Interaction Mechanisms Between Anionic Surfactant Micelles and Different Metal Ions in Aqueous Solutions

In its using or eliminating processes, surfactant solutions usually exhibit different behaviors because of the different species or concentrations of the encountered metal ions. Interactions between anionic surfactant (SDS) micellar solutions and several familiar metal salt solutions (Al₂(SO₄)₃, FeCl₃, CaCl₂ and MgCl₂) were investigated. Precipitates were formed in all systems except SDS‐MgCl₂ visually. Stoichiometric analysis reveals that, in SDS‐Al₂(SO₄)₃ system, the precipitation phenomenon is mainly owing to the effect of adsorption‐charge neutralization between Al³⁺ ions and SDS micelles; in SDS‐FeCl₃ system, bridge connection effect of Fe(OH)²⁺ ions among SDS micelles becomes the dominant mechanism; while in SDS‐CaCl₂ system, all SDS micelles are decomposed and solubility product of Ca(DS)₂ crystal results in the precipitation. SEM photographs of the precipitates can serve as additional vivid proofs of the above conclusion.     

Potentiometric Study of the Effect of Sodium Dodecylsulfate and Dioxane on the Hydrolysis of the Aluminum(III) Ion

Hydrolytic equilibria of the aluminum(III) ion were studied in the presence of a surfactant, sodium n-dodecylsulfate (SDS) and, separately, in mixed water + dioxane and water + dioxane + surfactant media at 298.15 K, by using potentiometric measurements with a glass electrode. The concentration of SDS was between 1.25 and 25.0 mmol-dm⁻³, whereas the volume percent of dioxane was varied from 10 to 50. The supporting strong electrolyte was 0.1 mol-dm⁻³ LiCl. A general least-squares treatment of the data indicates the formation of mononuclear hydrolytic complexes of the form Al(OH)_m^{3 − m} (m = 1–3) at all studied compositions. At lower concentrations of SDS (≤ 12.5 mmol-dm⁻³) it was necessary to include polynuclear hydrolytic complexes in the hydrolytic model. On increasing the concentration of SDS, the formation of polynuclear complexes is suppressed, and at the SDS concentration of 25.0 mmol-dm⁻³, only Al(OH)²⁺ and Al(OH)₂⁺ are observed in solution. At lower volume percentages of dioxane, the speciation involved polynuclear complexes in addition to mononuclear complexes. At dioxane concentrations higher than 20 vol% only mononuclear complexes are formed. The simultaneous presence of the SDS and dioxane as ionic medium modifiers produces only the mononuclear complexes Al(OH)²⁺ and Al(OH)₂⁺, which have significantly higher stability constants than in the pure ionic medium.     

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     

Flocculation of anionic surfactant micelles in the presence of hydrocarbons

Adsorptive micellar flocculation (AMF) is a surfactant-based separation process based on the flocculation of micelles of suitable anionic surfactants by Al³⁺. The micelles form large amorphous flocs which can be removed by filtration, which is attractive because soluble pollutants are captured by the floc, thus providing a simple separation method. As the primary aim of AMF is the removal of anionic pollutants from aqueous streams, it is important to investigate the influence of the various substances which may affect it. This article discusses the influence of the presence of insoluble hydrocarbons on the flocculation of micelles of the anionic surfactant dodecyl sulfate by Al³⁺. The ratio between surfactant remaining in solution and total surfactant and the ratio between Al³⁺ and surfactant in the flocculate are determined in systems composed of an aqueous solution containing a constant dodecyl sulfate concentration of 0.05 M and a variable Al³⁺ concentration and an organic phase (decane) with phase volume ratios of decane/water ranging from 0 (no decane) to 0.15. The flocculation is only slightly affected by the presence of decane for decane/solution ratios below 0.05, while the effect (lower flocculated fraction) is more marked above this ratio. Received: 25 October 1999/Accepted: 7 February 2000     

Effects of Cs and Na ions on the interfacial properties of dodecyl sulfate solutions

The competitive binding of counterions to anionic dodecyl sulfate ions in aqueous solutions of cesium dodecyl sulfate (CsDS) and sodium dodecyl sulfate (SDS) mixtures, which significantly influences the critical micelle concentration (cmc) and surface (or interfacial) tension of surfactant solutions, was investigated. The cmc and degree of counterion binding were obtained through electrical conductivity measurements. The curve of cmc versus the mole fraction of CsDS in the surfactant mixture was simulated by Rubingh's equations, which enabled us to estimate the interaction parameter in micelles (W _R) based on the regular solution approximation. The curve-fitting exhibited a slightly negative value (W _R=−0.1), indicating that the mixing (SDS+CsDS) enhances micelle formation owing to a greater interaction between surfactant molecules and counterions than in pure systems (SDS). On going from SDS, SDS:CsDS(75:25), SDS:CsDS(50:50), SDS:CsDS(25:75) to CsDS, interfacial tension at the hexadecane/surfactant-solution interface showed a negative deviation from the mixing rule (interaction parameter in adsorbed film W _A=−0.38), indicating the replacement of Na⁺ bound to anionic dodecyl sulfate by Cs⁺ ions owing to the stronger interaction between the Cs⁺ and the dodecyl sulfate ions. Droplet sizes of emulsion formed with hexadecane and aqueous dodecyl sulfate solutions were investigated using the light scattering spectrophotometer. The higher binding capacity of Cs⁺, having a smaller hydrated ionic size than Na⁺, also resulted in a negative deviation in emulsion droplet size in mixed systems. Received: 10 May 2000/Accepted: 11 August 2000     

Phase behavior and solution properties of sodium (3-dodecanoyloxy-2-hydroxy-propyl) succinate in water

Sodium (3-dodecanoyloxy-2-hydroxy-propyl) succinate (SLGMS) is a conjugated anionic surfactant in which a glycerol residue connects with a hydrophilic sodium succinate and dodecanoate. Aqueous micellar phase (W_m), hexagonal (H₁), bicontinuous cubic (V₁), and lamellar (L_α) phases are successively formed with increasing the surfactant concentration in a binary SLGMS-water system. The Krafft point is below 0 °C. The effective cross sectional area per surfactant molecule, a _s, in the H₁ phase is almost constant, 0.5 nm², and the shape of cylindrical micelle is almost unchanged with surfactant concentration. The cmc value of SLGMS measured by means of surface tension, electrical conductivity, and fluorescence probe methods is in the range of 4∼9 × 10⁻⁵ mol/l that is much lower than that of sodium dodecanoate, 2 × 10⁻² mol/l, or SDS, 8 × 10⁻³ mol/l. Hence, it is considered that the polar glycerol part in the SLGMS acts as a hydrophobic part. The solubilization of oil in the SLGMS solution is much higher than that in the SDS solution and this also suggests that the glycerol and succinic units act as lipophilic moieties. Received: 15 June 2000/Accepted: 27 July 2000     

Stability against aggregation of bovine casein micelles in the presence of acidic and alkaline gelatin

The effect of the presence of colloidal dispersed and molecular dispersed acidic (type A) and alkaline (type B) gelatins with similar molecular weight and size but different isoelectric points (7.9 and 4.9) on the stability against aggregation of bovine casein micelles was investigated by turbidimetric titration and laser techniques, over a wide range of biopolymers concentrations, gelatin/casein ratio in the initial mixture (0.03–20), pH (4.9–6.7) and ionic strength (10⁻³(milk salts)–1.0 NaCl), using glucono-δ-lactone (GL) as acidifier. Aggregates of acid gelatin A interact with the oppositely charged micellar casein at an ionic strength of around 10⁻³(milk salts) and pH 6.7 resulting in the formation of an electroneutral complex by ionic bonds between the carboxyl groups of casein and the amino groups of the gelatin molecules. The complexes obtained are polynuclear, the aggregation of which is not as sensitive to pH as that of free casein micelles. Aggregation of such complexes is the result of bridging flocculation. The “molar” ratio gelatin aggregates/casein micelles in the mixed aggregates is 4/1. The complexes are formed and stabilised via electrostatic interaction rather than through hydrogen bonds or hydrophobic interaction. In the presence of an excess of gelatin molecules in the initial mixture a charged gelatin–casein complex forms and some dissociation of casein micelles occurs and, as a consequence, soluble complexes are obtained. During the addition of alkaline gelatin B aggregates to the micellar casein solution and subsequent acidification of the mixture by GL, no effect of the presence of gelatin B on the stability of micellar casein was observed. Received: 28 March 2000 Accepted: 5 October 2000     

Synthesis and pH-sensitive micellization of doubly hydrophilic poly(acrylic acid)-b-poly(ethylene oxide)-b-poly(acrylic acid) triblock copolymer in aqueous solutions

A doubly hydrophilic triblock copolymer poly(acrylic acid)-b-poly(ethylene glycol)-b-poly(acrylic acid) (PAA-b-PEO-b-PAA) with M _w/M _n = 1.15 was synthesized by atom transfer radical polymerization of t-butyl acrylate (tBA), followed by acidolysis of the PtBA blocks. The pH-sensitive micellization of PAA-b-PEO-b-PAA in acidic solution was investigated by potentiometric titration, fluorescence spectrum, dynamic light scattering and zeta potential. The pK _a was 6.6 and 6.0 in deionized water and in 0.1 mol/L NaCl solution, respectively. The copolymer formed micelles composed of a weakly hydrophobic core of complexed PAA and PEO and a hydrophilic PEO shell in 1 mg/mL solution at pH < 5.5 due to hydrogen bonding. The critical micelle concentration was 0.168 mg/mL at pH 2.0. At pH < 4.5, steady and narrow distributed micelles were formed. Increasing pH to 5.0, unsteady and broad distributed micelles were observed. At pH > 5.5, the micelle was destroyed owing to the ionization of the PAA blocks.     

Interfacial and aggregation properties of aqueous sodium <Emphasis Type="Italic">N</Emphasis>-dodecanoylsarcosinate solutions at different pH

The pH dependence of an anionic surfactant, sodium N-dodecanoylsarcosinate (SLAS), has been studied by measuring interfacial tension, fluorescence, dynamic light scattering, etc., in aqueous solutions with phosphate and borate buffers. The interfacial tension (γ) of SLAS decreases remarkably with a pH decrease and is constant at pH > 7.3. The observed values for the critical micelle concentration (cmc) and the surfactant concentration at which its γ value is reduced by 20 mN/m from that of pure water (C ₂₀) decrease with a pH decrease, while those also become constant at pH > 6.5 and >7.3, respectively. On the other hand, the interfacial excess of SLAS increases at pH < 7.3. These interfacial behaviors have been further investigated by the addition of Tl⁺ which replaces Na⁺ of SLAS. The observed γ values of LAS⁻ with the different counter cations are in the order of H⁺ < Tl⁺ < Na⁺. In order to reveal aggregation properties of SLAS, the aggregation number (N _agg), the micropolarity, the hydrodynamic radius (R _h) of micelle, and the fluorescence anisotropy of Rhodamine B (r) have been evaluated at various pHs. The N _agg value shows a decreasing tendency with a pH increase. The I ₁/I ₃ ratio and the R _h values do not strongly depend on pH. The r value decreases until pH 7 and remains constant at pH > 7.0. These interfacial and micelle properties have been discussed in detail considering the electrostatic interaction and the molecular structures of the hydrophilic headgroup.     

The effect of the melting of the collagen-like gelatin aggregates on the stability against aggregation of the bovine casein micelles

The effect of the melting of the collagen-like acid and alkaline gelatin aggregates on the stability against aggregation of bovine casein micelles was investigated by turbidimetry, DSC and circular dichroism in the wide range of biopolymers concentrations, gelatin/casein ratio (R) in initial mixture (R=0.03–20), pH (4.9–6.7), ionic strength (I=10⁻³ to 1.0/NaCl/), and temperature (10°–70 °C), using glucono-δ-lactone (GL) as acidifier. At low ionic strength (10⁻³/milk salts/) and neutral pH interaction between gelatin molecules and casein micelles is suppressed significantly above 36 °C. The melting of the collagen-like acidic gelatin above this temperature shifts the pH at which the complex formation is maximal to the acidic range. The cause may be that some of the functional ionized groups of gelatin molecules are inaccessible due to the conformational changes. The presence of gelatin B molecules had no effect on the aggregation stability of micellar casein in the range 0.03<R<20. At very high total protein concentration (above 10%) depletion flocculation of casein micelles was observed. The reason for the very high stability of casein micelles in this case cannot be explained by volume exclusion. Received: 28 March 2000 Accepted: 5 October 2000     

Electrophoresis of colloidal α-alumina

Measurements of the electrophoretic mobility (u _E) of particles of colloidal α-alumina were made as a function of pH, electrolyte concentration and electrolyte type (NaCl, NaNO₃ and KCl) using two similar instrumental techniques. Significant differences (50% or less) in the values of u _E of particles in NaCl were obtained from the two instruments; however, the isoelectric points (IEPs) (the pH at which u _E=0), estimated from the two sets of measurements, occurred at 7.5 ± 0.3 and 7.8 ± 0.05 and were not significantly different. The latter estimate corresponds with those for particles in KCl and NaNO₃ of 8.05 ± 0.11 and 7.95 ± 0.18, respectively, made using the same instrument and indicate that the IEP was a weak function of electrolyte type. When cations acted as counterions (pH > IEP), the absolute magnitudes and the ranges of u _E with electrolyte concentration were found to be significantly less than when anions acted as counterions (IEP > pH). Estimates of the zeta potential (ζ), made using various procedures, showed variations of up to 25% at low ratios of electrical-double-layer thickness (κ ⁻¹) to particle radius (a) (κa∼10) and were of a similar scale to differences in u _E, but no significant variations (95% confidence) in ζ were obtained at high values (κa∼200). Received: 12 July 2000 Accepted: 17 October 2000     

Micellar behavior of acrylamide– octylphenylpoly(oxyethylene) acrylate copolymer in aqueous solution

The copolymer of acrylamide and octylphenylpoly(oxyethylene) acrylate macromonomer (AM-C₈PhEO₇Ac) was synthesized and characterized by IR and NMR spectroscopy. The molecular weight of the copolymer was determined to be 1.21 × 10⁵ by static light scattering. The weight contents of AM and macromonomer were determined to be 67.8 and 32.2%, respectively, by elemental analysis. The micellar behavior of the copolymer in aqueous solution was studied by UV spectroscopy and atomic force microscopy (AFM). The molecules of AM-C₈PhEO₇Ac copolymer form monomolecular micelles in the concentration range 4 × 10⁻⁶ – 3 × 10⁻⁵ g/ml and polymolecular micelles at concentrations above 3 × 10⁻⁵ g/ml according to the UV analysis. The AFM images indicate that the monomolecular micelles are globular with diameters of 70 nm and have a narrow size distribution. The polymolecular micelles can be globular or cylindrical depending on the concentration, and have a wide size distribution. Received: 10 February 1999 Accepted in revised form: 28 June 1999     

Polydisperse fractal aggregate formation in clay mineral and iron oxide suspensions, pH and ionic strength dependence

pH- and ionic-strength-dependent aggregation of permanently and conditionally charged clay mineral (montmorillonite) and iron oxide (magnetite) particles was investigated by means of dynamic light scattering and rheology. An indifferent electrolyte (NaCl) was used. The surface charging of solids was determined by acid–base titration. The point of zero charge (PZC) of magnetite seemed to be at pH 8.0 ± 0.1. The permanent negative charges on the basal plane of montmorillonite influence the interfacial distribution of H⁺ and Na⁺ ions. The pH dependence of the electrophoretic mobility showed directly the dominance of negative charges on montmorillonite lamellae independently of pH, while for magnetite the sign of the mobility reversed at pH ˜ 8.0. Montmorillonite particles formed stable suspensions; coagulation did not take place below 35 mM 1:1 electrolyte independently of pH. The aggregation of magnetite sol becomes significant near the pH of the PZC even at low ionic strength. Colloidal stability in composite systems was investigated at pH ˜ 4, where oxide and clay mineral particles are oppositely charged. At the lowest NaCl concentrations (1, 5 mM) the mixed systems remained stable and aggregation of oppositely charged particles could not be observed at all. Heterocoagulation of dissimilar particles needed a definite amount of dissolved electrolytes (about 8 mM). Mixed clay mineral and oxide systems are more sensitive to electrolyte under acidic conditions than those separately. Rheological investigation of the mixed clay mineral–oxide suspensions at pH ˜ 4 provided proof for the absence of attractive particle interaction at low ionic strength (2 mM). A physical network of oppositely charged particles could form only at higher salt concentration, for example, in the presence of 10 mM NaCl. The yield value of plastic systems showed a significant maximum at 1:15 magnetite/montmorillonite mass ratio. Received: 21 November 2000 Accepted: 20 December 2000     

Potentiometric and Multinuclear Magnetic Resonance Study of the Solution Equilibria Between Aluminium(III) Ion and L-Aspartic Acid

Summary. Solution equilibria between aluminium(III) ion and L-aspartic acid were studied by potentiometric, ²⁷Al, ¹³C, and ¹H NMR measurements. Glass electrode equilibrium potentiometric studies were performed on solutions with ligand to metal concentration ratios 1:1, 3:1, and 5:1 with the total metal concentration ranging from 0.5 to 5.0 mmol/dm³ in 0.1 mol/dm³ LiCl ionic medium, at 298 K. The pH of the solutions was varied from ca. 2.0 to 5.0. The non-linear least squares treatment of the data performed with the aid of the Hyperquad program, indicated the formation of the following complexes with the respective stability constants log β_p,q,r given in parenthesis (p, q, r are stoichiometric indices for metal, ligand, and proton, respectively): Al(HAsp)²⁺ (log β_1,1,1 = 11.90 ± 0.02); Al(Asp)⁺ (log β_1,1,0 = 7.90 ± 0.03); Al(OH)Asp⁰ (log β_1,1,−1 = 3.32 ± 0.04); Al(OH)₂Asp⁻ (log β_1,1−2 = −1.74 ± 0.08), and Al₂(OH) Asp³⁺ (log β_2,1,−1 = 6.30 ± 0.04). ²⁷Al NMR spectra of Al³⁺ + aspartic acid solutions (pH 3.85) indicate that sharp symmetric resonance at δ∼10 ppm can be assigned to (1, 1, 0) complex. This resonance increases in intensity and slightly broadens upon further increasing the pH. In Al(Asp)⁺ complex the aspartate is bound tridentately to aluminum. The ¹H and ¹³C NMR spectra of aluminium + aspartic acid solutions at pH 2.5 and 3.0 indicate that β-methylene group undergoes the most pronounced changes upon coordination of aluminum as well as α-carboxylate group in ¹³C NMR spectrum. Thus, in Al(HAsp)²⁺ which is the main complex in this pH interval the aspartic acid acts as a bidentate ligand with –COO⁻ and –NH₂ donors closing a five-membered ring.     

Electrocatalytic reduction of hydrogen peroxide at Prussian blue modified electrodes: a RDE study

Electrocatalytic reduction of hydrogen peroxide at Prussian blue modified electrode has been studied with rotating disk electrode in pH 5.5 and 7.3 solutions. It has been shown that the electrocatalytic cathodic reduction obeys Koutecky–Levich relationship at electrode potentials ranging from 0.1 to −0.4 V vs. Ag/AgCl for low concentrations of peroxide not exceeding 0.3 mM. Within this potential window, the calculated kinetic cathodic current ranges within the limits of 2.15–6.09 and 1.00–3.60 mA cm⁻² mM⁻¹ for pH 5.5 and 7.3, respectively. For pH 5.5 and 7.3 solutions, a linear slope of the dependence of kinetic current on electrode potential of −10.8 and −2.89 mA cm⁻² mM⁻¹ V⁻¹, respectively, has been obtained. At a higher concentration of peroxide, exceeding 0.6 mM, deviations from Koutecky–Levich relationship have been observed. These deviations appear more expressed at higher potentials and higher solution pH. The results obtained have been interpreted within the frame of two-step reaction mechanism, including (1) dissociative adsorption of hydrogen peroxide with the formation of OH radicals and (2) one-electron reduction of these radicals to OH⁻ anions. At a higher concentration of peroxide, and especially at a higher pH, the second process becomes rate limiting.     

Potentiometric and Multinuclear Magnetic Resonance Study of the Solution Equilibria Between Aluminium(III) Ion and L-Aspartic Acid

Solution equilibria between aluminium(III) ion and L-aspartic acid were studied by potentiometric, ²⁷Al, ¹³C, and ¹H NMR measurements. Glass electrode equilibrium potentiometric studies were performed on solutions with ligand to metal concentration ratios 1:1, 3:1, and 5:1 with the total metal concentration ranging from 0.5 to 5.0 mmol/dm³ in 0.1 mol/dm³ LiCl ionic medium, at 298 K. The pH of the solutions was varied from ca. 2.0 to 5.0. The non-linear least squares treatment of the data performed with the aid of the Hyperquad program, indicated the formation of the following complexes with the respective stability constants log β_p,q,r given in parenthesis (p, q, r are stoichiometric indices for metal, ligand, and proton, respectively): Al(HAsp)²⁺ (log β_1,1,1 = 11.90 ± 0.02); Al(Asp)⁺ (log β_1,1,0 = 7.90 ± 0.03); Al(OH)Asp⁰ (log β_1,1,−1 = 3.32 ± 0.04); Al(OH)₂Asp⁻ (log β_1,1−2 = −1.74 ± 0.08), and Al₂(OH) Asp³⁺ (log β_2,1,−1 = 6.30 ± 0.04). ²⁷Al NMR spectra of Al³⁺ + aspartic acid solutions (pH 3.85) indicate that sharp symmetric resonance at δ∼10 ppm can be assigned to (1, 1, 0) complex. This resonance increases in intensity and slightly broadens upon further increasing the pH. In Al(Asp)⁺ complex the aspartate is bound tridentately to aluminum. The ¹H and ¹³C NMR spectra of aluminium + aspartic acid solutions at pH 2.5 and 3.0 indicate that β-methylene group undergoes the most pronounced changes upon coordination of aluminum as well as α-carboxylate group in ¹³C NMR spectrum. Thus, in Al(HAsp)²⁺ which is the main complex in this pH interval the aspartic acid acts as a bidentate ligand with –COO⁻ and –NH₂ donors closing a five-membered ring.     

Influence of metal ions on the alkali-swelling behavior of carboxylated acrylic polymer latexes

The alkalization of carboxylated acrylic polymer latexes by sodium hydroxide gives rise to swelling of the particles. For a poly(n-butyl acrylate) latex copolymerized with 15 wt % methacrylic acid (MAA) and 7 wt % acrylonitrile the particle volume increases by a factor of 30. The alkali-swelling does not depend on the type of monovalent cation used in the base (LiOH, NaOH, KOH, NH₄OH). In contrast, when bivalent cation bases such as Ca(OH)₂ are employed no latex swelling is observed during neutralization because of ionic crosslinking of the copolymer chains. Crosslinking also takes place when the bivalent cations (Ca²⁺, Zn²⁺, Mg²⁺) are added as chlorides to dispersions with latexes previously swollen by sodium hydroxide. In these experiments the original size of the latexes is reached again at a molar ratio MAA: bivalent metal ion of 2:1, i.e. at charge compensation of the carboxyl groups. The shrinking behavior is almost independent of the type of bivalent metal ion used. On the other hand, it is more pronounced when trivalent cations such as Fe³⁺ are added. In general, the experiments demonstrate that the alkali swelling of acrylic latexes is dominated by electrostatic forces. Received: 18 August 1998 Accepted in revised form: 26 October 1998     

Cloning,Expression, and Characterization of a Wide-pH-Range Stable Phosphite Dehydrogenase from <Emphasis Type="Italic">Pseudomonas</Emphasis> sp. K in <Emphasis Type="Italic">Escherichia coli</Emphasis>

A phosphite dehydrogenase gene (ptdhK) consisting of 1,011-bp nucleotides which encoding a peptide of 336 amino acid residues was cloned from Pseudomonas sp. K. gene ptdhK was expressed in Escherichia coli BL21 (DE3) and the corresponding recombinant enzyme was purified by metal affinity chromatography. The recombinant protein is a homodimer with a monomeric molecular mass of 37.2 kDa. The specific activity of PTDH-K was 3.49 U mg⁻¹ at 25 °C. The recombinant PTDH-K exhibited maximum activity at pH 3.0 and at 40 °C and displayed high stability within a wide range of pHs (5.0 to 10.5). PTDH-K had a high affinity to its natural substrates, with K _m values for sodium phosphite and NAD of 0.475 ± 0.073 and 0.022 ± 0.007 mM, respectively. The activity of PTDH-K was enhanced by Na⁺, NH₄⁺, Mg²⁺, Fe²⁺, Fe³⁺, Co²⁺, and EDTA, and PTDH-K exhibited different tolerance to various organic solvents.     

Effect of pH and ionic strength on the interaction of humic acid with aluminium oxide

The effect of pH and neutral electrolyte on the interaction between humic acid/humate and γ-AlOOH (boehmite) was investigated. The quantitative characterization of surface charging for both partners was performed by means of potentiometric acid–base titration. The intrinsic equilibrium constants for surface charge formation were logK _a,1 ^int=6.7±0.2 and logK _a,2 ^int = 10.6±0.2 and the point of zero charge was 8.7±0.1 for aluminium oxide. The pH-dependent solubility and the speciation of dissolved aluminium was calculated (MINTEQA2). The fitted (FITEQL) pK values for dissociation of acidic groups of humic acid were pK ₁ = 3.7±0.1 and pK ₂ = 6.6±0.1 and the total acidity was 4.56 mmol g⁻¹. The pH range for the adsorption study was limited to between pH 5 and 10, where the amount of the aluminium species in the aqueous phase is negligible (less than 10⁻⁵ mol dm⁻³) and the complicating side equilibria can be neglected. Adsorption isotherms were determined at pH ∼ 5.5, ∼8.5 and ∼9.5, where the surface of adsorbent is positive, neutral and negative, respectively, and at 0.001, 0.1, 0.25 and 0.50 mol dm⁻³ NaNO₃. The isotherms are of the Langmuir type, except that measured at pH ∼ 5.5 in the presence of 0.25 and 0.5 mol dm⁻³ salt. The interaction between humic acid/humate and aluminium oxide is mainly a ligand-exchange reaction with humic macroions with changing conformation under the influence of the charged interface. With increasing ionic strength the surface complexation takes place with more and more compressed humic macroions. The contribution of Coulombic interaction of oppositely charged partners is significant at acidic pH. We suppose heterocoagulation of humic acid and aluminium oxide particles at pH ∼ 5.5 and higher salt content to explain the unusual increase in the apparent amount of humic acid adsorbed. Received: 20 July 1999 /Accepted in revised form: 20 October 1999     

Rheology of viscoelastic anionic micellar solutions in the presence of a multivalent counterions

The growth and structure of anionic micelles sodium dodecyl trioxyethylene sulfate (SDES) in the presence of a multivalent counter-ion, Al³⁺, were investigated by means of rheological methods and the technique of freeze-fracture transmission electron microscopy. It was found that wormlike micelles and network structures could be formed in SDES/AlCl₃ aqueous micellar solutions, according to the measurements of the zero-shear viscosity, the complex viscosity and the dynamic moduli (storage modulus and loss modulus), and the application of the Cox–Merz rule and a Cole–Cole plot. The cyclic shear test, the plateau modulus and the relaxation time were also studied to express the rheological properties of the wormlike micellar solutions. The structure was of a character of a nonlinear viscoelastic fluid and departed from the simple Maxwell model. Received: 23 November/2000 Accepted: 31 January 2001