Relationship between interaction forces and the structural compactness of depletion flocculated colloids

Insertion of anionic synthetic clay in lamellar surfactant phases

We describe the different mixed colloidal solutions that can be obtained when mixing equivalent quantities of a synthetic anionic clay to surfactants forming lamellar phases in the absence of added salt. The important quantity driving toward insertion or depletion is the osmotic pressure, of the lamellar phase and of the clay alone. Competition for water is the main driving force toward dispersion, inclusion or exclusion (phase separation). In the case of a nonionic surfactant (C 12 E 5 ) mixed with Laponite, undulations quenched by the surfactant-decorated clay lead to swelling; inclusion is not observed due to differences in rigidity. Long-range order is weakened leading eventually to the exclusion of surfactant in excess. In the case of a double anionic system (AOT-Laponite), electrostatic is dominant and the three regimes are encountered. In the catanionic case, admixing the double chain cationic lipid DDAB to the clay (in large charge excess), the platelets are coated by a positively charged bilayer. Long-range order is very efficiently dampened. From a low threshold (2% by weight), there is exclusion of a clay-poor collapsed lamellar phase, detected by the swelling of the main phase. The cationized clay does not interfere with the molecular force balance: the location of the critical point is unchanged. At high Laponite concentration, a very puzzling microstructure is observed. Some phase diagrams as well as representative SANS and SAXS data are extracted from the complete results concerning the lyotropic lamellar phase mixing problem available with all measures and evaluations of osmotic pressures in the PhD of the late Isabelle Grillo.

Monte Carlo Simulation Modeling of Nanoparticle-Polymer Cosuspensions

Creation of high-quality and novel polymer-particle nanocomposites to a large extent depends on understanding the behaviors of individual polymer chains and particles, especially at the mixing state in a liquid solvent. Simulations can help identify critical parameters and equations that govern the suspension behaviors. This study is the first attempt to understand the agglomeration processes of ZnO nanoparticle and poly(methyl methacrylate) polymer cosuspensions through a constant number Monte Carlo simulation. A modified Derjaguin-Landau-Verwey-Overbeek theory is used to describe the particle-particle interactions that lead to agglomeration. The average agglomerate size and number are measured as a function of suspension resting time, particle to polymer volume ratio, polymer chain length, and suspension drying. The agglomerate size increases persistently with the resting time and particle content increase, ranging from 1.2 μm for the 1 vol % particle content suspension to 4.6 μm for the 20 vol % particle content suspension after 30 min of suspension resting. The agglomerate size distribution for all of the particle contents follows a lognormal distribution. As the polymer chain length increases, agglomeration also becomes more severe. If drying is accounted for and thus the solids loading continually increases, the suspension becomes much more stable because of increases in viscosity and depletion stabilization.

Externally Triggered Oscillatory Structural Forces

This paper addresses triggering of oscillatory structural forces via temperature variation across an aqueous dispersion of thermoresponsive poly( N-isopropylacrylamide) (PNIPAM) nanogels confined between silica surfaces. Oscillatory structural forces are a well-known phenomenon in colloidal science, caused by interactions between molecules or colloids. Modulation of these forces usually requires changing the internal parameters of the dispersion, such as ionic strength, particle concentration, and surface charge, or changing the properties of the confining walls, such as surface roughness, potential, or elasticity. All of these parameters are usually fixed and can only be changed via exchange of the sample or the complete experimental setup. Here, a new approach is presented, combining the characteristics of smart materials with the properties of nanoparticles, using negatively charged PNIPAM nanogels. Aqueous dispersions of these nanogels express no oscillatory structural forces in the initial state (20 °C), below the volume phase transition temperature (32 °C). Heating (60 °C) reduces the nanogel size and leads to a more negative ζ-potential, which triggers the onset of oscillatory structural forces.

Tunable depletion potentials driven by shape variation of surfactant micelles

Depletion interaction potentials between micron-sized colloidal particles are induced by nanometer-scale surfactant micelles composed of hexaethylene glycol monododecyl ether (C_{12}E_{6}), and they are measured by video microscopy. The strength and range of the depletion interaction is revealed to arise from variations in shape anisotropy of the surfactant micelles. This shape anisotropy increases with increasing sample temperature. By fitting the colloidal interaction potentials to theoretical models, we extract micelle length and shape anisotropy as a function of temperature. This work introduces shape anisotropy tuning as a means to control interparticle interactions in colloidal suspensions, and it shows how the interparticle depletion potentials of micron-scale objects can be employed to probe the shape and size of surrounding macromolecules at the nanoscale.

Phase behaviour of a model colloid-polymer mixture at low colloid concentration

The phase behaviour of a colloid-polymer mixture is studied at very low colloid concentrations (below 0.5%). The size ratio between the polymer and the colloidal particles is around 0.09, so that the colloids experience short-range attractions. At these low volume fractions, fluid-crystal coexistence is found at moderate polymer concentrations and the kinetics of crystallization are analyzed by turbidimetry. At higher polymer concentrations, clustering of the particles occurs, but some of them remain in a diluted, gas, phase composed mainly of single particles. These states do not correspond to vapor-liquid coexistence, as shown by studying the density of the gas phase. Strong interactions induce flocculation of the system, producing fractal aggregates with dimension df ≈ 1.8, compatible with diffusion limited cluster aggregation (DLCA). These results are discussed in connection with the phase diagram of colloid-polymer mixtures, obtained at much higher colloid concentrations. For low colloid volume fractions, below ∼0.05%, no clustering of the particles is observed for any polymer concentration.

Aggregate morphology and aggregation rate constants for silica dispersions in the presence of salt and polyelectrolyte

It is shown that the addition, over suitable concentration ranges, of mixtures of (nonadsorbing) sodium poly(styrene sulfonate) and potassium chloride, to dispersions of silica particles in water, can lead to very large changes in the sediment height of the resulting aggregates, reflecting similarly large changes in particle packing density within the aggregates. It can also lead to aggregation rates which are considerably faster than the diffusion-controlled rates (by as much as a factor of 2.5), although this enhancement is reduced as the dispersion particle concentration is reduced.

Microscopic structure and elasticity of weakly aggregated colloidal gels

We directly probe the microscopic structure, connectivity, and elasticity of colloidal gels using confocal microscopy. We show that the gel is a random network of one-dimensional chains of particles. By measuring thermal fluctuations, we determine the effective spring constant between pairs of particles as a function of separation; this is in agreement with the theory for fractal chains. Long-range attractions between particles lead to freely rotating bonds, and the gel is stabilized by multiple connections among the chains. By contrast, short-range attractions lead to bonds that resist bending, with dramatically suppressed formation of loops of particles.

Rheological behavior and structural interpretation of waxy crude oil gels

A waxy crude oil which gels below a threshold temperature has been investigated under static and dynamic conditions, using a combination of rheological methods, optical microscopy, and DSC. Particular attention is given in this work to the influence of the mechanical history on gel strength and to describing the time-dependent rheological behavior. The gels display a strong dependence of the yield stress and moduli on the shear history, cooling rate, and stress loading rate. Of particular interest is the partial recovery of the gel structure after application of small stress or strain (much smaller than the critical values needed for flow onset) during cooling, which can be used to reduce the ultimate strength of the crude oil gel formed below the pour point. A second focus of this study is to further develop the physical interpretation of the mechanism by which wax crystallization produces gelation. Gelation of the waxy crude oil studied is suggested to be the result of the association between wax crystals, which produces an extended network structure, and it is shown that the system displays features common to attractive colloidal gels, for one of which, fumed silica (Aerosil 200) in paraffin oil, rheological data are reported. The colloidal gel model provides a simple and economical basis for explaining the response of the gelled oil to various mechanical perturbations and constitutes a fruitful basis from which to develop technologies for controlling the gelation phenomenon, as suggested by the rheological results reported.

Particle-induced phase separation in mixed polymer solutions

The influence of added colloidal particles on the phase separation of mixed aqueous polymer solutions is investigated. Two types of particles (polystyrene latex or silica) and different combinations of segregating polymers (dextran of varying molar mass combined with poly(ethylene oxide) (PEO) of varying molar mass, or Ucon, a copolymer of ethylene oxide and propylene oxide) were used. All systems displayed particle-induced instability effects, but the extent of the effect varied strongly between the various combinations and with the amount of added salt. Very large instability effects were seen in certain mixtures. Two mechanisms, both relying on the adsorption of at least one of the polymers to the particle surface, seem to operate. Close to the cloud-point curve of the particle-free polymer1/polymer2/water mixture, adsorption of PEO or Ucon to the particles gives rise to a capillary-induced phase separation. Close to the dextran/water axis of the phase diagram, the adsorbing polymer gives rise to a surface modification of the particles, which then interacts repulsively with the surrounding dextran solution.

Self-diffusion in dilute colloidal suspensions with attractive potential interactions

The colloidal short-time self-diffusivity D(s)(s)(phi) is significantly retarded relative to hard sphere suspensions for the case of interparticle potential interactions induced by a nonadsorbing polymer. A comparison of diffusing wave spectroscopy measurements with direct calculations of D(s)(s)(phi) demonstrates that depletion effects on structure explain the observed retardation. We show that coexistence boundaries place unexpectedly severe constraints on the amount of D(s)(s)(phi) retardation possible for stable suspensions. The measured retardation is demonstrated to be an indicator of suspension metastability.

Viscosity effect on the structural compactness of latex flocs formed under weak depletion attractions

Dilute aqueous dispersions of colloidal polystyrene latex spheres were flocculated by adding a nonadsorbing polymer sample, poly(acrylic acid). The structural compactness of the flocs thus formed was characterized in terms of their mass fractal dimension using the small-angle static light scattering technique. It was found that with low poly(acrylic acid) concentrations and thus weak depletion attraction forces, the dispersion medium viscosity had a marked effect on the floc structure. An increase in the viscosity led to formation of denser flocs. This was revealed in three sets of depletion flocculation experiments: (a) adjusting the background electrolyte concentration at a fixed level of poly(acrylic acid), (b) using water and 30% (w/w) glycerol as the respective solvents, and (c) inducing latex flocculation with two poly(acrylic acids) of different molecular weights at the respective critical polyacid concentrations. Direct force measurements were made with atomic force microscopy to isolate the influence of viscosity on floc structure from that of interparticle interaction energies. We conclude that the formation of denser flocs with increasing medium viscosity can be attributed to the reduced diffusivity of particles in the solution. The latter resulted in an enhanced rate of floc restructuring (through relaxation of attached particles) relative to floc growth.

Measuring forces with the AFM: polymeric surfaces in liquids

This review links together for the first time both the practicalities of force measurement and the work carried out to date on force detection between polymeric surfaces in liquids using the atomic force microscope (AFM). Also included is some of the recent work that has been carried out between surfactant surfaces and biologically coated surfaces with the AFM. The emphasis in this review is on the practical issues involved with force measurement between these types of surfaces, and the similarities and irregularities between the observed types of forces measured. Comparison is made between AFM and surface force apparatus (SFA) measurements, as there is a much longer history of work with the latter. Results indicate that forces between the surfaces reviewed here are a complicated mixture of steric-type repulsion, conformational behaviour on separation and long-range attraction, which is often ascribed to 'hydrophobic' forces. The origin of this latter force remains uncertain, despite its almost ubiquitous appearance in force measurements with these types of surfaces.

The Effect of Molecular Weight of Nonadsorbing Polymer on the Structure of Depletion-Induced Flocs

This work explores the structural compactness of depletion-induced particle flocs with respect to the molecular weight of nonadsorbing polymer flocculants. Small-angle static light scattering was used to monitor the structural characteristics of the flocs, which were formed by the addition of nonadsorbing poly(acrylic acids) to a stable colloidal polystyrene latex dispersion. It was found that the floc mass fractal dimension, considered to be a measure of structural compactness, was dependent upon both the molecular weight and the concentration of the polyacid. In particular, reducing the molecular weight of the polymer at a fixed polyacid concentration resulted in higher mass fractal dimensions, despite the highly polydisperse nature of the polymer samples. This structural behavior was attributed to the lower particle sticking efficiencies upon collision. This reduced sticking ability is the result of the shallowing in the secondary potential energy well with decreasing polymer chain length, which was directly supported by atomic force microscopy data. Our results suggest that the formation of a shallower attraction well with a lower molecular weight nonadsorbing polymer is the result of the insufficiency of the increased osmotic pressure to counter-balance the short-ranged nature of the depletion interaction.

On techniques for the measurement of the mass fractal dimension of aggregates

A review is presented of a number of techniques available for the characterisation of the structure of aggregates formed from suspensions of sub-micron particles. Amongst the experimental techniques that have been commonly used are scattering (light, X-ray or neutron), settling and imaging and these are the focus of this work. The theoretical basis for the application of fractal geometry to characterisation of flocs and aggregates is followed by a discussion of the strengths and limitations of the above techniques. Of the scattering techniques available, light scattering provides the greatest potential for use as a tool for structure characterisation even though interpretation of the scattered intensity pattern is complicated by the strong interaction of light and matter. Restructuring further complicates the analysis. Although settling has long been used to characterise particle behaviour, the absence of an accurate permeability model limits the technique as a means of determining the porosity of fractal aggregates. However, it can be argued that the determination of fractal dimension is relatively unaffected. The strength of image analysis lies in its ability to provide a great deal of information about particle morphology and the weaknesses lie in the difficulties with image processing and sample size as this is a particle counting technique. There are very few papers which compare the fractal dimension measured by more than one technique. Light scattering potentially provides a useful tool for checking settling results. However, further work is required to develop proper models for aggregate permeability and flow-through effects.