Slow Coagulation Research Articles

The Effect of Composition of Water–Ethanol Solutions on the Kinetics of Coagulation of Fused Quartz Suspensions

The effect of composition of water–ethanol solutions containing 0–96 vol % alcohol on the coagulation kinetics of dilute fused quartz suspensions is studied using flow ultramicroscopy technique. It is established that, in both water and ethanol containing 4 vol % water, freshly prepared quartz suspensions are stable with respect to aggregation; at ethanol concentrations from 10 to 90 vol %, an ultrafast coagulation takes place, with characteristic time (coagulation period) being essentially smaller compared to a value corresponding to the Smoluchowski theory of fast coagulation kinetics. It was shown that the aging of the aforementioned suspensions for more than 24 h results in slow coagulation at high and low alcohol content, but its ultrafast character is retained in the ethanol concentrations range of 40–48 vol %. The probable reasons for the ultrafast coagulation in the disperse systems specified are discussed.

Heterogeneity of colloidal particle networks analyzed by means of Minkowski functionals.

The heterogeneity and large scale connectivity of colloidal particle networks, which are generated by Brownian dynamics simulations, is examined. This is achieved by employing integral geometric measures in the form of the Minkowski functionals or quermassintegrals. It is found that these measures in conjunction with the parallel-body technique amount to a powerful tool to characterize the structure, going beyond the information contained in the pair-correlation function. The development of heterogeneities during network formation as well as their dependence on the volume fraction and the interaction potential is studied. In particular, it is found that slow coagulation enhances the heterogeneity of the network compared to fast coagulation.

Effect of coagulation conditions on the microfibrillar network of a rigid polymer

AbstractAn important element in the microstructure of high performance fibers and films fabricated from rigid polymers is an interconnected network of oriented microfibrils, the lateral size of which is about 10 nm. This study is an attempt to elucidate the mechanism by which the microfibrils are formed so that larger lateral dimensions can be achieved by suitable processing. Because this morphology emerges in the coagulation stage of the spinning process, we compared the microfibrillar network formed by drastically different coagulation conditions. Ribbons, spun from solution of poly(p‐phenylene benzobisthiazole) in polyphosphoric acid through a slit die, were coagulated either in the ordinary rapid process with water (timescale of seconds) or in a slow process with phosphoric acid (timescale of hours). The coagulated microfibrillar network was dried with supercritical CO2 for X‐ray scattering measurements and impregnated with epoxy resin for sectioning and imaging by TEM. Slow coagulation yields better‐aligned microfibrils of enhanced chain packing, but the lateral size of the microfibrils formed in both cases is similar, about 10 nm. Heat treatment increases the width of water‐coagulated microfibrils but not the acid‐coagulated ones. The observations do not support spinodal decomposition as the mechanism of microfibril formation during coagulation, as was previously suggested. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1087–1094, 2002

Local Structure Evolution in Particle Network Formation Studied by Brownian Dynamics Simulation

The effect of solid content and colloidal interactions on the structure of forming networks of colloidal particles is studied by Brownian dynamics simulation. The different situations are compared in terms of the pair distribution function and the distribution of nearest neighbors around each particle. The results indicate that, in fast coagulation, the higher solid contents lead to a freezing-in of the liquid structure. Nevertheless, this effect can be reduced substantially by the introduction of a shallow secondary minimum and an energy barrier in the interaction potential. However, the structures resulting from such slow coagulation show a substantial degree of porosity, larger than those produced at the same solid content but by fast coagulation. It is also shown how the porosity (defined on a few particle diameters) is reflected in the distribution of nearest neighbors around the center particle, i.e., the very local conformation in the particle network. Fractal analysis shows that, at the relatively high volume fractions considered in this study, no intermediate fractal regime exists.

Smoke Coagulation in Compartment Fire Modelling

The effect of coagulation on the total smoke particles concentration and mean particles volume fields is studied under the conditions of a compartment fire. An approximate analytical consideration of aerosol temporal behaviour accounting for the simultaneous action of aerosol generation, coagulation, and dilution is given, and two regimes of fast and slow coagulation are considered. A CFD model of compartment fire, including smoke aerosol generation and coagulation, is presented. The results of CFD simulation for a hydrocarbon burner sooting fire with smoke movement and coagulation in a two room compartment are analysed, where the regimes of coagulation and their key features predicted by means of analytical consideration are illustrated by numerical simulations. For a given geometry of the compartment, fuel type, and volumetric smoke generation rate, the coagulation rate was shown to be determined by the mean volume of the particles generated in the flame zone. In the limit of small particles generated (fast coagulation regime), the aerosol parameters far away the flame zone were found to be weakly dependent on those inside the flame zone due to a strong coagulation effect. In the opposite limit of large particles generated (slow coagulation regime), coagulation has no effect on the aerosol parameters. The approximate boundary between the two regimes has been defined.

動的光散乱法による単分散ラテックス粒子の凝集速度測定に関する基礎的研究

The experimental collision frequency factors of monodispersed polystyrene latex (PSL) particles in KCl solutions were measured by the use of the dynamic light scattering method. This method determines the cluster-size distribution by counting the number of different clusters during the coagulation process and therefore permits the determination of the rough collision frequency factors by the fitting curves for the data. The fitting curves were calculated on the assumption that the coagulation between two same-fold particles is dominant. Numerous studies have revealed significant discrepancies between the theoretical collision frequency factors that can be calculated from Smoluchowski and Fuchs formulation and the experimental results, especially in the slow coagulation region. In this paper's results, good qualitative agreement was found for the theoretical collision factors in the rapid coagulation, but in the slow coagulation, significant discrepancies were confirmed both qualitatively and quantitatively, especially for high-fold particles.

Thermal aggregation of patatin studied in situ.

In this work dynamic light scattering was used to study the thermal aggregation of patatin in situ, to elucidate the physical aggregation mechanism of the protein and to be able to relate the aggregation behavior to its structural properties. The dependence of the aggregation rates on the temperature and the ionic strength suggested a mechanism of slow coagulation, being both diffusion and chemically limited. The aggregation rate dependence on the protein concentration was in accordance with the mechanism proposed. The aggregation rates as obtained at temperatures ranging from 40 to 65 degrees C correlated well with unfolding of the protein at a secondary level. Small-angle neutron scattering and dynamic light scattering results were in good accordance; they revealed that native patatin has a cylindrical shape with a diameter and length of 5 and 9.8 nm, respectively.

Application of the Schulze–Hardy rule to haematite and haematite/humate colloid stability

The applicability of the Schulze–Hardy rule to haematite and haematite/humate mixtures has been investigated by measuring the rate of fast and slow coagulation after the addition of increasing concentrations of mono, di and trivalent ions. Haematite colloids follow the rule for mono, di and trivalent ions. Haematite colloids coated with humate generally follow the rule for mono and di valent ions with the exception of copper ions. Trivalent ions do not follow the Schulze–Hardy rule because of their relatively strong complexation with humate. The size of the counter ion has an effect on the de-stabilising ability of ions of the same charge.

Change of morphological properties in drawing water-swollen cellulose films prepared from organic solutions. a view of molecular orientation in the drawing process

Drawable water-swollen cellulose films were prepared by coagulating in water two different cellulose organic solution systems. The drawability of the water-swollen films was dependent on the rate of coagulation. Transparent films prepared by the slow coagulation showed good drawability and had a maximum draw ratio of 2.0. However, the drawn films maintained the highly noncrystalline state even after dried at 50°C under vacuum. X-ray analysis and polarized FT-IR measurements performed under a saturated deuterium oxide vapor of these dried drawn films, prepared by slow coagulation, showed that their noncrystalline regions (more than 80%) as well as crystalline regions (less than 20%) were highly oriented by the drawing process. Furthermore, meridional intensity curves in the X-ray diffraction exhibited interesting patterns even though the drawn sample was highly noncrystalline. In fact, they are quite different from those in regenerated cellulose II fibers. However, despite this increase in draw ratio and in the orientation of the chains, the number of crystalline domains in the films did not increase significantly. This may perhaps be attributed to the three-dimensional network structure resulting from the intermolecular hydrogen bonds between chains which are maintained through the drawing process and which can hinder the crystallization of cellulose. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 451–459, 1999

Equilibrium and Nonequilibrium Properties of Bulk and Nanosize Clusters of Carbon.

The detonation behavior of carbon-rich high explosives is affected by a slow coagulation of carbon atoms by diffusion and their possible transformation from one chemical bonding type to another. We have examined the applicability of the Brenner potential at high pressure and high temperature by molecular dynamics simulations and the stability and barrier height between graphitic and diamond clusters as a function of cluster size by quantum chemical calculations. The diffusion kinetics of carbon clusters have been examined by including a time-dependent surface correction to the Gibbs free energy of these clusters.

MODELING THE KINETICS OF CARBON COAGULATION IN EXPLOSIVES DETONATION

A typical insensitive high explosive such as LX-17 has a large carbon content. The detonation behavior of these explosives is affected by a slow coagulation of carbon atoms by diffusion and their possible transformation from one chemical bonding type to another. We have used the Brenner bond order potential to compute the melting line of diamond at high pressure and high temperature by molecular dynamics and Monte Carlo simulations, with the goal to refine the potential for the study of the kinetics of the graphite–diamond transition. The slow diffusion-controlled kinetics of carbon clusters has been examined by including a time-dependent surface correction to the Gibbs free energy of these clusters in the nonequilibrium CHEQ code. We also propose a new explosive burn model which incorporates a partial release of the heat of detonation in a fast reaction zone, followed by a diffusion-limited release of the remaining energy. Hydrodynamic applications of the new burn model to LX-17 show that computed expansion and compression results both agree closely with experimental data.

Spectroelectrochemical characterization of cobalt hexacyanoferrate films in potassium salt electrolyte

Abstract Cobalt(II) hexacyanoferrate(III, II) films show reversible electrochromic behavior in potassium salt electrolyte. Gold-covered foil was used as a conductive, optically transparent substrate onto which cobalt hexacyanoferrate films were deposited by a slow coagulation method. Both voltammetric and spectroelectrochemical results are consistent with the existence of two cobalt hexacyanoferrate forms, presumably KCoII1.5[FeII(CN)6] and K2CoII[FeII(CN)6]. The separate nature of the redox reactions of these forms is clearly evident from spectroelectrochemical measurements, particularly from voltabsorptometry, which involves monitoring of the time-derivative signal of absorbance as a function of the linearly scanned potential. Combination of voltabsorptometry with voltammetry allows determinations of molar absorptivity and film loading, and it permits changes of concentration of colored redox centers vs the applied potential to be monitored.

ELECTRO-OPTICAL STUDY OF OXIDE DISPERSIONS AND THEIR STABILIT

Oxide/solution interface is with the clear origin of the surface charge, obtained through ionization of surface hydroxyl groups. The oxide particles produced by flame hydrolysis present fractal aggregates built from nearly spherical particles with constant diameter (AI2O3, SiO2). In the process of acid hydrolysis particles with well defined form (spheres, rods, ellipsoids) and high monodispersity could be obtained (α-Fe2O3, α-β-FeOOH). Different types of oxides are investigated by the means of electric light scattering. An information on the electric surface properties of oxide/solution interface as well as in presence of various additives, particles dimensions and fractal structure is obtained. The process of slow coagulation in presence of bivalent electrolyte and surface active substance is studied electrooptically. The change of dynamic electric behaviour (induced dipole moment) in the processes of interaction is followed. Some considerations are made on the nonequillbium electric properties of the interface and the connection with suspension stability.

RAPID BROWNIAN COAGULATION IN DILUTE POLYDISPERSE EMULSIONS

ABSTRACT In this experimental investigation of rapid Brownian coagulation in dilute oil-in-water emulsions, the absence of deformation influence on droplet interaction and drainage is established. The result is in agreement with the theory of Brownian coagulation in miniemulsions proposed by the Borwankar-Ivanov group. As a first approximation the applicability of Smoluchowski theory to rapid coagulation in emulsions is established. Observing the weak influence of droplet deformation on rapid coagulation and taking into account Borwankar-Ivanov theory that the transition to charged droplets and lower electrolyte concentrations decreases the droplet deformation, we conclude that both slow coagulation and the reversible one cannot be influenced by droplet deformation in miniemulsions. A droplet surface is homogenous in contrast to solid particle surfaces. Thus emulsions can be used as a

Evaluation of stability ratio in oil-in-water emulsions

The kinetics for emulsion coagulation retarded by the electrostatic repulsion between droplets in the micron size range was studied. The rather high electrokinetic potential is provided by the presence of 10 −3 M NaOH. The enormous discrepancy between theory and experiment is shown to be analogous to that of solid suspensions. It was shown that Fuchs theory is invalid for emulsions where the existence of surface roughness or heterogeneity is completely excluded. To ensure that the electrostatic barrier is the single reason for the coagulation retardation, the use of ionic surfactant adsorption was avoided. This necessary condition causes an increase of the critical thickness h cr at which spontaneous disruption of the thin emulsion film occurs. The condition h cr < h max is formulated as necessary for the examination of slow coagulation theory on emulsions, where h max is the distance between the surface and the electrostatic energy maximum. For micron size droplets the predominance of Brownian coagulation over gravitational coagulation was ensured by the use of a dichlorodecane emulsion.

Charging and coagulation during flame synthesis of silica

Abstract The effect of charging during flame synthesis of silica particles is investigated. These particles are made by SiCl4 oxidation/hydrolysis in a premixed, CH4O2N2 flat flame. Unipolar and bipolar DC electric fields are created across the flame axis using various combinations of needle and plate electrodes. Needle electrodes create highly focused electric fieldds resulting in the onset of convection (iOnic wind) across the flame. As a result, these electric fields not only charge the newly formed particles, but they also reduce the flame temperature and the particle residence time at high temperatures. However, the field created with this configuration is not stable. Using a plate as one of the electrodes increases the stability of the electrc field. Using a plate/plate electrode configuration suppresses the onsset of the ionic wind, so the effect of field charging on particle growth may be separated from that of convection (ionic wind). A negative electric field creates the most drasctic effect on the characteristics of the product particles. The primary particles size decreases with increasing field strength regardless of polarity or electrode configuration. Repulsion of the charged particles resulted in electrostatic dispersion, slower coagulation and smaller primary particle sizes. Transmission electron micrographs show that the extent of agglomeration is greatly influenced by electrode polarity and configuration during electrically assisted flame synthesis of silica powders.

Role of interfacial potential in coagulation of cuprammonium cellulose solution

The electric potential, copper ion flux, and ammonia flux across the interface of cuprammonium cellulose solution (CCS) and various 1.0 equiv/Lelectrolyte solutions (ES) at 25°C were measured. The interfacial potentials were strongly negative (–10 to –35 mV) with H2SO4, HCI, and (NH4)2SO4 as ES, weakly positive (6 to 8 mV) with NaCl, KCl, LiCl, CsCL, and RbCl as ES, and strongly positive (19 to 34 mV) with KOH and NaOH as ES, generally showing values similar to the diffusion potentials for electrolyte solutions comprising ions of the same absolute charge. The ammonia flux (about 1 X 10-4 mol/cm2/s) was relatively unaffected by the interfacial potential, but the copper ion flux was clearly dependent on it. These results, together with the observed rates of CCS coagulation, indicate that the mechanism of the coagulation was largely determined by the interfacial potential, with strongly negative potential gradients accelerating the Cu2+ flux into the ES and CCS coagulation proceeding rapidly by Cu2+ removal, strongly positive potential gradients accelerating the Na+ flux into the CCS and coagulation proceeding rapidly via the formation of cellulose-Na+ complex, and the absence of a strong potential gradient capable of accelerating the ion flux resulting in slow coagulation by ammonia removal. It may therefore be possible to control the interfacial potential and the ion flux by the ES composition, and thus to influence the structure of regenerated cellulosic fibers and membranes. © 1996 John Wiley & Sons, Inc.

Role of interfacial potential in coagulation of cuprammonium cellulose solution

The electric potential, copper ion flux, and ammonia flux across the interface of cuprammonium cellulose solution (CCS) and various 1.0 equiv/Lelectrolyte solutions (ES) at 25°C were measured. The interfacial potentials were strongly negative (–10 to –35 mV) with H2SO4, HCI, and (NH4)2SO4 as ES, weakly positive (6 to 8 mV) with NaCl, KCl, LiCl, CsCL, and RbCl as ES, and strongly positive (19 to 34 mV) with KOH and NaOH as ES, generally showing values similar to the diffusion potentials for electrolyte solutions comprising ions of the same absolute charge. The ammonia flux (about 1 X 10-4 mol/cm2/s) was relatively unaffected by the interfacial potential, but the copper ion flux was clearly dependent on it. These results, together with the observed rates of CCS coagulation, indicate that the mechanism of the coagulation was largely determined by the interfacial potential, with strongly negative potential gradients accelerating the Cu2+ flux into the ES and CCS coagulation proceeding rapidly by Cu2+ removal, strongly positive potential gradients accelerating the Na+ flux into the CCS and coagulation proceeding rapidly via the formation of cellulose-Na+ complex, and the absence of a strong potential gradient capable of accelerating the ion flux resulting in slow coagulation by ammonia removal. It may therefore be possible to control the interfacial potential and the ion flux by the ES composition, and thus to influence the structure of regenerated cellulosic fibers and membranes. © 1996 John Wiley & Sons, Inc.

Surface Roughness and the Particle Size Effect on the Rate of Slow, Perikinetic Coagulation

In this paper we consider the effect of surface roughness on the rate of coagulation, and on the particle size dependence of the stability ratio W in slow coagulation in particular. Deviations from a smooth surface ("surface roughness") will affect coagulation rates, if and when their characteristic amplitudes, the interparticle distance, and the range of the interaction forces are commensurate. Colloidal particles will nearly always show a degree of surface roughness on the scale of the Debye length, which under coagulation conditions is on the order of 1 nm in the surrounding electrolyte. Representing surface irregularities by a function which scales with the interparticle distance, we have calculated the interaction with the DLVO theory. It is found that surface roughness must strongly affect the relation of W with the curvature of the particle surfaces. Computed log W - log c diagrams for various combinations of particle size and surface roughness suggest that the lack of sensitivity of log W - log c diagrams to the particle size, which is generally found in the experiments, can be due to surface roughness.

Surface Charge Relaxation during Coagulation

Transient deviations from the equilibrium surface charge density during the interaction of colloidal particles have been studied. Such deviations cause the process of particle encounter to become a non-first-order Markov process, complicating the analysis of colloid stability. Two methods are presented to calculate a modified colloid stability ratio, taking such deviations into account in an approximate way. These methods differ by their estimates of the time scale of the Brownian encounter and its dependence on the height of the energy barrier. Despite these differences, both methods show that double-layer dynamics can have major consequences for the stability ratio. However, the resulting dependences of the rate of slow coagulation on the particle radius are different for the two methods. This indicates that double-layer dynamics could explain the experimentally found insensitivity of the stability ratio to the particle size, provided the time scale of the encounter is strongly dependent on the height of the energy barrier. However, this proviso is unlikely to be satisfied. A simple statistical analysis indicates that the time scale of any individual encounter slightly decreases with growing barrier height!