Physico-Chemical Properties of Laponite®/Polyethylene-oxide Based Composites

This review aims to provide a literature overview as well as the authors' personal account to the studies of Laponite® (Lap)/Polyethylene-oxide (PEO) based composite materials and their applications. These composites can be prepared over a wide range of their mutual concentrations, they are highly water soluble, and have many useful physico-chemical properties. To the readers' convenience, the contents are subdivided into different sections, related with consideration of PEO properties and its solubility in water, behavior of Lap systems(structure of Lap-platelets, properties of aqueous dispersions of Lap and aging effects in them), analyzing ofproperties LAP/PEO systems, Lap platelets-PEO interactions, adsorption mechanisms, aging effects, aggregation and electrokinetic properties. The different applications of Lap/PEO composites are reviewed. These applications include Lap/PEO based electrolytes for lithium polymer batteries, electrospun nanofibers, environmental, biomedical and biotechnology engineering. Both Lap and PEO are highly biocompatible with living systems and they are non-toxic, non-yellowing, and non-inflammable. Medical applications of Lap/PEO composites in bio-sensing, tissue engineering, drug delivery, cell proliferation, and wound dressings are also discussed.

High resolution and fidelity 3D printing of Laponite and alginate ink hydrogels for tunable biomedical applications

The formulation of hydrogels that meet the necessary flow characteristics for their extrusion-based 3D printing while providing good printability, resolution, accuracy and stability, requires long development processes. This work presents the technological development of a hydrogel-based ink of Laponite and alginate and evaluates its printing capacity. As a novelty, this article reports a standardizable protocol to quantitatively define the best printing parameters for the development of novel inks, providing new printability evaluation parameters such as the Printing Accuracy Escalation Index. As a result, this research develops a printable Laponite-Alginate hydrogel that presents printability characteristics. This ink is employed for the reproducible manufacture of 3D printed scaffolds with versatile and complex straight or curved printing patterns for a better adaptation to different final applications. Obtained constructs prove to be stable over time thanks to the optimization of a curing process. In addition, the study of the swelling and degradation behavior of the Laponite and alginate 3D printed scaffolds in different culture media allows the prediction of their behavior in future in vitro or in vivo developments. Finally, this study demonstrates the absence of cytotoxicity of the printed formulations, hence, setting the stage for their use in the field of biomedicine.

2D Liquid-Crystallization-Driven Self-Assembly of Rod-Coil Block Copolymers: Living Growth and Self-Similarity

Liquid-crystallization-driven self-assembly (LCDSA) is an emerging methodology, which has been employed to construct controllable 1D nanostructures. However, 2D nanostructures via living LCDSA are rarely reported, and the complicated growth kinetics are not well-known. Herein, we perform Brownian dynamics (BD) simulations to investigate the 2D living growth of disklike micelles via LCDSA of rod-coil block copolymers. The 2D seeded-growth behavior is achieved by incorporating the unimers onto the edges of disklike seeds with smectic-like liquid-crystalline (LC) cores. The fluidity of such LC-like micellar cores is conducive to the chain adjustments of rod blocks during the 2D living growth process. The apparent growth rate and unique self-similarity kinetics are governed by the interplay between the variations in the growth rate coefficient and the reactive sites at the micelle edges. This work provides an in-depth understanding of the 2D living growth of micelles and guidance to construct well-defined 2D hierarchical nanostructures.

Influence of medium structure on the physicochemical properties of aging colloidal dispersions investigated using the synthetic clay LAPONITE®

Physical aging in colloidal dispersions manifests as a reduction in kinetic freedom of the colloids. In aqueous dispersions of charged clay colloids, the role of interparticle electrostatic interactions in determining the aging dynamics has been evaluated extensively. Despite water being the dispersion medium, the influence of water structure on the physicochemical properties of aging clay dispersions has, however, not been considered before. In this work, we use LAPONITE®, a model hectorite clay mineral that acquires surface charges when dispersed in water, to study the relative contributions of dispersion medium structure and interparticle electrostatic interactions on the physicochemical properties of aging hectorite clay dispersions. The structure of the dispersion medium is modified either by incorporating dissociating/non-dissociating kosmotropic (structure-inducing) or chaotropic (structure-disrupting) molecules or by changing dispersion temperature. Photon correlation spectroscopy, rheological measurements and particle-scale imaging are employed to evaluate the physicochemical properties of the dispersions. Our experiments involving incorporation of external additives demonstrate a strong influence of dispersion medium structure on the dispersion properties when the interparticle electrostatic interactions are weak. We introduce a new temperature dependent measurement protocol, wherein the temperature of the medium is fixed before adding the clay particles, to manipulate the hydrogen bonds in the aqueous medium in the absence of external additives. Accelerated aging, observed upon raising the temperature regardless of the experimental thermal histories, is attributed to increased interparticle electrostatic interactions as in the room temperature experiments with ionic additives. Our study identifies that in the presence of weak interparticle electrostatic interactions, changes in the physicochemical properties of charged clay dispersions can be driven by manipulating hydrogen bond populations in aqueous medium.

The structure-property relationship in LAPONITE® materials: from Wigner glasses to strong self-healing hydrogels formed by non-covalent interactions

Rheology, small-angle X-ray scattering (SAXS), and dynamic light scattering (DLS) analysis, zeta potential measurement, scanning electron microscopy (SEM), and micro-FTIR and absorbance spectroscopy were used to enlighten the controversial literature about LAPONITE® materials. Our data suggest that pristine LAPONITE® in water does not form hydrogels induced by the so-called "house of cards" assembly, but rather forms Wigner glasses governed by repulsive forces. Ionic interactions between anisotropic LAPONITE® nanodiscs, sodium polyacrylate and inorganic salts afforded hydrogels that were transparent, self-standing, moldable, strong, and biocompatible with shear-thinning and self-healing behavior. An extensive study on the role of salts in the gelification process dictates a trend that relates the valence of cations with the viscoelastic properties of the bulk material (G' values follow the trend, monovalent < divalent < trivalent). These hydrogels present G' values up to 5.1 × 104 Pa, which are considered high values for non-covalent hydrogels. Hydrogels crosslinked with sodium phosphate salts are biocompatible, and might be valid candidates for injectable drug delivery systems due to their shear-thinning behavior with rapid self-healing after injection.

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.

Laponite®: A key nanoplatform for biomedical applications?

Laponite® is a synthetic smectite clay that already has many important technological applications, which go beyond the conventional uses of clays in pharmaceutics and cosmetics. In biomedical applications, particularly in nanomedicine, this material holds great potential. Laponite® is a 2-dimensional (2D) nanomaterial composed of disk-shaped nanoscale crystals that have a high aspect ratio. These disks can strongly interact with many types of chemical entities (from small molecules or ions, to natural or synthetic polymers, to different inorganic nanoparticles) and are also easily functionalized and readily degraded in the physiological environment giving rise to non-toxic and even bioactive products. This review will highlight the potential of Laponite® as a nanomaterial in the fields of drug delivery, bioimaging, tissue engineering and regenerative medicine. New concepts, as well as novel innovative materials that stand out from the usual ones due to the unique properties of Laponite®, will also be presented and discussed.

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.

Isotopic Effect on the Gel and Glass Formation of a Charged Colloidal Clay: Laponite

The time evolution of both dynamic and static structure factors of a charged colloidal clay, Laponite, dispersed in both H₂O and D₂O solvents has been investigated through multiangle dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS) as a function of weight concentration. The aging phenomenology and the formation of arrested states, both gel and glass, are preserved in D₂O, while the dynamics is slowed down with respect to water. These findings are important to understand the role played by the solvent in the interparticle interactions and for techniques such as neutron scattering and nuclear magnetic resonance that allow for the extension of the accessible scattering vectors and time scales.

Phase Behavior of Aqueous Suspension of Laponite: New Insights with Microscopic Evidence

Investigating microstructure of suspensions with particles having anisotropic shape that share complex interactions is a challenging task leading to competing claims. This work investigates phase behavior of one such system: aqueous Laponite suspension, which is highly contested in the literature, using rheological and microscopic tools. Remarkably, we observe that over a broad range of Laponite (1.4 to 4 wt %) and salt concentrations (0 to 7 mM), the system overwhelmingly demonstrates all the rheological characteristics of the sol-gel transition leading to a percolated network. Analysis of the rheological response leads to fractal dimension that primarily depends on the Laponite concentration. We also obtain the activation energy for gelation, which is observed to decrease with increase in Laponite as well as salt concentration. Significantly, the cryo-TEM images of the postgel state clearly show the presence of a percolated network formed by interparticle bonds. The present work therefore conclusively establishes the system to be in an attractive gel state resolving a long-standing debate in the literature.

Sedimentation stability and aging of aqueous dispersions of Laponite in the presence of cetyltrimethylammonium bromide

This work discusses the sedimentation stability and aging of aqueous suspensions of Laponite in the presence of cetyltrimethylammonium bromide (CTAB). The concentration of Laponite was fixed at a constant level C(l)=2%wt, which corresponds to the threshold between equilibrium gel IG(1) and glass IG(2) states. The concentration of CTAB C(s) was within 0-0.3 %wt. In the presence of CTAB, the Laponite aqueous suspensions were unstable against sedimentation and separated into the upper and bottom layers (U and B layers, respectively). The dynamic light-scattering technique has revealed that addition of CTAB even at a rather small concentration, C(s)=0.0164 %wt (0.03 cation exchange capacity), induced noticeable changes in the aging dynamics of the U layer. It was explained by equilibration of CTAB molecules that were initially nonuniformly distributed between different Laponite particles. Accelerated stability analysis by means of analytical centrifugation with rotor speed ω=500-4000 rpm revealed three sedimentation regimes: continuous (I, C(s)<0.14 %wt), zonelike (II, 0.14<C(s)<0.2%wt), and gel-like (III, C(s)>0.2%wt). It was demonstrated that the B layer was "soft" in the zonelike regime. The increase of ω resulted in its supplementary compressing and collapse of "soft" sediment above certain critical centrifugal acceleration. The physical nature of the observed behavior, accounting for enhancement of hydrophobic interactions between Laponite particles, is discussed.

Size dependence of tracer diffusion in a laponite colloidal gel

Using a fluorescence recovery after photobleaching (FRAP) technique, we present measurements of probe diffusion in a colloidal glass-a Laponite suspension. By varying the probe size over 2 orders of magnitude, as well as the concentration of the colloidal glass, we evidence and quantify the deviations of the probe diffusivity from the bulk Stokes-Einstein expectations. These experiments suggest that the probe diffusion in the dynamically arrested Laponite structure is mainly controlled by the ratio between the probe size and the typical clay platelets interdistance. Comparing with a simple hindered diffusion mechanism, the reduction of tracer diffusion is discussed in terms of the hydrodynamic interaction of the probe with the Laponite structure. Finally, these results can be interpreted in terms of a scale dependent viscosity of the colloidal glass.

Dynamics and microstructure analysis of N-isopropylacrylamide/silica hybrid gels

The mechanical properties, gelation process, and microstructure of N-isopropylacrylamide (NIPAm)/silica hybrid gels were investigated by using mechanical measurements, dynamic light scattering (DLS), and contrast-variation small-angle neutron scattering (CV-SANS). The three different hybrid gels, containing an equal total volume fraction of silica particles of different sizes (NS_S: small = 20-30 angstroms radius; NS_M: medium = 40-55 angstroms; and NS_L: large = 100-150 angstroms), were employed for comparison. An obvious effect on the compressive properties due to the size of the silica particles was observed, with harder materials produced from particles of smaller size. The gelation process was strongly dependent on the competition between silica aggregation and NIPAm polymerization, as measured by using the DLS methods. In the case of NS_S, the rate of aggregation was larger than NIPAm polymerization, resulting in the formation of an aggregate-dominant microstructure. The aggregation rate for NS_M and NS_L, however, was comparable to the NIPAm polymerization rate, leading to simultaneous aggregation and polymerization. CV-SANS determined that the NIPAm adsorption layer on the silica surface was clearly affected on a microscopic scale by the silica particle sizes.

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.

Arrested state of clay-water suspensions: gel or glass?

The aging of a charged colloidal system has been studied by small-angle x-ray scattering, in the exchanged momentum range Q=0.03-5 nm(-1) , and by dynamic light scattering, at different clay concentrations (C(w)=0.6-2.8%) . The static structure factor S(Q) has been determined as a function of both aging time and concentration. This is the direct experimental evidence of the existence and evolution with aging time of two different arrested states in a single system simply obtained only by changing its volume fraction: an inhomogeneous state is reached at low concentrations, while a homogeneous one is found at high concentrations.

Thermal and colloidal behavior of amine-treated clays: the role of amphiphilic organic cation concentration

The modification of sodium montmorillonite (NaMMT) through the insertion of amphiphilic hexadecylammonium cations into the clay's interlayer spaces has been studied. Alkylammonium concentrations equivalent to 0.15-3.00 times the cation exchange capacity of the clay were used. The conformation of the surfactant cations in the confined space of the silicate galleries was investigated by X-ray diffraction analysis and scanning electron microscopy, while the organoclay's thermal stability was examined by thermogravimetric analysis. The clay's surface properties induced by the ion-exchange process were followed by measurements of the mineral's zeta potential as a function of pH and surfactant concentration, while the coagulation rates of organoclay suspensions in water and in chloroform were examined using dynamic light scattering. All the results are consistent with showing that the overall characteristics and thus the behavior of the modified MMT particles strongly depend on the alkylammonium surfactant concentration used in the modification process. This, however, has very important implications for any attempt to incorporate the organomodified MMT particles into different media for various applications such as polymer nanocomposite preparation.

Gels and glasses in a single system: evidence for an intricate free-energy landscape of glassy materials

In the free-energy landscape picture of glassy systems, their slow dynamics is due to a complicated free-energy landscape with many local minima. We show that for a colloidal glassy material multiple paths can be taken through the free-energy landscape. The evolution of the nonergodicity parameter shows two distinct master curves that we identify as gels and glasses. We show that for a range of colloid concentrations, the transition to nonergodicity can occur in either direction (gel or glass), accompanied by "hesitations" between the two. Thus, colloidal gels and glasses are merely global free-energy minima in the same free-energy landscape, and the paths leading to these minima can be complicated.

Cluster, glass, and gel formation and viscoelastic phase separation in aqueous clay suspensions

We have systematically investigated the phase diagram of clay particles in water to understand the relation between the local and macroscopic properties and the structures of clay suspensions. We focused, in particular, on sodium Cloisite (CNa) particles at concentrations typically used in nanocomposites (concentrations from 1 to 4 wt %) and at an extended range of ionic strengths (10(-5) to 10(-2) M NaCl). The suspensions have been characterized using rheology and a combination of scattering techniques (neutrons, X-rays, and light). We demonstrate the existence of a liquid cluster phase at low clay and intermediate salt concentrations and provide new insight into the nature of the solid-like dispersions at low and high ionic strengths.

Diagram of the aging dynamics in laponite suspensions at low ionic strength

We measure the dynamic structure factor (DSF) of probe particles embedded in an aging laponite suspension quenched by cessation of shear and the associated relaxation time tau as a function of wave vector q and aging time t(w). The different q dependences measured in the successive exponential and full aging regimes, respectively, tau approximately q(-2) and tau approximately q(-1.25), yield a weak positive q dependence for the aging time t(wc) corresponding to the crossover between the two regimes. This implies that the full aging behavior is first seen when investigating large length scales in the aging suspension. We propose a qualitative diagram of the aging dynamics and discuss the features of the DSF of the probes and of the matrix in the two aging regimes. Consistently with the idea that the full aging regime is first observed when probing large length scales, t(wc) is markedly shorter when the motion of the probes is tracked instead of the collective fluctuations of concentration in the matrix. The exponential aging regime is most probably related to the liquid-glass transition induced by the cessation of shear rather than to the aging of a glass.

More on the phase diagram of Laponite

The phase diagram of a charged colloidal system (Laponite) has been investigated by dynamic light scattering in a previously unexplored range of salt and clay concentrations. Specifically, the clay weight and salt molar concentrations have been varied in the ranges C(w) = 0.004 divided by 0.025 and C(s) = (1 x 10(-3) divided by 5 x 10(-3)) M, respectively. As in the case of free salt water samples (C(s) approximately = 1 x 10(-4) M), an aging dynamics toward two different arrested phases is found in the whole examined C(w) and C(s) range. Moreover a transition between these two different regimes is found. It is clear from these measurements that a revision of the phase diagram is necessary and a new "transition" line between two different arrested states is drawn.

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.

Nonaqueous suspensions of laponite and montmorillonite

It is shown how stable, nonaqueous suspensions of fully exfoliated smectite clays such as Laponite and montmorillonite can be obtained. Suspensions in toluene and in a branched aliphatic solvent (polydecene) were characterized using elemental analysis, rheometry, and small-angle X-ray scattering. For Laponite, stable suspensions were obtained by adsorbing a dichain poly(isobutylene) based stabilizer to the particles. This approach did not result in full exfoliation for montmorillonite particles, possibly because the stabilizer was able to connect individual clay sheets at the edges during the treatment process. Instead, a quaternary ammonium surfactant, dimethyl dioctadecylammonium bromide (DODAB), was first adsorbed to the clay. Subsequently, adsorption of the poly(isobutylene) based stabilizer to this pretreated clay resulted in fully exfoliated suspensions of montmorillonite. Suspensions of clay particles treated only with DODAB have been studied by several authors before and are included in this study for comparison. A detailed discussion of the suspension behavior of the different clay samples is given. The suspension routes presented here may enable further studies of the structure and flow behavior of suspensions of thin, flexible clay platelets as a function of aspect ratio.

Scaling of the kinetics of slow aggregation and gel formation for a fluorinated polymer colloid

The aggregation and gelation kinetics in moderately concentrated (0.004 </= phi(0) </= 0.1) colloidal dispersions of fluorinated polymer particles has been studied. The aggregation was adjusted to proceed slowly enough to allow a convenient characterization of the kinetics through static and dynamic light scattering on quenched and diluted samples. A population balance model based on second-order aggregation rates is developed to compute the time evolution of the cluster mass distribution, from which we calculate the values of the average radii and structure factor measured by light scattering, so as to allow a direct comparison between measured and calculated quantities. The model suggests the introduction of a dimensionless time which allows the scaling of all the aggregation data on unique master curves defined by only two parameters: the exponent of the power-law aggregation kernel, lambda, and the aggregate fractal dimension, d(f). The predicted master curves were observed experimentally, which confirms the validity of the aggregation model and allows the unique determination of the kinetic and structural parameters of the aggregation process. The cluster growth behavior, although significantly slower than DLCA, shows power-law kinetics rather than the exponential one typical of RLCA and the cluster structure is characterized by an unexpectedly small fractal dimension, d(f) = 1.7. The occurrence of gelation has been characterized using small amplitude oscillatory shearing to monitor the time evolution of the elastic modulus. It is found that also these curves, together with the gel time value, scale with the stability ratio of primary particles for a given solid volume fraction. We further use the model to calculate the cumulative occupied volume fraction of the growing aggregates and quantify in this way the increasing space filling, which is solid volume fraction dependent. The experimentally determined dimensionless gel times, which are also solid volume fraction dependent, scale then directly with the dimensionless time to reach a certain degree of the space filling in the model. This finding suggests that, like the aggregation kinetics, the gelation kinetics is governed by a second-order rate process.

Revised state diagram of Laponite dispersions

We propose a state diagram of charged disk-like mineral particle (Laponite) dispersions as a function of the Laponite concentration (C) and the concentration of added salt (C(s)), based on simple observation and light-scattering measurements. At low C or high C(s) the dispersions separate into two domains due to sedimentation of Laponite aggregates, while at high C and low C(s) they form homogeneous gels that do not flow upon tube reversal. The aggregation rate and the structure factor of the Laponite dispersions is determined with light scattering as a function of C and C(s). We discuss in detail the controversy on the origin of gelation of Laponite dispersions in the absence of added salt. We argue that aggregation rather than glass formation causes gelation.

Routes to gelation in a clay suspension

The gelation of water suspension of a synthetic clay (Laponite) has been studied by dynamic light scattering in a wide range of clay weight concentration (C(w)=0.003-0.031). At variance with previous determination, indicating a stable liquid phase for C(w)<C(*)(w) approximately 0.015-0.018, we find that gelation actually takes place in the whole examined C(w) range. We find also that C(*)(w) marks the transition between two different routes to gelation. We hypothesize that at low concentration Laponite suspension behaves as an attractive colloid and that the slowing down of the dynamics is attained by the formation of larger and larger clusters while at high concentration the basic units of the arrested phase could be the Debye Huckel spheres associated with single Laponite plates.

The interplay between screening properties and colloid anisotropy: towards a reliable pair potential for disc-like charged particles

The electrostatic potential of a highly charged disc (clay platelet) in an electrolyte is investigated in detail. The corresponding non-linear Poisson-Boltzmann (PB) equation is solved numerically, and we show that the far-field behaviour (relevant for colloidal interactions in dilute suspensions) is exactly that obtained within linearized PB theory, with the surface boundary condition of a uniform potential. The latter linear problem is solved by a new semi-analytical procedure and both the potential amplitude (quantified by an effective charge) and potential anisotropy coincide closely within PB and linearized PB, provided the disc bare charge is high enough. This anisotropy remains at all scales; it is encoded in a function that may vary over several orders of magnitude depending on the azimuthal angle under which the disc is seen. The results allow to construct a pair potential for discs interaction, that is strongly orientation dependent.

Dynamic light scattering studies of poly(ethylene oxide) adsorbed on Laponite: layer conformation and its effect on particle stability

Dynamic light scattering has been used to determine the hydrodynamic thickness of poly(ethylene oxide) (PEO) adsorbed on synthetic anisotropic clay particles (Laponite) as a function of molecular weight. The layer thicknesses, and their increase with molecular weight, indicate that the conformation of the adsorbed layer is very compact and is much smaller than those normally observed for polymer adsorption on flat interfaces. The aggregation kinetics of the polymer coated particles in 5 mM NaCl was analyzed in a quantitative manner, revealing that the potential barrier to aggregation is strongly enhanced when polymer is present.

Interactions between laponite and microbial biofilms in porous media: implications for colloid transport and biofilm stability

Quartz sand columns and sand-filled microscope flow cells were used to investigate the transport characteristics of the clay colloid laponite, and a biofilm-forming bacterium, Pseudomonas aeruginosa SG81. Separate experiments were performed with each particle to determine their individual transport characteristics in clean sand columns. In a second set of experiments, bacterial biofilms were formed prior to introduction of the clay colloids. In the independent transport experiments, bacteria and laponite each conformed to known physicochemical principles. A sodium chloride concentration of 7 x 10(-2) M caused complete retention of the laponite within the sand columns. P. aeruginosa SG81 was generally less influenced by ionic strength effects; it showed relatively low mobility at all ionic strengths tested and some (albeit reduced) mobility when introduced to the columns in 1M NaCl, the highest concentration tested, but nevertheless showed reproducible trends. Under conditions favourable to laponite retention and biofilm stability (7 x 10(-2) MNaCl), laponite suspensions were able to remobilise a portion of the attached bacterial biomass. At low ionic strength, the profile of laponite elution was also altered in the presence of a P. aeruginosa biofilm. These observations suggest that while a reduction in ionic strength has a dominant influence on the mobilisation of biological and inorganic colloids, the presence of laponite and biomass can have a distinct influence on the mobility of both types of colloids. Since these events are likely to occur in subsurface environments, our results suggest that colloid-biofilm interactions will have implications for colloid-bound contaminant transport and the remobilisation of pathogens.

Influence of pyrophosphate or polyethylene oxide on the aggregation and gelation of aqueous laponite dispersions

The influence of pyrophosphate or polyethylene oxide (PEO) on the aggregation and gelation of dispersions of model disklike clay particles (Laponite) is studied using light scattering and rheology. Pyrophosphate adsorbs onto the positively charged rim and inhibits bond formation between the rim and the negatively charged faces of the particles. At low concentrations of pyrophosphate the aggregation of Laponite is only retarded, without significant modification of the structure of the aggregates and gels. The decrease of the aggregation rate can be explained by an increase of the energy barrier to the formation of bonds in proportion to the pyrophosphate concentration. Addition of a large amount of pyrophosphate leads to the breakup of Laponite aggregates and gels. PEO adsorbs onto the Laponite particles and inhibits aggregation by steric hindrance. The reduction of the aggregation rate depends on the molar mass and is maximal at about 1000 g/mol. Higher molar mass PEO bridges between the particles and leads to the formation of clusters or a weak gel immediately after mixture.

Aging processes and scale dependence in soft glassy colloidal suspensions

The aging behavior of colloidal suspensions of laponite, a model synthetic clay, is investigated using light scattering techniques. In order to measure the complete dynamic structure factor as a function of time and of wave vector, we have developed an original optical setup using a multispeckle technique for simple light scattering. We have thus measured the correlation of the scattered light intensity as a function of the age of the sample t(w) for various concentrations. For sufficiently concentrated samples, we observe a two-stage relaxation process. The fast relaxation is diffusive, stationary, and reminiscent of the liquidlike behavior observed in less concentrated samples. The slow relaxation behavior, however, is more complex. It exhibits two successive regimes as the sample ages. In the first regime, the decay time tau(a) increases exponentially with t(w) as long as tau(a)<t(w). In the second one, "full aging" is observed in which tau(a) is proportional to t(w). In this second regime, the relaxation of concentration fluctuations are hyperdiffusive, scaling as exp[-(t/tau(a))(beta)] with beta=1.35+/-0.15. In addition, the spatial dependence of this relaxation time scales as tau(a)(q) approximately q(-x) with x approximately 1.3.