Thermodynamics of supersaturated solutions: From ternary electrolyte+solute+H2O to binary solute+H2O systems

In our previous paper, a new experimental technique based on the potentiometric method was developed to obtain thermodynamic activity data (in form of activity ratio) particularly for ternary electrolyte+nonelectrolyte+H₂O solutions supersaturated with the nonelectrolyte. In this paper, a rigorous thermodynamic approach is proposed to derive activity data for binary nonelectrolyte+H₂O solutions in the supersaturated region, via cell potentials of an electrolyte tracer and solubility data of the nonelectrolyte. An analysis of thermodynamic consistency is presented to evaluate the reliability and accuracy of the measured data. A systematic experimental investigation of three ternary electrolyte+nonelectrolyte+H₂O and two binary nonelectrolyte+H₂O systems is reported in both under-saturated and supersaturated regions (up to a supersaturation level of approximately 27%). Interesting thermodynamic behaviors are observed and their significance is discussed, with the role of thermodynamics in exploration of crystallization phenomena highlighted.     

Ternary diffusion in aqueous NaCl + MgCl2 solutions at 25°C

A conductimetric method has been used to measure ternary diffusion coefficients for aqueous NaCl + MgCl₂ solutions at 25°C. The measurements were made at total salt concentrations from 0.01–0.5 M for NaCl : MgCl₂ ratios 1 : 3, 1 : 1 and 3 : 1. Binary thermodynamic, viscosity and mobility data for NaCl + water and MgCl₂ + water solutions are used to predict ternary diffusion coefficients for the NaCl + MgCl₂ solutions. Qualitative agreement is obtained with optical and conductimetric data for ionic strengths up to 2 M.     

Osmotic coefficients of polyelectrolyte solutions, measurements and correlation

Osmotic coefficient data for aqueous sodium polyanetholesulfonic acid, sodium polyacrylate and polydiallyl dimethylammonium chloride solutions were determined at 298 K by employing the isopiestic method. The measured osmotic coefficients increase with increasing concentration in the experimental concentration range (0.1-1.5 m). A molecular thermodynamic model developed previously for polyelectrolyte solutions has been used to fit the experimental data. The concentration dependence of the osmotic coefficients can be described satisfactorily.     

Diffusivity of protein in aqueous solutions

The diffusion coefficient of lysozyme, a globular protein, was measured at various conditions as functions of lysozyme concentration, salt concentration, and solution ‘age’ in concentrated, saturated, and supersaturated solutions, employing Gouy interferometry. Distilled water, 0.05 M potassium phosphate buffer, and 0.1 M sodium acetate buffer solutions with 0, 2, 4, and 5 wt% NaCl were used as solvents. The pH of lysozyme solutions in distilled water was 4.75 due to the self-buffering capacity of lysozyme. The pH’s of the lysozyme solutions in the potassium phosphate and sodium acetate buffers were adjusted to 6.8 and 4.0, respectively. The experimental temperature was 25°C. In a salt-free system, the concentration dependent diffusion of lysozyme showed typical electrolyte diffusion behavior, while a salt-polyelectrolyte system exhibited the behavior of a non-electrolyte. Diffusion results in the supersaturated region showed a little effect of concentration or solution ‘age’ al a fixed NaCl concentration. A rapid decline in diffusion coefficient with increasing NaCl concentration in the supersaturated region, however, was observed.     

Thermodynamic modeling of CO2 solubility in aqueous solutions of NaCl and Na2SO4

A thermodynamic model based on the electrolyte NRTL activity coefficient equation and PC-SAFT equation-of-state is developed for CO₂ solubility in aqueous solutions of NaCl and Na₂SO₄ with temperature up to 473.15 K, pressure up to 150 MPa, and salt concentrations up to saturation. The Henry's constant parameters of CO₂ in H₂O and the characteristic volume parameters for CO₂ required for pressure correction of Henry's constant are identified from fitting the experimental gas solubility of CO₂ in pure water with temperature up to 473.15 K and pressure up to 150 MPa. The NRTL binary parameters for the CO₂-(Na⁺, Cl⁻) pair and the CO₂-(Na⁺, SO₄²⁻) pair are regressed against the experimental VLE data for the CO₂-NaCl-H₂O ternary system up to 373.15 K and 20 MPa and the CO₂-Na₂SO₄-H₂O ternary system up to 433.15 K and 13 MPa, respectively. Model calculations on solubility and heat of solution of CO₂ in pure water and aqueous solutions of NaCl and Na₂SO₄ are compared to the available experimental data of the CO₂-H₂O binary, CO₂-NaCl-H₂O ternary and CO₂-Na₂SO₄-H₂O ternary systems with excellent results.     

Determination of the activity coefficient of neodymium in liquid aluminium by potentiometric methods

The activity coefficient of neodymium in liquid aluminium phase has been determined potentiometrically in the temperature range of 973-1073 K. To the author's knowledge, no data on this parameter has been published yet.Three different electrochemical methods have been tested: the cyclic voltammetry technique, the coulometric additions method and the direct use of an Al-Nd alloy. In addition, an experimental set-up has been designed which allows working with small amounts of solvent (30 g). The molten eutectic mixture CaCl₂-NaCl (52-48 mol%) has been selected as the electrolyte.From the results obtained, the variation of the activity coefficient of Nd in Al(l) as a function of the temperature can be expressed as follows: log γ_Nd(Al) = 9.81 − 17134/T(K), in the range 973-1073 K. It has been found a good agreement between the activity coefficient values obtained from the different methods tested. Hence, it can be stated that either of the techniques used allows determining reliable values for the activity coefficient.     

Effects of cold work and sensitization treatment on the corrosion resistance of high nitrogen stainless steel in chloride solutions

The effects of cold work and sensitization treatment on the microstructure and corrosion resistance of a nickel-free high nitrogen stainless steel (HNSS) in 0.5 M H₂SO₄ + 0.5 M NaCl, 3.5% NaCl and 0.5 M NaOH + 0.5 M NaCl solutions have been investigated by microscopic observations, electrochemical tests and surface chemical analysis. Cold work introduced a high defect density into the matrix, resulting in a less protective passive film as well as reduced corrosion resistance for heavily cold worked HNSS in a 3.5% NaCl solution. No obvious degradation in corrosion resistance took place in a 0.5 M H₂SO₄ + 0.5 M NaCl solution, possibly due to the stability of the passive film in this solution. Sensitized HNSSs showed reduced corrosion resistance with increasing cold work level in both 3.5% NaCl and 0.5 M H₂SO₄ + 0.5 M NaCl solutions due to a reduction in the anti-corrosion elements in the matrix during the cold work-accelerated precipitation process. The cold work and sensitization treatment had no influence on the corrosion resistance of the HNSS in the 0.5 M NaOH + 0.5 M NaCl solution even though the property of the passive film changed. The effects of cold work and sensitization treatment on the characteristics of passive films formed in the three solutions are discussed.     

Thermodynamics of ion conductivity of alkali halides across a polystyrene-based titanium arsenate membrane

The electric conductance of ion across titanium arsenate membrane has been recorded. Aqueous solutions of KCl, NaCl and LiCl were used. The conductance values of titanium arsenate membrane have been found to increase with increase in concentrations as well as temperature (10-50 °C) in these cases. The conductance values of electrolytes follow the sequence for the cations: K⁺ > Na⁺ > Li⁺. Negative ΔS values are considered to indicate formation of bond between the permeating species and the membrane material. The physico-chemical characterization of the hybrid material was established by XRD, TGA and simultaneous SEM studies.     

Calcium oxalate crystallization on concrete heterogeneities

The precipitation of calcium oxalate monohydrate (COM) as a waterproofing material for concrete constructions is a promising approach. The kinetics of spontaneous precipitation of the calcium oxalate from supersaturated solutions in batch reactors was investigated. The precipitation took place in supersaturated solutions of equal concentrated solutions of sodium oxalate and calcium nitrate and was monitored when COM precipitated from the solutions as a function of time. The experiments were done both in unseeded and in solutions seeded with COM crystals and also with concrete grains (Portland cement). COM formed exclusively in all cases. From the analysis of the kinetics data, it was concluded that the prevalent mechanism for COM precipitation was surface diffusion. Satisfactory fit of the data to the polynuclear model yielded values for the surface energy ca. 30 mJ m⁻². The relatively low values obtained suggested mainly heterogeneous nucleation. The rates of COM precipitation increased with increasing amounts of inoculating COM seed crystals suggesting significant contribution of seeded growth in the overall process. Inoculation with concrete grains resulted in lower rates possibly due to the release of inhibiting species in the supersaturated solutions from the partially dissolving substrate.     

TERNARY NaCl+LiCl+H2O MIXED ELECTROLYTE SYSTEM: DETERMINATION OF ACTIVITY COEFFICIENTS BASED ON POTENTIOMETRIC METHOD

The mean activity coefficients for NaCl in a ternary electrolyte system were determined by the potentiometric method, at 25°C, using a solvent polymeric (PVC) sodium-selective membrane electrode (Na⁺ ISE), containing N,N'-dibenzyl-N,N'-diphenyl-1,2-phenylenedioxydiacetamide as ionophore, and combined with an Ag/AgCl electrode. The potentiometric measurements were performed at the same ionic strengths in different series of mixed salt solutions, each characterized by a fixed salt molal ratio r (where r = m₁/m₂ = 1, 10, 50, 100). The nonideal behavior of the ternary NaCl(m₁) + LiCl(m₂) + H₂O electrolyte system was described based on the Pitzer ion-interaction model for mixed salts over the ionic strength ranging from 0.01 up to about 4 mol/kg. Two- and three-particle Pitzer interaction parameters for a mixed electrolyte system were determined based on potentiometric data, and the critical role of potentiometric selectivity coefficient (K₁₂) of ISE as limiting factor in the potentiometric measurements was analyzed.     

Poly(N-ethylaniline) coatings on 304 stainless steel for corrosion protection in aqueous HCl and NaCl solutions

Poly(N-ethylaniline) (PNEA) coatings were grown by potentiodynamic synthesis technique on 304 stainless steel (SS) alloy from 0.1 M of N-ethylaniline (NEA) in 0.3 M oxalic acid solution. Characterization of adhesive and electroactive PNEA coatings was carried out by cyclic voltammetry, FT-IR spectroscopy and scanning electron microscopy (SEM) techniques. The protective properties of PNEA coatings on SS were elucidated using linear anodic potentiodynamic polarization, Tafel and electrochemical impedance spectroscopy (EIS) test techniques, in highly aggressive 0.5 M HCl and 0.5 M NaCl solutions. Linear anodic potentiodynamic polarization test results proved that PNEA coating improved the degree of protection against pitting corrosion in HCl and NaCl solutions. Tafel test results showed that PNEA coating appears to enhancement protection for SS in 0.5 M NaCl and 0.5 M HCl solutions. However, according to long-term EIS results, PNEA coating is better for the protection of SS electrodes during the long immersion period in NaCl compared to that in HCl medium.     

Characteristics and potential applications of a novel forward osmosis hollow fiber membrane

There has been a resurgence of interest in forward osmosis (FO) as a potential means of desalination, dewatering and in pressure retarded osmosis, which Sidney Loeb was advocating over 3 decades ago. This paper describes the characteristics and potential applications of a newly developed FO hollow fiber membrane, which was fabricated by interfacial polymerization on the inner surface of a polyethersulfone (PES) hollow fiber. This FO membrane presents excellent intrinsic separation properties, with a water flux of 42.6 L/m² h using 0.5 M NaCl as the draw solution and DI water as the feed with the active layer facing the draw solution orientation at 23 °C. The corresponding ratio of salt flux to water flux was only 0.094 g/L, which is superior to all other FO membranes reported in the open literature. To evaluate different application scenarios, various NaCl solutions (500 ppm (8.6 mM), 1 wt.% (0.17 M) and 3.5 wt.% (0.59 M)) were used as the feed water to test the performance of the FO membrane. The membrane can achieve a water flux of 12.4 L/m² h with 3.5 wt.% NaCl solution as the feed and 2 M NaCl as the draw solution, suggesting it has good potential for seawater desalination.     

Enhanced electrochemomechanical activity of polyaniline films towards high pH region: contribution of Donnan effect

Enhancement of electrochemomechanical deformation (ECMD) and expansion of ECMD activity towards high pH have been simultaneously achieved in the electrolytes equilibrated with HCl and NaCl solutions for polyaniline (PANI) film. The maximum deformation has been reached to 6.7% in the mixture of 3 M HCl and 3 M NaCl equilibrated at pH 3 at ambient temperature. By comparing the fact that the ECMD magnitude is 3.2% in 1 M HCl equilibrated at pH 0.5, the simple judicious selection of electrolyte condition leads to the enhancement of ECMD magnitude by around two times.The concentration dependence on both electrical conductivity and absorption spectra elucidates the increase of protonation ratio as a function of electrolyte concentration. The results indicate that the high concentration retains both the electrochemical and ECMD activities in PANI film towards high pH region, which is the enhanced functionality of PANI supported by Donnan effect.     

Effect of spinning temperature and blend ratios on electrospun chitosan/poly(acrylamide) blends fibers

We report the formation of non-woven fibers without bead defects by electrospinning blend solutions of chitosan and polyacrylamide (PAAm) with blend ratios varying from 75 wt% to 90 wt% chitosan using a modified electrospinning unit wherein polymer solutions can be spun at temperatures greater than ambient up to 100 °C. Electrospinning at elevated temperature leads to further expansion of the processing window, by producing fibers with fewer defects at higher chitosan weight percentage in the blends. Effects of varying blend ratios, spinning temperatures, and molecular weights on fiber formation were studied and optimum conditions for formation of uniform non-woven fiber mats with potential applications for air and water filtration were obtained. Uniform bead-less fiber mats with fiber diameter as low as 307 ± 67 nm were formed by spinning 90% chitosan in blend solutions at 70 °C.     

Experimental compressibilities and average intermolecular distances of poly(ethylene glycol) molecular masses 2000-8000 Da in aqueous solution

Small-Angle Neutron Scattering (SANS) is a powerful, nondestructive technique that can measure simultaneously macroscopic compressibilities of solutions and overall shapes of macromolecular solutes, as well as their intermolecular structural correlations. We have conducted SANS experiments on aqueous solutions of polyethylene glycols (PEGs) with nominal molecular masses 2000, 4000, and 8000 Da over the q-range 0.03-0.30 Å⁻¹ [q = (2π/λ)sin θ]. By incorporating accurate background subtraction and short extrapolations of the intermolecular structure factor S(q) down to q = 0, the isothermal compressibility can be measured. The results indicate a significant and systematic dependence of the solutions' compressibility on both molecular mass and concentration of PEG, unlike the solutions' osmotic pressures and activity of the water. This implies that the structure of water in the vicinity of PEG is considerably altered relative to the bulk state even though the activity coefficient of water remains nearly invariant in this range. Graphs of S(q) for 3% w/w to 17% w/w solutions each show a gradual rise from the low-q side to a broad plateau, which indicates weak intermediate-range correlations between oligomers that are probably associated with soft, repulsive, solvent-mediated PEG-PEG interactions. Since both the water and PEG change structures from their neat forms, any quantitative assignment of changes in partial volumes must necessarily be arbitrary. However, the linear change in compressibility with PEG concentration below ∼7% w/v can be said to indicate a composite solution, which parallels the behavior of composite solids.     

Synthesis of nanocrystalline 8 mol% yttria stabilized zirconia by the oleate complex route

Nanocrystalline 8 mol% yttria stabilized zirconia (YSZ) powder has been synthesized by the oleate complex route. Oleate complexes of zirconium and yttrium were formed in the hexane rich layer by the reaction of sodium oleate with zirconyl chloride and yttrium chloride at the interface of the two ternary solutions in water–ethanol–hexane system. The zirconyl oletae and yttrium oleate complexes on heating decomposed to oxide through the formation of carbonate intermediates. The powder obtained by calcination at 600 °C for 2 h was cubic YSZ with surface area of 42 m²/g. The YSZ powder contained primary particles of ∼300 nm size and the primary particles were aggregate of crystallites of 5–10 nm. The compacts prepared from the YSZ powder were sintered to ∼99% TD (theoretical density) at 1400 °C. The sintered YSZ had a low average grain size of 0.73 μm.     

Impedance spectroscopy study of saturated mortar samples

Mortar samples saturated with different solutions (deionised water, 0.5 M NaCl and 1 M NaCl) are studied to quantify microstructural changes induced by weathering of concrete in near-neutral solutions containing chlorides. The study is based on physical and electrochemical techniques. Mercury intrusion porosimetry is employed to quantify porosity and pore size distribution, and impedance spectroscopy as a non-destructive technique able to give information on structural changes. SEM imaging, XRD, and EDX characterisation allow elucidating the causes of the observed structural changes.Portlandite leaching tends to increase porosity. Nevertheless, the leaching process towards near-neutral solutions makes resistivity to increase. The presence of chlorides induces Friedel's salt formation, a slow process associated to the development of small size pores (around 10 nm).Impedance spectroscopy in the frequency range 10⁻² to 10² MHz is successfully employed to follow Friedel's salt formation and pore network development in mortars.     

Additivity rules for the prediction of the density of aqueous solutions containing mixed-type solutes

Densities of ternary aqueous solutions of mixed-type solutes have been measured at 25°C to high concentration. Two methods, the one isopycnotic (same density) and the other based on ionic strength, have been used to predict the densities of these solutions. The isopycnotic method proved to be the more accurate. The two methods have been applied in reverse to calculate densities of supersaturated binary aqueous solutions of NaCl and KCl. The methods give concordant results for each salt Deviation parameters for the methods are presented, with a discussion of their use.     

Additivity rules for the prediction of the density of aqueous solutions containing mixed-type solutes

Densities of ternary aqueous solutions of mixed-type solutes have been measured at 25°C to high concentration. Two methods, the one isopycnotic (same density) and the other based on ionic strength, have been used to predict the densities of these solutions. The isopycnotic method proved to be the more accurate. The two methods have been applied in reverse to calculate densities of supersaturated binary aqueous solutions of NaCl and KCl. The methods give concordant results for each salt Deviation parameters for the methods are presented, with a discussion of their use.