Steric stabilization of iron (III) hydroxide sols by various amino acids

The dispersion and coagulation phenomena of iron(III) hydroxide sols were investigated as a function of pH in the absence and presence of amino acids. The amino acids used were glycine,L--alanine,DL--amino-n-butyric acid,L-valine,L-leucine,L- isoleucine,L-glutamic acid andL-arginine. The turbidity measurements of the iron-(III) hydroxide sols, which were prepared by pouring an aqueous iron(III) chloride solution into boiling distilled water, were carried out using a spectrophotometer with an addermixer device and an automatic recording system. The zeta potentials of sol particles were obtained by ultra-microelectrophoresis. The change in turbidity of the sol, as a measure in stability of the sol, increased with increasing pH in the region of pH 2–8, and reached a maximum at the isoelectric point of the particles. The coagulation at the isoelectric point was prevented by adding amino acids, and the stabilization had an optimum point at concentrations which depended upon the kinds of amino acids. The remarkable dispersing effect of amino acids which occurred near the isoelectric point of the particles at the suitable concentration of the ammo acids may be due to the steric protection by amino acid adsorbed. The protective action was explained according to a modified DLVO theory, the modification for London-van der Waals force being applied in order to take the effect of the adsorption layer into account.     

Catalase monolayers at the air/water interface I. Reversibilities and irreversibilities

/A-isotherms of catalase monolayers established at the air/water-interface are discussed quantitatively on the basis of molecular data: A relationship between a critical value of the surface pressure, the corresponding molecular area, and the molecular dimension of the molecules at the interface is proposed. It is shown that the unfolding of molecules at the water surface is pH-dependent. For each pH-value there is a distinct degree of unfolding; the molecules keep their globular state at neutral pH. Establishment at the surface of bulk solutions corresponding to globular and partly unfolded states, respectively, catalase molecules keep their original configuration on changing the pH-value of bulk-phase. The monolayers are confirmed to show reversibility with regard to lateral changes of state as well as irreversibility with respect to desorption of molecules.A model is proposed to explain the nature of the critical/A-value occurring in the/A-isotherms: on compression beyond _c, molecular segments are transferred from the surface into the bulkphase via a subsurface layer. From the experiments it is concluded that the surface pressure is determined, not only by the surface itself, but also by this subsurface layer.     

“Cooperative” Effects in the Destruction of Foam Column. Comparison of the Behavior of Polyhedral Two- and Three-Dimensional Foams

The kinetics of the destruction of polyhedral monolayer and bulk (three-dimensional) foams from solutions of nonionic surfactants and protein (lysozyme) was studied at various pressure drops Pand constant temperaturetand at various temperatures at P= const in order to elucidate the role of cooperative effects in a foam stability. It was established that lifetimes _p of monolayer and bulk foams prepared from surfactants at t= const decrease with an increase in pressure drop; however, the values of _p are always larger for the monolayer than for bulk foams. Avalanche-like destruction typical of foam prepared from surfactant solutions, which is observed at certain pressure drops in foam liquid phase and temperature, was not disclosed in the monolayer foam formed from the same solutions. For foams with structured adsorption layers (the foam from lysozyme solution), the lifetimes and character of destruction practically do not differ for monolayer and bulk foams. It was established that cooperative effect appears in very thin foam layers beginning with the bubble bilayer.     

Temperature dependent dielectric relaxation study of ethylene glycol-water mixtures

Using the picosecond time domain reflectometry method, dielectric relaxation measurements for 13 ethylene glycol (EG)-water mixtures have been studied from 0 to 40°C. The dielectric relaxations in the mixtures show a Debye-type behavior, whereas the relaxation in pure EG can be described by the Cole-Cole model. The static dielectric constant ₀, the relaxation time and the dielectric constant at high frequency have also been determined at various temperatures. The dielectric relaxation data suggests that there is no tendency to form hydrogen bonds with the addition of water to EG unlike other alcoholic systems but this tendency becomes increasingly important with decreasing temperature. The activation energy decreases with increased water content in the mixture as expected.     

Lyotropic effect in surface charge,electrokinetics, and coagulation of a rutile dispersion

Potentiometric, electrokinetic, and coagulation experiments with a rutile dispersion in the pH region above the point of zero charge exhibit an inverse lyotropic sequence for counterions: Li⁺>K⁺>Cs⁺. The potentiometric and electrokinetic data were interpreted by a surface complexation model assuming the Stern-Gouy-Chapman structure of the interfacial layer, which yielded the values of inner layer capacitances,C, and the intrinsic equilibrium constants,K _ass ⁰ , characterizing the specificity of each counterion. These parameters were used to explain the order of lyotropic sequences in the adsorption, coagulation, and electrokinetic phenomena.     

Mechanism of demulsification of oil-in-water emulsion in the centrifuge

Summary The mechanism of demulsification of 30% Nujol-70 % water emulsions with various surfactants has been studied using a Coulter Counter to monitor the particle size at various depths in the cream layer as a function of time of centrifugation. It was found that the cream layer is stratified with varying sizes of oil globules along the centrifuge cell. Coalescence of the oil globules occurs throughout the body of the cream as well as at the bulk oil-cream interface at high concentrations of Tween 80, and Sponto 221 emulsions whereas the bulk oil-cream interface is found to be the major site of coalescence at lower concentrations of Tween 80 and Gantrez AN 119 emulsions. The globule size at the bulk oil-cream interface reaches a plateau (steady state particle size), and subsequently the rate of oil separation decreases considerably in all cases. The steady state particle size is a function of the concentration of Tween 80, the particle size increasing with increase in concentration of Tween 80.With 10 figures     

Adsorption characteristics of synthesized mordenite membranes

Nitrogen adsorption at 77 K was measured for a self-supporting mordenite membrane synthesized on and detached from, a polytetrafluoroethylene (PTFE) substrate, and for a simultaneously formed powder. The adsorption by the membrane saturated an amount which was about half that found for adsorption by the powder. To examine this difference in adsorption characteristics, both of powder and membrane were measured for adsorption for water, methanol,n-hexane andtert-butyl alcohol. For methanol,n-hexane andtert-butyl alcohol, the powder and membrane both showed the same adsorption behavior as for nitrogen. When water molecules were adsorbed, however, there was no difference in saturated adsorption amount between powder and membrane, suggesting the presence of deformed pores in the rectangular crystal layer of the mordenite membrane. The deformed pores are assumed to occur as gaps among crystallites which displaced along theab planes of the rectangular crystals, forming the rectangular crystal layer. The channel bonding sites resulting from such displacement were found to serve as pore walls against adsorbable molecules which have a cross-sectional area greater than that of the water molecule. and so to hinder their entrance into the deformed pores. X-ray diffraction analysis revealed that the rectangular crystals have no periodic, continuous structure along thec-axis; they show a slight distortion of the crystallite shift from the plane perpendicular to thec-axis.     

Structural analysis of water adsorbed in silica gel

The melting point, T _f of water in a pore decreases as the surface area to pore volume ratio of the pore decreases. Analysis of water absorbed in the pores of silica gels using differential scanning calorimetry (DSC) and dielectric relaxation spectroscopy (DRS) shows that the thickness of the bound, non-freezing water layer adjacent to the pore surface increases as its temperature increases, but that it is independent of the surface silanol concentration, [Si_sOH]. In contrast, the thickness decreases as the cylindrical pore radius r _H decreases. Thus, the increase in the bound water thickness from 0.45 nm for gels with r _H =1.2 nm to 1.2 nm for gels with r _H =7.5 nm is due to the increase from –53°C to –7°C of the temperature (e.g., the melting point T _f ) at which the bound water thickness was measured, and not due to the increase in t _H or the decrease in [Si_sOH]. The T _f of bulk water measured in a DSC was –0.3°C. The boiling point T _v of bulk water measured in a DSC was 81.3°C. T _v increased to 94°C in 7.5 nm pores and to 109°C K in 1.2 nm pores.     

The specific heat of surface layers

Summary The well-known relation ofW. Thomson for the latent heat of surface expansion was obtained by applying the second law to a cyclic surface expansion and heating process. A simple derivation of an expression for the excess heat capacity of the surface layer is obtained by applying the first law to the same cyclic process.If we do not include the amount of potential energy converted to heat,A /TdT in the definition of the excess heat capacity, the resultant relation between the specific heat of the surface layers and their excess heat capacity is of the same form as that expressing the latent heat of vaporization in terms of the differential heat capacities of the liquid and vapor phases.Numerical calculation shows that the excess heat capacity for the surface layer of water amounts to 24 percent of the heat capacity of the molecular layer in the bulk liquid phase.     

ESR study on the stability of W/O gel-emulsions

W/O gel-emulsions (high-internal-phase-volume-ratio emulsions) form in water (or brine) /tetraethyleneglycol dodecyl ether/heptane system above the HLB (hydrophile-lipophile balance) temperature of the system. A salt, which largely decreases cloud temperature in a water-nonionic surfactant system, makes the surfactant film rigid and the gel-emulsions hence become very stable. The effect of aded salt on the apparent order parameter S, and the isotropic hyperfine splitting constant a _N in gel-emulsions was determined by the ESR spin probe method using 5-doxyl stearic acid as the spin probe. The apparent order parameter S, and the isotropic hyperfine splitting constant a _N increase with increasing salinity in Na₂SO₄, CaCl₂, and NaCl systems. It is considered that the surfactant molecules are tightly packed in these systems and this tendency is highly related to the stability of gel-emulsions. The salt dehydrates the hydrophilic moiety of surfactant and hence the lateral interactions of surfactant molecular layer at the water-oil interface increases.The observed difference in the apparent order parameter between the ordinary emulsions and the gel-emulsions suggests that most of the surfactant molecules are adsorbed at the oil-water interface (the surface of the water droplet) in gel-emulsions.     

The coagulation of Ca3Al2(OH)12 in aqueous electrolyte solutions

Summary Coagulation rate measurements of Ca₃Al₂(OH)₁₂ in different aqueous electrolyte solutions (NaOH, NaNO₃, Ca(OH)₂) show that, at concentrations lower than 0.1 M NaNO₃ and 0.3 M NaOH respectively, the stability increases with increasing ionic strength. This fact cannot be explained as either primary or secondary coagulation according to the DLVO theory. However, polymeric ions such as poly-aluminate ions, present in the liquid phase can cause the observed coagulation behaviour at these concentrations. At NaNO₃ concentrations higher than 0.1 M NaNO₃ the coagulation behaviour can be explained as primary coagulation according to the DLVO theory assuming = withA ₁₍₂₎ = 0.03 X 10^–20J. Assuming a distance between the plane with average potentialgs and of 0.3 to 0.4 nm,A ₁₍₂₎ =0.28 -0.30 x 10^–20J depending on the conditions between these two layers.

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Zusammenfassung Koagulationgeschwindigkeitsmessungen an Ca₃ Al₂(OH)₁₂-Suspensionen zeigen, daß in wässerigen Elektrolytlösungen (NaOH, NaNO₃, Ca(OH)₂) bei Konzentrationen niedriger als 0.1 M NaNO₃ bzw. 0.3 M NaOH die Stabilität größer wird, wenn die Elektrolytkonzentration zunimmt. Weder primäre noch sekundäre Koagulation nach der DLVO-Theorie kann diesen Effekt erklären. Die Anwesenheit von polymeren Ionen (z. B. Polyaluminationen) in der Lösung kann das beobachtete Koagulationsverhalten bei diesen Konzentrationen aber deuten. Bei NaNO₃-Konzentrationen größer als 0.1 M kann das Koagulationsverhalten erklärt werden als primäre Koagulation nach der DLVO-Theorie mit der Annahme, daß= und A1(2)=0.03 X 10^–20 J. Nimmt man an, daß der Abstand zwischen den Flächen mit durchschnittlichen Potentialen und etwa 0.3–0.4 nm beträgt, so wird A1(2)=0.28–0.30 x 10^–20 J, abhängig vom Zustand zwischen diesen Flächen.

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With 8 figures and 1 table     

Formation of cobalt(II), nickel(II) and copper(II) chloro complexes in alcohols and the Irving-Williams order of stabilities

Summary Co^II chloro complexes were studied in MeOH at 25 °C and at constant ionic strength of 1 mol dm^–3. Formation of three complexes is postulated for which the overall stability constants are calculated: log ₁ = 1.2, log ₂ = 1.7 and log ₃ = 1.4. The electronic spectra and the formation curves of the identified species are presented for the first time in this medium. The results are compared with those obtained in other alcohols and increasing stability with increasing molecular weight of the solvents is established. Further comparative study showed that the maximum stability of the chloro complexes is found with the Cu^II ion as the central atom. This confirms the Irving-Williams order of stabilities for the first transition metal complexes in this alcoholic medium and the result is explained in terms of the second ionization potential of the elements.     

Trace elements in the water cycle of the Šalek valley,Slovenia, using INAA

Three watersheds were studied by sampling bulk precipitation deposition, seepage water at 50 cm soil depth and spring water. As the main analytical method for determination of trace elements and heavy metals in water samples, thek ₀-based method of INAA was used. The results showed an increased content and concentration range of trace elements in precipitation, soil water and spring water in the vicinity of the otanj Thermal Power Plant. We demonstrated that thek ₀-based method of INAA as a multielement nondestructive technique is a highly suitable approach to determining some toxic trace elements in environmental studies of the water cycle.     

Effect of solvent on reactions of coordination complexes. Part 12: Kinetics and mechanism of chloride substitution reactions ofcis-(chloro)(2-aminoethanol)bis(ethylenediamine) cobalt(III) andcis-(chloro)(1-aminopropan-2-ol)-bis(ethylenediamine) cobalt(III) ions in acidic and basic ethylene glycol-water media

Summary The loss of chloride ion from the title complexes resulted in the predominant formation of the chelated amino-alcohol productscis-[Co(en)₂(NH₂CH₂CH(X)O/H)]^2+/3+ (X=H or Me). The kinetics of chloride release were investigated in aqueous ethylene glycol (EG) media (0 to 80% by wt of EG) at 40°–65°C in acidic media and at 20°–35°C in basic media. The rate constants decreased linearly with increasing mol fraction of the cosolvent. The plots of log kversus D _s ^–1 (S_s=bulk dielectric constant, k=first order or second order rate constants) were essentially linear with negative slopes for the reactions in an acidic medium, and tended to be curved for the base catalysed reactions. The activation enthalpies and entropiesversus X_EG (X_EG=mol fraction of EG) plots indicated extrema which might be associated with the effects of the solvent structural changes on these thermodynamic parameters. The observed solvent isotope effect at 50°C, [HClO₄]=0.010 mol dm^–3 for Cl^– release was lower than the value for the aquation ofcis-[Co(en)₂(alkylamine)Cl]²⁺ complexes reported in the literature. This is consistent with the lack of direct solvent molecule participation in the actual act of substitution at the cobalt(III) centre, as expected for a true intramolecular reaction.Part-11: A. C. Dash and J. Pradhan,Ind. J. Chem.,29A, 167 (1990).     

Foam and Wetting Films from Aqueous Cetyltrimethylammonium Bromide Solutions: Electrostatic Stability1

Foam films and wetting films on quartz formed from aqueous solutions of cetyltrimethylammonium bromide (CTAB) are investigated in a wide range of surfactant concentrations in the presence of background electrolyte (5 × 10^–4 mol dm^–3 NaCl). Foam and wetting films are convenient models for the study of symmetric (free thin liquid films) and asymmetric (thin liquid films on solid substrate) films with the same air/solution interface. Microinterferometric methods of assessment of foam and wetting films are used which allow precise determination of the film thickness. Determined are the values of the potential ₀ of the diffuse electrical layer at the solution/air interface (applying the method of equilibrium foam films) and the potential ₁ at the solution/quartz interface (applying the method of capillary electrokinetics). These values are used to analyze the stability of the films studied in terms of the DLVO theory. A conclusion drawn is that both kinds of films studied are stabilized by electrostatic interaction forces. It is shown that with increasing CTAB concentration, a charge reversal occurs at both the solution/air and solution/quartz interfaces which determines the stability/instability conditions of the foam and wetting films. Concentration ranges where both kinds of films produce stable (equilibrium) films are found. There are also concentration ranges where the films either rupture or are metastable (quasi-equilibrium). The CTAB concentration ranges, which provide the formation of unstable (rupturing and metastable) and stable films, are different for symmetric (foam) and asymmetric (wetting) thin liquid films. It is only at high CTAB concentrations (higher that >2 × 10^–4 mol dm^–3) that both cases render formation of stable equilibrium films. These studies give direct experimental indications that the electrostatic interactions between identical or different interfaces can differ when the surfactant concentration is varied.     

Conductance studies in amide-water mixtures. II. Sodium salts inN,N-dimethylformamide-water mixtures at 35°C

Conductance data for sodium nitrite, chloride, and acetate in water andN,N-dimethylformamide (DMF)-water mixtures (74.82D42.48) for the concentration range 0.001–0.04N, as well as the densities, viscosities, and dielectric constants of the solvent mixtures at 35°C, are reported. The data have been analyzed by the Fuoss (1975) equation. The existence of a maximum in the viscosity at a 13 mole ratio of DMF and water is indicated. The Walden products for all the three salts pass through a maximum while the equivalent conductances show a minimum with increasing DMF content. The maxima in the Walden product are attributed to the dehydration of ions by the cosolvent (DMF).Part I:Indian J. Chem. 14A, 1015 (1976).Deceased.     

Effect of pressure on the conductivities of HCl and KCl in water at 0°C

The electrical conductivities of hydrochloric acid and potassium chloride in water have been measured in the concentration range of 3×10 ^–4 –10 ^–3 moles-dm ^–3 at 0°C up to 3500 bar. The limiting molar conductance (₀) for HCl increases with increasing pressure, while ₀(KCl) has a maximum around 1700 bar. The excess conductance of hydrogen ion [ ₀ ^E =₀(HCl)–₀(KCl)] increases with increasing pressure. Its pressure dependence indicates that the reorientation of water molecules, which is the rate-determining step in the proton jump, becomes faster at higher pressure. This anomaly is attributed to the distortion with pressure of the hydrogen bonds in water.     

Thermodynamics of Complexation of Ag+ with 18-Crown-6 in Water-Dimethyl Sulfoxide Mixtures

The stability of complexes and enthalpy of interaction of Ag⁺ ions with 18-crown-6 in waterdimethyl sulfoxide (DMSO) mixtures were determined by calorimetric titration in the range of mole fractions XDMSO from 0.0 to 0.97 at 298.15 K. With increasing concentration of the nonaqueous component in the solvent to XDMSO 0.3, the stability of the complex ion [AgL]⁺ increases, which is followed by a decrease in logK(AgL⁺) to 0.35 plusmn 0.15 at XDMSO 0.97. The exothermic effect of the reaction shows a similar trend. The presence of the extremum in the logK-XDMSO and _r H-XDMSO dependences is explained by the competition of two solvation contributions: destabilization of the ligand with decreasing water content in the solvent and formation of strong solvation complexes of Ag⁺ with DMSO.     

Characterization of solute-membrane interactions by TMA-measurements

On the basis of relationships derived by Oplatka et al. [1] correlating the derivative of tensile stress with respect to temperature for a polymer strip at constant length, (/T) _t , with the derivative of entropy with respect to strip length, (S/l)_T, relationships are derived relating the derivation of strip length with respect to temperature under constant tensile stress, (l/T), with the derivative of entropy with respect to tensile load, (S/) _T . The developed relationships are applied to evaluate TMA (thermomechanical analysis) measurements performed with asymmetric and homogeneous cellulose acetate membranes where the membrane strips were kept in deionized water. Moreover, TMA measurements were performed with homogeneous CA membranes while maintaining the membrane strips in bathing solutions of different, but in each instance, constant electrolyte concentrationc _s (e.g., LiCl, LiNO₃, Li₂SO₄, CaCl₂; 0 c _s saturation). Under constant tensile stress, , as-cast asymmetric CA membranes, which are kept in deionized water, first elongate with increasing temperature at temperatures up to about 50°C. However, they shrink with a further increase of temperature in the temperature range 50°C up to about 90°C, whereas creeping starts at temperatures beyond 90°C, leading again to an elongation of the strip with a further increase of temperature. The negative apparent coefficient of thermal expansion, existing between 50° to 90°C, is consistent with shrinking phenomena observed with asymmetric CA membranes [2]. In addition, it correlates well with both an entropy decrease with increasing tensile stress at constant strip length and an entropy increase with decreasing strip length at constant tensile stress. Contraction of the strip with increasing temperature at constant tensile load might be due to increasing coil formation of polymer molecules rendered possible by disintegration of joints such as H-bonds between polymer chains. Depending on the electrolyte of the bathing solution and its concentration, homogeneous CA membranes exhibit positive and negative apparent linear coefficients of thermal expansion under constant tensile stress.