Anisotropic viscoelastic phase separation in polydisperse hard rods leads to nonsticky gelation.

Spinodal demixing into two phases having very different viscosities leads to viscoelastic networks-i.e., gels-usually as a result of attractive particle interactions. Here, however, we demonstrate demixing in a colloidal system of polydisperse, rod-like clay particles that is driven by particle repulsions instead. One of the phases is a nematic liquid crystal with a highly anisotropic viscosity, allowing flow along the director, but suppressing it in other directions. This phase coexists with a dilute isotropic phase. Real-space analysis and molecular-dynamics simulations both reveal a long-lived network structure that is locally anisotropic, yet macroscopically isotropic. We show that our system exhibits the characteristics of colloidal gelation, leading to nonsticky gels.

Controlling the Rheology of Montmorillonite Stabilized Oil-in-Water Emulsions.

The rheology of hexadecane-in-water emulsions stabilized by montmorillonite platelets was investigated. In these systems excess particles form a network in the continuous phase which strongly dictates their rheological behavior. The emulsions were modified by the addition of NaCl and Na4P2O7 to the continuous phase at varying concentrations. Remarkably, changes of up to 3 orders of magnitude in elastic modulus and yield stress of the emulsions were achieved. The droplets retained long-term coalescence stability after the addition of NaCl or Na4P2O7 and even after the removal of the continuous phase network. The latter finding shows that the droplets are primarily stabilized by the formation of a solid barrier at the interface. These emulsions are therefore highly versatile formulation materials with an exceptional degree of stability and tunability.

Steady-state droplet size in montmorillonite stabilised emulsions.

The formation of hexadecane-in-water emulsions stabilised by montmorillonite platelets was studied. In this system the platelets form a monolayer around the droplets and the droplet size decreases with increasing platelet volume fraction. However, the number of platelets present exceeds that required for monolayer coverage. The kinetics of emulsification were investigated and coalescence of droplets during turbulent mixing was found to continue even after the droplets had reached their ultimate size. Non-spherical droplets, resulting from arrested coalescence, were not observed suggesting that particles may be desorbing from the interface during the turbulent flow. A kinetic model based on a competition between droplet break-up and coalescence, mediated by particle adsorption and desorption, reproduces experimental trends in droplet diameter. The model can be used to predict the most efficient formulation to minimise droplet diameters for given materials and mixing conditions and sheds light on the processes occurring during emulsification in this system.

Three-Phase Coexistence in Colloidal Rod-Plate Mixtures.

Aqueous suspensions of clay particles, such as montmorillonite (MMT) platelets and sepiolite (Sep) rods, tend to form gels at concentrations around 1 vol %. For Sep rods, adsorbing sodium polyacrylate to the surface allows for an isotropic-nematic phase separation to be seen instead. Here, MMT is added to such Sep suspensions, resulting in a complex phase behavior. Across a range of clay concentrations, separation into three phases is observed: a lower, nematic phase dominated by Sep rods, a MMT-rich middle layer, which is weakly birefringent and probably a gel, and a dilute top phase. Analysis of phase volumes suggests that the middle layer may contain as much as 6 vol % MMT.

Controlling Clusters of Colloidal Platelets: Effects of Edge and Face Surface Chemistries on the Behavior of Montmorillonite Suspensions.

The structural and rheological consequences of adsorbing pyrophosphate anions to the edges and polyetheramines to the faces of montmorillonite platelets in aqueous suspension were investigated. Oscillatory rheology and scattering experiments showed that the two surface treatments act in different regions of the phase diagram and that this can be attributed to modifications of local particle interactions resulting in changes to the behavior and morphology of platelet clusters. The polyetheramine was found to neutralize surface charge, reducing electrostatic repulsion between platelets and therefore allowing them to come into closer proximity. This reduces the effective volume fraction of the clusters and reverses jamming in low ionic strength arrested phases. Conversely, the adsorption of pyrophosphate was found to introduce a high concentration of negative charge to the particle edge, resisting the formation of bonded percolating gels at high ionic strength. The two separate surface chemistries can be applied in parallel with no adverse effects and thus have the potential to be applied to dual functionalization of two-dimensional colloids such as platelets. This has implications for finer formulation design where targeted rheology modification could be achieved by careful selection of chemistry at one surface accompanied by an additional function at the other.

Isotropic-nematic phase transition of polydisperse clay rods.

Rod-like colloidal particles are known to display an isotropic-nematic phase transition on increase of concentration, as predicted already by Onsager. Both natural clay particles and synthetic rods tend to be polydisperse, however, and the question arises how to allow for this in comparing experimental observations with theory. Experimental data for a wide range of samples (both from the literature and the new results) have been collated, with aspect ratios ranging from 14 to 35. As a characteristic, the concentration is taken where half of the sample volume is nematic. Experimental data agree well with predictions for monodisperse finite aspect ratio rods. However, compared to these predictions, the width of the transition (taken as the ratio of isotropic and nematic limiting concentrations) is noticeably broadened. Still, in most cases, the transition can be characterised by a linear increase of the nematic phase volume with sample concentration. The transition width is in broad agreement with theoretical predictions for infinitely thin rods.

Competition between polymers for adsorption on silica: a solvent relaxation NMR and small-angle neutron scattering study.

The competition between poly(vinylpyrrolidone) and poly(ethylene oxide) for adsorption at the silica surface was studied by solvent relaxation nuclear magnetic resonance and small-angle neutron scattering. The additive nature of the NMR relaxation rate enhancement was used to observe changes in the train layer when the two polymers were in direct competition for an increasing weight percentage of silica. PVP is shown to displace preadsorbed PEO from the particle surface, and this was observed for a range of PVP molecular weights. SANS measurements were found to give detailed information on the adsorption of the polymers in the separate systems; however, only qualitative information on the adsorption of the polymers could be obtained from the mixed samples. At a total polymer concentration of 0.4% w/v with 1.1% w/v silica, the SANS data were consistent with PVP adsorbing at the surface and dPEO remaining in solution, in agreement with the NMR data.

Microcapsules composed of cross-linked organoclay.

Polyelectrolyte-modified montmorillonite particles were used to stabilize oil-in-water Pickering emulsions, which were then bound together by an oil-soluble cross-linker to obtain microcapsules. It was determined how the morphology and rigidity of the microcapsules changed as polyelectrolyte and cross-linker concentrations were varied. Well-defined microcapsules could be formed by using a moderate concentration of polyelectrolyte, and the higher the cross-linker concentration, the more rigid the microcapsules. Dried microcapsules were observed using SEM, and it was shown that the clay platelets lie flat next to each other on the microcapsule surface, forming an armor-like structure.

Colloidal particles in competition for stabilizer: a solvent relaxation NMR study of polymer adsorption and desorption.

The competitive adsorption of poly(vinylpyrrolidone) onto silica and alumina-modified silica particles was studied using solvent relaxation nuclear magnetic resonance. The additive nature of the measured relaxation rate enabled predictions to be made of the relaxation rate in different polymer adsorption scenarios. Preferential adsorption of the poly(vinylpyrrolidone) onto the unmodified silica particles occurred when there was insufficient polymer in the system to coat the entire available surface area. Desorption was also found to occur when the polymer was initially adsorbed upon the alumina-modified particle and silica particles were added.

Two-stage phase separation in ternary colloid-polymer mixtures.

Whilst binary colloid-polymer mixtures have been studied in detail over the past few decades, here the first results are presented on a ternary mixture involving two particle sizes. Novel and unusual phase separation kinetics are found, with a liquid phase separating from an aggregate phase.

Identification of ß-carotene oxidation products produced by bleaching clay using UPLC-ESI-MS/MS.

The removal of plant pigments such as ß-carotene is an aspect of vegetable oil processing often desired by the food and pharmaceutical industries. Adsorption of ß-carotene to acid-activated clay (AAC) is a well-established method for purification. Despite this, the removal mechanism of ß-carotene is not well understood. UPLC-MS/MS analysis of surface compounds extracted from ß-carotene-AAC (BC-AAC) complexes show that AAC acts as an oxidiser. Oxidation products detected included canthaxanthin and 3',4'-didehydro-ß-caroten-4-one. AAC had surface water exchanged with an 18O labelled water and was then exposed to ß-carotene. Carotenoids labelled with 18O were produced from this reaction, suggesting surface water is necessary for ß-carotene removal.

Adsorption of polyetheramines on montmorillonite at high pH.

Adsorption of a series of polyetheramines on montmorillonite in aqueous suspension was investigated by a range of methods: elemental analysis, atomic absorption spectroscopy, measurement of pH, conductivity and electrophoretic mobility, and small-angle X-ray scattering. Adsorption proceeds through an ion exchange mechanism. The maximum surface coverage attained is equivalent to about 40% of the cationic exchange capacity of the clay. Adsorption of the poly(oxypropylene) block adjacent to the amine group onto the clay surface may contribute to this. Surprisingly the adsorption takes place at pH conditions well above the pK(a) of the amine surfactants, where they are not protonated in the bulk solution. The surface coverage as a function of molar mass broadly agrees with predictions assuming adsorbed polymers adopt a densely packed mushroom configuration at the clay surface.

The phase behavior of dispersions of silica particles in mixtures of polystyrene and dimethylformamide.

Depletion-induced phase separation in mixtures of charged silica particles and nonadsorbing polymer near theta conditions (polystyrene in dimethylformamide) was studied. The colloid-polymer size ratio q was varied through the particle size and the electrical double layer thickness (kappa(-1)) through addition of lithium chloride (LiCl). The dependence of the phase boundaries, and the nature of the separated phases, on q and kappa is reported and is found to be in good agreement with recent theoretical predictions (Gogelein, C.; Tuinier, R. Eur. Phys. J., E 2008, 27, 171-184).

Scaling the structure factors of protein limit colloid-polymer mixtures.

The scaling of the phase boundaries and structure factors of protein limit colloid-polymer mixtures has been investigated through the addition of large nonadsorbing polymer chains to a solution of small microemulsion droplets. The colloid-polymer size ratio has been varied between 10 and 16 by changing the microemulsion droplet size; the phase boundaries were shown previously to observe theoretical scaling relations very well [Langmuir 2009, 25 (7), 3944-3952]. These thermodynamic scaling relations are now shown to also hold extremely well for the individual and cross-term partial structure factors. The structure factors for systems with different size ratios occupying the same point in scaled phase space show extremely good agreement. The properties and stability of these mixtures are governed by the polymer mesh.

Competing active and passive interactions drive amoebalike crystallites and ordered bands in active colloids.

Swimmers and self-propelled particles are physical models for the collective behavior and motility of a wide variety of living systems, such as bacteria colonies, bird flocks, and fish schools. Such artificial active materials are amenable to physical models which reveal the microscopic mechanisms underlying the collective behavior. Here we study colloids in a dc electric field. Our quasi-two-dimensional system of electrically driven particles exhibits a rich and exotic phase behavior exhibiting passive crystallites, motile crystallites, an active gas, and banding. Amongst these are two mesophases, reminiscent of systems with competing interactions. At low field strengths activity suppresses demixing, leading to motile crystallites. Meanwhile, at high field strengths, activity drives partial demixing to traveling bands. We parametrize a particulate simulation model which reproduces the experimentally observed phases.

Effect of suspended clay particles on isotropic-nematic phase transition of liquid crystal.

Sterically stabilised nano-platelets were prepared by treating montmorillonite clay with both a surfactant and a polymeric stabiliser. These nano-platelets formed stable suspensions in a thermotropic liquid crystal, 5CB. This is in marked contrast with previous work on preparing liquid crystal suspensions of either spheres, which formed gels on cooling the solvent into the nematic phase, or clay platelets stabilised only with low molecular weight surfactant, which tended to aggregate. In the isotropic state of the liquid crystal, static light scattering showed that the clay nano-platelets were freely suspended, and no aggregation was detected even after repeated temperature cycling into and out of the nematic phase. Small-angle X-ray scattering showed that the clay was delaminated nearly completely in the liquid crystal, with some stacks of a few clay nano-platelets having formed. Differential scanning calorimetry of the liquid crystal/clay suspensions showed a small but non-monotonic shift of the transition temperature compared to that of the pure liquid crystal. This behaviour is similar to that of liquid crystal confined in porous media, with an initial increase of the transition temperature on adding clay being ascribed to the effect of surface anchoring facilitating the formation of the nematic phase, whereas a decrease at higher clay concentrations (or equivalently, for smaller pores) is ascribed to confinement effects frustrating the formation of the nematic phase. This interpretation is supported by polarising light microscopy which showed the nematic domain size becoming smaller on increasing the clay concentration.

Colloid-polymer mixtures in the protein limit.

This review discusses the structure and phase behaviour of mixtures of colloidal particles and non-adsorbing polymers in the protein limit of large polymers and small colloids. The vast majority of work on colloid-polymer mixtures has been concerned with the colloid limit of large colloidal particles and small polymer chains. In this regime, the diameter of the colloidal particles, , is larger than the characteristic size of the polymer-taken as twice their radius of gyration, . The opposite limit, of size ratios , is called the protein limit due to the common practice of adding polymer to protein solutions in order to aid protein crystallisation. Theoretical predictions for systems in the protein limit are considered briefly and then the main focus is on recent experimental studies of mixtures in the protein limit.

Stabilisation of water-in-water emulsions by montmorillonite platelets.

The formation of water-in-water emulsions from the aqueous two phase system containing polyethylene oxide and pullulan, stabilised by montmorillonite platelets, was investigated. A novel approach of preparing the emulsions at non-equilibrium polymer concentrations was successfully utilised to control viscosity during mixing and allow the use of low energy emulsification methods. Polyethylene oxide adsorbed to the platelets much more strongly than pullulan favouring the formation of pullulan-in-polyethylene oxide emulsions which remained stable for a period of weeks. Polarising microscopy and small angle light scattering were used to show that droplets were most likely stabilised against coalescence by the adsorption of randomly oriented aggregates of platelets and against creaming by the formation of chains of droplets bridged by the adsorbed aggregates. Montmorillonite platelets were therefore shown to stabilise water-in-water emulsions and their preference for emulsion type was driven by the adsorption of the polymers to the particle surface.

The role of initiator on the dispersibility of polystyrene microgels in non-aqueous solvents.

Non-aqueous microgel particles are commonly synthesised in water, dried, and then redispersed in non-aqueous solvents. An important factor to consider when synthesising such particles is the initiator, which can determine how well the particles disperse in solvents. Polystyrene microgel particles were made with three different initiators. When a neutral, oil soluble initiator (azobisisobutyronitrile) was used the particles dispersed in toluene as well as cyclohexane and decalin. In contrast, anionic, water-soluble initiators (potassium persulfate or azobis(4-cyanovaleric acid)) created particles that only redispersed in toluene and not the other two solvents. Of the three considered, toluene is the best solvent for polystyrene and also has the highest polarizability, making it most effective at redispersing particles with polar or ionisable functional groups. Zeta potential and conductivity measurements, however, did not detect a direct relationship between particle charging and redispersibility. Oil soluble initiators result in "inside out" polymerisation where the initiator groups are buried inside the growing particle, whereas water-soluble initiators result in "outside in" polymerisation, with the polar initiator groups residing on the particle surface. By tailoring the ratio between water and oil soluble initiators, it may be possible to synthesise microgel particles with uniform or designed charge profiles from the core to the surface.

Isotropic-nematic phase transition in aqueous sepiolite suspensions.

Aqueous suspensions of sepiolite clay rods in water tend to form gels on increase of concentration. Here it is shown how addition of a small amount (0.1% of the clay mass) of a common stabiliser for clay suspensions, sodium polyacrylate, can allow the observation of an isotropic-nematic liquid crystal phase transition. This transition was found to move to higher clay concentrations upon adding NaCl, with samples containing 10(-3) M salt or above only displaying a gel phase. Even samples that initially formed liquid crystals had a tendency to form gels after several weeks, possibly due to Mg(2+) ions leaching from the clay mineral.