Multiscale-linking simulation of irreversible colloidal deposition in the presence of DLVO interactions

An efficient multiscale-linking algorithm, based on the self-consistent integration of Brownian dynamics simulation of particle trajectories with the solution of the continuum-level conservation equation for particle concentration subject to an adaptive Neumann boundary condition that accounts for the blocking effect of deposition, is developed. The algorithm has been already validated in the case of deposition of noninteracting hard spheres [R.V. Magan, R. Sureshkumar, Multiscale Model. Simul. 2 (2004) 475]. In this study, the above algorithm is extended to incorporate particle interactions modeled by the DLVO theory. The simulations are used to identify a time scale at which the deposition process transitions from a power-law to an asymptotic regime. Detailed characterization of the two regimes is provided for a wide range of ionic strength, particle surface charge density, bulk volume fraction, and substrate potential values. The radial distribution functions obtained for various ionic strengths can be collapsed into a master curve when the radial distance is normalized with respect to a characteristic length scale of inter-particle repulsion. Moreover, simulation results suggest a rescaled, uniformly valid soft random sequential adsorption (RSA) model. Simulation results for the kinetics and monolayers structure compare favorably with experimental data, without the use of adjustable parameters. Comparison with other dynamic simulation techniques shows that while their predictions are qualitatively similar, notable quantitative differences exist especially for small ionic strengths.     

Electrostatic calculations on colloids with planar and spherical charge distributions

A model of the electrical double layer has been derived by assuming that a fraction of double layer counter-ions remains associated with the charged surface (Nature 1984 310: 396). The magnitude of this fraction, i.e. the degree of counter-ion association, can be derived from experimental data. E.g. the electrostatic repulsions in clay gels can be quantitatively explained with the degree of counter-ion association that is independent of plate separation. The dependence of the degree of counter-ion association on bulk ionic strength is obtained from Lubetkin'set al data (Philos Trans R Soc London (1984) A311133), which permits the derivation of a new limiting law that predicts large and ionic strength independent repulsive forces at close surface separations. Such forces have been usually attributed to hydration forces. Within the DLVO framework the theory perdicts reasonable values of the equilibrium separations of swelling clays with divalent counter-ions. The Stern potentials of clays are also consistently accounted for. New calculations for spherical surfaces predict that Stern potentials can have maxima at certain ionic strengths. These maxima, which are often observed in the case of polymer latices, disappear in the limit of infinite radius.     

Activity measurements in the ternary system NaBr+NaClO4+H2O at 25°C

The activity coefficients of sodium bromide in the ternary system NaBr+NaClO₄+H₂O were determined at 25°C and constant ionic strength of 0.1, 0.5, 1, 2, and 3 mol-kg^?1 from emf of the cell without, liquid junction $$ISE - Na|NaBr(m_A ), NaClO_4 (m_B ), AgCl_{(s)} 1 Ag$$ The experimental activity coefficients were comparatively analyzed by using the Harned, Scatchard, Pitzer and Lim-HOLL treatments. All these methods are adequate for the analysis of the experimental data. The results have been compared with those of Lanier for the system: NaCl+NaClO₄+H₂O. The Gibbs excess energy of mixing was obtained and qualitatively interpreted in terms of ionic interactions.     

Preparation of Ultrafine TATB and the Technology for Crystal Morphology Control

The ultrafine 1,3,5‐triamino‐2,4,6‐trinitrobenzene (TATB) has been prepared by using solvent and non‐solvent method, and the influencing factors in close relationship with the grain size and crystal morphology control such as categories and dosage of surfactants, volume ratio of solvent to non‐solvent have also been considered in this paper. It showed that these factors had remarkable effect on the crystal morphology, particle size and agglomeration during the crystallization process. By using 0.095% (mass percentage) ionic surfactant (S) as the additive and using spray‐drops feeding device as the dropping equipment, 1.06 g TATB raw materials have been refined into free‐running ellipsoid and spherical TATB grains with the grain size from 30 to 50 nm. By using 0.014% (mass percentage) non‐ionic surfactant (P) as the additive, spherical TATB grains with the particle diameter of 50 nm and with narrow particle‐size distribution have also been obtained. It was shown by the characterizations that the ultrafine particle of TATB had better heat resisting evenness and its 5 seconds ignition point is advanced by 7.5 K.     

Preparation and characterization of carboxylic-containing poly(methyl methacrylate) nanolatexes

Poly(methyl methacrylate) (PMMA)-based latex particles bearing carboxylic groups at the surface were prepared via emulsion polymerization. The polymerization recipe and process were optimized in order to target monodisperse particles with diameters around 100 nm. The polymerizations were performed using 4,4-azobis(4-cyanopentanoic) acid (ACPA) as initiator and sodium dodecyl sulphate (SDS) as surfactant. The polymerization conversion was determined by both gas chromatography and gravimetry. The final latexes were characterized with respect to particle size, size distribution, surface charge density, electrokinetic properties (i.e. electrophoretic mobility vs pH and ionic strength) and colloidal stability (i.e. coagulation rate constants vs pH and stability factor vs ionic strength).     

Thermodynamic properties of the dissociation of several α-oxooximes

The changes in free energy, enthalpy and entropy for the dissociation of several -oxooximes (phenylglyoxal aldoxime, glyoxylanilide oxime,N-ethyl-glyoxylanilide oxime and 1-phenyl-1,2-propanedione-2-oxime) have been determined in water-ethanol medium (50% v/v) at 0.5M ionic strength (NaNO₃) and 25 °C. The changes in dissociation free energy were calculated from the pK _a values determined by glass-electrode potentiometry, and refined by using the programs MINIGLASS and SUPERQUAD. The dissociation enthalpies were determined by direct thermometric titration, and refined by using the programs MINITERM and MULTITERM.     

Transport and Deposition of Alumina Nanoparticles in Water Saturated Porous Media: An Experimental Study

Transport and deposition of nanoparticles (NPs) have drawn great attention in different fields of engineering in recent years because of their potential risk to the environment. In this study, mobility of aluminum oxide NP, one of the most popular metal oxide NPs, was studied in synthetic saturated porous media. The impacts of ionic strength using monovalent (NaCl) and divalent (MgCl₂) salt solution, pH, and NPs concentrations on nano-alumina mobility were investigated. The experimental results showed that the transport of nano-alumina was influenced by ionic strength; the highest mobility was observed at the 0.01 M solution and decreased by increasing ionic strength. Moreover, the suspension containing alumina NPs presented the best mobility behavior in the acidic solution (pH = 2) in comparison with neutral and basic solutions. Consequently, transport of NPs with particle size distribution lower than 100 nm through porous media was facilitated at the acidic, low ionic strength condition. In addition, faster elution occurred when the influent concentration was increased from 0.1 wt% to 0.3 wt%. Finally, it is expected that ionic strength, pH of solution, and NPs concentration will be key parameters to control the nano-alumina mobility.     

Diffusion in concentrated dispersions: a study with fiber-optic quasi-elastic light scattering (FOQELS)

A fiber-optic, quasi-elastic light-scattering instrument is described using single-mode fiber optical components, including a novel slanted exit face optode. The setup operates with homodyne signal detection. It enables the characterization of diffusion processes in concentrated dispersions up to volume concentrations of 50%.The performance of the instrument is exemplified with results obtained from latex spheres with diameters of 226 nm and 404 nm at volume fractions from =0.01 to =0.5. The correlation functions are analyzed according to the second order cumulants method and the Contin-procedure yielding an average and a distribution function of the short-time self-diffusion coefficient,D _eff ^s , respectively.At high ionic strength the concentration dependence ofD _eff ^s /D ₀ is found to be in close agreement with theoretical predictions based on a multi-body interaction model of hard spheres up to =0.45. With decreasing ionic strength the negative slope of the virial expansion tends to increase, presumably due to enhanced repulsive electrostatic interactions.The described technology offers new experimental means for on-line remote control sensing of particle size in concentrated disperse systems.Presented at the 34. Hauptversammlung der Kolloidgesellschaft e.v., Bochum, Oct. 1–4, 1989     

Ermittlung von Reaktionsgeschwindigkeitskonstanten aus relaxationskinetischen Meßergebnissen,gewonnen bei verschiedenen Ionenstärken

Zusammenfassung Das Dimerisierungsgleichgewicht geladener Moleküle 2M ^zD ^2z wurde als einfaches Modellsystem gewählt, um — unter Verwendung von bei verschiedenen Ionenstärken gewonnenen relaxationskinetischen Meßergebnissen — die Bestimmung ionenstärkenunabhängiger Reaktionsgeschwindigkeitskonstanten und der thermodynamischen Gleichgewichtskonstante zu demonstrieren.

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Evaluation of rate constants utilizing relaxation data obtained at variable ionic strength

The monomer-dimer equilibrium of charged molecules 2M ^zD ^2z has been used as a simple model to demonstrate how ionic strength independent rate parameters and the thermodynamic equilibrium constant can be determined from relaxation kinetic data obtained at variable ionic strength.

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Mit 3 Abbildungen     

Ionic Strength Dependence of Formation Constants and Complexation of Glutamine with Dioxovanadium(V)

The dependence of L-glutamine protonation and its complexation with dioxovanadium(V) on ionic strength (I) is reported in sodium perchlorate solution as a background salt. The measurements have been performed at 25 ± 0.1°C and various ionic strengths in the range 0.1 to 1.0 mol/l, using a combination of potentiometric and spectrophotometric techniques. The overall analysis of the present and the previous data dealing with the determination of stability constants at different ionic strengths allowed us to obtain a general equation, by which a formation constant determined at a fixed ionic strength can be calculated, with a good approximation, at another ionic strength, if 0.1 I 1.0 mol/l (NaClO₄).     

Characterization and stability of TiO2 nanoparticles in industrial dye stuff effluent

Stability studies were conducted in different solutions (deionized water (DI), NaCl, CaCl₂, and MgCl₂) at different pH. Agglomeration and zeta potential were influenced by ionic strength, type of electrolyte, and the presence of dye stuff. The Derjaguin–Landau–Verwey–Overbeek (DLVO) theory was used to analyze the stability and/or agglomeration of the nanoparticles in the different solutions. Repulsive or attractive forces stipulated by the DLVO theory were used to quantitatively discuss the results. The increase in ionic strength increased agglomeration which was linked to pH_pzc, as there were minimal electrostatic repulsions at the pzc, yet the attractive van der Waals forces were dominant. Addition of the dye stuff significantly decreased the agglomeration as the dye stuff changed the overall zeta potential of TiO₂ nanoparticles to negative across the entire pH which improved stability as there were particle–particle repulsions. Monovalent and divalent cations were compared and Ca²⁺ increased the mean diameter of nanoparticles as it effectively decreased the EDL of the nanoparticles, thus enhancing agglomeration. The DLVO theory was successful at explaining, in terms of the interaction energies between nanoparticles, the phenomena that caused either agglomeration or stability of the as-synthesized TiO₂ nanoparticles in the different solutions.     

“Emulsifier-free” emulsion copolymerization of styrene and butylacrylate: particle size control with synthesis parameters

 This paper presents the synthesis of film-forming particles with controlled diameters. The copolymerization of styrene and butylacrylate is realized by emulsion synthesis in the presence of potassium sulfopropylmethacrylate (SPM), an ionogenic monomer and sodium bicarbonate. The initiator, ammonium persulfate, and the SPM ensure the stabilization of particles. After the particles are washed, their diameters are measured by Dynamic Light Scattering (DLS) with a Zetasizer 3 (Malvern). The particle diameters are controlled by SPM concentration and the ionic strength of the medium. An increase of ionic strength, at constant [SPM], increases diameters, though an increase of [SPM], at constant ionic strength, decreases diameters. The experimental results are interpreted by two different modelizations, either by log–log equations like in the literature, or by semi-log and exponential equations. The latter ones give better modelization. Received: 18 July 1997 Accepted: 3 March 1998     

The second dissociation constant of sulfuric acid at various temperatures by the conductometric method

The thermodynamic second dissociation constant K₂ for sulfuric acid has been determined by conductivity measurements of aqueous sulfuric acid solutions at various temperatures. The data are treated by using two different methods developed with two independent assumptions due to Noyes et al. and Shedlovsky. Both methods require the knowledge of relevant ionic conductivities, which may be calculated from the Onsager limiting law. The values for K₂ obtained with these two methods show excellent agreement. The value of 0.0103 mol-L^–1 at 25°C agrees with the best literature value of 0.0102 mol-L^–1 within the experimental uncertainty, as also does the enthalpy of dissociation which is derived from the derivative of the temperature coefficient of K₂.     

Size-based characterization of nanometric cationic maghemite particles using capillary zone electrophoresis

Size-sorted maghemite (gamma-Fe(2)O(3)) particle populations of number mean solid diameters ranging from 6 to 10 nm were suspended and directly characterized in their stabilizing acidic, citrated or basic aqueous media using CZE coupled with UV detection. Analytical conditions were optimized in order to ensure reliable mobility measurements of these ferrofluids in their anionic and cationic forms. Particular interest has been paid to the investigation of the positively charged ferrofluids since cationic colloids have received little attention so far. A strategy for capillary wall modification was chosen in order to prevent particle adsorption while preserving high analytical performances. The influence of experimental conditions such as particle volume fraction, injection volume, electric field strength and electrolyte nature on electrophoretic profiles and measured electrophoretic mobilities was evaluated. A size-dependent electrophoretic mobility was demonstrated and discussed in terms of the ratio of the particle radius to Debye length with reference to existing models (Henry, etc.). Although these nanometric particle distributions lie in a very narrow size range, partial separation was obtained with selectivity varying as a function of electrolyte ionic strength.     

Die Acidität von Silanolgruppen. Vorläufige Mitteillung

The intrinsic acidity constant K_int. of surface silanol groups has been determined by coulometric titration of silicagel at 25° in solutions of the constant ionic strength 0,1M consisting primarily of sodium perchlorate. The experimental data are consistent with      

Kinetic Salt Effect in the 18O-Exchange Reaction between Bromate and Water

The kinetic salt effect in the ¹⁸O-exchange reaction between bromate ion and water has been investigated at 60° in a range of ionic strength from 0.02M < I < 1.9M using NaNO₃ or NaClO₄ as inert salts. From the experimental data the following relation was deduced for the uncatalysed reaction path In the H⁺-catalysed reaction path the Brønsted-Davies equation was obeyed up to I ? 0.1M At I > 0.8M the sign of Δ logk₂/Δ I^1/2 was positive. The theoretical interpretation of these results is consistent with the mechanistic evidence obtained previously from the rate law and the solvent isotope effects.     

Ionic strength dependence of formation constants. Part II: potentiometric study of the copper(II)-malonate-glycinate system in the range 0.01≤I≤1.0

Summary The ternary copper(II)-malonate-glycinate system has been investigated by potentiometric measurements using a glass electrode, at 37° and at different ionic strengths (0.01I1.0 mol dm^–3), with KNO₃ as background salt. Complex formation between K⁺ and malonate has also been taken into account, and the parameters for the dependence on ionic strength have been obtained for each formation constant. A general equation, valid for all the complexes formed in this system, for the dependence, log K=f(I), is also proposed. The stabilization parameters of the ternary complex are also analyzed as a function of the ionic strength.     

Transport and retention of TiO2 rutile nanoparticles in saturated porous media under low-ionic-strength conditions: measurements and mechanisms

The mechanisms governing the transport and retention kinetics of titanium dioxide (TiO(2), rutile) nanoparticle (NP) aggregates were investigated in saturated porous media. Experiments were carried out under a range of well-controlled ionic strength (from DI water up to 1 mM) and ion valence (NaCl vs CaCl(2)) comparable to the low end of environmentally relevant solution chemistry conditions. Solution chemistry was found to have a marked effect on the electrokinetic properties of NP aggregates and the sand and on the resulting extent of NP aggregate transport and retention in the porous media. Comparable transport and retention patterns were observed for NP aggregates in both NaCl and CaCl(2) solutions but at much lower ionic strength with CaCl(2). Transport experimental results showed temporal and spatial variations of NP aggregate deposition in the column. Specifically, the breakthrough curves displayed a transition from blocking to ripening shapes, and the NP retention profiles exhibited a shift of the maximum NP retention segment from the end toward the entrance of the column gradually with increasing ionic strength. Additionally, the deposition rates of the NP aggregates in both KCl and CaCl(2) solutions increased with ionic strength, a trend consistent with traditional Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Upon close examination of the results, it was found that the characteristics of the obtained transport breakthrough curves closely followed the general trends predicted by the DLVO interaction-energy calculations. However, the obtained NP retention profiles were found to deviate severely from the theory. We propose that a NP aggregate reconformation through collision between NP aggregates and sand grains reduced the repulsive interaction energies of NP-NP and NP-sand surfaces, consequently accelerating NP deposition with transport distance and facilitating approaching NP deposition onto NPs that had already been deposited. It is further suggested that TiO(2) NP transport and retention are determined by the combined influence of NP aggregate reconformation associated with solution chemistry, travel distance, and DLVO interactions of the system.     

The Particle In-Flight Characteristics in Plasma Spraying Process Measured by Phase Doppler Anemometry (PDA)

Detailed measurements of particle in-flight characteristics have been carried out using a PDA system for benchmarking as well as to provide further information to aid the development of simulation models. The parameters studied included four conditions of primary gas flow rate and carrier gas flow rate. The particle velocities, diameters, and the corresponding volume flux at different locations were obtained. Due to the one port particle injection arrangement, it was noted that particles in general sprayed with an angle deviated from the nozzle axis Z_n, to the opposite side of the powder feeder port. The particles would also deviate from the spraying cone axis with a divergence angle (). The deviation and divergence angles were examined under different plasma spraying conditions. The measurement data rates at different cross-sectional planes were also obtained so as to compare the results derived from the volume flux measurement and the actual coating on a substrate at the equivalent standoff distance. It was found that the spraying area obtained from the measurement-data-rate increased with downstream distance and a linear relationship between spraying area and distance was also established. Comparing the integrated results, it was noted that the spraying areas derived from the measurement data rate were close to the actual spraying areas obtained from the coordinate measurement machine (CMM) results.     

The primary electroviscous effect of prolate silica sols

The intrinsic viscosity and the dynamic mobility of four silica sols have been measured as a function of the ionic strength. It was found that intrinsic viscosity decreased with increasing ionic strength, which we attribute to the primary electroviscous effect. The geometry and the charge of the particles were fitted using experimental viscosity, light scattering, and dynamic mobility data, where the intrinsic viscosity measured at the highest ionic strength for a given sol was used as input data in our analysis. Further, the boundary element (BE) method was used to calculate the primary electroviscous effect and electrophoretic mobility of charged prolate ellipsoids. These calculations were then compared with experimental data, and the primary electroviscous effect was subtracted from the intrinsic viscosity at a given ionic strength, which led to a slightly altered geometry of the particles. This revised geometry was used as input data using the BE method, and the procedure was repeated iteratively until agreement was obtained at high ionic strength. In general, good agreement between theory and experiment was found.