Laponite-Based Nanomaterials for Drug Delivery

Laponite is a clay-based material composed of synthetic disk-shaped crystalline nanoparticles with highly ionic, large surface area. These characteristics enable the intercalation and dissolution of biomolecules in Laponite-based drug delivery systems. Furthermore, Laponite's innate physicochemical properties and architecture enable the development of tunable pH-responsive drug delivery systems. Laponite's coagulation capacity and cation exchangeability determine its exchange capabilities, drug encapsulation efficiency, and release profile. These parameters are exploited to design highly controlled and efficacious drug delivery platforms for sustained drug release. In this review, they provide an overview of how to design efficient delivery of therapeutics by leveraging the properties and specific interactions of various Laponite-polymer composites and drug moieties.

Impacts of negatively charged colloidal clay particles on photoisomerization of both anionic and cationic azobenzene molecules

Aqueous clay colloids influence the photoisomerization kinetics of both of the anionic and cationic azobenzene molecules although the guest species has been limited to cationic or polar molecules because of the intrinsic negative electric charges of the clay particles.

Hierarchical self-assembly, spongy architecture, liquid crystalline behaviour and phase diagram of Laponite nanoplatelets in alcohol-water binary solvents

Hydrophobicity and solvation of different charged species are among the various key factors that regulate the self-assembly of colloids, and macromolecules in their suspensions. In this paper, we demonstrate a method to tune the interaction potential and the resulting phase behaviour and microstructure of the states that form by using a combination of Laponite nanoplatelets and alcohols in water. This allows us to exquisitely control the self-assembly process of Laponite nanoplatelets. A new class of soft materials, called nanoclay-organogels, is studied systematically for their aging behaviour, microscopic structure and mechanical properties. Real space imaging techniques depicted spongy architecture with nano and micron size pores inside the gel matrix indicating the hierarchical self-assembly of the nanoplatelets in the aqueous solutions of polar organics. We have extensively examined the dispersion stability, aggregation, gelation and liquid crystalline behaviour of Laponite nanoplatelets in different alcohol (methanol, ethanol, 1-proponaol and ethylene glycol, and glycerol)-water binary solvents, thereby proposing a generalized description of nanoclay in alcoholic solutions, which is poorly probed and marginally understood in the literature. A phase diagram of Laponite® in alcohol solutions is proposed, which clearly demarcates regions of isotropic sol, unstable sol, isotropic gel, nematic/birefringent gel, glass, flocculated sedimentation and liquid crystalline structures.

2D Nanoclay for Biomedical Applications: Regenerative Medicine, Therapeutic Delivery, and Additive Manufacturing

Clay nanomaterials are an emerging class of 2D biomaterials of interest due to their atomically thin layered structure, charged characteristics, and well-defined composition. Synthetic nanoclays are plate-like polyions composed of simple or complex salts of silicic acids with a heterogeneous charge distribution and patchy interactions. Due to their biocompatible characteristics, unique shape, high surface-to-volume ratio, and charge, nanoclays are investigated for various biomedical applications. Here, a critical overview of the physical, chemical, and physiological interactions of nanoclay with biological moieties, including cells, proteins, and polymers, is provided. The state-of-the-art biomedical applications of 2D nanoclay in regenerative medicine, therapeutic delivery, and additive manufacturing are reviewed. In addition, recent developments that are shaping this emerging field are discussed and promising new research directions for 2D nanoclay-based biomaterials are identified.

Microstructure and Soft Glassy Dynamics of an Aqueous Laponite Dispersion

Synthetic hectorite clay Laponite RD/XLG is composed of disk-shaped nanoparticles that acquire dissimilar charges when suspended in an aqueous medium. Owing to their property to spontaneously self-assemble, Laponite is used as a rheology modifier in a variety of commercial water-based products. In particular, an aqueous dispersion of Laponite undergoes a liquid-to-solid transition at about 1 vol % concentration. The evolution of the physical properties as the dispersion transforms to the solid state is reminiscent of physical aging in molecular as well as colloidal glasses. The corresponding soft glassy dynamics of an aqueous Laponite dispersion, including the rheological behavior, has been extensively studied in the literature. In this feature article, we take an overview of recent advances in understanding soft glassy dynamics and various efforts taken to understand the peculiar rheological behavior. Furthermore, the continuously developing microstructure that is responsible for the eventual formation of a soft solid state that supports its own weight against gravity has also been a topic of intense debate and discussion. In particularly, extensive experimental and theoretical studies lead to two types of microstructures for this system: an attractive gel-like or a repulsive glass-like structure. We carefully examine and critically analyze the literature and propose a state (phase) diagram that suggests an aqueous Laponite dispersion to be present in an attractive gel state.

TEMPO-oxidised cellulose nanofibrils; probing the mechanisms of gelation via small angle X-ray scattering

The structure of dispersions of TEMPO-oxidised cellulose nanofibrils (OCNF), at various concentrations, in water and in NaCl aqueous solutions, was probed using small angle X-ray scattering (SAXS). OCNF are modelled as rod-like particles with an elliptical cross-section of 10 nm and a length greater than 100 nm. As OCNF concentration increases above 1.5 wt%, repulsive interactions between fibrils are evidenced, modelled by the interaction parameter νRPA > 0. This corresponds to gel-like behaviour, where G' > G'' and the storage modulus, G', shows weak frequency dependence. Hydrogels can also be formed at OCNF concentration of 1 wt% in 0.1 M NaCl(aq). SAXS patterns shows an increase of the intensity at low angle that is modelled by attractive interactions (νRPA < 0) between OCNF, arising from the screening of the surface charge of the fibrils. Results are supported by ζ potential and cryo-TEM measurements.

Nanoengineered Colloidal Inks for 3D Bioprinting

Nanoengineered hydrogels offer the potential to design shear-thinning bioinks for three-dimensional (3D) bioprinting. Here, we have synthesized colloidal bioinks composed of disk-shaped two-dimensional (2D) nanosilicates (Laponite) and poly(ethylene glycol) (PEG). The addition of Laponite reinforces the PEG network and increases viscosity, storage modulus, and network stability. PEG-Laponite hydrogels display shear-thinning and self-recovery characteristics due to rapid internal phase rearrangement. As a result, a range of complex patterns can be printed using PEG-Laponite bioinks. The 3D bioprinted structure has similar mechanical properties compared to the as-casted structure. In addition, encapsulated cells within the PEG-Laponite bioink show high viability after bioprinting. Overall, this study introduces a new class of PEG-Laponite colloidal inks for bioprinting and cell delivery.

Assembly of large area crack free clay porous films

A method for making inverse opal-like porous clay films that are crack-free over a large area (on the scale of square centimeters).

Clay-Alcohol-Water Dispersions: Anomalous Viscosity Changes Due to Network Formation of Clay Nanosheets Induced by Alcohol Clustering

Clay-alcohol-water ternary dispersions were compared with alcohol-water binary mixtures in terms of viscosity and optical absorbance. Aqueous clay dispersions to which lower alcohols (ethanol, 1-propanol, 2-propanol, and tert-butanol) were added exhibited significant viscosity anomalies (maxima) when the alcohol content was 30-55 wt %, as well as optical absorbance anomalies (maxima). The maximum viscosity (η_max) depended strongly on the clay content and varied between 300 and 8000 mPa·s, making it remarkably high compared with the viscosity anomalies (2 mPa·s) observed in alcohol-water binary mixtures. The alcohol content at η_max decreased as the hydrophobicity of the alcohol increased. The ternary dispersions with viscosity anomalies exhibited thixotropic behaviors. The effects of other hydrophilic solvents (glycols) and other kinds of clays were also clarified. Based on these findings and the average particle size changes, the viscosity anomalies in the ternary dispersions were explained by alcohol-clustering-induced network formation of the clay nanosheets. It was estimated that 0.9, 1.7, and 2.5 H₂O molecules per alcohol molecule were required to stabilize the ethanol, 2-propanol, and tert-butanol, respectively, in the clay-alcohol-water dispersions.

Printing small dots from large drops

Printing of droplets of pure solvents containing suspended solids typically leads to a ring stain due to convective transport of the particles toward the contact line during evaporation of the solvent. In mixtures of volatile solvents, recirculating cells driven by surface tension gradients are established that lead to migration of colloidal particles toward the center of the droplet. In favorable cases, a dense disk of particles forms with a diameter much smaller than that of the droplet. In the latter stages of drying, convective transport of the particles radially toward the contact line still occurs. Two strategies are described to fix the distribution of particles in a compact disk much smaller than the initial diameter of the drying droplet. First, a nanoparticulate clay is added to induce an evaporation-driven sol-gel transition that inhibits convective flow during the latter stages of drying. Second, a nonadsorbing polymer is added to induce depletion flocculation that restricts particle motion after the particles have been concentrated near the center of the droplet. The area of the resulting deposit can be as little as 10% of the footprint of the printed droplet.

Smectite clay--inorganic nanoparticle mixed suspensions: phase behaviour and rheology

Smectite clay minerals and their suspensions have long been of both great scientific and applications interest and continue to display a remarkable range of new and interesting behaviour. Recently there has been an increasing interest in the properties of mixed suspensions of such clays with nanoparticles of different size, shape and charge. This review aims to summarize the current status of research in this area focusing on phase behaviour and rheological properties. We will emphasize the rich range of data that has emerged for these systems and the challenges they present for future investigations. The review starts with a brief overview of the behaviour and current understanding of pure smectite clays and their suspensions. We then cover the work on smectite clay-inorganic nanoparticle mixed suspensions according to the shape and charge of the nanoparticles - spheres, rods and plates either positively or negatively charged. We conclude with a summary of the overarching trends that emerge from these studies and indicate where gaps in our understanding need further research for better understanding the underlying chemistry and physics.

Control of the particle distribution in inkjet printing through an evaporation-driven sol-gel transition

A ring stain is often an undesirable consequence of droplet drying. Particles inside evaporating droplets with a pinned contact line are transported toward the periphery by radial flow. In this paper, we demonstrate how suspensions of laponite can be used to control the radial flow inside picoliter droplets and produce uniform deposits. The improvement in homogeneity arises from a sol-gel transition during evaporation. Droplets gel from the contact line inward, reducing the radial motion of particles and thus inhibiting the formation of a ring stain. The internal flows and propagation of the gelling front were followed by high-speed imaging of tracer particles during evaporation of the picoliter droplets of water. In the inkjet nozzle, the laponite network is broken down under high shear. Recovery of the low shear viscosity of laponite suspensions was shown to be fast with respect to the lifetime of the droplet, which was instrumental in controlling the deposit morphology. The radial and vertical particle distributions within dried deposits were measured for water droplets loaded with 1 and 5 wt % polystyrene spheres and various concentrations of laponite. Aggregation of the polystyrene spheres was suppressed by the addition of colloidal silica. The formulation can be tuned to vary the deposit profile from a ring to a pancake or a dome.

Rheological study of two-dimensional very anisometric colloidal particle suspensions: from shear-induced orientation to viscous dissipation

In the present study, we investigate the evolution with shear of the viscosity of aqueous suspensions of size-selected natural swelling clay minerals for volume fractions extending from isotropic liquids to weak nematic gels. Such suspensions are strongly shear-thinning, a feature that is systematically observed for suspensions of nonspherical particles and that is linked to their orientational properties. We then combined our rheological measurements with small-angle X-ray scattering experiments that, after appropriate treatment, provide the orientational field of the particles. Whatever the clay nature, particle size, and volume fraction, this orientational field was shown to depend only on a nondimensional Péclet number (Pe) defined for one isolated particle as the ratio between hydrodynamic energy and Brownian thermal energy. The measured orientational fields were then directly compared to those obtained for infinitely thin disks through a numerical computation of the Fokker-Plank equation. Even in cases where multiple hydrodynamic interactions dominate, qualitative agreement between both orientational fields is observed, especially at high Péclet number. We have then used an effective approach to assess the viscosity of these suspensions through the definition of an effective volume fraction. Using such an approach, we have been able to transform the relationship between viscosity and volume fraction (ηr = f(φ)) into a relationship that links viscosity with both flow and volume fraction (ηr = f(φ, Pe)).

Phase inversion of emulsions containing a lipophilic surfactant induced by clay concentration

Emulsions stabilized by clay particles and sorbitan monooleate (Span 80) were investigated, and an abnormal phase inversion was observed by increasing the concentration of clay particles in the aqueous phase. At a fixed concentration of Span 80 in the oil phase, the emulsions were oil-in-water (o/w) when the concentration of clay particles in the aqueous phase was low. Surprisingly, the emulsion inverted to water-in-oil (w/o) when the concentration of the hydrophilic clay particles was increased. On the basis of the results of rheological measurements and laser-induced fluorescent confocal microscopy observation, we suggest that this phase inversion is induced by the gel structures formed at high concentration of clay particles. The effects of clay concentration on the stability and the droplet size of these emulsions were also investigated.

Aging of rotational diffusion in colloidal gels and glasses

We study the rotational diffusion of aging Laponite suspensions for a wide range of concentrations using depolarized dynamic light scattering. The measured orientational correlation functions undergo an ergodic to nonergodic transition that is characterized by a concentration-dependent ergodicity-breaking time. We find that the relaxation times associated with rotational degree of freedom as a function of waiting time, when scaled with their ergodicity-breaking time, collapse on two distinct master curves. These master curves are similar to those previously found for the translational dynamics; the two different classes of behavior were attributed to colloidal gels and glasses. Therefore, the aging dynamics of rotational degree of freedom provides another signature of the distinct dynamical behavior of colloidal gels and glasses.

Aqueous suspensions of natural swelling clay minerals. 2. Rheological characterization

We report in this article a comprehensive investigation of the viscoelastic behavior of different natural colloidal clay minerals in aqueous solution. Rheological experiments were carried out under both dynamic and steady-state conditions, allowing us to derive the elasticity and yield stress. Both parameters can be renormalized for all sizes, ionic strength, and type of clay using in a first approach only the volume of the particles. However, applying such a treatment to various clays of similar shapes and sizes yields differences that can be linked to the repulsion strength and charge location in the swelling clays. The stronger the repulsive interactions, the better the orientation of clay particles in flows. In addition, a master linear relationship between the elasticity and yield stress whose value corresponds to a critical deformation of 0.1 was evidenced. Such a relationship may be general for any colloidal suspension of anisometric particles as revealed by the analysis of various experimental data obtained on either disk-shaped or lath- and rod-shaped particles. The particle size dependence of the sol-gel transition was also investigated in detail. To understand why suspensions of larger particles gel at a higher volume fraction, we propose a very simplified view based on the statistical hydrodynamic trapping of a particle by an another one in its neighborhood upon translation and during a short period of time. We show that the key parameter describing this hydrodynamic trapping varies as the cube of the average diameter and captures most features of the sol-gel transition. Finally, we pointed out that in the high shear limit the suspension viscosity is still closely related to electrostatic interactions and follows the same trends as the viscoelastic properties.

Rheo-SAXS investigation of shear-thinning behaviour of very anisometric repulsive disc-like clay suspensions

Aqueous suspensions of swelling clay minerals exhibit a rich and complex rheological behaviour. In particular, these repulsive systems display strong shear-thinning at very low volume fractions in both the isotropic and gel states. In this paper, we investigate the evolution with shear of the orientational distribution of aqueous clay suspensions by synchrotron-based rheo-SAXS experiments using a Couette device. Measurements in radial and tangential configurations were carried out for two swelling clay minerals of similar morphology and size, Wyoming montmorillonite and Idaho beidellite. The shear evolution of the small angle x-ray scattering (SAXS) patterns displays significantly different features for these two minerals. The detailed analysis of the angular dependence of the SAXS patterns in both directions provides the average Euler angles of the statistical effective particle in the shear plane. We show that for both samples, the average orientation is fully controlled by the local shear stress around the particle. We then apply an effective approach to take into account multiple hydrodynamic interactions in the system. Using such an approach, it is possible to calculate the evolution of viscosity as a function of shear rate from the knowledge of the average orientation of the particles. The viscosity thus recalculated almost perfectly matches the measured values as long as collective effects are not too important in the system.

Aqueous suspensions of natural swelling clay minerals. 1. Structure and electrostatic interactions

In this article, we present a general overview of the organization of colloidal charged clay particles in aqueous suspension by studying different natural samples with different structural charges and charge locations. Small-angle X-ray scattering experiments (SAXS) are first used to derive swelling laws that demonstrate the almost perfect exfoliation of clay sheets in suspension. Using a simple approach based on geometrical constraints, we show that these swelling laws can be fully modeled on the basis of morphological parameters only. The validity of this approach was further extended to other clay data from the literature, in particular, synthetic Laponite. For all of the investigated samples, experimental osmotic pressures can be properly described by a Poisson-Boltzmann approach for ionic strength up to 10(-3) M, which reveals that these systems are dominated by repulsive electrostatic interactions. However, a detailed analysis of the Poisson-Boltzmann treatment shows differences in the repulsive potential strength that are not directly linked to the structural charge of the minerals but rather to the charge location in the structure for tetrahedrally charged clays (beidellite and nontronites) undergoing stronger electrostatic repulsions than octahedrally charged samples (montmorillonites, laponite). Only minerals subjected to the strongest electrostatic repulsions present a true isotropic to nematic phase transition in their phase diagrams. The influence of ionic repulsions on the local order of clay platelets was then analyzed through a detailed investigation of the structure factors of the various clay samples. It appears that stronger electrostatic repulsions improve the liquidlike positional local order.

Irreversible aging dynamics and generic phase behavior of aqueous suspensions of Laponite

We study the aging behavior of aqueous suspension of Laponite having 2.8 wt % concentration using rheological tools. At various salt concentration all the samples demonstrate orientational order when observed using crossed polarizers. In rheological experiments we observe inherent irreversibility in the aging dynamics which forces the system not to rejuvenate to the same state in the shear melting experiment carried out at a later date since preparation. The extensive rheological experiments carried out as a function of time elapsed since preparation demonstrate the self-similar trend in the aging behavior irrespective of the concentration of salt. We observe that the exploration of the low-energy states as a function of aging time is only kinetically affected by the presence of salt. We estimate that the energy barrier to attain the low-energy states decreases linearly with increase in the concentration of salt. The observed superposition of all the elapsed time and the salt-concentration-dependent data suggests that the aging that occurs in low salt concentration systems over a very long period is qualitatively similar to the aging behavior observed in systems with high salt concentration over a shorter period.

Liquid Crystalline Behavior and Related Properties of Colloidal Systems of Inorganic Oxide Nanosheets

Inorganic layered crystals exemplified by clay minerals can be exfoliated in solvents to form colloidal dispersions of extremely thin inorganic layers that are called nanosheets. The obtained “nanosheet colloids” form lyotropic liquid crystals because of the highly anisotropic shape of the nanosheets. This system is a rare example of liquid crystals consisting of inorganic crystalline mesogens. Nanosheet colloids of photocatalytically active semiconducting oxides can exhibit unusual photoresponses that are not observed for organic liquid crystals. This review summarizes experimental work on the phase behavior of the nanosheet colloids as well as photochemical reactions observed in the clay and semiconducting nanosheets system.

Multiple nonergodic disordered states in Laponite suspensions: a phase diagram

We study the time evolution of different Laponite suspensions from a low-viscosity ergodic state to a viscoelastic nonergodic state over a wide range of volume fractions and salt contents. We find that the evolution of nonergodicity parameter (Debye-Waller factor) splits into two branches for all the samples, which correspond to two distinct dynamically arrested states. At moderately high salt concentrations, on the other hand, a third nonergodic state appears that is different from the above two nonergodic states. Measurement of the conductivity of Laponite solutions in pure water shows that the contribution of counterions in the ionic strength is considerable and their role should be taken into account in interpretations of aging dynamics and the phase diagram. Based on these data and available data in the literature, we propose a (nonequilibrium) phase diagram for Laponite suspensions.

Gelation on the microscopic scale

Particle-tracking methods are used to study gelation in a colloidal suspension of Laponite clay particles. We track the motion of small fluorescent polystyrene spheres added to the suspension, and obtain the micron-scale viscous and elastic moduli of the material from their mean-squared displacement. The fluorescent spheres move subdiffusively due to the microstructure of the suspension, with the diffusive exponent decreasing from close to one at early times to near zero as the material gels. The particle-tracking data show that the system becomes more heterogeneous on the microscopic scale as gelation proceeds. We also determine the bulk-scale moduli using small-amplitude oscillatory shear rheometry. Both the macroscopic and microscopic moduli increase with time, and on both scales we observe a transition from a primarily viscous fluid to an elastic gel. We find that the gel point, determined as the time at which the viscous and elastic moduli are equal, is length-scale dependent--gelation occurs earlier on the bulk scale than on the microscopic scale.

Rheological behaviour of aqueous suspensions of laponite: new insights into the ageing phenomena

In this paper, the ageing behaviour of suspensions of laponite with varying salt concentration is investigated using rheological tools. It is observed that ageing is accompanied by an increase in the complex viscosity. The creep experiments performed at various ages show damped oscillations in the strain. The characteristic time scale of the damped oscillations, the retardation time, shows a prominent decrease with increasing age of the system. However, this dependence weakens with an increase in the salt concentration, which is known to change the microstructure of the system from glass like to gel like. We postulate that a decrease in the retardation time can be represented as a decrease in the viscosity (friction) of the dissipative environment surrounding the arrested entities that oppose elastic deformation of the system. We believe that ageing in colloidal glass leads to a greater ordering that enhances relative spacing between the constituents, thereby reducing the frictional resistance. However, since a gel state is inherently different in structure (fractal network) to that of a glass state (disordered), ageing in the gel does not induce ordering. Consequently, we observe an inverse dependence of retardation time on age, which becomes weaker with an increase in the salt concentration. We analyse these results from the perspective of ageing dynamics of both glass and gel states of laponite suspensions.

Thermodynamic and structural properties of phospholipid langmuir monolayers on hydrosol surfaces

Measurements of Langmuir pressure/area isotherms, rheology, grazing incidence X-ray diffraction (GIXD), and grazing incidence diffuse X-ray scattering out of the specular plane (GIXOS) have been used to investigate the influence of a hydrosol containing charged mineral nanoparticles on the thermodynamic and structural properties of a DPPC monolayer. The mineral adsorption layer that is formed via electrostatic interaction underneath the lipid layer alters the thermodynamic properties of the phospholipid monolayer in terms of maximal achievable compression, compressibility, and phase behavior. Modifications appear in the latter case as a coolinglike effect. Rheology measurements of the bulk viscoelastic properties revealed a stabilizing effect of the transient bulk network on the surfactant layer. The lipid chain lattice is found to be reorganized and adapted to the internal atomic structure of the mineral particles. A model for the superposition of Bragg rods from the lipid chains and the minerals is applied to separate these scattering contributions. In the vicinity of the mineral particles, the (2) reflection for DPPC on a liquid substrate was found, indicating strongly suppressed fluctuations at the surface. An estimation of the Debye-Waller factor associated with the lipid layer organization is used to quantify the damping of fluctuations within the lipid matrix due to the rigidifying and stabilizing effect of the mineral particles.

Invalidity of deriving interparticle distance in clay–water systems using the experimental structure factor maximum obtained by small-angle scattering

Small-angle X-ray scattering (SAXS) has been widely used to investigate the organization of clay colloids in response to the particle concentration and ionic strength of the suspending medium. In such investigations, measuring the interparticle distance and/or spacing is usually attempted. In random or short-range ordered clay-water systems, the interparticle distances are often derived from the experimental structure factor maximum; however, the validity of such practice has never been theoretically or experimentally evaluated. The experimental structure factors of several clay-water systems with and without polyphosphate treatment to block the edge charges of clay particles were obtained from SAXS data in order to understand the physical meaning of this property. The results show that the polyphosphate treatment eliminated the experimental structure factor maximum and that the particle concentration effects were correlated with the depression on the curve in a random clay-water system (e.g., illite and laponite). For clay particles with greater anisotropy (i.e., montmorillonite), polyphosphate treatment enhanced the ordering of clay layers at high particle concentrations forming long-range ordered crystals showing Bragg reflections. In this ordered system, distinctive and symmetrical peaks representing the interparticle spacing were obtained by using a Fourier transform of the scattering curves. Thus, we conclude that the experimental structure factor maximum is induced by the edge-face oriented interactions, which may not be in direct contact as in a house-of-cards structure, and the position of the maximum should not be interpreted as an averaged interparticle distance in a clay-water system unless particles orient along the same direction.

Kinetics of ergodic-to-nonergodic transitions in charged colloidal suspensions: aging and gelation

There are two types of isotropic disordered nonergodic states in colloidal suspensions: colloidal glasses and gels. In a recent paper [H. Tanaka, J. Meunier, and D. Bonn, Phys. Rev. E 69, 031404 (2004)], we discussed the static aspect of the differences and the similarities between the two. In this paper, we focus on the dynamic aspect. The kinetics of the liquid-glass transition is called "aging," while that of the sol-gel transition is called "gelation." The former is primarily governed by repulsive interactions between particles, while the latter is dominated by attractive interactions. Slowing down of the dynamics during aging reflects the increasing cooperativity required for the escape of a particle from the cage formed by the surrounding particles, while that during gelation reflects the increase in the size of particle clusters towards the percolation transition. Despite these clear differences in the origin of the slowing down of the kinetics between the two, it is not straightforward experimentally to distinguish them in a clear manner. For an understanding of the universal nature of ergodic-to-nonergodic transitions, it is of fundamental importance to elucidate the differences and the similarities in the kinetics between aging and gelation. We consider this problem, taking Laponite suspension as an explicit example. In particular, we focus on the two types of nonergodic states: (i) an attractive gel formed by van der Waals attractions for high ionic strengths and (ii) a repulsive Wigner glass stabilized by long-range Coulomb repulsions for low ionic strengths. We demonstrate that the aging of colloidal Wigner glass crucially differs not only from gelation, but also from the aging of structural and spin glasses. The aging of the colloidal Wigner glass is characterized by the unique cage-forming regime that does not exist in the aging of spin and structural glasses.

Phase diagrams of Wyoming Na-montmorillonite clay. Influence of particle anisotropy

Natural Na-Wyoming montmorillonite was size fractionated by successive centrifugation. Polydisperse particles with average sizes of 400, 290, and 75 nm were then obtained. As the structural charge of the particles belonging to three fractions (determined by cationic exchange capacity measurements) is the same, such a procedure allows studying the effect of particle anisotropy on the colloidal phase behavior of swelling clay particles. Osmotic stress experiments were carried out at different ionic strengths. The osmotic pressure curves display a plateau whose beginning systematically coincides with the sol/gel transition determined by oscillatory stress measurements. The concentration corresponding to the sol/gel transition increases linearly with particle anisotropy, which shows that the sol/gel transition is not directly related to an isotropic/nematic transition of individual clay particles. Indeed, a reverse evolution should be observed for an I/N transition involving the individual clay particles. Still, when observed between crossed polarizer and analyzer, the gel samples exhibit permanent birefringent textures, whereas in the "sol" region, transient birefringence is observed when the samples are sheared. This suggests that interacting clay particles are amenable to generate, at rest and/or under shear, large anisotropic particle associations.

Nonergodic states of charged colloidal suspensions: repulsive and attractive glasses and gels

Two types of isotropic disordered nonergodic states exist in colloidal suspensions: glasses and gels. The difference between the two is that the nonergodicity, or elasticity, of gel stems from the existence of a percolated network, while that of glass stems from caging effects. Despite this clear difference in the origin of nonergodicity, it is not straightforward to distinguish the two states in a clear manner. Taking a Laponite suspension as an explicit example, we propose a general phase diagram for charged colloidal systems. It follows that a transition from the glass to the gel state can be induced by changing the interparticle interactions from predominantly repulsive to attractive. This originates from the competition between electrostatic Coulomb repulsion and van der Waals attraction. If the repulsion dominates, the system forms a Wigner glass, while in a predominantly attractive situation it forms a gel. In the intermediate region, where both repulsive and attractive interactions play roles, it may form an attractive glass.

Synchrotron SAXS/WAXD and rheological studies of clay suspensions in silicone fluid

Suspensions of two commercial smectite clays, montmorillonite KSF and montmorillonite K10, in a low-viscosity silicone oil (Dow Corning 245 Fluid) were studied by simultaneous synchrotron small-angle X-ray scattering (SAXS)/wide-angle X-ray diffraction (WAXD) techniques and rheological measurements. In the 0.5% (w/v) KSF clay suspension and two K10 clay suspensions (0.5% and 1.0%), WAXD profiles below 2theta=10.0 degrees did not display any characteristic reflection peaks associated with the chosen montmorillonite clays, while corresponding SAXS profiles exhibited distinct scattering maxima, indicating that both clays were delaminated by the silicone oil. In spite of the large increase in viscosity, the clay suspensions exhibited no gel characteristics. Dynamic rheological experiments indicated that the clay/silicone oil suspensions exhibited the behavior of viscoelasticity, which could be influenced by the type and the concentration of the clay. For the K10 clay suspensions, the frequency-dependent loss modulus (G") was greater in magnitude than the storage modulus (G') in the concentration range from 0.5 to 12.0%. The increase in the clay concentration shifted the crossover point between G' and G" into the accessible frequency range, indicating that the system became more elastic. In contrast, the KSF clay suspension exhibited lower G' and G" values, indicating a weaker viscoelastic response. The larger viscoelasticity response in the K10 clay suspension may be due to the acid treatment generating a higher concentration of silanol groups on the clay surface.

Dissociation of thixotropic clay gels

Laponite dispersions in water, at moderate ionic strength and high pH, are thixotropic: depending on previous history, they can be fluids or gels. The mechanisms of the fluid-gel and gel-fluid transitions have been examined through ionic analysis of the aqueous phase, static light, and small-angle neutron scattering, rheological experiments, and centrifugation. The results indicate that the particles attract each other in edge-to-face configurations. These attractions cause the particles to gather in microdomains, which subsequently associate to form very large fractal superaggregates, containing all the particles in the dispersion. A gel state is obtained when the network of connections is macroscopic. This network is destroyed by the application of sufficient strain, but it heals at rest. The addition of peptizers weakens the edge-to-face attractions, and makes the healing times much slower.

Interpretation of small-angle x-ray scattering data from dilute montmorillonite suspensions using a modified Guinier approximation

Smectites are a group of 2:1-layer phyllosilicate minerals that have been extensively studied by small-angle x-ray scattering (SAXS) because of their industrial and environmental significance. In previous studies, a Guinier plot has been used to obtain the radius of gyration of the clay particles, from which geometric information of the particle structure is derived. Using an indirect Fourier transform to treat SAXS data from a dilute montmorillonite suspension, a negative electron contrast at the clay-water interface is observed. This electron inhomogeneity has violated the assumption underlying the application of the Guinier plot, which requires particles to have a uniform electron density. The presence of this inhomogeneity explains the inability of previous studies to correctly determine particle dimensions using the Guinier plot. Using this model of the clay-water interface, a modified Guinier plot has been derived and was experimentally verified. The calculation shows the presence of negative electron contrast at montmorillonite-water interfaces, which is in accordance with the results from the indirect Fourier transform method. This approximation has the potential to predict the geometric information for similar colloids studied by small-angle scattering.

Scaling Behavior of the Rheological Properties of Montmorillonite Suspensions: Correlation between Interparticle Interaction and Degree of Flocculation

In this work we investigate some aspects of the rheological behavior of sodium montmorillonite (NaMt) suspensions in the pH range 3 to 9, of NaCl concentrations between 10(-3) and 10(-1) M, and of solid concentrations between 5 and 11% w/v. Three kinds of experiments were performed: steady-state viscometry, oscillatory test, and creep recovery. The physical quantities of interest were the yield stress sigma(y) of the suspensions, the elastic rigidity modulus G', and the instantaneous elastic compliance. Furthermore, G' was obtained from oscillatory tests in three different experiments: determination of the viscoelastic linear region, oscillograms, and the gelation process. All quantities were found to scale with the concentration of solids, C, according to a power law of the form Y=k(y)C(n). The exponents n were found to change from approximately 3 to approximately 6 when the pH was increased from 3 to 9 (at constant ionic strength 10(-2) M), although values as high as 10 were estimated when the NaCl concentration was reduced to 1 mM. Such values of n correlate well with the characteristics of the edge-to-face (E-F), edge-to-edge (E-E), and face-to-face (F-F) interparticle interactions. The minimum values of n correspond to maximum E-F attractions, whereas the largest n are associated with strong F-F repulsions. Copyright 2001 Academic Press.