Effects of salt on the temperature and pressure responsive properties of poly(N-vinylisobutyramide) aqueous solutions

 We examined the effects of salt on the lower critical solution temperature (LCST) and lower critical solution pressure (LCSP) of aqueous solutions of poly (N-vinylisobutyramide), polyNVIBA, and compared them with those on poly(N-isopropylacrylamide), polyNIPAAm. We found that the addition of salt (such as Na₂SO₄, NaCl, or KCl) decreased the LCST of aqueous polyNVIBA from 45 °C to below 20 °C, almost linearly with the salt concentrations and dependent on the type of salt. We observed a similar concentration-dependent decrease in LCST for polyNIPAAm. When KI or NaSCN was added to each aqueous polymer solution, some smaller increases in LCST were observed at relatively low salt concentrations; higher concentrations of salt gave an almost linear decrease in LCST. As for LCSP, the addition of most types of salt lowered the transition pressure, but the effects were much more dependent on the type and the valence of the salt (especially of anion) in both polymers. Salt with divalent anion showed a larger decrease in LCSP, but those with mono valent anion showed a relatively small decrease, even showed a slight increase at lower salt concentrations in the case of polyNVIBA. Salt with I^- or SCN^- showed evident increases in LCSP up to 1 M and was maintained higher than the control even at 2 M. We discuss the interactions of the amide group in the side chains of polymers and water and their perturbation by ions. Received: 13 November 1997 Accepted: 22 January 1998     

Colloidal magnesium aluminum hydroxide and heterocoagulation with a clay mineral. I. Properties of colloidal magnesium aluminum hydroxide

Magnesium aluminum hydroxide, the most important member of layered double hydroxides, was peptized by intense washing. The particle diameter, 70–130 nm, depended on the temperature of aging the parent material. The electrophoretic mobility of the particles decreased with increasing pH, from 3.7 × 10⁻⁸ m²/Vs at pH 5 to 0.5 × 10⁻⁸ m²/Vs at pH 12.3. An isoelectric point at pH∼7 was reached with the addition of 87 mmol/l NaSCN, 3 mmol/l Na₂SO₄ and Na₂CO₃, and 0.7 mmol/l Na₂HPO₄. The critical coagulation concentration for the 2% (w/w) dispersion was 88 mmol/l NaCl, 1.8 mmol/l Na₂CO₃, 1.4 mmol/l Na₂SO₄, and 1.2 mmol/l Na₂HPO₄ at pH∼7. The 2% dispersion at pH∼7 showed an almost Newtonian flow behavior. Yield values were developed after salt addition. The 2% dispersion reached a yield value of 2 Pa at 100 mmol/l NaCl, 3 Pa at 100 mmol/l Na₂SO₄, and 5 Pa at 100 mmol/l Na₂CO₃. Sodium phosphate in comparison with the other salts showed a liquefying effect. The yield value increased to 3 Pa at 1–10 mmol/l Na₂HPO₄ and decreased to 0.5 Pa at 100 mmol/l Na₂HPO₄. Received: 28 February 2001 Accepted: 8 March 2001     

In situ conductivity measurements of LiMn2O4 thin films during lithium insertion/extraction by using interdigitated microarray electrodes

Thin films of LiMn₂O₄ have been prepared by RF magnetron sputtering on interdigitated microarray electrodes. In situ conductivity–potential profiles and cyclic voltammograms during extraction/insertion processes of Li ions were obtained simultaneously in nonaqueous and aqueous electrolyte solutions (1 M LiClO₄/propylene carbonate and 1 M LiCl/water). The electronic conductivity of Li_{1– x} Mn₂O₄ was found not to show metallic transition and maintain its semiconducting state during the extraction/insertion of Li ion. A slight decrease in conductivity was observed with increasing the anodic potential, i.e., with increasing x (lithium extraction) and recovered reversibly when the potential returned to the cathodic side (re-insertion of Li ions). Similar results were obtained in both aqueous and nonaqueous electrolyte solutions. Received: 17 June 1997 / Accepted: 2 January 1998     

Preparation and characterization of Ca1 − x Ce x MnO3 perovskite electrodes

The electrochemical properties of Ca_1 − x Ce _x MnO₃ perovskite-type oxide electrode have been investigated by cyclic voltammetry in Na₂SO₄ aqueous solutions with pH 14. The structural and morphological characterizations have also been investigated and the information used to interpret the electrochemical behavior. An estimation of the electrode’s capacitance and roughness factor has been obtained by means of cyclic voltammetry. The specific capacitance and consequently the roughness factor values are affected by the presence of Ce ions in the oxide. These findings are in agreement with the increase of the oxide-specific surface area by the introduction of Ce ion. The open-circuit potential and the voltammetric patterns are dependent on the presence of Ce ion in the electrodes and support that the surface electrochemistry of the perovskite oxide electrodes is governed by the Mn⁴⁺–Mn³⁺ redox couple.     

On the mechanism initiating bursting oscillatory patterns during the pitting corrosion of a passive rotating iron-disc electrode in halide-containing sulphuric acid solutions

Bursting current oscillations (trains of current spikes interrupted by a steady-state passive current) were found to occur during the iron anodic polarization in halide-containing sulphuric acid solutions. These oscillations occur at potentials located within a partially passive state of the iron. The partially passive state appears at potentials more positive than the unstable passive-active transition region of the current-potential curve of the Fe|2 M H₂SO₄ system for halide concentrations approximately ranged between 20 and 50 mM. The bursting oscillations appear after a certain induction period which ranged from minutes to several hours. Experiments were conducted in which an external resistance was connected in series between the ground and the working electrode for the Fe|2 M H₂SO₄ + 30 mM Br⁻ system. The results demonstrate that the appearance of oscillations is closely related to an IR-drop mechanism operating during the pitting corrosion of Fe induced by halides. When an appropriate value of the external resistance was imposed in the system the induction period was entirely eliminated. Bursting oscillations appear at once after setting the potential at the desired value. These results provide strong evidence that during the anodic electrodissolution of iron in halide-containing sulphuric acid solutions bursting current oscillations will be induced even though the electrode seems to be in the partially passive state. This is expected to occur after the passage of an induction period during which the system acquires a critical value of the ohmic potential drop. Received: 20 October 1997 / Accepted: 2 January 1998     

Bi5Nb3O15 as a photocatalyst: photocatalytic and photoelectrochemical studies

Bi₅Nb₃O₁₅ was prepared from a stoichiometric mixture of Bi₂O₃ and Nb₂O₅ at 300–500 °C. The prepared photocatalyst was characterized by diffuse reflection spectrum (DRS), X-ray diffraction (XRD), scanning electron microscopy (SEM) and particle size analysis. The band gap, crystal structure and average grain size were determined from the above methods to be 3.25 eV, distorted pyrochlore and 4–5 μm respectively. The photoelectrochemical behavior of hydrogen-reduced Bi₅Nb₃O₁₅ was investigated in 0.1 M Na₂SO₄ and using the Fe(CN)₆ ^3−/4− redox couple for measuring the current-voltage characteristics. The cyclic voltammetric studies revealed that the onset potential for photocurrent generation existed at −0.45 V, which is more negative to water reduction level at pH 7.0, and that of the photocurrent at 1.0 V was observed as 0.58 mA/cm². Photocatalytic hydrogen production has been achieved by using Bi₅Nb₃O₁₅ as a photocatalyst in presence of methyl viologen. The quantum yield for hydrogen production for this system was found to be 0.83. All the studies clearly indicated that Bi₅Nb₃O₁₅ has potential in solar energy conversion. Received: 22 May 1997 / Accepted: 18 September 1997     

Impedance study of adsorption of iodide ions at Cd(0001) and Bi(111) electrode from various solutions with constant ionic strength

Adsorption of iodide ions at the Bi(111) and Cd(0001) electrodes from the aqueous solutions with constant ionic strength 0.1x M KI + 0.1(1−x) M KF and 0.1x M KI + 0.033(1−x) M K₂SO₄ has been studied by impedance spectroscopy. It was found that, to a first approximation, the classical Frumkin–Melik–Gaikazyan equivalent circuit with the slow diffusion-like and adsorption steps can be applied for fitting the experimental impedance data for iodide ions adsorption on Bi(111) and Cd(0001) from aqueous solutions with constant ionic strength. The modified Grafov–Damaskin circuit can be used in the region of electrode potentials, where parallel faradic processes (electroreduction of protons, oxygen traces) are probable. The more complicated Ershler equivalent circuit, taking into account the slow diffusion-like, adsorption and charge transfer steps, is not applicable for characterization of the adsorption process of I⁻ at Bi(111) and Cd(0001) electrodes.     

Characterization of Silicate Complexes in Aqueous Sodium Sulfate Solutions by FAB-MS

When the sodium ion (Na⁺) concentration is increased above 0.5 mol-dm⁻³ (M), the concentrations of dissolved silica in aqueous sodium chloride (NaCl) and sodium nitrate (NaNO₃) solutions decrease because of the salting out effect. On the other hand, the concentration of the dissolved silica in aqueous sodium sulfate (Na₂SO₄) solutions increases monotonously as the concentration of Na⁺ is increased above 0.5 M. The purpose of this study is to determine the reasons why the salting-out effect is not observed in Na₂SO₄ solutions. FAB-MS (Fast Atom Bombardment Mass Spectrometry) was used to sample directly the silica species dissolved in aqueous Na₂SO₄, NaCl, and NaNO₃ solutions. In the FAB-MS spectra of these solutions, the peak intensity ratios of the linear tetramer to the cyclic tetramer largely increased for Na⁺ concentrations between (0.1 and 1) M. This shows that some characteristics of the Na₂SO₄ solutions are similar to those of the NaCl and NaNO₃ solutions. In Na₂SO₄ solutions, however, when the concentration of Na⁺ is higher than 1 M, the peak intensity of the dimer is much higher than those of the other silicate complexes. In Na₂SO₄ solutions, the SO₄²⁻ ion undergoes partial hydrolysis to form HSO⁻₄ and OH⁻ is produced. In particular, in the range where the concentration of SO₄²⁻ is high, the pH of the solution increases slightly. This higher pH yields more dimers from the hydrolysis of silicate complexes. This increase in dimer production agrees with the observation that silica dissolves in sodium hydroxide (NaOH) solutions mainly as a dimer when the concentration of NaOH is less than 0.1 M. In Na₂SO₄ solutions at high concentrations, a salting-out effect is not observed for silica. This is due to the increase in the concentration of OH⁻, which accelerates the hydrolysis of silica and results in dimer formation.     

Ion-specific swelling behavior of poly(vinyl alcohol) gel prepared by γ-ray irradiation

Poly(vinyl alcohol) gel was prepared by γ-ray irradiation of an aqueous solution of the polymer and its swelling behavior in solutions of the alkali-metal and alkaline-earth-metal salts was investigated. The gel deswelled in solutions containing strongly hydrated anions and swelled in solutions containing strongly hydrated cations. The degree of swelling of the gel was in the following order: SO₄ ²⁻<Cl⁻<Br⁻ ≅ NO₃ ⁻<I⁻ for the anions and K⁺<Na⁺<Li⁺ and Sr²⁺<Ca²⁺<Mg²⁺ for the cations. These results were interpreted in terms of interactions between the polymer and the ions through the hydration layers. Received: 6 November 2000 Accepted: 24 May 2001     

The electrochemical determination of ammonium based on the selective inhibition of the low-spin iron(II)/(III) system of Prussian blue

A new method is described for the determination of ammonium in aqueous solutions with electrodes modified by Prussian blue (PB). The specific voltammetric response of PB-modified electrodes to ammonium ions is used for their analytical determination. In the presence of ammonium ions, a concentration-dependent inhibition of the low-spin iron(II/III) system of PB occurs. Only thallium and rubidium ions cause similar inhibition. A useful electrochemical determination method is thus available for detecting ammonium ions in the presence of frequently interfering potassium and sodium ions. Paraffin-impregnated graphite electrodes modified with a mechanically transferred PB layer and bulk-modified PB-composite electrodes are studied. The method is applicable within a concentration range which extends from 4 × 10⁻⁵ mol/l to 10⁻² mol/l NH₄ ⁺. The composite electrode is used in an electrochemical flow-through system in conjunction with the Kjeldahl method. Received: 21 April 1997 / Accepted: 28 May 1997     

Effect of complexing agent (1,10-phenanthroline) on ADP and KDP crystals

1,10-Phenanthroline (Phen) as a new additive was added into the solutions of KH₂PO₄ (KDP) and NH₄H₂PO₄ (ADP) in a small amount (∼2.5·10⁻³ M L⁻¹). The crystals were grown from the aqueous solutions of pH ∼4.5 at constant temperature by solvent evaporation technique. It leads to an increase in metastable zone width and assists the bulk growth process. The growth rate of crystals in the presence of Phen decreases considerably with an increase in impurity concentration (∼2.5·10⁻² M L⁻¹). Not much variation is observed in FTIR and XRD of pure and doped ADP/KDP. It appears that the growth promoting effect (GPE) of Phen is caused by the adsorption of the organic additive on the prism faces of ADP/KDP crystals. Higher optical transmittance is observed in the presence of the dopant. Detailed microhardness studies of ADP crystals reveal the anisotropy in the hardness behaviour. Scanning electron microscope (SEM) photographs exhibit the effectiveness of the impurity in changing the surface morphology of ADP/KDP crystals. Contrary to expectations, Phen depresses the NLO efficiency of ADP/KDP, suggesting that the molecular alignments in the presence of Phen results in cancellation effects disturbing the non-linearity.     

Electrodeposition of Platinum Nanoparticles on Multi-Walled Carbon Nanotubes for Electrocatalytic Oxidation of Methanol

Platinum (Pt) nanoparticles were deposited at the surface of well-aligned multi-walled carbon nanotubes (MWNTs) by potential cycling between +0.50 and −0.70 V at a scanning rate of 50 mV · s⁻¹ in 5 mM Na₂PtCl₆ solution containing 0.1 M NaCl. The electrocatalytic oxidation of methanol at the nanocomposites of Pt nanoparticles/nanotubes (Pt_nano/MWNTs) has been investigated using 0.2 M H₂SO₄ as supporting electrolyte. The effects of various parameters, such as Pt loading, concentration of methanol, medium temperature as well as the stability of Pt_nano/MWNTs electrode, have been studied. Compared to glassy carbon electrode, carbon nanotube electrode significantly enhances the catalytic efficiency of Pt nanoparticles for methanol oxidation. This improvement in performance is due not only to the high surface area and the fast electron transfer rate of nanotubes but also to the highly dispersed Pt nanoparticles as electrocatalysts at the tips and the sidewalls of nanotubes.     

Experimental Study on Viscosity of Colloidal Silica in Aqueous Electrolytic Solutions

The relative viscosity of colloidal silica dispersion in aqueous electrolytic solutions as the function of volume fraction of dry particles in the solutions has been experimentally determined in this work, in order to study the effects of pH and electrolytes (Na₂SO₄ and AlCl₃) on the hydration of the silica surfaces in the solutions. The results have shown that the maximum relative viscosity of the silica dispersion and the strongest hydration of the silica in aqueous solutions appeared at neutral pH, while the stronger the acidity and the alkalinity of the aqueous solution, the weaker the hydration. In the presence of the electrolytes (Na₂SO₄ and AlCl₃), the relative viscosity of the silica dispersion reduced and the hydration of the silica in aqueous solutions became weak. The higher the concentration of the electrolytes, the weaker the hydration, indicating that the destabilization of the colloidal silica dispersion in aqueous solutions might be realized through adding the high-valence electrolytes to weaken the hydration of the particle surfaces (hydration forces between the particles). Also, it has been shown that the negative zeta potentials of the colloidal silica in aqueous solutions greatly reduced in the presence of the electrolytes. Therefore, the high-valence electrolytes (Na₂SO₄ and AlCl₃) as the coagulant of colloidal silica in aqueous solutions might be originated from that the presence of the electrolytes simultaneously reduces the electrical double layer repulsive force and the hydration repulsive forces between the particles in aqueous solutions.     

The surface characteristics of polycrystalline cobalt electrooxidised in sulfuric acid solutions

The oxidation of cobalt electrodes has been carried out by means of cyclic voltammetry and coulometry under controlled potential in sulfuric acid solutions of different concentrations. The electrochemical scanning tunneling microscope/scanning tunneling microscope (ECSTM/STM) systems constructed by the authors and scanning electron microscopy (SEM) with the SEM-EDX system of surface analysis of the elements have been used. The procedure applied in this work made it possible to observe the fragments of the same surface by means of SEM and ECSTM/STM. The most typical images for a polycrystalline Co electrode with a ±10% accuracy at the scales of 4800 nm × 4800 nm and 100 nm × 100 nm are presented and the results are discussed. In a diluted electrolyte (0.1 M), irregular forms of a stable cobalt oxide with Co:O ratio ∼1:1 appear. Unreproducible results have been obtained in a 1.0 M H₂SO₄ solution. Compact and relatively regular layers of cobalt oxide of the same ratio have been obtained in 0.1 M H₂SO₄, as well as in 10.0 M sulfuric acid solution, under controlled oxidation potential at the passivation range. Received: 6 January 1999 / Accepted: 5 May 1999     

Two-Phase Aqueous Extraction of Chromium and its Application to Speciation Analysis of Chromium in Plasma

 This work proposes a new extraction method for chromium based on the two-phase aqueous system isopropyl alcohol-ammonium sulfate-ammonium thiocyanate (i-PrOH-(NH₄)₂SO₄-NH₄SCN), and the related experimental conditions are optimized. The results show that chromium (III) can be quantitatively extracted under the selected conditions: 4 mL of i-PrOH, 200 μL of 2 mol/L sulfuric acid, 1 mL of 4 mol/L NH₄SCN and 3 mL of saturated (NH₄)₂SO₄ solution (V_total=10 mL). Application of the proposed method to speciation analysis of plasma chromium was also investigated and satisfactory results were obtained. Received May 22, 1999. Revision November 8, 1999.     

The effect of salts on the conformation and microstructure of poly(N‐isopropylacrylamide) (PNIPA) in aqueous solution

We present an experimental investigation of the conformation and microstructure of Poly(N‐isopropylacrylamide) (PNIPA) in aqueous solution in the presence of salts. As a model, a strong salting–out salt (Na₂SO₄) and a strong salting–in salt (NaSCN) were chosen. Light scattering measurements show that Na₂SO₄ decreases the radius of gyration of PNIPA compared to its value in water, whereas NaSCN increases it. Moreover, the NaSCN solution was found to be a better solvent for PNIPA compared to water, whereas Na₂SO₄ solution is worse. Small‐angle neutron scattering measurements of semidilute PNIPA solutions, at temperatures well below the phase‐transition temperature, exhibit the behavior predicted by the model of dynamic concentration fluctuations characterized by a single correlation length. Excess scattering at low angles is observed in salt solutions at temperatures that are near, yet below, the phase‐transition temperature. This may indicate intrachain heterogeneities on the scale of 6–8 nm. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3713–3720, 2004     

Swelling behaviors of polyelectrolyte hydrogels containing sulfonate groups

Series of maleate monoester and diester monomers based on poly(ethylene glycol) monomethyl ether (MPEG) were copolymerized using the ionizable 2‐acrylamido‐2–methyl propane sulfonic acid (AMPS) via different dose rate of electron‐beam irradiation (40–150 kGy). The crosslinking of the copolymers were carried out in aqueous acidic solutions at pH 1 or in the presence of 1% N,N‐methylene bisacrylamide (MBA) as crosslinking agent. The final equilibrium water content and swelling capacities for the prepared hydrogels were determined in aqueous solutions at pH 1, 6.8, and 12 and in aqueous salt solutions at 298 K. Swelling equilibria for prepared hydrogels were determined in different molar salt solutions of NaCl, KCl, CaCl₂, Na₂SO₄, K₂SO₄, and CaSO₄. The swelling ratios of gels in pure water and in the salt solutions were found to depend on the counterion species in the increasing sequence of Ca²⁺, Na⁺ and K⁺. Copyright © 2002 John Wiley & Sons, Ltd.     

Influence of finishing oil on structure and properties of multi-filament fibers from cellulose dope in NaOH/urea aqueous solution

Cellulose multi-filament fibers have been spun successfully on a pilot plant scale, from a cellulose dope in 7 wt% NaOH/12 wt% urea aqueous solution pre-cooled to −12 °C. Coagulation was accomplished in a bath with 10 wt% H₂SO₄/12 wt% Na₂SO₄ and then 5 wt% H₂SO₄ aqueous solution. By using different finishing oil, including H₂O, 4% glycerol aqueous solution, 2% polyvinyl alcohol (PVA) aqueous solution, 2% polyethylene glycol octyl phenylether (OP) aqueous solution, mobol and 2%glycerol/1%PVA/1%OP aqueous solution (PGO), we prepared six kinds of the cellulose multi-filaments, with tensile strength of 1.7–2.1 cN/dtex. Their structure and properties were investigated with scanning electron microscope (SEM), ¹³C NMR solid state, wide-angle X-ray diffraction (WAXD) and tensile testing. The cellulose fibers treated with PGO possessed higher mechanical properties and better surface structure than others. Interestingly, although the orientation of the cellulose multi-filaments is relatively low, the tensile strength of the single-fiber was similar to that of Lyocell. It was worth noting that the dyeability of the multi-filament fibers was superior to viscose rayon.     

Morphological studies of the mechanism of pit growth of pure aluminum in sulfate ion- or nitrate ion-containing 0.1 M NaCl solutions

The mechanism of pit growth of pure aluminum (Al) in sulfate ion (SO₄ ^2–)- or nitrate ion (NO₃ ^–)-containing 0.1 M sodium chloride solutions has been studied in terms of the morphological changes of artificial pits using potentiodynamic polarization experiment, potentiostatic current transient technique and optical microscopy. The increase in SO₄ ^2– and NO₃ ^– ion concentrations in NaCl solution raised the pitting potential E _pit of pure Al, which is ascribed to the impediment to pit initiation on pure Al by the addition of SO₄ ^2– or NO₃ ^– ions. From the potentiostatic current transients of artificial pits in aqueous 0.1 M NaCl solution, the average value of the pit current was observed to increase with increasing SO₄ ^2– ion concentration, whereas that value of the pit current in the presence of NO₃ ^– ions increased up to ca. 0.4 M NO₃ ^– ion concentration and then decreased abruptly with increasing NO₃ ^– ion concentration. From observations of the morphologies of the pits, it appears that the pit grows preferentially in the lateral direction or in the downward direction by adding SO₄ ^2– or NO₃ ^– ions to aqueous 0.1 M NaCl solution, respectively. Based upon the experimental results, two different pit growth mechanisms by anion additives can be proposed: (1) pit growth by the preferential attack of both SO₄ ^2– and Cl^– to the pit wall in SO₄ ^2–-containing solutions; (2) pit growth by the creation of an aggressive environment at the pit bottom up to 0.4 M NO₃ ^– ion concentration due to the lower mobility of NO₃ ^– than Cl^– in NO₃ ^–-containing solutions. Electronic Publication     

水系混合离子全电池LiMn2O4//NaTi2(PO4)3

分别以LiMn_2O_4,NaTi_2(PO_4)_3为正负极,1 mol·L~(-1) Li_2SO_4和0.5 mol·L~(-1) Na_2SO_4的混合水溶液为电解液组装成一种水系混合离子全电池。分别将正负极材料在3种不同水相电解液(1 mol·L~(-1) Li_2SO_4、0.5 mol·L~(-1)Na_2SO_4以及1 mol·L~(-1) Li_2SO_4+0.5 mol·L~(-1)Na_2SO_4混合电解液)中进行循环伏安和恒流充放电测试,结果发现,LiMn_2O_4在上述电解液中仅有Li~+的脱出/嵌入而Na~+由于半径较大而不参与该过程,NaTi_2(PO_4)_3在3种电解液中Li+、Na+均参与嵌入/脱嵌过程,且Li~+和Na~+的嵌入/脱出峰电位相差不大,分别为-0.82和-0.64 V,-0.95和-0.75 V;全电池在265 mA·g~(-1)电流密度下平均放电电压为1.55 V,充放电比容量分别为100.1和74.9 m Ah·g~(-1)。