Showing papers in "Colloid Journal in 2001"

The Use of Digital Processing of Video Images for Determining Parameters of Sessile and Pendant Droplets

Abstract: Methods used at different stages of digital processing of video images in studies of surface phenomena are considered. A segmentation method making it possible to detect several objects in an image field with a nonuniform illuminance is proposed; this method is based on dynamic thresholding. Various methods of determining the characteristics of the capillary system under consideration by fitting the parameters of Laplace's equation are discussed. The factors affecting the accuracy of parameter determination for sessile and pendant droplets are analyzed. On the basis of model calculations, the deviations (due to digitization of the interface profile during its digital processing) of the resultant surface tension and contact angle from exact values are estimated. Some experimental applications of the techniques described in the paper are discussed.

Concentration polarization and specific selectivity of membranes in pulse mode

Abstract: Nonstationary concentration polarization of membranes was analyzed theoretically. Desalination and transport of single- and double-charged ions through MK-40 cation-exchange membrane was studied in the stationary and pulse modes with the fixed voltage. Main generalities in the changes of the pH and conductivity of desalinated solution as well as concentrations of single- and double-charged ions as the functions of solution flow velocity, voltage, frequency and off-duty ratio of pulses were established.

A Study of the Redistribution Conditions of Carbon Black Particles from the Bulk to the Interface in Heterogeneous Polymer Blends

Abstract: The redistribution conditions of carbon black particles to the interface between polymers were studied based on the measurement of the value of superadditive conductivity of several heterogeneous polymer blends filled with powdered carbon black. The redistribution of carbon black from the polymer phase that has low ability to wet the powder proceeds efficiently only to the boundary with polymer having better wetting ability. Significant differences in the conditions of particle localization at the boundaries between polymer phases and between two low-molecular-weight liquids arising from the specific features of polymer adsorption were demonstrated.

Low-Frequency Dielectrophoresis and the Polarization Interaction of Uncharged Spherical Particles with an Induced Debye Atmosphere of Arbitrary Thickness

Abstract: The electroosmotic component of the mutual dielectrophoresis is accounted for in the theory of the polarization interaction of uncharged particles. Previously, for the case when the polarization charge layer thickness is small as compared with the particle radius, it was shown that the electro-osmosis exerts significant effect on the motion of a particle in a nonuniform field (external or induced by a neighboring particle). The electro-osmosis in the induced Debye atmosphere of arbitrary thickness is considered in this paper. A class of systems characterized by the formation of transverse linear aggregates of particles is determined. The aggregates are weakly conducting media with nonconducting particles but more polar than the medium. The possibility of the occurrence of transverse electrocoagulation structures, which was originally obtained by us using the heuristic approach, is supported by results obtained in the solution of the electrohydrodynamic problem of the long-range interaction of polarized particles. The results obtained confirm high efficiency of the heuristic method used.

Thermodynamic and Kinetic Foundations of the Micellization Theory: 4. Kinetics of Establishment of Equilibrium in a Micellar Solution

Abstract: A system of the kinetic equations of the material balance for the concentrations of surfactant monomers and micelles in a micellar nonionic surfactant solution was formulated The equilibrium state of a materially isolated micellar solution was analyzed The system of the kinetic equations of the material balance of a micellar solution was solved The total time of the establishment of equilibrium in a micellar solution was determined It was shown that this time increases or (typically) decreases with an increase in micelle concentration, depending on the degree of micellization

Thermodynamic and Kinetic Foundations of the Micellization Theory: 5. Hierarchy of Kinetic Times

Abstract: The characteristic kinetic times of micellization in the solution of a nonionic surfactant: the times of establishment of quasi-equilibrium concentrations of molecular aggregates in micellar, subcritical, and overcritical regions, times of establishment of quasi-equilibrium concentrations of molecular aggregates in the near-critical region of their sizes, the average time between two successive acts of emission of surfactant monomers by a micelle, the average value of micelle lifetime, the time of establishment of quasi-stationary mode of matter exchange between the solution and molecular aggregate, as well as the times of fast and slow relaxation in a solution were analyzed. The hierarchy of these times disclosing complex multistage kinetic process of micelle formation and decomposition and the establishment of equilibrium in the micellar solution was revealed. It was shown that this hierarchy is provided by the small parameters of the kinetic theory. The inverse problem of micellization kinetics was discussed; this problem allows us to find the characteristics of the formation work for micellar aggregate from the experimental data on the relaxation time of micellar solution.

Properties of Aqueous Suspensions of Highly Dispersed Silica in the Presence of Polyvinylpyrrolidone

Abstract: The effect of polyvinylpyrrolidone (PVP) on stability, rheological, electrokinetic and electrophysical properties of aqueous dispersions of A-300 pyrogenic silica was studied. The suspensions were shown to be most stable at PVP concentrations in the range from 0.12 to 0.24 wt % that was provided by structure-related mechanical factors. In the presence of PVP, the value of ζ-potential decreases, the ac conductivity increases, and rheological properties vary similarly to variations of oxide dispersity.

Modified Tilting-Plate Method for Measuring Contact Angles

Abstract: The additivity of the dynamic curvature of a thermocapillary depression and the static curvature of liquid meniscus is experimentally confirmed. A high sensitivity of response shape to the static curvature of liquid surface is used to improve the tilting-plate method. The results of measuring contact angles by this method are well consistent with the data obtained by the sessile drop method. The behavior of meniscus is analyzed in the tilting of a plate with various positioning of its rotation axis in relation to the liquid–gas interface. The applicability limits of the modified method are indicated.

Synthesis of Barium Sulfate Nanoparticles in Water-in-Oil Microemulsion Systems

Abstract: Micellar solutions of poly(ethylene glycol) octylphenyl ether and n-hexyl alcohol in cyclohexane were used to study the formation of water-in-oil microemulsion system and to synthesize barium sulfate nanoparticles. Barium sulfate particles (the average diameter is 10 nm) were obtained by mixing two microemulsion systems containing Ba2+and SO2–4ions. It was shown that the sizes of BaSO4particles measured by the dynamic light scattering and electron microscopy are in good agreement with each other.

Kinetics of the initial stage of gelation in chitosan solutions containing glutaric aldehyde : Viscometric study

Abstract: The effects of glutaric aldehyde concentration C GA(in the range from 0.07 to 0.2 wt %) and temperature T(from 20 to 65°C) on the kinetics of an increase in the effective viscosity ηeffof aqueous solutions of chitosans (C CH= 2.4 wt %, pH 3.6, and ionic strength I= 0.05) with molecular masses equal to 33 × 104and 55 × 104and the degrees of substitution at N-acetyl and amino groups of 0.08 and 0.90, respectively were studied. The kinetic curves ηeff (t ) exhibited the induction periods of a relatively slight increase in the viscosity of chitosan–glutaric aldehyde mixed solutions with time followed by the periods of a drastic increase in the viscosity. The former periods correspond to the formation of gel clusters; the latter periods, to the gelation over the whole volume of a solution. Optimal ranges of the parameters C GAand Twere determined for the preparation of spherical gel particles of slightly cross-linked chitosan, which are used for the sorption of heavy metals from low-concentrated solutions.

A Study of the Effect of Linear Tension Effect on the Polystyrene Microsphere Wettability with Water

Abstract: A technique aimed at measuring contact angles for microparticles and suitable for determining the linear tension is developed. The technique is based on determining the "equilibrium" position of the microsphere (where the sum of forces acting on the microsphere is zero) at the liquid-gas interface. This position is unambiguously determined from the experimental force-distance curve obtained for the microparticle-interface interaction. The measurements are performed using an original setup based on the operating principle of the atomic force microscope. The advancing and receding contact angles of water are measured for individual polystyrene microspheres with radii from 1 to 5 μm. The contact angles are shown to increase with a decreasing microsphere radius, whereas the hysteresis of the contact angle decreases. The results indicate the existence of a negative linear tension in the case of the rough surface of polystyrene microspheres, which is deformed along the three-phase contact line.

Redispersion of Highly Disperse Powder of Titanium Dioxide in Aqueous Medium

Abstract: Based on an analysis of interparticle interaction in highly disperse powders, a parameter is suggested allowing one to evaluate the tendency of dispersed particles to the formation of aggregates, the aggregation coefficient. It was shown that the particles of metal oxide powders with radius smaller than 10 μm are present in their aggregated state. It was found that redispersion of highly disperse titanium dioxide powder in the aqueous medium is incomplete. The incomplete redispersion may be caused by the roughness of particles and/or the formation of the regions of entrapped air that are not destroyed by capillary pressure.

Synthesis and Some Properties of Sols Prepared by Hydrolysis of Lanthanum Nitrate

Abstract: The procedure for synthesis of hydrosols by hydrolysis of lanthanum nitrate was developed. The range of pH values was established corresponding to the maximum aggregation stability of sols. The photon-correlation spectroscopy was applied to determine the average hydrodynamic radius of sol particles, equal to 50 nm. The thresholds of fast coagulation of sol in the presence of sodium nitrate and sodium sulfate were determined.

Interparticle correlations and magnetic properties of concentrated ferrocolloids

Abstract: A statistical mechanical model was proposed for describing magnetic properties of highly concentrated ferrocolloids and dielectric properties of polar liquids. The model is based on a relationship between the magnetization and the pair correlation function of a spatially homogeneous ferroparticle system. The advantage of the proposed method is that the determination of the pair distribution function, with an accuracy of the powers of the concentration nk and the powers of the interparticle dipole-dipole interaction potential Udk, leads to an expression for the magnetization of the order of nk+1 and Udk+1, respectively. Relations for the magnetization and the magnetic susceptibility of a ferrofluid and also for the permittivity of a polar liquid taking into account interparticle correlations with an accuracy of terms ∼Ud2 were derived. The results obtained within the framework of this model agree closely with the numerical calculations and experimental data.

Adsorption of Cationic Surfactants on Highly Dispersed Silica

Abstract: Comparative study was made of adsorption of cationic surfactants, decamethoxine, ethonium, and cetyltrimethylammonium bromide, on a highly dispersed amorphous silica at different pH values. Contrary to the adsorption of monoquaternary ammonium compound, cetyltrimethylammonium bromide, the adsorption of bis(quaternary ammonium) compounds, ethonium and decamethoxine, was found to be caused mainly by electrostatic interaction. The adsorption of antiseptic decamethoxine is reversible at pH 6; this may be used to design carriers capable of controlled drug release into the surrounding medium.

Study of Retention Ability of Nucleopore Membranes

Abstract: The influence of pore sizes of nucleopore membranes and their shapes on the retention of polystyrene particles of 40–100 nm in size was studied. The interrelation between structural parameters of the nucleopore membranes found by various methods and the efficiency of filtration of calibrated particles were analyzed. The potential and limitations of the procedure for determining the efficiency of filtration of nanosized latex particles was demonstrated; the procedure was based on UV absorption spectroscopy.

Stability of Aqueous Dispersions of the Hydrated Titanium Dioxide Prepared by Titanium Tetrachloride Hydrolysis

Abstract: The aggregation stability of aqueous dispersions of the hydrated anatase and rutile samples in the neutral and alkaline pH regions is related to the electrostatic stabilization factor and is described in terms of the classical DLVO theory. The abnormally high stability of the dispersions with respect to 1 : 1 and 1 : 2 coagulating electrolytes, which is exhibited in the acidic pH region, is explained by the existence of well-developed gel layers at the particle surfaces. The specific role in the formation of such layers is played by the sulfate ions.

Electrosurface Properties of Hydr(oxides) and Oxide Nanostructures in 1 : 1 Electrolyte Solutions: 2. Electrokinetic Characteristics of Boehmite, Goethite, and Silicon Dioxide

Abstract: The electrophoretic mobility of particles of silicon dioxide, aluminum hydroxide, and iron hydroxide was measured as a function of pH in NaCl and KCl background solutions. Isoelectric points for the investigated objects were determined. Electrokinetic potentials were calculated with allowances for the particle shape and the polarization of the electrical double layer within the framework of the Overbeek–Boes–Wiersema model.

Dynamic Contact Angles of Water in Ultrathin Capillaries

Abstract: Velocities of motion V of advancing meniscus of water in quartz capillaries with radii from 45 to 270 nm was directly measured using an optical microscope. In the case, when the meniscus advanced over the wetting film that is remained after the previous meniscus receding, hysteresis was not observed, and the wetting was complete. When the meniscus advanced over the yet unwetted surface, the dynamic contact angle θd greatly depended on V, this dependence was the more pronounced, the smaller the r value. As the velocity V increases to 10–3 cm s–1, the value of θd rises to 60°–70° reaching the plateau. Preliminary adsorption of water vapors on the capillary surface markedly decreases the values of θd. The results obtained cannot be explained in terms of hydrodynamic and barrier theories of the contact angle. It was assumed that the controlling factor is the kinetics of vapor adsorption on the capillary surface in front of advancing meniscus.

Aggregation Stability of Colloidal Silica Sol–Polystyrene Latex Mixtures

Abstract: The aggregation stability of silica hydrosol in the presence of polystyrene latex was studied. Electrolytes added to the system were demonstrated to promote the heteroadagulation of silica particles on the surface of polystyrene globules. Moreover, the aggregation stability of the mixed dispersion increased in an acidic region (heteroadagulation stabilization), but decreased in an alkaline region. It was disclosed that the formation of heteroadagulation complex particles in the acidic region did not decrease the sedimentation stability of the mixed dispersion and was accompanied by a decrease in its optical density.

Silsesquioxane Films on Water Surface

Abstract: The properties of films formed on the water surface by rigid molecules of organosilicon compounds, oligomeric silsesquioxanes containing phenyl and ethyl side groups, are studied. These compounds are shown to form insoluble stable layers. The properties of the monomolecular films of rigid macromolecules are similar to those of the monolayers of flexible-chain molecules, and the former can be considered as two-dimensional solutions. In contrast to flexible chains, the collapse of the monolayers of rigid molecules, which is formally considered as the first-order phase transition, is accompanied by the formation of an anisotropic lamellar structure, which can be referred to as the mesomorphic state of the substance.

Droplet motion induced by diffusion of soluble surfactant to the external medium : Experiment

Abstract: Experimental examinations were performed for the motion of liquid droplets suspended under the condition of neutral buoyancy (Plateau technique) in the second liquid in the presence of surfactant transfer from droplets to the surrounding liquid or surfactant transfer to the droplet from the external medium. It was shown that, at a certain initial surfactant concentration, spontaneous droplet motion arises, which has the oscillation character. When the initial concentration exceeds a certain critical value, almost steady-state translational droplet motion occurs in addition to oscillation displacements. In the case of surfactant transfer from the surrounding liquid to the droplet, only oscillation displacements of a droplet with larger amplitude take place.

Three-Dimensional Aspect of the Surface Tension: An Approach Based on the Total Pressure Tensor

Abstract: The conditions of mechanical equilibrium were considered, and the generalized notion of the surface tension at an arbitrarily curved surface layer was analyzed on the basis of the total nondiagonal pressure tensor including the external fields and anisotropic even in the bulk phases. It was shown that the transverse surface tension can be eliminated using the selection of a dividing surface; however, in the general case, this surface does not exhibit the properties of the tension surface. On the whole, three-dimensional and nondiagonal character of the tensors of excess surface stresses determined by the integration over the volume and the cross section of the surface layer is retained at any selection of the dividing surface.

Spectroturbidimetry of Liposome Suspensions

Abstract: Spectroturbidimetric method of the determination of the average size and thickness of the shell in polydisperse suspensions of liposome particles was discussed. The method is based on measuring the wave exponent of suspension (the logarithmic slope of turbidity spectrum) and specific turbidity (turbidity per unit mass concentration of dispersed substance). The solution of inverse problem was performed using the exact calculation of the characteristics of light scattering for polydisperse spherical bilayer particles with allowance for spectral dependence of optical constants. Practical realization of this method is illustrated by the experimental determinations of the structural parameters of liposomes prepared from egg lecithin.

Birefringence in Concentrated Ferrocolloids

Abstract: Magnetic birefringence in concentrated kerosene-based magnetite colloids was experimentally studied. It was shown that field, frequency, and temperature dependences of a signal can be satisfactorily described already in terms of one-particle model, however with obligatory allowance for particle polydispersity. The role of aggregates in the evolution of this effect was shown to be insignificant.

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

Abstract: 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 ϕ0 of the diffuse electrical layer at the solution/air interface (applying the method of “equilibrium foam films”) and the potential ϕ1 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.

Mechanism of Reversible Thixotropic Transformation in Concentrated Suspensions of Monodisperse Spherical Silica Particles: Analysis in Terms of Colloidal Crystal Formation and Growth

Abstract: The mechanism of thixotropic transformation of concentrated suspension of monodisperse spherical silica particles into gel made of blocks with regular structure was considered in terms of colloidal crystal formation and the Alder–Hoover model of phase transition. The kinetics of linear growth of colloidal crystals as structural blocks was experimentally studied. This process was shown to have the activation nature determined by the diffusion of structural units at the interface and can be described in terms of Frenkel's kinetic theory of liquids.

Effect of Phase Transitions on Adsorption Properties of Thin Ferroelectric Poly(vinylidene fluoride-co-trifluoroethylene) Films

Abstract: The adsorption isotherms of water, oxygen, and hexane molecules at the surface of poly(vinylidene fluoride-co-trifluoroethylene) films 30 and 5 monolayers thick obtained by the Langmuir–Schaefer technique were measured at various temperatures using quartz crystal microbalance. An effect of a ferroelectric phase transition occurring at a temperatures from 363 to 388 K on the adsorption activity of the films of the both thicknesses was disclosed. The highest adsorption was observed at a temperature T≈ 380 K. In the case of the superthin copolymer film 5 monolayers thick, one more maximum of the adsorbability was detected at T≈ 300 K that corresponded to a low-temperature phase transition, which is typical of only the copolymer films thinner than 30 monolayers. The effects observed were explained by the facilitation of an adsorbate diffusion into the copolymer film upon its structural rearrangement caused by the phase transition. The results of this study allowed us to propose a new method for the determination of structural phase transitions based on studying isotherms of molecular adsorption from the gaseous phase.

Contribution of chemical and physical processes to the formation of alkaline silicate suspensions and their properties

Abstract: The mechanism of the formation of alkaline silicate suspensions was proposed based on the role of chemical interactions of hydroxides, silicates, carbonates, and polyphosphates of alkali metals, soluble in dispersion medium, with one another and with the surfaces of natural disperse minerals. The experimental data indicating that these interactions in alkaline silicate suspensions proceed mainly in the pH 10–11 range were summarized. Based on developed notions, practical recommendations taking into account chemical interactions were suggested for preparing the alkaline silicate suspensions with predetermined properties.

Sorption of Anionic Polysaccharides and Bovine Serum Albumin on a Macroporous Glass

Abstract: Concentration and time dependences of the sorption of alginic acid and κ-carrageenan on an MPS-1000 macroporous glass were studied. The character of pH effect on the sorption of these polysaccharides was established. For a number of sorbents prepared by the preliminary adsorption of polysaccharides on an MPS-1000 glass, the dependence of the sorption of bovine serum albumin (BSA) on the surface content of alginic acid and κ-carrageenan was studied. The model of adsorption in the silica–polysaccharide–protein system accounting for the interaction between polysaccharide ionic groups and active sites at the sorbent surface and protein molecules was proposed. Acid-base properties of silica modified with polysaccharides were investigated by the potentiometric titration, and the results supporting the validity of the proposed model were obtained.