Nonaqueous suspensions of laponite and montmorillonite

A synthetic and transparent clay removes Microcystis aeruginosa efficiently

Clay-algae flocculation is a promising method to remove harmful algal blooms (HABs) in aquatic ecosystems. Many HAB-generating species, such as Microcystis aeruginosa (M. aeruginosa), a common species in lakes, produce toxins and harm the environment, human health, and the economy. Natural clays, such as bentonite and kaolinite, and modification of these clays have been applied to mitigate HABs by forming large aggregates and settling down. In this study, we aim to examine the impact of laponite, a commercially available smectite clay that is synthetic, transparent, compatible with human tissues, and degradable, on removing HABs. We compare the cell removal efficiencies (RE) of laponite, two natural clays, and their polyaluminum chloride (PAC)-modified versions through clay-algae flocculation experiments. Our results show that the optimum concentrations of laponite, bentonite, kaolinite, PAC-modified bentonite, and PAC-modified kaolinite to remove 80 % of the M. aeruginosa cells from the water column are 0.05 g/L, 2 g/L, 4 g/L, 2 g/L and 0.3 g/L respectively. Therefore, to achieve the same cell removal efficiency, the amount of laponite needed is 40 to 80 times less than bentonite and kaolinite, and 6 times less than PAC-modified kaolinite. We demonstrate that the superior performance of laponite clay is because of its smaller particle size, which increases the encounter rate between cells and clay particles. Furthermore, experiments using water samples from Powderhorn Lake confirmed laponite's effectiveness in mitigating HABs. Our price analysis also suggests that this commercially-available clay, laponite, can be used in the field at a relatively low cost.

Temperature-Insensitive Nonpolar Suspensions of Polyoxyethylene Alkyl Ether-Grafted Silica Nanoparticles

Nonpolar suspensions of organically modified particles exhibit a strong temperature sensitivity owing to the high-temperature-induced desorption/decomposition and the low-temperature-induced disorder/order conformational transition of the modifiers. This strong temperature sensitivity limits their applications, such as lubricants and oil-based drilling fluids, which require the suspensions to operate over a wide temperature range (e.g., 0-200 °C). We hypothesize that the introduction of a flexible ethylene oxide (EO) chain into the modifiers can disrupt the low-temperature-induced ordered conformation to improve the stability of the nonpolar suspensions. In this article, nonpolar suspensions with temperature insensitivity in the range of 5-160 °C were obtained via the covalent modification of silica NPs and the introduction of EO chains into the modifier molecules. Here, octadecyl-grafted silica NPs (C18-SiO2) and polyoxyethylene alkyl ether-grafted silica NPs (AEOn-SiO2) were synthesized and subsequently dispersed in mineral oil. The rheological properties of each suspension at different temperatures were evaluated, and the thermal stability of AEOn-SiO2 in mineral oil was investigated along with the conformational changes of the grafted chains. In the temperature range of 5-160 °C, the apparent viscosity and gel strength of the C18-SiO2 suspension changed dramatically, whereas the AEOn-SiO2 suspensions exhibited constant rheological properties over this temperature range. This temperature insensitivity of AEOn-SiO2 suspensions is attributed to the excellent thermal stability of AEOn-SiO2 in mineral oil and the disordered conformation of the EO chains upon cooling. This study provides a novel approach to preparing temperature-insensitive nonpolar suspensions, which have potential applications in the petroleum and lubricant industries.

Rheological modification of partially oxidised cellulose nanofibril gels with inorganic clays

This study aimed to quantify the influence of clays and partially oxidised cellulose nanofibrils (OCNF) on gelation as well as characterise their physical and chemical interactions. Mixtures of Laponite and montmorillonite clays with OCNF form shear-thinning gels that are more viscous across the entire shear range than OCNF on its own. Viscosity and other rheological properties can be fine-tuned using different types of clay at different concentrations (0.5-2 wt%). Laponite particles are an order of magnitude smaller than those of montmorillonite (radii of 150 Å compared to 2000 Å) and are therefore able to facilitate networking of the cellulose fibrils, resulting in stronger effects on rheological properties including greater viscosity. This work presents a mechanism for modifying rheological properties using renewable and environmentally-friendly nanocellulose and clays which could be used in a variety of industrial products including home and personal care formulations.

Modified montmorillonite clay microparticles for stable oil-in-seawater emulsions

Environmentally benign clay particles are of great interest for the stabilization of Pickering emulsions. Dodecane-in-synthetic seawater (SSW) emulsions formed with montmorillonite (MMT) clay microparticles modified with bis(2-hydroxyethyl)oleylamine were stable against coalescence, even at clay concentrations down to 0.1% w/v. Remarkably, as little as 0.001% w/v surfactant lowered the hydrophilicity of the clay to a sufficient level for stabilization of oil-in-SSW emulsions. The favorable effect of SSW on droplet size reduction and emulsion stability enhancement is hypothesized to be due to reduced electrostatic repulsion between adsorbed clay particles and a consequent increase in the continuous phase (an aqueous clay suspension) viscosity. Water/oil (W/O) emulsions were inverted to O/W either by decreasing the mass ratio of surfactant-to-clay (transitional inversion) or by increasing the water volume fraction (catastrophic inversion). For both types of emulsions, coalescence was minimal and the sedimentation or creaming was highly correlated with the droplet size. For catastrophic inversions, the droplet size of the emulsions was smaller in the case of the preferred curvature. Suspensions of concentrated clay in oil dispersions in the presence of surfactant were stable against settling. The mass transfer pathways during emulsification of oil containing the clay particles were analyzed on the droplet size/stability phase diagrams to provide insight for the design of dispersant systems for remediating surface and subsurface oceanic oil spills.

Modifying the structure and flow behaviour of aqueous montmorillonite suspensions with surfactant

Colloidal suspensions of plate-like particles undergo a variety of phase transitions. The predicted isotropic/nematic transition is often pre-empted by a sol/gel transition, especially in suspensions of the most commonly used natural swelling clay montmorillonite (MMT). A number of factors, including charge interactions, flexibility and salt concentration, may contribute to this competition. In this study, the effect of surfactant adsorption on suspensions of MMT was studied using rheology, small-angle X-ray scattering, static light scattering and optical microscopy. The addition of a polyetheramine surfactant reduced the moduli of the system and shifted the sol/gel transition to a much higher clay concentration, compared with suspensions of bare clay particles. Yet, scattering data revealed no change in suspension structure on length scales up to around a micrometre. Primary aggregates remain at this length scale and no nematic phase is formed. There is, however, a change in structure at large length scales (of order 20 μm) where light scattering indicates the presence of string-like aggregates that disappear on addition of surfactant. Microscope images of dried suspensions also revealed a string-like structure. The dried strings show strong birefringence and may consist of concentric cylinders, self-assembled from clay sheets.

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.

Sol-gel transition in dispersions of layered double-hydroxide nanosheets

Surfactant-intercalated layered double-hydroxide solid Mg-Al LDH-dodecyl sulfate (DDS) undergoes rapid and facile delamination to its ultimate constituent, single sheets of nanometer thickness and micrometer size, in a nonpolar solvent such as toluene to form stable dispersions. The delaminated nanosheets are electrically neutral because the surfactant chains remain tethered to the inorganic layer even on exfoliation. With increasing volume fraction of the solid, the dispersion transforms from a free-flowing sol to a solidlike gel. Here we have investigated the sol-gel transition in dispersions of the hydrophobically modified Mg-Al LDH-DDS in toluene by rheology, SAXS, and (1)H NMR measurements. The rheo-SAXS measurements show that the sharp rise in the viscosity of the dispersion during gel formation is a consequence of a tactoidal microstructure formed by the stacking of the nanosheets with an intersheet separation of 3.92 nm. The origin and nature of the attractive forces that lead to the formation of the tactoidal structure were obtained from 1D and 2D (1)H NMR measurements that provided direct evidence of the association of the toluene solvent molecules with the terminal methyl of the tethered DDS surfactant chains. Gel formation is a consequence of the attractive dispersive interactions of toluene molecules with the tails of DDS chains anchored to opposing Mg-Al LDH sheets. The toluene solvent molecules function as molecular "glue" holding the nanosheets within the tactoidal microstructure together. Our study shows how rheology, SAXS, and NMR measurements complement each other to provide a molecular-level description of the sol-gel transition in dispersions of a hydrophobically modified layered double hydroxide.

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.

Bulk phase behavior of binary hard platelet mixtures from density functional theory

We investigate isotropic-isotropic, isotropic-nematic, and nematic-nematic phase coexistence in binary mixtures of circular platelets with vanishing thickness, continuous rotational degrees of freedom, and radial size ratios lambda up to 5. A fundamental measure density functional theory, previously used for the one-component model, is presented and results are compared against those from Onsager theory as a benchmark. For lambda<or=1.7 the system displays isotropic-nematic phase coexistence with a widening of the biphasic region for increasing values of lambda . For size ratios lambda>or=2, we find that demixing into two nematic states becomes stable and an isotropic-nematic-nematic triple point can occur. Fundamental measure theory gives a smaller isotropic-nematic biphasic region than Onsager theory and locates the transition at lower densities. Furthermore, nematic-nematic demixing occurs over a larger range of compositions at a given value of lambda than found in Onsager theory. Both theories predict the same topologies of the phase diagrams. The partial nematic order parameters vary strongly with composition and indicate that the larger particles are more strongly ordered than the smaller particles.

Dichroism in dye-doped colloidal liquid crystals

Nematic liquid crystals were obtained in sterically stabilized suspensions of rodlike particles of sepiolite clay, with an average length up to 900 nm and aspect ratio up to 40. In agreement with computer simulations for hard spherocylinders, the isotropic-nematic transition shifted to lower volume fractions with increasing aspect ratio. However, the coexistence gap was broadened noticeably due to particle polydispersity. The sepiolite crystal structure includes channels filled with zeolitic water, which can be replaced by indigo dye molecules. The indigo molecules are constrained inside the zeolitic channels to be aligned along the long axes of the rods. As a result, the colloidal nematic phase showed a marked dichroism, with an order parameter up to 0.5 for magnetically aligned samples, similar to typical values for dye-doped thermotropic liquid crystals.

Edge-modification of laponite with dimethyl-octylmethoxysilane

This study examines the edge modification of laponite, with a monoalkoxy silane, dimethyl-octylmethoxysilane. The influence of ultrasonics, aging time and silane concentration on the resultant materials is examined. The silylated clays are characterized by XRD, IES, TGA, and Si NMR. The amount of grafted silane was increased by increasing the ratio of silane to clay, sonication of the reaction mixture and ageing the reaction mixture for no more than 24 h to avoid removal of grafted silane due to equilibrium effects.

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.

Nonaqueous suspensions of surface-modified kaolin

A range of different stabilizers have been used to render natural kaolin clay particles hydrophobic and dispersible in nonpolar solvents such as heptane. Both silanol and aluminol groups are known to be present at the kaolin surface. Use of a Hammett indicator showed that silanes would not neutralize the acidic aluminol sites, whereas amines would neutralize these sites. Both types of stabilizer adsorbed chemically onto the clay. In addition, a combined silane + amine treatment and a polyisobutylene-based stabilizer with a succinimide/amine head group (SAP230) were also considered. Both would neutralize the acid sites. The final sediment density after settling under gravity was used to gauge suspension stability, which varied with the kaolin surface treatment as silanes < amines < silane + amine < SAP230. This behavior was very similar for suspensions in heptane and in a higher molecular weight branched alkane, polydecene. This trend of increasing stability correlated very well with an increase in surface coverage of the stabilizing moieties, a decrease in particle size found using small-angle light scattering, and a decrease in Bingham yield stress obtained by fitting rheological data.

Isotropic-nematic phase transition of nonaqueous suspensions of natural clay rods

A novel model system for studying the behavior of hard colloidal rods is presented, consisting of sterically stabilized particles of natural sepiolite clay. Electron microscopy and scattering results confirmed that the organophilic clay particles were individual, rigid rods when dispersed in organic solvents. With a length-to-diameter ratio of approximately 27, the particles showed nematic ordering for volume fractions phi > 0.06. Polarizing microscopy revealed that the phase separation process involved nucleation, growth, and coalescence of nematic domains. The phase volumes and particle concentrations in the coexisting phases were determined. The dependence of these quantities on the total concentration of the suspension agrees well with Onsager's [Ann. N. Y. Acad. Sci. 51, 627 (1949)] isotropic-nematic phase transition theory extended to bidisperse and polydisperse rod systems, and with previous experimental results for rigid rodlike particles. Particle size distributions were obtained by analyzing transmission electron microscopy images. A significant fractionation with respect to rod length (but not diameter) was observed in the coexisting isotropic and nematic phases. The relative polydispersity of both daughter phases was distinctly smaller than that of the parent suspension. The phase behavior of these daughter fractions agrees well with the predictions for hard spherocylinders of corresponding aspect ratios. An isotropic-nematic-nematic phase equilibrium was seen to develop in phase separated samples after 1 month standing and is ascribed to the effect of polydispersity and possibly gravity. The second nematic phase appearing is dominated by very long rods.

Density functional theory for colloidal mixtures of hard platelets, rods, and spheres

A geometry-based density-functional theory is presented for mixtures of hard spheres, hard needles, and hard platelets; both the needles and platelets are taken to be of vanishing thickness. Geometrical weight functions that are characteristic for each species are given, and it is shown how convolutions of pairs of weight functions recover each Mayer bond of the ternary mixture and hence ensure the correct second virial expansion of the excess free-energy functional. The case of sphere-platelet overlap relies on the same approximation as does Rosenfeld's functional for strictly two-dimensional hard disks. We explicitly control contributions to the excess free energy that are of third order in density. Analytic expressions relevant for the application of the theory to states with planar translational and cylindrical rotational symmetry--e.g., to describe behavior at planar smooth walls--are given. For binary sphere-platelet mixtures, in the appropriate limit of small platelet densities, the theory differs from that used in a recent treatment [L. Harnau and S. Dietrich, Phys. Rev. E 71, 011504 (2004)]. As a test case of our approach we consider the isotropic-nematic bulk transition of pure hard platelets, which we find to be weakly first order, with values for the coexistence densities and the nematic order parameter that compare well with simulation results.

Effective interaction of charged platelets in aqueous solution: investigations of colloid laponite suspensions by static light scattering and small-angle x-ray scattering

We study dilute aqueous solutions of charged disklike mineral particles (laponite) by a combination of static light scattering (SLS) and small-angle x-ray scattering (SAXS). Laponite solutions are known to form gels above a certain critical concentration that must be described as nonequilibrium states. Here we focus on the investigation by SLS and SAXS at concentrations below gelation (c<0.016 g/L) and at low concentrations of added salt (0.001M and 0.005M). Thus, we have obtained the scattering function of single Laponite platelets as well as the structure factor describing their interaction at finite concentration. A detailed analysis of the combined sets of data proves that the solutions are in a well-defined equilibrium state. Moreover, this analysis demonstrates the internal consistency and accuracy of the scattering functions obtained at finite concentrations. We find that laponite particles interact through an effective pair potential that is attractive on short range but repulsive on longer range. This finding demonstrates that Laponite solutions exhibit only a limited stability at the concentration of added salt used herein. Raising the ionic strength to 0.005M already leads to slow flocculation as is evidenced from the enhanced scattering intensity at smallest scattering angles. All data strongly suggest that the gelation occurring at higher concentration is related to aggregation.

Analysis of the Degree of Nematic Ordering within Dense Aqueous Dispersions of Charged Anisotropic Colloids by 23Na NMR Spectroscopy

Aqueous dispersions of Laponite, a synthetic clay neutralized by sodium counterions, are used as a model of charged anisotropic colloids to probe the influence of the shape of the particle on their organization within a macroscopic nematic phase. Because of the large fraction of condensed sodium counterions in the vicinity of the clay particle, (23)Na NMR is a sensitive probe of the nematic ordering of the clay dispersions. We used line shape analysis of the (23)Na NMR spectra and measurements of the Hahn echo attenuation to quantify the degree of alignment of the individual clay particles along a single nematic director. As justified by simple dynamical simulations of the interplay between the sodium quadrupolar relaxation and its diffusion through the porous network limited by the surface of the clay particles, we probe the degree of ordering within these clay nematic dispersions by measuring the variation of the apparent (23)Na NMR relaxation rates as a function of the macroscopic orientation of the clay dispersion within the magnetic field.