Cocklebur-shaped colloidal dispersions

Unique cocklebur-shaped colloidal dispersions were prepared using a combination of a nanoextruder applied to the aqueous solution containing methyl methacrylate (MMA) and n-butyl acrylate (n-BA) with azo-bis-isobutyronitrile (AIBN) or potassium persulfate (KPS) initiators and stabilized by a mixture of sodium dioctyl sulfosuccinate (SDOSS) and 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (DCPC) phospholipid. Upon extrusion and heating to 75 degrees C, methyl methacrylate/n-butyl acrylate (MMA/nBA) colloidal particles containing tubules pointing outward were obtained as a result of DCPC phospholipids present at the particle surfaces. The same cocklebur-shaped particles were obtained when classical polymerization was used without a nanoextruder under similar compositional and thermal conditions, giving a particle size of 159 nm. However, when Ca(2+) ions are present during polymerization, cocklebur morphologies are disrupted. Because DCPC tubules undergo a transition at 38 degrees C, such cocklebur morphologies may offer numerous opportunities for devices with stimuli-responsive characteristics.     

Polymer modified colloidal dispersions

The effect of added polyvinyl alcohol (PVA) and polyvinyl pyrrolidone (PVP) on a dispersion of polystyrene particles stabilised with grafted methoxy PEG chains is discussed. PVA adsorbed more strongly on the particles compared to PVP. Polymer addition led to stable mixtures in the case of PVA compared to depletion phase separation observed in the case of PVP. Rheological measurements showed thickening of the dispersion and absence of any structure in the case of PVA, in contrast to structure formation in the case of PVP due to depletion phase separation. A weak gel state was reached for ca. 7% w/w PVP. The observed behaviour is in accord with the relative propensity of PVA and PVP to interact with the particle surface, the grafted chains and their solubility in water. The solvency of the free polymer chains dominated the overall behaviour while the contribution from the incompatibility between free and grafted chains was conterbalanced by differences in the free polymer adsorption on the particles.     

Computer simulation of colloidal dispersions

This paper discusses recent applications of statistical mechanics to dispersions with particular emphasis on the computer simulation of the dynamic properties.Fundamental to any computation on a colloidal dispersion is the knowledge of the potential of mean force for at least a pair of suspended particles. At low-to-moderate particle concentrations for stable dispersions, statistical mechanical calculations based on the normal DLVO pair potential produce reasonable agreement with experiment for a number of equilibrium properties of simple latex dispersions. This phenomenon indicates that under these conditions the DLVO pair potential is a reasonable effective pair potential. However, recent Monte Carlo simulations and experimental measurements with liquids of spherical molecules suggest that the force between a pair of dispersed particles at very small separation may differ significantly from that predicted by DLVO theory.The computation of dynamic properties of dispersions involves problems not encountered in the above equilibrium calculations. In particular, one must include the effects of indirect hydrodynamic as well as direct interactions among the particles. This computation may be easily accomplished at moderately low particle concentrations and the results of such calculations are able to give a very detailed analysis of the results of Photon Correlation Spectroscopy measurements on ion exchanged polystyrene latex suspensions at low concentration. These computations also, once again, emphasize the usefulness of DLVO pair potentials as effective pair potentials for systems of strongly interacting particles.     

The microrheology of colloidal dispersions

Summary A theory of doublet formation, the first step in coagulation of dilute monodisperse spherical sols subjected to shearing motion, in which Brownian motion dominates the effects of shear is presented. Contrary to Smoluchowski's predictions, it is found that the increase in the rate of coagulation due to shear is not proportional to the shear rateG, but toG _1/2 with the proportionality constant depending on the perikinetic capture efficiency, the translational diffusion constant and sphere radius. With non-spherical particles the increase rate due to shear also varies with G_1/2 and depends, in addition, on particle size and shape.

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Zusammenfassung Eine Theoriefür Doppelkugelformationen als der erste Schritt in der Koagulation von verdünnten, monodispersen, kugelförmigen Solen wird präsentiert. Die Sole sind -einer Scherströmung ausgesetzt, in der die Brownsche Molekularbewegung über die Effekte der Scherbeanspruchung dominiert. Im Gegensatz zu Smoluchowskis Voraussagen wurde gefunden, daß die Steigung im Ausmaß der Koagulation, hervorgerufen durch die Scherung, nicht einfach dem GeschwindigkeitsgradientenG, sondernG _1/2 proportional ist. Die Proportionalitätskonstante hängt vom perikinetischen Einfangwirkungsgrad, der translatorischen Diffusionskonstanten und dem Kugelradius ab. Mit nicht-kugeligen Teilchen variiert das Koagulationsausmaß auch mitG _l/2, hängt aber zusätzlich von Teilchengröße und Gestalt ab.

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List of Symbols ai constant of integration - d distance at whichf _t changes sign - diffusion tensor - D ,D functions describing radial and tangential components of diffusion - function defined in [39] - f(r ^*),f (r ^*) function representing radial part ofn, belonging to eigen value - g(, ) function representing tangential part ofn - i –1 - J, J _B,J _S flux, due to Brownian motion, due to shear - k Boltzmann constant - k _i constant of integration - K(r ^*) function defined in [28] - l, m integers - n, normalized number concentration, value in outer expansion - N; N ₀ number concentration of single particles; at infinity - O (r ^*) of orderr ^* - P operator defined in [14] - P ^|m|(cos) associated Legendre polynomials - Q constant defined in [34] - Pe _t ^1/2 r^* - R radius of imaginary sphere around reference sphere - operator defined in [13] - T absolute temperature - U velocity vector - Pe _{t 1/2}x _i - z _l function defined in [22] - ; ^* defined in [20 a] ; + - _p; _{p p} ^* perikinetic capture efficiency(/2); _p ^*/2 - _l, ₂ collision efficiencies - ₃ 2₂/3x03C0; - ; ^* constant defined in [39b]; 2 - constant defined in [27] - gamma function - percentage increase in capture frequency due to shear - arbitrarily small distance - eigen value - differential operatorM With 3 figuresThis work was supported by Grant MA-4012 of the Medical Research Council of Canada     

Depletion flocculation in colloidal dispersions

The destabilisation of colloidal dispersions by free, non-adsorbing polymer molecules in solution is reviewed from both a theoretical and experimental perspective. Consideration is given to the behaviour of polymers in solution in relation to the theories proposed by Flory, de Gennes, and Scheutjens and Fleer. Similarly, the theories describing the behaviour of polymers at or near an interface are also examined together with an outline of their relevance to the depletion interaction. The depletion interaction itself is also comprehensively reviewed, taking into account the different segment density profile theories currently in the scientific literature. Experimental reports of the depletion phenomena are described, covering both hard sphere and soft sphere systems, with both the direct and indirect evidence of the depletion interaction being considered.     

Chemiluminescence in the ozonation of C60 aqueous dispersions

Two types of chemiluminescence (CL) arising upon ozonation of crystalline, amorphous, and molecular aqueous dispersions of C₆₀ prepared in different ways were discovered and studied. The weak long-wavelength CL-1 (λ_max > 650 nm) is due to thermocatalytic decomposition of ozone on the surface of fullerene micro- and nanoparticles. The bright short-wavelength CL-2 (λ_max = 570 nm) is caused by generation of electronically excited states of the products of C₆₀ oxidation with ozone. CL-1 appears upon ozonation of aqueous dispersions of C₆₀ consisting of surface-hydrated crystalline micro- and nanoparticles by low concentrations of ozone. CL-2 is exhibited upon ozonation of nano-sized C₆₀ aqueous dispersions and colloid solutions, which contain C₆₀ molecules surrounded by a strong aqueous shell and their associates, by higher concentrations of ozone. Owing to shielding by the hydration shell, C₆₀ fullerene in aqueous dispersions is much less reactive towards ozone and forms oxidation products of different composition as compared with C₆₀ in organic solvents.     

Development of effective Stokesian dynamics method for ferromagnetic colloidal dispersions (cluster-based Stokesian dynamics method)

We have developed a new Stokesian dynamics (SD) method for nondilute colloidal dispersions, which enables us to reduce drastically the computation time. To verify the validity of the present method, which is called the "cluster-based SD method," three-dimensional simulations of a ferromagnetic colloidal dispersion have been carried out for a simple shear flow. The correlation function and viscosity have been evaluated to compare the results obtained by the present method with those obtained by the ordinary SD method and by the method of ignoring hydrodynamic interactions between particles. The results obtained here are summarized as follows. The transient properties from an initial state obtained by the present method agree well with those obtained by the ordinary method, even if a radius r(clstr), which defines the cluster formation, is taken as a small value such as r(clstr)=1.2d (d is the particle diameter). Also, the equilibrium properties such as the pair correlation function and viscosity obtained by the present cluster-based method are in satisfactory agreement with those obtained by the ordinary SD method. Furthermore, the cluster-based method drastically reduces the computation time to about one-fourteenth to one-seventieth that of the ordinary method. It is clear from these results that the cluster-based SD method is significantly superior to the ordinary SD method for ferromagnetic colloidal dispersions for which a large model system such as N=1000 or 10,000 is indispensable in simulations.     

Light scattering - diagnostic methods for colloidal dispersions

The increasing demand from the colloid research community for quick and noninvasive experimental techniques as well as the rapid progress of modern optics and electronics have led to a considerable expansion in the field of light scattering. This review introduces the basic concepts with some emphasis on novel approaches like the study of interacting particle systems, multiple scattering techniques, and non ergodic samples. Particular attention is then devoted to recent experimental progress towards more compact and rugged instruments.     

Large-scale structures in sheared colloidal dispersions

Research into the structures induced by flow in various colloidal systems has started to focus in the past few years on large-scale structures and inhomogeneous flow phenomena. Some novel experimental observations using rheo-optical or NMR imaging techniques, as well as theoretical developments, have unraveled a wide variety of flow-induced structural phenomena, including shear banding and the development of structural anisotropy in weakly aggregated dispersions and near-critical systems.     

Theory of rheology of colloidal dispersions

This brief review contains a survey of recent literature on theory of rheology of colloidal dispersions. Areas of active research are highlighted, such as approximations for many body interactions in weakly sheared dispersions and the flow-induced microstructural distortions in more strongly sheared dispersions. The former approach seeks to capture the viscosity increase on increasing particle concentration, whereas the latter explains shear thinning and shear thickening.     

Equation of state of colloidal dispersions

We present a comparison of experimentally and theoretically determined osmotic pressures for various colloidal dispersions. Experimental data is collected from several different silica and polystyrene dispersions. The theoretical pressure determinations are based on the primitive model combined with the cell model, and the physical quantities are calculated exactly using Monte Carlo simulations in the canonical and grand canonical ensemble. The input to the simulations in terms of colloidal particle size, surface charge density, and so forth are taken directly from experiments, and the approach does not contain any adjustable parameters. The agreement between theory and experiment is very good without any fitting parameters, showing that the simplifications behind the primitive model and the cell model are physically sound. The results reveal a surprising correspondence between the equations of state in spherical and planar geometries, indicating that the particle shape is of secondary importance in dispersions dominated by repulsive interactions. For one of the silica dispersions, we have also investigated how various monovalent counterions influence the swelling properties. Within experimental error, we are unable to detect any ion specificity, which is further support for the theoretical models used.     

Interfacial-chemistry mediated behavior of colloidal talc dispersions

Interfacial chemistry and rheological behavior of talc suspensions as a function of pH, talc solid content, and type and concentration of ions have been investigated using electrokinetic measurements, rheology, settling behavior, and solution analysis. Zeta potential measurements show a strong dependence on the pH history of the talc suspension that only occurs when the surface area (solid content) of the talc is high. Particle interactions measured through dispersion yield stress measurements show a similar dependence. Talc is a magnesium silicate mineral and the dependence seen in the electrokinetic properties in this study has been attributed to Mg(II) dissolution at low pH, and has been confirmed by solution analysis. At high solid content (>20 wt%), pronounced Mg(II) ion dissolution occurs at low pH values. Formation and adsorption of electropositive Mg(II) hydrolysis products occurs at high pH (>9), and these lead to zeta potential reduction and, at high solid contents, charge reversal. Particle interactions reflect the surface chemistry behavior. Consequently, for a freshly prepared suspension at high pH, the yield stress is lower compared to after the pH has been taken to 5 and subsequently increased.     

Structure of polymer fluids in colloidal dispersions

We have calculated some structural properties of a fluid of hard sphere polymer chains about a variable sized central hard sphere with the Monte Carlo method in the canonical ensemble. We have additionally calculated these structural properties with an integral equation based on density functional theory. The integral equation theory gives good agreement with the simulations at all but the highest densities.     

Metal speciation dynamics in colloidal ligand dispersions

In this work we propose a dynamic metal speciation theory for colloidal systems in which the complexing ligands are localized on the surface of the particles; i.e., there is spatial heterogeneity of binding sites within the sample volume. The differences between the complex formation and dissociation rate constants of complexes in colloidal dispersions and those in homogeneous solutions originate from the differences in kinetic and mass transport conditions. In colloidal systems, when the effective rate of dissociation of the surface complexes becomes fully diffusion controlled, its value is defined via the geometrical parameters of the particle. We assess the extent to which the conventional approach of assuming a homogeneously smeared-out ligand distribution overestimates the lability of surface complexes in colloidal ligand dispersions. The validity of the theory is illustrated by application to binding of lead and cadmium by carboxyl modified latex particles: our approach correctly predicts the formation/dissociation rate constants, which differ by several orders of magnitude from their homogeneous solution counterparts.     

Preparation and properties of colloidal iron dispersions

We systematically study the properties of dispersions of iron-based colloids synthesized in a broad size range by thermal decomposition of ironcarbonyl using different stabilizing surfactants. The synthesis results in stable dispersions of monodomain magnetic colloids. Our particles appear to consist of an amorphous Fe(0.75)C(0.25) alloy. Sizes of particles coated with modified polyisobutene or oleic acid can be easily controlled in the 2-10 nm range by varying the amounts of reactants. Extensive characterization with various techniques gives particle sizes that agree well with each other. In contrast to dispersions of small particles, which consist of single colloids, dynamic aggregates are present in dispersions of larger particles. On exposure to air, an oxide layer forms on the particle surface, consisting of a disordered Fe(III) oxide.     

Dynamic light scattering from concentrated colloidal dispersions

From the scattered intensity as a function of scattering wave number, local ordering in concentrated latex dispersions in benzene is found. From the combination of intensity and photon correlation experiments follows the existence of strong hydrodynamic interactions between the dispersed particles.     

Approximate analytic expression for the stability ratio of colloidal dispersions

Approximate analytic expressions are derived for the stability ratios of dispersions of spherical colloidal particles with and without viscous interactions between particles on the basis of the DLVO theory of the potential energy of the electrostatic and van der Waals interactions between two approaching particles. The obtained approximate stability expressions agree with exact numerical results with negligible errors and are applicable irrespective of the magnitude of the potential maximum unlike the previous approximate stability expressions, which are applicable only when the potential maximum is much greater than the thermal energy.