activation energy 26±2 kJ mol−1 ) and the counterion surface diffusion coefficient to (4.8±0.9)×10−10 m2 s−1 at 25 °C (a factor 2.8 smaller than in an infinitely dilute aqueous electrolyte solution). Furthermore, the flexibility of the cylindrical aggregates in the investigated E phase (aggregate volume fraction 0.27) is quantified in terms of an orientational order parameter SDC≊0.9.

Freezing potentials arising on solidification of dilute aqueous solutions of electrolytes

A theory of the Workman-Reynolds effect [E.J. Workman and S.E. Reynolds, Phys. Rev. 78 (1950) 254] - the difference in electric potentials between dilute aqueous solution (water with impurities) and an ice crystal growing from the solution - is developed. The effect arises from inequality between distribution coefficients for solute cations and anions. At the beginning of solidification, the predominantly trapped ions from in ice a space charge layer parallel to the ice-water interface. An excess of counterions accumulates in the water next to the interface. The impurity cations and anions in ice then are neutralized by highly mobile OH - and H 3O + groups (ionization defects) supplied by thermal dissociation of H 2O molecules in ice. Uncompensated ionization defects remaining after neutralization are moved by the electric field to the growing interface, encountering and neutralizing ions of solute. Thus the space charge layer of impurity follows the growth front. At the same time the oppositely charged layer of counterions is pushed ahead of the interface. The potential difference between the two layers - here considered as “capacitor plates” - is the freezing potential. It is calculated taking into account a current of OH - and H 3O + ions from water to ice. Our theory predicts that the freezing potential arises only when the product of growth rate and impurity concentration at the front exceeds a critical value that depends on pH, at which point the potential increases with the growth rate up to tens and hundreds of volts. The calculations presented in this paper qualitatively explain the experimental dependence of the transient potential difference on crystal thickness, rate of crystal growth, initial solute concentration, and the pH of water. In the case of instability of the ice crystallization front the theory predicts a rapid neutralization of the electrical charge in ice, a decrease of the crystallization potential, and a substantial change in acidity of the water remaining unfrozen. This change in pH is proportional to the logarithm of the ratio of the total ice-water interface area and the unfrozen water volume. Chemical reactions caused by this change in pH are called freezing (crystallization) hydrolysis. The experiments described demonstrate freezing hydrolysis of potassium ferricyanide (a reduction of K 3Fe(CN) 6 to K 4Fe(CN) 6) in a frozen NaCl-K 3Fe(CN) 6-H 2O solution.

Modeling the thermodynamic properties of sodium chloride in steam through extended corresponding states

Recent precise data on anomalous behavior of apparent molar properties of electrolyte solutions in near-critical steam have raised important questions as to how the thermodynamic properties of these systems should be described. Current Gibbs free energy models fail for highly compressible solutions. Here, a Helmholtz free energy formulation is presented as a first step in modeling compressible dilute aqueous electrolyte solutions. Comparisons are made with the known critical line, coexistence curves, apparent molar volumes, and heat capacities of NaCl in steam, and conclusions presented on improving the model.

On theory of ionic volumes in dilute aqueous solutions of electrolytes

For purposes of calculation of densities of aqueous solutions of strong electrolytes and of their mixtures, theoretical substantiation of the formerly outlined empirical model of apparent molar ionic volumes φ in solution is presented. According to the model, the hydration sheaths of ions consisting of radially close-packed H2O molecules having effective molar volume V’w the same for all ions, are on their contact with the adjacent structure of liquid water surrounded by a layer of excess voids. This layer can bee substituted in the model by continuous gap of which d0, which again is common to all the ions and is temperature-independent. Using experimental φ values of twenty-seven strong electrolytes together with the data on electrolytic transport of water on ions, V’w = 12 cm3 mol-1 and d0 = (40 ± 2) pm were found for mono- and divalent ions in 1 mol dm-3 solutions, independently of ionic charges and crystallographic radii. The exception are small ions Li+ and Na+, the volumes of which – if interpreted on the basis of the model – correspond to hydration sheaths formed by a cluster of voluminous ice-like structure. An anomaly in this respect has been encountered also in the case of NH+4 ion.