Colloidal stability of chitosan-modified poly(methyl methacrylate) latex particles

Experiments of coagulation kinetics were used to study the influence of the electrolyte concentration on the colloidal stability of cationic poly(methyl methacrylate) latex particles with various degrees of chitosan modification. For the chitosan-free latex products prepared by various levels of 2,2′ azobis(2-amidinopropane) dihydrochloride (V-50) at constant pH, the critical coagulation concentration (ccc) increases with increasing V-50 concentration, due to the enhanced particle surface charge density. On the other hand, the chitosan-modified latex products at constant pH do not exhibit very different values of ccc. This result is attributed to the counterbalance between two opposite effects related to the grafted chitosan, that is, the increased particle surface charge density and the enhanced shift of the particle's shear plane toward the aqueous phase with the chitosan content. The ccc of the latex products with various degrees of chitosan modification decreases significantly when the pH increases from 3 to 7. This is because the degree of ionization of the surface amino groups (the particle surface charge density) decreases with increasing pH. As a result, the stability of the colloidal system decreases significantly with increasing pH. The apparent Hamaker constant and diffuse potential were obtained from the coagulation kinetics data. These two parameters along with the zeta potential and particle size data for the latex samples taken immediately after the end of the coagulation experiments were also used to study the effect of ionic strength on the colloidal stability of the latex particles. Received: 10 October 1998 Accepted in revised form: 16 December 1998     

Electrophoretic mobility of a colloidal particle with constant surface charge density

When the electrophoretic mobility of a particle in an electrolyte solution is measured, the obtained electrophoretic mobility values are usually converted to the particle zeta potential with the help of a proper relationship between the electrophoretic mobility and the zeta potential. For a particle with constant surface charge density, however, the surface charge density should be a more characteristic quantity than the zeta potential because for such particles the zeta potential is not a constant quantity but depends on the electrolyte concentration. In this article, a systematic method that does not require numerical computer calculation is proposed to determine the surface charge density of a spherical colloidal particle on the basis of the particle electrophoretic mobility data. This method is based on two analytical equations, that is, the relationship between the electrophoretic mobility and zeta potential of the particle and the relationship between the zeta potential and surface charge density of the particle. The measured mobility values are analyzed with these two equations. As an example, the present method is applied to electrophoretic mobility data on gold nanoparticles (Agnihotri, S. M.; Ohshima, H.; Terada, H.; Tomoda, K.; Makino, K. Langmuir 2009, 25, 4804).     

The effect of surface modification on the stability characteristics of poly(N-isopropylacrylamide) latices under Brownian and flow conditions

The stability of thermoreversible microgel particles of poly(N-isopropylacrylamide) having carboxylate surface charge groups has been studied in the presence of electrolyte and non-adsorbing polymer. Methylation of the surface charge groups leads to a decrease in the electrophoretic mobility of the particles and also the interparticle electrostatic repulsive potential, resulting in the material becoming more susceptible to flocculation. The Hamaker constant of the microgel particles increases with the decrease in the hydrodynamic size of the particles following heating. This brings about an increase in the van der Waals attractive energy which results in the particles aggregating in the presence of sufficient electrolyte. Under conditions of flow through membranes, where shearing forces are operative, the flocculation observed following the heating of the dispersions results in the blockage of pores.     

Stability of the polymerizable surfactant stabilized latex particles during semibatch emulsion polymerization

The effect of the polymerizable surfactant, sodium dodecyl allyl sulfosuccinate (JS-2), on the stability of polybutyl acrylate latex particles during semibatch emulsion polymerization was investigated in this work. Experimental data show that the ionic strength is the most important parameter in determining the latex stability during the reaction. Both the amount of coagulum produced by intensive coagulation and percentage of the particle volume change (ΔV) caused by limited flocculation increase with increasing electrolyte concentration. The parameter Δ V increases significantly when the concentration of JS-2 in the initial reactor charge ([JS-2]_i) increases. The amount of coagulum increases rapidly when the agitation speed is increased from 400 to 800 rpm. Experiments of coagulation kinetics were carried out to study the stability of latex products toward added salts. The experimental data show that the chemical stability of the latex product increases with increasing pH. Furthermore, the critical coagulation concentration and diffuse potential increase with increasing [JS-2]_i. It is postulated that the increasing electrostatic attraction force between two approaching particles due to the increased [JS-2] _i can increase the apparent magnitude of Hamaker constant.     

Colloid particle and protein deposition - electrokinetic studies

Recent developments in the electrokinetic determination of particle, polyelectrolyte and protein deposition at solid/electrolyte interfaces, are reviewed. In the first section basic theoretical results are discussed enabling a quantitative interpretation of the streaming current/potential and microelectrophoretic measurements. Experimental results are presented, pertinent to electrokinetic characteristics of simple (homogeneous) surfaces such as mica, silica and various polymeric surfaces used in protein studies. The influence of the ionic strength, background electrolyte composition and pH is discussed, and the effective (electrokientic) charge of these interfaces is evaluated. In the next section, experimental data obtained by streaming potential measurements for colloid particle mono- and bilayers are presented and interpreted successfully in terms of available theoretical approaches. These results, obtained for model systems of monodisperse colloid particles are used as reference data for discussion of more complicated experiments performed for polyelectrolyte and protein covered surfaces. Results are discussed, obtained for cationic polyelectrolytes (PEI, PAH) and fibrinogen adsorbing on mica, interpreted quantitatively in terms of the theoretical approach postulating a heterogeneous 3D charge distribution. The Gouy-Chapman model, based on the continuous charge distribution proved inadequate. Interesting experimental data are also discussed, obtained by electrophoretic methods in the case of protein adsorption on colloid latex particles. In the last section, supplementary results on particle deposition on heterogeneous surfaces produced by controlled protein adsorption are discussed. Quantitative relationships between the amount of adsorbed protein, zeta potential of the interface and the particle coverage are specified. Possibility of evaluating the heterogeneity of protein charge distribution is pointed out. The anomalous deposition of colloid particles on protein molecules bearing the same sign of zeta potential, which contradicts classical DLVO theory, is interpreted in terms of the fluctuation theory. It is concluded that theoretical and experimental results obtained for model colloid systems and flat interfaces can be effectively used for interpretation of protein adsorption phenomena, studied by electrophoresis. In this way the universality of electrokinetic phenomena is underlined.     

Flow ultramicroscopic study of interparticle interaction in suspensions and kinetics of reversible aggregation

The kinetics of coagulation leading, in the long run, to the establishment of the aggregation equilibrium is studied by the flow ultramicroscopy method with allowance for the probability of aggregate formation and disintegration. The case of a slight aggregation is considered when the doublet-to-singlet concentration ratio in a disperse system is low. An equation characterizing the time dependence of the average sizes of aggregates is derived. The equation is analyzed and methods are proposed for determining the repulsive barrier and the depth of the energy minimum characterizing the potential of interparticle pair interaction from experimental data on coagulation kinetics. The case of long-range coagulation is investigated. The effects of particle size, Hamaker’s constant, and electrolyte concentration in a dispersion medium on the probability of disaggregation are estimated in terms of the theory of surface forces. Limits of the flow microscopy method in the determination of the secondary energy minimum value are considered.     

Approximate analytic expressions for the electrophoretic mobility of spheroidal particles

Approximate analytic expressions are derived for the electrophoretic mobility of spheroidal particles (prolate and oblate) carrying low zeta potential in an electrolyte solution under an applied tangential or transverse electric field. The present approximation method, which is based on the observation that the electrophoretic mobility of a particle is determined mainly by the distortion of the applied electric field by the presence of the particle. The exact expression for the equilibrium electric potential distribution around the particle, which can be expressed as an infinite sum of spheroidal wave functions, is not needed in the present approximation. The electrophoretic mobility values calculated with these approximate expressions for spheroidal particles with constant surface potential or constant surface charge density are in excellent agreement with the exact numerical results of previous reports with the relative errors less than about 4%.     

Dynamic electrophoretic mobility of spherical colloidal particles in a salt-free medium

A theory is proposed for the dynamic electrophoretic mobility mu(omega) of spherical colloidal particles in a salt-free medium containing only counterions in an oscillating electric field of frequency omega. The dynamic mobility depends on the frequency omega of the applied electric field and on the particle volume fraction as well as on the particle surface charge. It is found that as in the case of the static electrophoretic mobility mu(0) in salt-free media, there is a certain critical value of the particle surface charge separating two cases, that is, the low-surface-charge case and the high-surface-charge case (in the latter case the counterion condensation takes place near the particle surface). For the low-surface-charge case, the dynamic mobility agrees with that of a sphere in an electrolyte solution in the limit of very low electrolyte concentrations kappaa-->0 (Hückel's limit), where kappa is the Debye-Hückel parameter and a is the particle radius. For the high-surface-charge case, however, the dynamic mobility becomes constant independent of the particle surface charge, because of the counterion condensation effects. A simple expression for the ratio mu(omega)/mu(0) applicable for all cases is given.     

Electrostatics and electrophoresis of engineered nanoparticles and particulate environmental contaminants: Beyond zeta potential-based formulation

Colloidal nano/micro-(bio)particles carry an electrostatic charge in aqueous media, and this charge is critical in defining their stability, (bio)adhesion properties, or toxicity toward humans and biota. Determination of interfacial electrostatics of these particles is often performed from zeta potential estimation using the electrophoresis theory by Smoluchowski. The latter, however, strictly applies to the ideal case of hard particles defined by a surface charge distribution under the strict conditions of particle impermeability to electrolyte ions and to flow. Herein, we review sound theoretical alternatives for capturing electrokinetic and therewith electrostatic features of soft colloids of practical interest defined by a 3D distribution of their structural charges and by a finite permeability to ions and/or flow (e.g., bacteria, viruses, nanoplastics, (bio)functionalized particles or engineered nanoparticles). Reasons for the inadequacy of commonly adopted hard particle electrophoresis models when applied to soft particulate materials are motivated, and analytical expressions that properly capture their electrophoretic response are comprehensively reviewed.     

Estimation of the Hamaker constants by sedimentation field-flow fractionation

van der Waals forces are one of several forces that control the adhesion between two materials. These forces are important to quantify in adhesion studies because they are always present and are always attractive. The major problem in calculating the van der Waals interaction between colloidal particles is that of evaluating the Hamaker constant. Hence, an accurately determined Hamaker constant for a given material is needed when interfacial phenomena such as adhesion are discussed in terms of the total potential energy between a particle and a substrate. In this paper, a new simple and accurate methodology for the estimation of the Hamaker constant is introduced. The results are in good agreement with those values found in literature.     

Colloidal stability of sulfonated polystyrene model colloids. Correlation with electrokinetic data

In this work we describe the colloid stability of functionalized latexes: two sulfonated polystyrene model colloids with the same particle size and different surface charge densities. The critical coagulation concentration (ccc) was determined in the presence of two electrolytes (11 and 22), being around 0.75 M and 0.075 M, respectively. By the DLVO theory the electrokinetic and colloid stability data were correlated to calculate the Hamaker constant at both experimental conditions. By comparing the experimental and theoretical values of the Hamaker constant, it is possible to get more information about the colloidal stabilization mechanism of functionalized latexes. In the case of sulfonated latexes, the electrostatic and seric contributions occur, with different influence for each latex depending of their surface electric charge.     

Electrophoretic mobility of a soft particle in a salt-free medium

A theory is presented for the electrophoretic mobility mu of dilute spherical soft particles (i.e., polyelectrolyte-coated particles) in salt-free media containing only counterions. As in the case of other types of particles (rigid particles and liquid drops) in salt-free media, there is a certain critical value of the particle charge separating two cases, the low-surface-charge case and the high-surface-charge case. For the low-charge case, the mobility is proportional to the particle charge and coincides with that of a soft particle in an electrolyte solution in the limit of very low electrolyte concentrations kappa-->0 (Hückel's limit), where kappa is the Debye-Hückel parameter. For the high-charge case, however, mu becomes essentially constant, independent of the particle charge, due to the counterion condensation effect.     

Experimental and theoretical evidence of overcharging of calcium silicate hydrate

Electrokinetic measurements such as electrophoresis may show an inversion of the effective surface charge of colloidal particle called overcharging. This phenomenon has been studied by various theoretical approaches but up to now very few attempts of confrontation between theory and experiment have been conducted. In this work we report electrophoretic measurements as well as Monte Carlo simulations of the electrokinetic potential for the surface of calcium silicate hydrate (CSH), which is the major constituent of hydrated cement. In the simulations, the surface charge of CSH nanoparticles in equilibrium with the ionic solution is determined by a single site characteristic and electrostatic interactions between all explicit charges at the surface and in the electric double layer. We will show that ordinary electrostatic interactions are enough to describe all experimental observations. Actually, an excellent agreement is found between experimental and simulated results without any fitting parameter, both with respect to surface titration and electrokinetic behaviour. The agreement extends over a wide range of electrostatic coupling, from a weakly charged surface with mainly monovalent counter-ions to a highly charged one with divalent counter-ions.     

Transient electrophoresis in a suspension of charged particles with arbitrary electric double layers

The startup of electrophoretic motion in a suspension of spherical colloidal particles, which may be charged with constant zeta potential or constant surface charge density, due to the sudden application of an electric field is analytically examined. The unsteady modified Stokes equation governing the fluid velocity field is solved with unit cell models. Explicit formulas for the transient electrophoretic velocity of the particle in a cell in the Laplace transforms are obtained as functions of relevant parameters. The transient electrophoretic mobility is a monotonic decreasing function of the particle-to-fluid density ratio and in general a decreasing function of the particle volume fraction, but it increases and decreases with a raise in the ratio of the particle radius to the Debye length for the particles with constant zeta potential and constant surface charge density, respectively. On the other hand, the relaxation time in the growth of the electrophoretic mobility increases substantially with an increase in the particle-to-fluid density ratio and with a decrease in the particle volume fraction but is not a sensitive function of the ratio of the particle radius to the Debye length. For specified values of the particle volume fraction and particle-to-fluid density ratio in a suspension, the relaxation times in the growth of the particle mobility in transient electrophoresis and transient sedimentation are equivalent.     

Electrophoretic mobility of latex particles: effect of particle size and surface structure

The influence of particle size on the electrophoretic mobility of negatively charged latex particles was examined by a comparison between theory and experiment. Theoretical values for the dependence of the mobility on electrolyte concentration were calculated by a modified White–O’Brian model (Hidalgo-Alvarez et al., Adv. Coll. Interf. Sci. 67 (1996) 1) which enables the consistent calculation of the zeta (ζ) potential. For three polystyrene latexes of different size but similar surface charge density the measured mobilities increased with increasing radius for the electrolyte range under consideration. The theoretical calcalations resulted in a qualitatively correct prediction of the experimental data. The experimental comparison of the mobilities of hydrophobic and hydrophilic particles of similar size and surface charge density lead to the conclusion that hydrophilic surfaces lower the electrophoretic mobility. The same theoretical model was able to describe correctly this observed behavior by assuming a greater distance of the plane of shear. The effect of a spatial distribution of the charges was examined by characterizing an electrosterically stabilized latex. Contrary to all standard latices with surface charges this latex didn't show any mobility maximum as a function of electrolyte concentration.     

Electrophoresis and electric conduction in a salt-free suspension of charged particles

The electrophoresis and electric conduction of a suspension of charged spherical particles in a salt-free solution are analyzed by using a unit cell model. The linearized Poisson-Boltzmann equation (valid for the cases of relatively low surface charge density or high volume fraction of the particles) and Laplace equation are solved for the equilibrium electric potential profile and its perturbation caused by the imposed electric field, respectively, in the fluid containing the counterions only around the particle, and the ionic continuity equation and modified Stokes equations are solved for the electrochemical potential energy and fluid flow fields, respectively. Explicit analytical formulas for the electrophoretic mobility of the particles and effective electric conductivity of the suspension are obtained, and the particle interaction effects on these transport properties are significant and interesting. The scaled zeta potential, electrophoretic mobility, and effective electric conductivity increase monotonically with an increase in the scaled surface charge density of the particles and in general decrease with an increase in the particle volume fraction, keeping each other parameter unchanged. Under the Debye-Hückel approximation, the dependence of the electrophoretic mobility normalized with the surface charge density on the ratio of the particle radius to the Debye screening length and particle volume fraction in a salt-free suspension is same as that in a salt-containing suspension, but the variation of the effective electric conductivity with the particle volume fraction in a salt-free suspension is found to be quite different from that in a suspension containing added electrolyte.     

Interaction between an ion-penetrable particle and a solid particle. II. Criteria for heterocoagulation

The potential energy of the total interaction between two spherical colloidal particles of different nature is calculated, i. e., of an ion-penetrable particle and an ion-impenetrable solid particle having a constant surface potential or constant surface charge density. The criteria for heterocoagulation are derived. The obtained results suggest a possibility of selective coagulation in the mixed system.     

Interaction between silica nanoparticles in binary liquid mixtures: the effect of polarity on adhesion

In order to explore the relationship between selective liquid sorption and the stability of dispersed silica particles, we carried out studies in ethanol–cyclohexane binary mixtures as well as in partially miscible 1-butanol–water mixtures. In ethanol–cyclohexane, adsorption excess isotherms were determined on hydrophilic and hydrophobic aerosil particles. The volume and the thickness of the adsorption layer were derived from the isotherms. Knowing the layer thickness and Hamaker constants, interparticle interaction potentials were calculated at various mixture compositions. At low ethanol concentration, where hydrophilic surfaces are considerably enriched in ethanol, interparticle interaction is enhanced and the high shear stress calculated from rheological measurements indicates the development of a three-dimensional network of aggregated particles. In contrast, hydrophobic aerosil particles in ethanol–cyclohexane approach each other without ensuing aggregation because interparticle interactions are weak, a fact well demonstrated by rheological measurements. It was also established that interactions between silica particles with hydrophilic surfaces are weak in butanol–water mixtures. Since water is preferentially adsorbed on the surface of hydrophilic particles to the azeotropic composition (\( x_{\rm{1}}^{\rm{a}} \)=0.62), within this wide composition range the Hamaker constant of the interfacial layer is identical with that of water.     

Electrokinetic behavior and colloidal stability of polystyrene latex coated with ionic surfactants

This work is focused on analyzing the electrokinetic behavior and colloidal stability of latex dispersions having different amounts of adsorbed ionic surfactants. The effects of the surface charge sign and value, and the type of ionic surfactant were examined. The analysis of the electrophoretic mobility (mu(e)) versus the electrolyte concentration up to really high amounts of salt, much higher than in usual studies, supports the colloidal stability results. In addition, useful information to understand the adsorption isotherms was obtained by studying mu(e) versus the amount of the adsorbed surfactant. Aggregation studies were carried out using a low-angle light scattering technique. The critical coagulation concentrations (ccc) of the particles were obtained for different surfactant coverage. For latex particles covered by ionic surfactants, the electrostatic repulsion was, in general, the main contribution to the colloidal stability of the system; however, steric effects played an important role in some cases. For latices with not very high colloidal stability, the adsorption of ionic surfactants always improved the colloidal stability of the dispersion above certain coverage, independently of the sign of both, latex and surfactant charge. This was in agreement with higher mobility values. Several theoretical models have been applied to the electrophoretic mobility data in order to obtain different interfacial properties of the complexes (i.e., zeta potential and density charge of the surface charged layer).     

DESTABILIZATION OF OIL-IN-WATER EMULSION WITH INORGANIC ELECTROLYTES IN THE ABSENCE AND IN THE PRESENCE OF SODIUM DODECYL SULFATE

ABSTRACT

The coagulation concentrations of Na⁺, Ba²⁺, and La³⁺ in the absence of SDS were analyzed in terms of the DLVO theory. The Hamaker constant obtained was larger by one order of magnitude than that available in the literature on the basis of the Lifshitz theory. An explanation of this discrepancy was given by taking into account the hydrophobic interaction between oil droplets in aqueous media. Coagulation of decane particles with Na in the presence of SDS (1 × 10^?3M) was interpreted qualitatively in terms of the DLVO theory in which the adsorbed layer effect was taken into account. The critical adsorption density of SDS at which no coalescence of aml1sion takes place was found to be 3.3 × 10^?10mol cm^?2. Electrokinetic adsorption densities of SDS were found to be smaller than those obtained from the interfacial tension vs. concentration data.