Charge-stabilized Dispersions Research Articles

Microfluidic osmotic compression of a charge-stabilized colloidal dispersion: Equation of state and collective diffusion coefficient.

We show, using a model coupling mass transport and liquid theory calculations for a charge-stabilized colloidal dispersion, that diffusion significantly limits measurement times of its equation of state (EOS), osmotic pressure vs composition, using the osmotic compression technique. Following this result, we present a microfluidic chip allowing one to measure the entire EOS of a charged dispersion at the nanoliter scale in a few hours. We also show that time-resolved analyses of relaxation to equilibrium in this microfluidic experiment lead to direct estimates of the collective diffusion coefficient of the dispersion in Donnan equilibrium with a salt reservoir.

Zeta Potential and Colloidal Stability Predictions for Inorganic Nanoparticle Dispersions: Effects of Experimental Conditions and Electrokinetic Models on the Interpretation of Results

In this work, a set of experimental electrophoretic mobility (μe) data was used to show how inappropriate selection of the electrokinetic model used to calculate the zeta potential (ζ-potential) can compromise the interpretation of the results for nanoparticles (NPs). The main consequences of using ζ-potential values as criteria to indicate the colloidal stability of NP dispersions are discussed based on DLVO interaction energy predictions. For this, magnetite (Fe3O4) NPs were synthesized and characterized as a model system for performing electrokinetic experiments. The results showed that the Fe3O4 NPs formed mass fractal aggregates in solution, so the ζ-potential could not be determined under ideal conditions when μe depends on the NP radius. In addition, the Dukhin number (Du) estimated from potentiometric titration results indicated that stagnant layer conduction (SLC) could not be neglected for this system. The electrokinetic models that do not consider SLC grossly underestimated the ζ-potential values for the Fe3O4 NPs. The DLVO interaction energy predictions for the colloidal stability of the Fe3O4 NP dispersions also depended on the electrokinetic model used to calculate the ζ-potential. The results obtained for the Fe3O4 NP dispersions also suggested that, contrary to many reports in the literature, high ζ-potential values do not necessarily reflect high colloidal stability for charge-stabilized NP dispersions.

Stabilizing Colloidal Particles against Salting-out by Shortening Surface Grafts.

A dramatic improvement is reported in the stability of colloidal particles when stabilizing surface grafts are systematically shortened from small polymers to single monomers. The colloidal dispersions consist of fluorinated latex particles, exhibiting a weak van der Waals attraction, with grafted steric layers of poly(ethylene glycol) (PEG) of different chain lengths. Using an effective salting-out electrolyte, Na2CO3, particle aggregates are detected above a threshold salt concentration that is independent of the particle concentration. The results are interpreted in terms of a sudden onset of nondispersibility of single particles, triggered by the solvent not completely wetting particle surfaces. By decreasing the PEG chain length, the threshold salt concentration is found to increase sharply. For grafts with just a single ethylene glycol group, dispersions remain stable up to exceedingly high concentrations of Na2CO3. However, on removal of the surface coverage altogether, the classical stability behavior of charge-stabilized dispersions is recovered. The behavior can be captured by a simple model that incorporates effective polymer-solvent interactions in the presence of an electrolyte.

Ultrafiltration of charge-stabilized dispersions at low salinity.

We present a comprehensive study of cross-flow ultrafiltration (UF) of charge-stabilized suspensions, under low-salinity conditions of electrostatically strongly repelling colloidal particles. The axially varying permeate flux, near-membrane concentration-polarization (CP) layer and osmotic pressure profiles are calculated using a macroscopic diffusion-advection boundary layer method, and are compared with filtration experiments on aqueous suspensions of charge-stabilized silica particles. The theoretical description based on the one-component macroion fluid model (OCM) accounts for the strong influence of surface-released counterions on the renormalized colloid charge and suspension osmotic compressibility, and for the influence of the colloidal hydrodynamic interactions and electric double layer repulsion on the concentration-dependent suspension viscosity η, and collective diffusion coefficient Dc. A strong electro-hydrodynamic enhancement of Dc and η, and likewise of the osmotic pressure, is predicted theoretically, as compared with their values for a hard-sphere suspension. We also point to the failure of generalized Stokes-Einstein relations describing reciprocal relations between Dc and η. According to our filtration model, Dc is of dominant influence, giving rise to an only weakly developed CP layer having practically no effect on the permeate flux. This prediction is quantitatively confirmed by our UF measurements of the permeate flux using an aqueous suspension of charged silica spheres as the feed system. The experimentally detected fouling for the largest considered transmembrane pressure values is shown not to be due to filter cake formation by crystallization or vitrification.

Polymorph selection in the crystallization of hard-core Yukawa system

Colloid-colloid interactions in charge-stabilized dispersions can to some extent be represented by the hard-core Yukawa model. The crystallization process and polymorph selection of hard-core Yukawa model are studied by means of smart Monte Carlo simulations in the region of face-centered-cubic (fcc) phase. The contact value of hard-core Yukawa potential and the volume fraction of the colloids are fixed, while the Debye screening length can be varied. In the early stage of the crystallization, the precursors with relatively ordered liquid structure have been observed. Although the crystal structure of thermodynamically stable phase is fcc, the system crystallizes into a mixture of fcc and hexagonal close-packed (hcp) structures under small Debye screening length since the colloidal particles act as effective hard spheres. In the intermediate range of Debye screening length, the system crystallizes into a mixture of fcc, hcp, and body-centered-cubic (bcc). The existence of metastable hcp and bcc structures can be interpreted as a manifestation of the Ostwald's step rule. Until the Debye screening length is large enough, the crystal structure obtained is almost a complete fcc suggesting the system eventually reaches to a thermodynamically stable state.

On the calculation of the structure of charge-stabilized colloidal dispersions using density-dependent potentials

The structure of charge-stabilized colloidal dispersions has been studied through a one-component model using a Yukawa potential with density-dependent parameters examined with integral equation theory and Monte Carlo simulations. Partial thermodynamic consistency was guaranteed by considering the osmotic pressure of the dispersion from the approximate mean-field renormalized jellium and Poisson–Boltzmann cell models. The colloidal structures could be accurately described by the Ornstein–Zernike equation with the Rogers–Young closure by using the osmotic pressure from the renormalized jellium model. Although we explicitly show that the correct effective pair-potential obtained from the inverse Monte Carlo method deviates from the Yukawa shape, the osmotic pressure constraint allows us to have a good description of the colloidal structure without losing information on the system thermodynamics. Our findings are corroborated by primitive model simulations of salt-free colloidal dispersions.

Investigation of the formation, structure and release characteristics of self-assembled composite films of cellulose nanofibrils and temperature responsive microgels

The possibility of forming self-organized films using charge-stabilized dispersions of cellulose I nanofibrils and microgel beads of poly-(N-isopropylacrylamide-co-acrylic acid) copolymer is presented. The build-up behavior and the properties of the layer-by-layer (LbL)-constructed films were studied using quartz crystal microbalance with dissipation (QCM-D) and ellipsometry. The morphology of the formed films was also characterized using atomic force microscopy (AFM) imaging. The applied methods clearly demonstrated the successful LbL-assembly of the monodisperse microgels and nanofibrils. The in situQCM-D measurements also revealed that contrary to the polyelectrolyte bound microgel particles, the nanofibrils-bound gel beads preserve their highly swollen state and do not suffer a partial collapse due to the lack of interdigitation of the oppositely charged components. To probe the accessibility of the gel beads in the formed films, the room temperature (∼25 °C) loading and release of a fluorescent dye (FITC) was also investigated. The incorporation of the cellulose nanofibrils into the multilayer resulted in an open structure that was found easily penetrable for the dye molecules even at constant room temperature, which is in sharp contrast with previously reported systems based on synthetic polyelectrolytes. The amount of dye released from the multilayer films could be fine-tuned with the number of bilayers. Finally, the thermoresponsivity of the films was also shown by triggering the burst release of the loaded dye when the film was collapsed.

Direct Observations of Three Nucleation/Growth Processes of Charge-Stabilized Dispersion Polymerizations with Varying Water/Methanol Ratios

This article presents observations of three polymerization modes of a self-developed cation-charge-stabilized styrene/water/methanol dispersion polymerization system: (1) a water/methanol (20/80) system, corresponding to a typical dispersion polymerization mode where the particle nucleation occurred in the solution phase and growth in the particle phase; (2) a pure CH(3)OH system, including a first nucleation in the solution phase with growth by absorption of the small particles and polymers formed in this phase, and a secondary nucleation with growth in the particle phase, when high molecular weight copolymers appeared in the solution phase; and (3) a water/methanol (5/95) system, similar to the conventional dispersion polymerization mode during the first 90 min, with subsequent epitaxial growth. Interestingly, the metastable state of the nucleation stage, including minuscule 6-nm particles, their aggregates, and the aggregating process, was first observed experimentally. By quantitatively following the relationship of the deposited molecular weight and the nucleation/growth process in the three systems, it was proposed that the molecular weight of the deposited polymer had to reach a specific high value before they could absorb or capture monomer to form smooth/spherical nuclei or particles.

Self-Organized Films from Cellulose I Nanofibrils Using the Layer-by-Layer Technique

The possibility of forming self-organized films using only charge-stabilized dispersions of cellulose I nanofibrils with opposite charges is presented, that is, the multilayers were composed solely of anionically and cationically modified microfibrillated cellulose (MFC) with a low degree of substitution. The build-up behavior and the properties of the layer-by-layer (LbL)-constructed films were studied using a quartz crystal microbalance with dissipation (QCM-D) and stagnation point adsorption reflectometry (SPAR). The adsorption behavior of cationic/anionic MFC was compared with that of polyethyleneimine (PEI)/anionic MFC. The water contents of five bilayers of cationic/anionic MFC and PEI/anionic MFC were approximately 70 and 50%, respectively. The MFC surface coverage was studied by atomic force microscopy (AFM) measurements, which clearly showed a more dense fibrillar structure in the five bilayer PEI/anionic MFC than in the five bilayer cationic/anionic MFC. The forces between the cellulose-based multilayers were examined using the AFM colloidal probe technique. The forces on approach were characterized by a combination of electrostatic and steric repulsion. The wet adhesive forces were very long-range and were characterized by multiple adhesive events. Surfaces covered by PEI/anionic MFC multilayers required more energy to be separated than surfaces covered by cationic/anionic MFC multilayers.

Generic behavior of the hydrodynamic function of charged colloidal suspensions

We discuss the generic behavior of the hydrodynamic function H(q) and diffusion function D(q) characterizing the short-time diffusion in suspensions of charge-stabilized colloidal spheres, by covering the whole fluid regime. Special focus is given to the behavior of these functions at the freezing transition specified by the Hansen-Verlet freezing rule. Results are presented in dependence on scattering wavenumber q, effective particle charge, volume fraction, salt concentration, and particle size, by considering both the low-charge and high-charge branch solutions of static structure factors. The existence of two charge branches leads to the prediction of a re-entrant melting-freezing-melting transition for increasing particle concentration at very low salinity. A universal limiting contour line is derived for the principal peak height value of H(q), independent of particle charge and diameter, and concentration and salinity, which separates the fluid from the fluid-solid coexistence region. This line is only weakly dependent on the value of the structure factor peak height entering the Hansen-Verlet rule. A dynamic freezing criterion is derived in terms of the short-time cage diffusion coefficient, a quantity easily measurable in a scattering experiment. The higher-dimensional parameter scans underlying this study make use of the fast and highly efficient deltagamma-scheme in conjunction with the analytic rescaled mean spherical approximation input for the static structure factor. Our results constitute a comprehensive database useful to researchers performing dynamic scattering experiments on charge-stabilized dispersions.

Directed self-assembly of colloidal crystals by dielectrophoretic ordering observed with small angle neutron scattering (SANS)

We report the first in situ measurements of directed self-assembly (DSA) of colloidal crystal structures by dielectrophoresis (DEP) observed with Small Angle Neutron Scattering (SANS). Development of a new sample environment allows for real time observation of electric field driven crystallization of sub-micron particles. SANS measurements for charge-stabilized colloidal dispersions of two different particle sizes as a function of electric field strength and frequency identify the order-disorder transition. The results validate a proposed master scaling curve for particle ordering developed for micron particles and thereby extends the scaling range to include sub-micron particles. This new experimental method enables the possibility of studying electric field induced alignment and ordering of colloidal nanoparticles.

Thermomechanical properties of graphite nanofibers/poly(methyl methacrylate) composites

Aligned chemical vapor deposition (CVD)-grown graphite nanofibers (GNFs) were used as a conductive filler in a poly(methyl methacrylate) (PMMA) system to investigate the effect of GNF content on the thermo-mechanical properties of GNFs/PMMA composites. In the result, an electrical percolation threshold for the composites was formed between 1 and 2 wt% GNF contents, which depended on high aspect ratio and good electrical conductivity of GNFs enabling them to percolate systems at low volume fraction. All of the composites contained individually dispersed GNFs after initial shear-intensive stirring. Negative surface charges on the GNFs led to charge-stabilized dispersions. Above the melting temperature of PMMA, the GNFs re-aggregated on application of elevated temperatures and/or modest shear forces. Additionally, the composites exhibited higher thermal and impact properties and lower thermal shrinkage compared with those of the neat PMMA. Consequently, it was demonstrated that GNFs had positive impacts on the thermo-mechanical properties of polymer composites and that they had the potential to be effectively utilized to enhance the structural performance of composites.

Spontaneous Oil-in-Water Emulsification Induced by Charge-Stabilized Dispersions of Various Inorganic Colloids

Charge-stabilized dispersions of inorganic colloids are shown to induce spontaneous emulsification of hydrophobic (TPM) molecules to stable oil-in-water emulsions, with monodisperse, mesoscopic oil droplet diameters in the range of 30-150 nm, irrespective of the polydispersity of the starting dispersions. The results for cobalt ferrite particles and commercial silica sols extend our first study (Sacanna, S.; Kegel, W. K.; Philipse, A. Phys. Rev. Lett. 2007, 98, 158301) on spontaneous emulsification induced by charged magnetite colloids and show that this type of self-assembly is quite generic with respect to the composition of the nanoparticles adsorbing at the oil-water interface. Moreover, we provide additional experimental evidence for the thermodynamic stability of these mesoemulsions, including spontaneous oil dispersal imaged by confocal microscopy and monitored in situ by time-resolved dynamic light scattering. We discuss the possibility that thermodynamic stability of the emulsions is provided by the negative tension of the three-phase line between oil, water, and adsorbed colloids.

Measurement of self-diffusion constant with two-dimensional X-ray photon correlation spectroscopy

The X-ray photon correlation spectroscopy technique probes the slow dynamics of disordered materials, overcoming the limitations of using photon correlation spectroscopy with coherent visible light. It extends the accessible range of the modulus of the scattering vector to short wavelength density fluctuations and is not sensitive to multiple scattering. We measure here experimentally the short-time self-diffusion coefficient D_{\rm S} of a charge-stabilized colloidal dispersion. It is in contradiction with theoretical models including many-body hydrodynamic interactions.

Diameter and Metallicity Dependent Redox Influences on the Separation of Single-Wall Carbon Nanotubes

Recently, it has become possible to separate and/or enrich fractions of single-wall carbon nanotubes (SWNTs) according to type (or otherwise termed "metallicity") and diameter (d(t)). Exposure of acid-treated SWNTs to amines has shown such separation. In this contribution, we describe the underlying mechanism for this separation and provide a better description of the physicochemical properties of charge-stabilized SWNT dispersions in polar aprotic media, such as N,N-dimethylformide (DMF). With the establishment of the reversible nature of the redox chemistry, SWNT(n+) + (n/2)H2O <==> SWNT + nH(+) + (n/4)O2, amine-induced pH changes as well as variations in H2O and O2 concentration in DMF are shown to cause differential partial-reduction trends according to d(t) and metallicity. At a pH of 10, the (n,m)-SWNTs that resist complete reduction to their undoped state remain in suspension while the rest that lose their charges populate the precipitate. These d(t)- and metallicity-dependent redox and separation trends are modeled based on the Gibbs free energy and charge loss as it pertains to the (n,m)-dependent SWNT integrated density of states (I(DOS)) across the corresponding pH-induced redox jump. At a given redox potential, the relative placement of the van Hove singularities and continuum determines the amount of charge left on various (n,m)-SWNTs that governs their relative dispersion stability in DMF.

Diffusion and microstructural properties of solutions of charged nanosized proteins: Experiment versus theory

We have reanalyzed our former static small-angle x-ray scattering and photon correlation spectroscopy results on dense solutions of charged spherical apoferritin proteins using theories recently developed for studies of colloids. The static structure factors S(q), and the small-wave-number collective diffusion coefficient D(c) determined from those experiments are interpreted now in terms of a theoretical scheme based on a Derjaguin-Landau-Verwey-Overbeek-type continuum model of charged colloidal spheres. This scheme accounts, in an approximate way, for many-body hydrodynamic interactions. Stokesian dynamics computer simulations of the hydrodynamic function have been performed for the first time for dense charge-stabilized dispersions to assess the accuracy of the theoretical scheme. We show that the continuum model allows for a consistent description of all experimental results, and that the effective particle charge is dependent upon the protein concentration relative to the added salt concentration. In addition, we discuss the consequences of small ions dynamics for the collective protein diffusion within the framework of the coupled-mode theory.

Solid–liquid transition of charge-stabilized colloidal dispersions: a single-component structure-function approach

We have extended the Raveché–Mountain–Streett one-phasecriterion that governs the freezing of Lennard-Jones systems to a hard-core repulsive Yukawa-model (HCRYM) system. We find in the framework of the Rogers–Young (RY) approximation for an Ornstein–Zernike integral equation that an HCRYM fluid freezes when the ratio α = g(rmin)/g(rmax), where rmax is the distance corresponding to the maximum in the radial distribution function g(r) and rmin is the distance corresponding to the subsequent minimum in g(r), is approximately 0.215. To describe the freezing of charge-stabilized colloidal dispersions in electrolytes, which consist of colloidal macroions,electrolyte small ions, and solvent molecules, we employ the single-component model in which the colloidal particles interact through the effective screened Coulomb potential of Belloni. Whenthe macroion surface effective charge number is taken as an adjustable parameter, the theoretical freezing line predicted by the RY g(rmin)/g(rmax) = 0.215 Raveché–Mountain–Streett one-phase criterion is in very good agreement with the corresponding experimental data.PACS Nos.: 61.25.Em, 61.20.Gy

Freezing of Charge-Stabilized Colloidal Dispersions

The Rogers-Young approximation for the Ornstein-Zernike integral equation is combined with the Hansen-Verlet one-phase criterion for freezing to predict freezing of a hard core repulsive Yukawa model (HCRYM) fluid. Comparison of theoretical predictions with corresponding computer simulation data discloses the superiority of the Rogers-Young approximation over the hypernetted chain approximation and the rescaled mean spherical approximation for freezing. Then, the Rogers-Young approximation combined with the Hansen-Verlet one-phase criterion is employed for the freezing of many-component charge-stabilized colloidal dispersions, which consist of colloidal macroions, electrolyte small ions, and solvent molecules and are modeled as a single-component charged hard core macroion interacting through a screened Coulomb potential. The theoretically predicted freezing line with the macroion surface charge number being assumed as an adjustable parameter is in very good agreement with the corresponding experimental data. The reason why, by the empirical Hansen-Verlet structure function approach, the single-component coarse-grained effective potential is valid for the freezing description of the many-component charge-stabilized colloidal solutions but not valid for the case of asymmetric binary hard sphere mixtures is discussed.

Liquid-glass re-entrant behavior in a charge-stabilized colloidal dispersion

Using the static structure factor S( q) calculated in the rescaled mean spherical approximation and in conjunction with the idealized mode-coupling theory, we determine the loci of the liquid-glass transition phase boundary for a salt-free suspension of charged colloids prepared in different counterions concentrations. For the simplest deionised charge-stabilized colloidal liquid, our calculations demonstrate the possibility of observing, in restrictive regions of the phase diagram, the liquid⇄glass⇄liquid⇄glass re-entrant transition. The latter disappears at a lower concentration of counterions and, when this arises, only the glass⇄liquid⇄glass is observed. We study this counterion-concentration-dependent re-entrant phenomenon by analyzing the non-ergodic Debye–Waller factor, S( q) and their corresponding spatial counterparts.

Viscosity of bimodal charge-stabilized polymer dispersions

Monodisperse electrostatically stabilized polymer latices with particles diameters of 310 and 120 nm were synthesized and used to prepare binary mixtures with bimodal size distribution. The zero shear viscosity of these bimodal, charge stabilized polymer latices was studied at different salt concentrations. In contrast to hard sphere colloidal suspensions, no minimum in viscosity was found as a function of the mixing ratio of small and large particles. Instead the viscosity increased when the fraction of small particles increased, which is due to the direct Coulomb interaction. In order to compare the results with data from hard sphere systems, we used an effective volume fraction φeff. The experimentally determined volume fraction at the divergence of the zero shear viscosity is compared with the hard sphere value in order to define φeff. The effective volume of the particles was then used to calculate the effective volume fraction of the binary mixtures containing small and large particles. When using φeff, a minimum of the viscosity was found at a composition of ∼30% of small particles similar to the behavior of bimodal hard-sphere suspensions. The volume fraction at maximum packing could be calculated by theoretical models only at high salt concentration. The models underestimate the maximum volume fraction at low salt concentration.