Water Electrolyte Solutions Research Articles

Effects of pressure on aqueous chemical equilibria at subzero temperatures with applications to Europa

Pressure plays a critical role in controlling aqueous geochemical processes in deep oceans and deep ice. The putative ocean of Europa could have pressures of 1200 bars or higher on the seafloor, a pressure not dissimilar to the deepest ocean basin on Earth (the Mariana Trench at 1100 bars of pressure). At such high pressures, chemical thermodynamic relations need to explicitly consider pressure. A number of papers have addressed the role of pressure on equilibrium constants, activity coefficients, and the activity of water. None of these models deal, however, with processes at subzero temperatures, which may be important in cold environments on Earth and other planetary bodies. The objectives of this work were to (1) incorporate a pressure dependence into an existing geochemical model parameterized for subzero temperatures (FREZCHEM), (2) validate the model, and (3) simulate pressure-dependent processes on Europa. As part of objective 1, we examined two models for quantifying the volumetric properties of liquid water at subzero temperatures: one model is based on the measured properties of supercooled water, and the other model is based on the properties of liquid water in equilibrium with ice. The relative effect of pressure on solution properties falls in the order: equilibrium constants( K) > activity coefficients (γ) > activity of water (a w). The errors (%) in our model associated with these properties, however, fall in the order: γ > K > a w. The transposition between K and γ is due to a more accurate model for estimating K than for estimating γ. Only activity coefficients are likely to be significantly in error. However, even in this case, the errors are likely to be only in the range of 2 to 5% up to 1000 bars of pressure. Evidence based on the pressure/temperature melting of ice and salt solution densities argue in favor of the equilibrium water model, which depends on extrapolations, for characterizing the properties of liquid water in electrolyte solutions at subzero temperatures, rather than the supercooled water model. Model-derived estimates of mixed salt solution densities and chemical equilibria as a function of pressure are in reasonably good agreement with experimental measurements. To demonstrate the usefulness of this low-temperature, high-pressure model, we examined two hypothetical cases for Europa. Case 1 dealt with the ice cover of Europa, where we asked the question: How far above the putative ocean in the ice layer could we expect to find thermodynamically stable brine pockets that could serve as habitats for life? For a hypothetical nonconvecting 20 km icy shell, this potential life zone only extends 2.8 km into the icy shell before the eutectic is reached. For the case of a nonconvecting icy shell, the cold surface of Europa precludes stable aqueous phases (habitats for life) anywhere near the surface. Case 2 compared chemical equilibria at 1 bar (based on previous work) with a more realistic 1460 bars of pressure at the base of a 100 km Europan ocean. A pressure of 1460 bars, compared to 1 bar, caused a 12 K decrease in the temperature at which ice first formed and a 11 K increase in the temperature at which MgSO 4·12H 2O first formed. Remarkably, there was only a 1.2 K decrease in the eutectic temperatures between 1 and 1460 bars of pressure. Chemical systems and their response to pressure depend, ultimately, on the volumetric properties of individual constituents, which makes every system response highly individualistic.

Static Forces, Structure and Flow Properties of Complex Fluids in Highly Confined Geometries

The Surface Forces Apparatus has been successfully used to measure the static and dynamic forces between surfaces across ultra-thin films of water and aqueous electrolyte solutions, and--more recently--polyelectrolyte-coated and articular cartilage surfaces in various solutions including hyaluronan, lubricin, and synovial fluid. The results give new insights into the lubricating action of biological lubricants such as synovial fluid and hyaluronan (a polysaccharide in synovial fluid), and biological surfaces such as phospholipid bilayers and cartilage surfaces. Contrary to earlier indications of long-range water-structuring at biological surfaces, more recent measurements clearly show that the viscosity of physiologically concentrated water (saline) is bulk-like beyond the first 1 or 2 layers from a single surface, and beyond 4-6 layers in thin films between two surfaces (the structure and forces may, however, be affected to larger distances). This implies that most structural, interaction force, and viscosity-related phenomena are determined--not only by the properties of the solvent (water)per se--but also by the surfaces and the water, ions, solutes, and macromolecules (proteins, polymers) exposed or adsorbed at the surfaces and, to a lesser degree, dissolved in the solvent. However, sometimes it is difficult to make a clear differentiation, e.g., one could consider hydration or surface-bound 'structured' water as part of the surface or as part of the intervening water between the two surfaces.

Properties of Hydrogels Derived from Cationic Analogues of Bile Acid: Remarkably Distinct Flowing Characteristics

A new class of cationic bile acid derivatives was synthesized to gel water and aqueous electrolyte solutions. Investigations on these hydrogels are carried out at different length scales by a combination of physical techniques. Each of these hydrogels exhibits unique characteristics, thus providing a spectrum of thermal, optical, and mechanical properties. X-ray crystallographic investigation of the single crystals of two of the gelators shows significant differences in the solid-state packing. X-ray scattering experiments indicate that the gel state consists of a different morph than that in the solid. Electron microscopic investigations of the xerogels reveal the fibrous nature of the gel structure. These fibers are associated mainly through bundling processes. A detailed rheological study reveals significant differences in the mechanical properties of the three hydrogels. The storage modulus varies in the range $(0.2-2) \times 10^5$ Pa at C = 2 wt% for these systems. The exponents of the scaling of the rheological parameters with the concentration for two of the systems agree well with those expected for cellular solids or strongly interacting colloidal gels. A third system exhibits a singular behavior with the energy of interaction between the colloidal flocs increasing with the concentration.

Surface Potential at the Gas–Nonaqueous Solution Interface

Published data on the values of surface potentials of water and aqueous electrolyte solutions at the gas–solution interface obtained by different experimental and theoretical methods are reviewed and compared. Special attention was paid to the analysis of surface potentials at the gas–water interface. The value of water surface potential calculated on the basis of the method of volta-potential difference was compared with the similar values collected by other methods. The data obtained make it possible to conclude that the value of the water surface potential is equal to +0.10 V that, at present, seems to be the most reasonable and correct value. Positive sign of this value proves the fact that the water dipoles are oriented by the negative pole toward the gaseous phase, whereas by the positive pole, toward the liquid phase.

Refractive Indices of Water and Aqueous Electrolyte Solutions under High Magnetic Fields

The refractive index of water was measured at atmospheric pressure under magnetic fields up to 10 T and found to increase by ∼0.1% with increasing magnetic field strength. In contrast, the refractive index of saturated aqueous electrolyte solutions decreased under the magnetic field. In the case of n-hexane, any change of the optical property by the magnetic field was not found. A possible explanation is that the lifetime of the hydrogen bond is prolonged due to the electron delocalization of a water dimer under the magnetic field.

NMR Evidence of Weak Continuous Transitions in Water and Aqueous Electrolyte Solutions

Nuclear magnetic resonance chemical shifts are used to examine the perturbations in water structure that occur with concentration changes in aqueous KF, KCl, and LiOH solutions. Changes in the slope of ion chemical shifts as a function of solute concentration can be explained by changes in water structure. The equilibrium shift in water structure occurs as a result of changes in the hydrogen bond strength. The changes in hydrogen bond strength are a result of changes in electrolyte concentration and electron delocalization throughout the liquid. The location of the changes in slope with concentration is temperature dependent. A correlation of the changes in slope of chemical shifts to minima in specific heat capacity suggests the occurrence of a weak continuous transition in the solution structure at the critical concentration corresponding to the specific heat capacity minimum. By extrapolation the experiments reported here imply that there is a weak continuous transition associated with the heat capacity minimum for pure water. There must also be a structural relaxation time in the liquid associated with this transition. The results of these experiments provide confirmation for the model of aqueous solutions we recently proposed in which the solution is composed of regions of pure water and regions of liquid crystalline electrolyte hydrates. The subphase composed of structurally perturbed water is the part of the system that participates in the weak continuous phase transition that is evidenced by the NMR chemical shifts. In complete agreement with earlier Raman experiments it appears that the entire solution is a single electronic whole with exquisite electronic delocalization between the water and liquid crystalline subphases so that the ionic nuclei experience the electronic effects of the transition in the water subphase.

Effect of salts on water viscosity in narrow membrane pores

The effect of various salts on the viscosity, and by implication structure, of water in polymeric membrane pores of radius ∼1.69 nm and low charge density has been studied. Permeation of pure water and various electrolyte solutions was analyzed using the Hagen–Poiseuille equation expressed in a ratio form to exclude membrane-specific quantities such as pore radius and length. The analysis produced viscosity ratios of electrolyte to pure water inside the membrane pores. Comparing the viscosity ratios inside the pores with their bulk counterparts showed that confinement significantly increased the sensitivity of water structure to the presence of ions. It has been found that, in relative terms in the pores, Cl − was a strong structure breaker, K + was a moderately strong structure breaker, Na + was a weak structure breaker, SO 4 2− was a weak structure maker, and Mg 2+ was a strong structure maker. Predictive modeling of membrane separation performance would benefit from such effects being taken into account in cases where the pore ion concentrations may be high.

Osmotic behavior of a Nafion membrane in methanol–water electrolyte solutions

In this paper, an experimental study was carried out in order to investigate the osmotic transport of methanol–water electrolyte solutions through a Nafion membrane. The experimental data indicated that the Nafion membrane showed the typical anomalous osmotic behavior of charged membranes. The influence of some relevant parameters, such as electrolyte concentration difference, weight fraction of methanol on solution, and nature of cation was considered. The results showed that the osmotic volume flow was decreased with the presence of methanol on solvent, but did not alter the anomalous osmotic behavior of the membrane.

Viscosities of concentrated electrolyte solutions

The variation of the viscosities of unassociated simple binary strong electrolyte solutions in water as a function of concentration and temperature have been modelled using a simple empirical expression of the type: log( η/ η o ) = B c + D c 2, where B = B 1 + B 2 t and D = D 1 + D 2 t. Such equations, containing only four adjustable parameters, fitted the viscosity data with average residuals < 0.9 % for a variety of electrolytes up to high concentrations (0 ≤ c/mol kg −1 ≤ 12) and over a wide temperature range (5 ≤ t/°C ≤ 90). Addition of a fifth term, B 3 t 2, was necessary for the accurate description of electrolyte viscosities at higher temperatures (≤ 150°C). Viscosities of ternary strong electrolyte solutions at constant ionic strength over the temperature range 25 ≤ t/°C ≤ 90 were reasonably well accounted for (with maximum residuals < 8 %) by assuming a simple ionic strength-based pro-rata additivity, analogous to Young's rule for thermodynamic properties. Departures from additivity appear to be largely related to the differences in the viscosities of the (binary) end-member solutions. Incorporation of a simple linear correction more than halved the residuals. The ‘Young's rule’ approach has been used to calculate the viscosities of the hypothetical pure NaAl(OH) 4(aq) solutions and of NaOH/NaAl(OH) 4 mixtures.

Experimental Study of Small-Scale Spatial Variation in Filtration Coefficient Using Tracer Method

The procedure for studying the nonuniformity of filtration flows in capillary-porous media was developed. Time dependences of local filtration coefficients averaged over the area and their spatial variation during the flow of water and aqueous electrolyte solutions were studied on soil samples of undisturbed structure using tracer method. It was found that the transformation of a pore space structure accompanied with reverse variations in pore sizes occurs because of the action of disjoining pressure in thin films depending on the ion nature. Relaxation times corresponding to the times necessary to attain equilibrium within interlayer gaps of clay minerals were determined. The conclusion was made that if the system volume is constant, the variations in the pore space structure take place because of decrease in the relative fraction of macropores and their sizes.

Triangular Phase Diagrams in the Phospholipid−Water−Alcohol System for the Preparation of Lipid Vesicles (Liposomes) Using the Coacervation Technique

The effect of alcohol and electrolytes on simple coacervation in the phospholipid−water−alcohol system is described in detail. The optimal coacervation conditions for formation of lipid vesicles in the phospholipid−water−alcohol system were examined by the triangular phase diagram. Three different alcohols, methanol, ethanol, and 1-propanol, were used as a lipid solvent. Water and various electrolyte solutions such as sodium chloride, ammonium chloride, magnesium chloride, calcium chloride, and sodium sulfate were used as a poor solvent for the lipids. When methanol and 1-propanol were used as lipid solvents, the entire phase resulted in a transparent, highly viscous gel phase. On the other hand, the profile of the coacervate precipitation of the phospholipid was observed when ethanol was used as a lipid solvent. Moreover, when 1-propanol was used as a lipid solvent, a large volume of water phase was required to induce the coacervation in the systems in comparison with methanol or ethanol. The coacervation was stimulated by electrolytes according to Hofmeister's series.

Current–Voltage Curves for a Cation-Exchange Membrane in Methanol–Water Electrolyte Solutions

The current-voltage curves for a cation-exchange membrane separating two equal methanol–water electrolyte solutions were determined under different experimental conditions. From these curves, the values of the limiting current density, IL, were determined. The influence of the volume percentage of methanol in the solvent on the results was analyzed. The presence of methanol in the solutions was shown not to affect the shape of the current–voltage curves typical of aqueous solutions. However, the system resistance and the values of the limiting current density were both greatly influenced by the content of methanol. Thus, the value of the resistance increased and the limiting current decreased with increasing methanol content of the solution.

Effect of Alkali Halides Upon Photocurrent Due to Emission of Electrons from Dropping Mercury Electrode Into Water

When dropping mercury electrode (DME) dipped in ultrapure water, thoroughly deaerated with pure hydrogen, is irradiated with UV light, we can observe a cathodic photocurrent in the non-faradaic region of the polarographic curve, in the absence of scavengers of hydrated electrons. Newly we have followed the effect on photocurrent of initial very small additions of alkali metal chlorides to pure water. By repeated experiments, we could show that first additions of the alkali metal halides increase the photocurrent and that the already known suppression of photocurrent by addition of halides begins only after the halide concentration exceeds ca 10-4 mol l-1. We could show for the first time that in ultrapure water and dilute electrolyte solutions, the photocurrent follows a modified seven-halves law. The extent of the positive effect of small additions of alkali metal halides on photocurrent depends on the nature of each alkali metal cation. The reason for this specificity is discussed.

Properties of Water Solutions of Electrolytes and Nonelectrolytes

Apparent large deviations of water solutions from ideal behavior are eliminated by taking account of the number of water molecules binding to solute sufficiently strongly (13.0 ± 1.5 kcal mol-1) as to be removed from the “bulk” solvent. Freezing point, boiling point, vapor pressure, and osmotic pressure measurements of electrolyte solutions of chlorides, bromides, and iodides are treated successfully, as are those of nonelectrolytes, for up to 50 wt % solute and concentrations hundreds of time greater than those over which Debye−Huckel theory applies. D−H theory focused on the behavior of ions; this work focuses on the nature of water. The nature of “free” water as solvent is changed by electrolytes. No evidence is found for ion pairs up to eutectic points. Hydration numbers (h) obtained are in agreement with many from more elaborate techniques. From freezing points, h values are as follows: H+, 6.7 ± 0.7; Li+, 6.6 ± 0.6; Na+, 3.9 ± 0.5; K+, 1.7 ± 0.5; NH4+, 1.8 ± 0.5; Mg2+, 13 ± 2; Ca2+, 12 ± 2; Sr2+, 1...

Phenomenological Properties and Phase Behavior of Benzylalkyldimethylammonium Salts in the Presence of Benzene and Electrolyte Solutions

The solubilities of benzylalkyldimethylammonium salts (BADAX) in benzene were characterized by the specific Krafft boundaries, depending on their molecular structures. BADAX affords not only the Krafft boundary but also the immiscibility gap in water or aqueous electrolyte solutions. BADAX formed aggregates in aqueous solution or in benzene when its concentration was above the critical solubility concentration (csc) of the Krafft boundary. It is assumed that the aggregate structure varies successively with the change in electrolyte concentration or oil−water ratio. The phase behavior of the mixture of BADAX, benzene, water, and electrolyte was examined as a function of temperature or a function of component composition. The conditions under which a microemulsion phase appears or coexists with other phases are clarified, and the change in aggregate structure is discussed, on the basis of the solubility and immiscibility gap.

Forces between polystyrene surfaces in water–electrolyte solutions: Long-range attraction of two types?

A great deal of effort has recently been focused on the experimental studies of the phenomenon of long-ranged attraction between identically charged colloidal (polystyrene) particles immersed in an electrolyte. The theoretical validation suggested the need for revision of the existing and established colloidal theories assuming the observed attraction is of electrostatic origin. We, however, demonstrate that for a number of reasons (first of all hydrophobicity and roughness of particles) the Derjaguin–Landau–Verwey–Overbeek (DLVO) behavior should not be expected for polystyrene surfaces. Indeed, the force measurements with an atomic force microscope-related set-up suggest that even within one pair of the interacting surfaces, attractive interaction of various types can be observed. There is usually a difference between the first approach and the later ones. The first approach is characterized by a short-range jump into a contact. Depending on conditions (electrolyte concentration, previous contacts of surfaces, etc.) there exists a late attraction of two types between polystyrene surfaces. The force of the first type is characterized by an abrupt jump from the maximum of a repulsive force, which is typically of longer-range than on the first approach. This is most likely due to submicroscopic bubbles trapped and/or formed due to previous contacts (and separation) of the surfaces. The attraction of the second type is weak and exponentially decaying.

Electrical double layer on the Cd–Ga liquid alloy in solutions of surface-inactive electrolytes in water, methanol and propylene carbonate

Abstract A novel electrode, liquid Cd (0.3 at.%)–Ga eutectic alloy, was fabricated and studied. Its electrical double layer characteristics were shown to differ markedly from those of a Ga electrode; they are practically identical to those of the Cd electrode. It was found that, in the absence of a metal–solvent chemisorption interaction, the electron capacitance of the Cd–Ga electrode approaches the electron capacitance of the Ga, In–Ga and Tl–Ga electrodes, but differs from that of the Hg electrode. The Cd–Ga electrode resembles the In–Ga electrode in its lyophilic nature. The value of the chemisorption potential drop of solvents (water, propylene carbonate and methanol), obtained with the Cd–Ga electrode, follows the rule characteristic of the Ga, In–Ga and Tl–Ga electrodes, namely, the chemisorption potential drop of a solvent correlates with the metal work function (in vacuo). Mercury does not fall into this pattern, which was explained by the longer distance of the solvent dipoles to the ionic lattice in this metal. The chemisorption interaction of the solvents with the Cd–Ga electrode, like Ga and In–Ga electrodes, is shown to increase with the solvent donor number.

Electrical Properties and Thermal Stability of Electrically Conductive Composites

Electrically conductive polypyrrole(PPy)-poly(N-methylpyrrole) (PNM P) composites were prepared by electrochemical polymerization of the corresponding monomers at a platinum electrode surface in 0.036M sodium dodecylsulfate(NaDS)/distilled water(H2O) electrolyte solution system. Two types of composites were prepared and characterized by TGA, SEM, and 4-probe meter.

Four‐photon Raman spectroscopy of intermolecular vibrations in water solutions of electrolytes

Four‐photon Raman spectroscopy of intermolecular vibrations in water solutions of electrolytes

Four-photon Raman spectroscopy of intermolecular vibrations in water solutions of electrolytes

The influence of electrolyte concentration on the water libration spectrum was studied by four-photon polarization spectroscopy. The results were compared with the data on stretching O–H vibrations observed in spontaneous Raman spectroscopy in water solutions of electrolytes, and a qualitative coincidence of these results was found. It is shown that the weakening of hydrogen bonds at low electrolyte concentrations and the strengthening of coupling between water molecules and ions at high concentrations shift the libration resonances to high and low wavenumbers, respectively. Copyright © 2000 John Wiley & Sons, Ltd.