Dilute Aqueous Salt Solutions Research Articles

Interactions between γ-alumina surfaces in water and aqueous salt solutions

Particle agglomeration is relevant to numerous industrial applications and consumer products. The present work explores interactions between and agglomeration of gamma (γ)-alumina nanoparticles in pure water and dilute aqueous salt solutions. To characterize surface- and salt-specific effects, potential of mean force (PMF) profiles between γ-alumina surfaces ([110] and [100] facets) are extracted using classical molecular dynamics (MD) simulations. Supporting experiments are conducted using dynamic light scattering (DLS) to investigate agglomeration at the macroscale. The ion pairs considered are sodium chloride, ammonium acetate, barium nitrate, and barium acetate; sampling a broad range of the Hofmeister series. As particle surfaces approach contact, free-energy fluctuations of the PMF profiles reflect structural adjustments of the intervening aqueous phase. We extract values for the cohesive energy from the MD results, and parse the resultant effective pair interactions into van der Waals and electrostatic contributions. Molecular scale findings from simulations correlate with hydrodynamic radii of γ-alumina nanoparticles, obtained from DLS experiments. The results highlight the applicability of molecular simulations to identify the origins of macroscale observables.

Structural and dynamic properties of some aqueous salt solutions.

Aqueous salt solutions are utilized and encountered in wide-ranging technological applications and natural settings. Towards improved understanding of the effect of salts on the dynamic properties of such systems, dilute aqueous salt solutions (up to 1 molar concentration) are investigated here, via experiments and molecular simulations. Four salts are considered: sodium chloride, for which published results are readily available for comparison, ammonium acetate, barium acetate and barium nitrate, for which published data are scarce. In the present work, molecular dynamics (MD) simulations are conducted to quantify viscosity and water self-diffusion coefficients, together with rheometry and Pulsed Field Gradient Spin Echo (PFGSE)-NMR experiments for validation. Simulation predictions are consistent with experimental observations in terms of trend and magnitude of salt-specific effects. Combining insights from the approaches considered, an interpretation of the results is proposed whereby the capacity of salts to influence bulk dynamics arises from their molecular interfacial area and strength of interaction with first hydration-shell water molecules. For the concentration range investigated, the interpretation could be useful in formulating aqueous systems for applications including the manufacturing of advanced catalysts.

Cation-specific interactions of protein surface charges in dilute aqueous salt solutions: a combined study using dielectric relaxation spectroscopy and Raman spectroscopy

We exploited glycine as a zwitterionic model system to experimentally probe the cation specific interaction of protein surface charges in dilute (≤0.25 mol L-1) aqueous solutions of four biologically relevant inorganic salts, NaCl, KCl, MgCl2 and CaCl2, via dielectric relaxation spectroscopy (DRS) and Raman spectroscopy. Glycine is the simplest building block of proteins and it exposes the same charged groups (carboxylate and ammonium) to the solvent that dominate the protein-water interface. As a counter ion, we selected Cl- due to its biological importance. For all systems, we performed simultaneous fitting of the real (ε') and imaginary (ε″) parts of the dielectric functions, assuming a multimodal relaxation model, obtained from concentration dependent dielectric measurements at ∼293 K. We observe a reduction of the dielectric amplitude for the glycine relaxation while the corresponding time constant shows only small (<7%) deviations compared to aqueous glycine solutions. We propose that the observed reduction in dielectric amplitude is due to a reduction of the effective dipole moment (µeff) of zwitterionic glycine caused by the interaction of glycine with the ion even at very low (0.05 M) salt concentrations. The interaction between divalent metal ions and zwitterionic glycine is increased compared to the monovalent cation-zwitterion interaction; a finding that is also supported by Raman spectroscopy. Our combined dielectric relaxation and Raman spectroscopic study indicates that ion-glycine interactions are weak and mediated by the solvent. Cation-specificity of protein surface charges is also observed in dilute salt solutions (≤0.25 mol L-1), where electrostatic interactions dominate.

Salt concentration dependence of ionic conductivity in ion exchange membranes

Electric field driven ion transport in ion exchange membranes, often quantified by membrane resistance or ionic conductivity, is important for membrane-based technologies such as electrodialysis, batteries, fuel cells, etc. Various methods for measuring membrane ionic conductivity have been reported in the literature, but no widely accepted, standard protocol exists. Consequently, conflicting ionic conductivity results for widely studied commercial ion exchange membranes have been reported, leading to confusion regarding, for example, the salt concentration dependence of membrane ionic conductivity, especially for membranes equilibrated with dilute aqueous salt solutions. In this study, we report a simple, fast, reliable technique for measuring ionic conductivity based on direct contact between the membrane and electrodes. The technique was used to measure ionic conductivity values for a series of commercial ion exchange membranes as a function of external solution NaCl concentration (ranging from 0.001 to 5M). The salt concentration dependence of membrane ionic conductivity was rationalized within the Nernst-Einstein framework. At low external solution salt concentrations (< 0.3M), ionic conductivity values were essentially constant since mobile ion concentrations in the membranes approached a constant value (i.e., the fixed charge group concentration). At high salt concentrations (> 0.3M), ionic conductivity values increased with increasing salt concentration for three of the membranes, presumably due to increased ion sorption owing to weaker Donnan exclusion, and decreased for one membrane, presumably due to decreased ion diffusion coefficients resulting from osmotic deswelling.

Size and Origins of Long-Range Orientational Water Correlations in Dilute Aqueous Salt Solutions.

Long-range ordering of water around solutes has been repeatedly invoked as the key to its biological function. Recently, it has been shown that in an 8 mM aqueous NaCl solution the orientational correlation between water molecules extends beyond 8 nm. This was interpreted as arising from ion-induced long-range effects on the water collective hydrogen-bond interactions. Each ion was suggested to affect >10 000 water molecules, leading to a picture involving nanoscopic "ordered domains". Using molecular dynamics simulations, we show that the very small long-range tail in the correlation function is caused primarily by pairs of water molecules belonging to different ions' hydration shells and can be made to practically disappear by rearranging the ionic positions. This means that the ion-induced water orientational ordering in millimolar salt solutions cannot be separated from ion-ion interaction effects, for which the Debye-Hückel screening length shrinks to less than 1 nm at physiological ionic strengths.

Mechanisms of Acceleration and Retardation of Water Dynamics by Ions

There are fundamental and not yet fully resolved questions concerning the impact of solutes, ions in particular, on the structure and dynamics of water, which can be formulated as follows: Are the effects of ions local or long-ranged? Is the action of cations and anions on water cooperative or not? Here, we investigate how the reorientation and hydrogen-bond dynamics of water are affected by ions in dilute and concentrated aqueous salt solutions. By combining simulations and analytic modeling, we first show that ions have a short-ranged influence on the reorientation of individual water molecules and that depending on their interaction strength with water, they may accelerate or slow down water dynamics. A simple additive picture combining the effects of the cations and anions is found to provide a good description in dilute solutions. In concentrated solutions, we show that the average water reorientation time ceases to scale linearly with salt concentration due to overlapping hydration shells and structural rearrangements which reduce the translational displacements induced by hydrogen-bond switches and increase the solution viscosity. This effect is not ion-specific and explains why all concentrated salt solutions slow down water dynamics. Our picture, which is demonstrated to be robust vis-a-vis a change in the force-field, reconciles the seemingly contradictory experimental results obtained by ultrafast infrared and NMR spectroscopies, and suggests that there are no long-ranged cooperative ion effects on the dynamics of individual water molecules in dilute solutions.

In Situ Liquid Cell Electron Microscopy of the Solution Growth of Au–Pd Core–Shell Nanostructures

Using in situ liquid cell electron microscopy we investigate Pd growth in dilute aqueous Pd salt solutions containing Au nanoparticle seeds. Au-Pd core-shell nanostructures are formed via deposition of Pd(0), generated by the reduction of chloropalladate complexes by radicals, such as hydrated electrons (eaq(-)) induced by the electron beam in the solution. The size and shape of the Au seeds determine the morphology of the Pd shells, via preferential Pd incorporation in low-coordination sites and avoidance of extended facets. Analysis of the Pd incorporation on Au particles at different distances from a focused electron beam provides a quantitative picture of the growth process and shows that the growth is limited by the diffusion of eaq(-) in the solution.

Solvation at nanoscale: Alkali-halides in water clusters

The solvation of alkali-halides in water clusters at nanoscale is studied by photoelectron spectroscopy using synchrotron radiation. The Na 2p, K 3p, Cl 2p, Br 3d, and I 4d core level binding energies have been measured for salt-containing water clusters. The results have been compared to those of alkali halide clusters and the dilute aqueous salt solutions. It is found that the alkali halides dissolve in small water clusters as ions.

Electrochemical catalytic behavior for platinum functionalized TiO2 nanotube arrays in PEM fuel cells

ABSTRACTConventional carbon electrode supports for platinum used in proton exchange membrane (PEM) fuel cell assemblies have issues related to carbon corrosion at typical cell operating and transient conditions. This corrosion gives rise to the evolution of greenhouse gases such as CO2, eventually degrading the carbon support and causing a loss of the catalyst specific area necessary to achieve the desired electrochemical performance. In this study, preliminary results are presented for Pt-functionalized TiO2 nanotube arrays as cathode catalyst supports for PEM fuel cells. The electrochemically synthesized TiO2 nanotube arrays were functionalized by different weight % of Pt via a solution-based approach using a dilute aqueous salt solution of hexachloroplatanic acid. Electron-beam based characterization techniques were used to study the structural and morphological features of the as-synthesized TiO2 nanotube arrays and functionalized Pt/TiO2 nanotube arrays. The electrochemical performance of the functionalized TiO2 nanotube arrays was studied by using cyclic voltammetry.

Interfacial Energy of Polypeptide Complex Coacervates Measured via Capillary Adhesion

A systematic study of the interfacial energy (γ) of polypeptide complex coacervates in aqueous solution was performed using a surface forces apparatus (SFA). Poly(L-lysine hydrochloride) (PLys) and poly(L-glutamic acid sodium salt) (PGA) were investigated as a model pair of oppositely charged weak polyelectrolytes. These two synthetic polypeptides of natural amino acids have identical backbones and differ only in their charged side groups. All experiments were conducted using equal chain lengths of PLys and PGA in order to isolate and highlight effects of the interactions of the charged groups during complexation. Complex coacervates resulted from mixing very dilute aqueous salt solutions of PLys and PGA. Two phases in equilibrium evolved under the conditions used: a dense polymer-rich coacervate phase and a dilute polymer-deficient aqueous phase. Capillary adhesion, associated with a coacervate meniscus bridge between two mica surfaces, was measured upon the separation of the two surfaces. This adhesion enabled the determination of the γ at the aqueous/coacervate phase interface. Important experimental factors affecting these measurements were varied and are discussed, including the compression force (1.3-35.9 mN/m) and separation speed (2.4-33.2 nm/s). Physical parameters of the system, such as the salt concentration (100-600 mM) and polypeptide chain length (N = 30, 200, and 400) were also studied. The γ of these polypeptide coacervates was separately found to decrease with both increasing salt concentration and decreasing polypeptide chain length. In most of the above cases, γ measurements were found to be very low, <1 mJ/m(2). Biocompatible complex coacervates with low γ have a strong potential for applications in surface coatings, adhesives, and the encapsulation of a wide range of materials.

Statistical mechanics of sum frequency generation spectroscopy for the liquid–vapor interface of dilute aqueous salt solutions

We demonstrate a theoretical description of vibrational sum frequency generation (SFG) at the boundary of aqueous electrolyte solutions. This approach identifies and exploits a simple relationship between SFG lineshapes and the statistics of molecular orientation and electric field. Our computer simulations indicate that orientational averages governing SFG susceptibility do not manifest ion-specific shifts in local electric field, but instead, ion-induced polarization of subsurface layers. Counterbalancing effects are obtained for monovalent anions and cations at the same depth. Ions held at different depths induce an imbalanced polarization, suggesting that ion-specific effects can arise from weak, long-ranged influence on solvent organization.

Perforated Vesicles as Intermediate Structures in the Transition from Vesicles to Micelles in Dilute Aqueous Systems Containing Long Chain Alcohols and Ionic Surfactants

Morphologies of aggregates in mixtures of long chain alcohols (n-decanol, n-dodecanol, and oleyl alcohol) and cationic surfactants (cetylpyridinium chloride, cetyltrimethylammonium bromide, and chloride) in dilute aqueous salt solutions were investigated by cryogenic transmission electron microscopy. In all cases elongated micellar structures transformed into vesicles with increasing alcohol concentration. Perforated bilayer structures were clearly identified in systems with oleyl alcohol and CTAB, in particular in 0.100 M NaBr. Increasing the salt concentration to 0.200 M other aggregates dominated composed of threadlike, probably branched, and interconnected micelles. A gradual evolution of morphologies was observed with increasing alcohol content at the lower salt concentration: thread-like micelles (developing branching to some extent), perforated vesicles, coexisting perforated and smooth bilayer structures, only smooth bilayers, and finally also inverted strucutures.

Deep desalination of water by evaporation through polymeric membranes

Transport properties and structural features of a number of already known and new types of polymeric membranes in desalination of aqueous salt solutions by membrane distillation and pervaporation were studied. The possibility of obtaining distilled water in a single stage by pervaporation from dilute or concentrated aqueous salt solutions was examined for the example of membranes fabricated from celluloses of varied origin (wood, cotton, bacterial).

Characterization of nanofiltration and reverse osmosis membrane performance for aqueous salt solutions using irreversible thermodynamics

A novel protocol is developed to characterize nanofiltration (NF) and reverse osmosis (RO) membranes that employ the Spiegler–Kedem membrane-transport model based on irreversible thermodynamics coupled with a Film Theory description of the concentration polarization. The novel aspect of this characterization protocol is extracting the concentration polarization boundary-layer thickness as well the three membrane transport parameters from permeation data. This methodology is applied to four dilute aqueous salt systems for a Dow Filmtech NF-255 membrane and one dilute aqueous salt system for a Dow Filmtech BS-30 membrane that were studied using a wellmixed flat sheet membrane permeation cell. Interestingly, the total volumetric flux and intrinsic rejection can be described quite well over the entire range of studied feed concentration, flow rates, and transmembrane pressures using constant membrane transport coefficients in the Spiegler–Kedem model. However, it should be emphasized that the conclusions drawn from this study are restricted to the dilute aqueous salt solutions that were employed. Some prior studies of aqueous salt systems have found it necessary to employ concentration-dependent transport coefficients to describe the permeation behavior. It is possible that the more exacting method for determining the thickness of the concentration boundary-layer thickness that is used in this study properly accounts for volumetric flow rate effects that have been attributed to nonconstant membrane transport coefficients. A design correlation for the intrinsic rejection of an NF or RO membrane as a function of the total volumetric flux is developed that encompasses the full range of concentrations used in the experiments, flow rates, and transmembrane pressures for a specified binary aqueous salt system. This design correlation formalism in principle can be used to characterize any NF or RO membrane irrespective of the type of membrane contactor employed.

Solubility of sodium soaps in aqueous salt solutions

The solubility of sodium soaps in dilute aqueous salt solutions has been systematically investigated by direct visual phase behavior observations. The added electrolytes, including simple inorganic salts and bulky organic salts, influence the solubility of sodium soaps in water, as represented by the varied soap Krafft point. Two inorganic salts, sodium chloride and sodium perchlorate, demonstrate a “salting-out” property. On the other hand, tetraalkylammonium bromides show an excellent ability to depress the soap Krafft point and enhance the soap solubility in water. With increasing the tetraalkylammonium ionic size, the degree of “salting-in” of soaps in water increases. However, solubility of pure tetraalkylammonium bromide in water decreases as the length of the alkyl chains increases. Furthermore, in the ternary water-tetrapentylammonium bromide (TPeAB)-sodium myristate (NaMy) system, we observed an upper cloud point phenomenon, which greatly shrinks the 1-phase micellar solution region in the phase diagram. This miscibility gap, together with the organic salt solubility limitation, restricts the use of tetraalkylammonium bromides with alkyl chains longer than 4 carbon atoms as effective soap solubility enhancement electrolytes. We also found that for sodium soap with a longer hydrocarbon chain, more tetrabutylammonium salt is required to reduce the soap Krafft point to room temperature.

Influence of the ionic strength on the degree of the form asymmetry p of poly-2-acrylamido-2-methylpropanesulfonic acid macromolecule in dilute aqueous-salt solutions

Dilute aqueous-salt solutions of macromolecular flexible-chain polyelectrolyte, poly-2-acrylamido-2-methylpropanesulfonic acid, were studied by the method of flow birefringence, viscometry, and light scattering. The degrees of the form asymmetry p of poly-2-acrylamido-2-methylpropanesulfonic acid macromolecules determined by this method for ionic strengths I = 1-0.05 M were compared with the main results of theoretical and experimental studies of the p-I dependence.

Solubility and reactivity of peroxyacetyl nitrate (PAN) in dilute aqueous salt solutions and in sulphuric acid

The loss rates of PAN in several dilute aqueous salt solutions (NaBr, Na 2SO 3, KI, NaNO 2, FeCl 3, and FeSO 4) and in sulphuric acid were measured at 279 K with a simple bubbler experiment. They are little different from that in pure water. For 5 M sulphuric acid hydrolysis and solubility were determined in the temperature range of 243–293 K. The hydrolysis rate k h=3.2×10 −4 s −1 at 293 K is close to that in water. The observed temperature dependence of the Henry's Law constant H =10 − 6.6±0.6 exp((4780±420)/T) M atm −1 leads to enthalpy and entropy of solvation Δ H solv=−39.7±3.5 kJ mol −1 and Δ S solv=−126±11 J mol −1 K −1, respectively.

The structure and phase equilibria of salt-water solution at supercritical conditions

We study the thermodynamic properties of the supercritical water and dilute aqueous salt solutions using molecular dynamics simulation, The sodium and chloride ions are found to show strong pairing at conditions corresponding to those of supercritical water oxidation, We also present the first prediction of the solubility of sodium chloride in supercritical water obtained directly from intermolecular interactions using molecular simulations.

Hindered diffusion of a single sphere very near a wall in a nonuniform force field

Total internal reflection microscopy is used to monitor Brownian fluctuations in the elevation of a single 10 μm sphere, levitated 0.05 to 0.35 μm above a glass plate by double-layer repulsion in a dilute aqueous salt solution. An analysis reveals that the initial slope of the autocorrelation function of the scattering intensity yields the diffusion coefficient evaluated at the weighted-average elevation in which the product of the Boltzmann factor and the square of the scattering intensity for that elevation are used as the weighting factor. When the sphere is very close to the wall, its mobility is expected to be significantly reduced owing to hydrodynamic interactions with the wall. Diffusion coefficients as small as 1.5% of the Stokes–Einstein value in the bulk were measured. Using Brenner’s relation between mobility and elevation [Chem. Eng. Sci. 16, 242 (1961)], the average elevation was inferred from the ratio of the average diffusion coefficient to its bulk value. The result is compared to the elevation obtained from the brightness of the sphere. Good agreement is obtained between the elevations inferred from the two independent measurements.

Interaction and adsorption of polyelectrolytes on mica

Interaction and adsorption of different polyelectrolytes onto mica was studied using the surface force technique. For highly charged polyelectrolytes in dilute aqueous salt solution, the adsorbed l ...