Revised state diagram of Laponite dispersions

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.

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.

Evaporation of an emulsion trapped in a yield stress fluid

The present work deals with emulsions of volatile alkanes in an aqueous clay suspension, Laponite, which forms a yield stress fluid. For a large enough yield stress (i.e. Laponite concentration), the oil droplets are prevented from creaming and the emulsions are thus mechanically stabilized. We have studied the evaporation kinetics of the oil phase of those emulsions in contact with the atmosphere. We show that the evaporation process is characterized by the formation of a sharp front separating the emulsion from a droplet-free Laponite phase, and that the displacement of the front vs. time follows a diffusion law. Experimental data are confronted to a diffusion-controlled model, in the case where the limiting step is the diffusion of the dissolved oil through the aqueous phase. The nature of the alkane, as well as its volume fraction in the emulsion, has been varied. Quantitative agreement with the model is achieved without any adjustable parameter and we describe the mechanism leading to the formation of a front.

Sol-gel and isotropic/nematic transitions in aqueous suspensions of natural nontronite clay. Influence of particle anisotropy. 2. Gel structure and mechanical properties

After size-selection, the phase behavior of aqueous suspensions of nontronite clay was analyzed by osmotic pressure measurements, rheological experiments, and small-angle X-ray scattering. All the measurements confirm that for ionic strength < or =10(-3) M/L, the system is purely repulsive. By combining results from osmotic pressure measurements and X-ray scattering, it appears that the pressure of the system can be well-described using a simple Poisson-Boltzmann treatment based on the interaction between charged infinite parallel planes. In terms of rheological properties, even if the status of the sol/gel transition remains partially unclear as the number density of particles at the sol-gel transition exhibits a -2 power dependence with average particle size, the yield stress and elasticity of the gels can be easily renormalized for all particle sizes on the basis of the volume of the particles. Furthermore, rheological modeling of the flow curves shows that for all the particles, an approach based on excluded volume effects captures most features of nontronite suspensions. Still, the high shear flow properties of the suspensions that reveal a strong orientation of particles in the flow are affected by electrostatic interactions. This study then shows that the rich phase behavior of clay minerals, notably the fact that some clay minerals display an isotropic/nematic transition while others exhibit a sol-gel transition, requires a full understanding of all the interactions in the system that can only be achieved by working on well-characterized size-selected samples.

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.

Interaction of nanometric clay platelets

The free energy of interaction between two nanometric clay platelets immersed in an electrolyte solution has been calculated using Monte Carlo simulations as well as direct integration of the configurational integral. Each platelet has been modeled as a collection of charged spheres carrying a unit charge the face of a platelet contains negative charges, and the edge, positive charges. The calculations predict that a configuration of "overlapping coins" is the global free energy minimum at intermediate salt concentrations (10-100 mM). A second weaker minimum, corresponding to the well-known "house of cards" configuration, also appears in this salt interval. At low salt concentrations the electrostatic repulsion dominates, while at intermediate concentrations electrostatic interactions alone can create a net attraction between the platelets. At sufficiently high salt content (>200 mM), the van der Waals interaction takes over and the net interaction becomes attractive at essentially all separations. From the calculated free energy and its derivative, we can derive a yield stress and elasticity modulus in fair agreement with experiment. The roughness of the platelets affects the quantitative behavior of the free energy of interaction but does not alter the results in a qualitative way. From the variation of the free energy of interaction, we would tentatively describe the phase behavior as follows: At low salt, the interaction is strongly repulsive and the dispersion should appear as a solid ("repulsive gel"). With increasing salt concentration, the repulsion is weakened and a liquid phase appears ("sol"). A further increase of the salt content leads a second solid phase ("attractive gel") governed by attractive interactions between the platelets. Finally, at sufficiently high salinity, the clay precipitates due to van der Waals forces.

Dispersions of internally liquid crystalline systems stabilized by charged disklike particles as pickering emulsions: basic properties and time-resolved behavior

The present paper reports on dispersions of internally liquid crystalline particles, formed from monoglyceride and oil mixtures, stabilized with discrete disklike particles of Laponite clay. Small-angle X-ray scattering (SAXS) was used to probe the presence of dispersed particles as well as their internal liquid crystalline structure. The data were compared to scattering results of reference systems, namely, from the bulk as well as from well-defined particles formed with a polymer as the emulsifier. The submicrometer sizes of the various particles could be derived using dynamic light scattering (DLS). The possible mechanisms involved in the stabilization of each of the different phases by the Laponite platelets, including the role of the residual salt, are discussed. Time-resolved experiments were performed over 60 days in order to follow the evolution of both the internal structure and size of the particles. In particular, we discuss the peculiar behavior of the sample without added oil, where the cubosomes transform into hexosomes over time. The effect of the high pH induced by the Laponite platelets in water, which could result in a hydrolysis of the monoglycerides, was shown to be responsible for the observed cubosome-to-hexasome transition, as well as for the decrease in the lattice parameters.

Sol/gel and isotropic/nematic transitions in aqueous suspensions of natural nontronite clay. Influence of particle anisotropy. 1. Features of the i/n transition

The phase behavior of a natural nontronite clay was studied for size-selected particles by combining osmotic pressure measurements, visual observations under polarized light, and rheological experiments. In parallel, the positional and orientational correlations of the particles were analyzed by small-angle X-ray scattering. Aqueous suspensions of nontronite exhibit a true isotropic/nematic (I/N) transition that occurs before the sol/gel transition, for ionic strengths below 10(-3) M/L. In this region of the phase diagrams, the system appears to be purely repulsive. The I/N transition shifts toward lower volume fractions for increasing particle anisotropy, and its position in the phase diagram agrees well with the theoretical predictions for platelets. SAXS measurements reveal the presence of characteristic interparticular distances in the isotropic, nematic, and gel phases. The swelling law (separation distance vs swelling law) exhibits two regimes. For high volume fractions, the swelling law is one-dimensional as in layered systems and reveals the presence of isolated platelets. At lower volume fraction, distances scale as phi(-1/3), indicating isotropic volumic swelling. Finally, the experimental osmotic pressure curves can be satisfactorily reproduced by considering the interparticle distances between two charged planes whose effective charge is around 10% of the structural charge.

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.

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.

Phase transitions of pyrogenic silica suspensions: a comparison to model laponite

Pyrogenic silica is often used as a thickening agent in paints, pastes, adhesives, or resins. Other applications include, e.g., abrasives in chemical mechanical planarization in the microelectronics industry. In all these applications it is essential to control the state of dispersion. Sometimes, phase transitions from the liquid to the solid state are required while in other cases they have to be completely avoided for the whole shelf life. The nature and influencing parameters of the fluid-solid transition for pyrogenic silica have not been investigated so far. Most investigations deal with the phase transitions of small clay particles such as laponite. Here, we dedicate our interest to the behavior of pyrogenic silica suspensions with varying specific surface area and ionic background concentration. To get an impression of the phase transition behavior we compare our results to model laponite suspensions. We apply dynamic light scattering measurements in the backscattering regime to minimize multiple scattering contributions from concentrated pyrogenic silica suspensions. Further on we exert a decomposition of the measured autocorrelation functions into an ergodic and nonergodic contribution. The analysis of the ergodic spectrum yields two different gelation kinetics for both systems, laponite and pyrogenic silica. For laponite these are in accordance with earlier investigations. The kinetics depend on the ionic background and the solids content of the suspensions. Additionally, we used dynamic extinction spectroscopy to follow the phase transitions of pyrogenic silica on a macroscale.

Model for cage formation in colloidal suspension of laponite

In this paper we investigate glass transition in aqueous suspension of synthetic hectorite clay, laponite. We believe that upon dispersing laponite clay in water, the system comprises of clusters (agglomerates) of laponite dispersed in the same. Subsequent osmotic swelling of these clusters leads to an increase in their volume fraction. We propose that this phenomenon is responsible for slowing down of the overall dynamics of the system. As clusters fill up the space, the system undergoes glass transition. Along with the mode coupling theory, the proposed mechanism rightly captures various characteristic features of the system in the ergodic regime as it approaches glass transition.

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.

Pickering miniemulsion polymerization using Laponite clay as a stabilizer

Solid-stabilized, or Pickering, miniemulsion polymerizations using Laponite clay discs as stabilizer are investigated. Free radical polymerizations are carried out using a variety of hydrophobic monomers (i.e., styrene, lauryl (meth)acrylate, butyl (meth)acrylate, octyl acrylate, and 2-ethyl hexyl acrylate). Armored latexes, of which the surfaces of the particles are covered with clay discs, are obtained, as confirmed by scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). Overall polymerization kinetics of the Pickering miniemulsion polymerizations of styrene were investigated via gravimetry. Comparison with the bulk polymerization analogue clearly shows compartmentalization. Moreover, retardation effects up to intermediate monomer conversions are observed; they are more prominent for the smaller particles and are ascribed to the Laponite clay. A model is presented that allows for the prediction of the average particle size of the latexes produced as a function of the amounts of monomer and Pickering stabilizers used. It shows that under specific generic conditions the number of clay discs used correlates in a linear fashion with the total surface area of the latex particles. This is a direct result of the reversibility of the Laponite clay disc adhesion process under the emulsification conditions (i.e., sonication) used.

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.

Aging in a Laponite colloidal suspension: A Brownian dynamics simulation study

The authors report Brownian dynamics simulation of the out-of-equilibrium dynamics (aging) in a colloidal suspension composed of rigid charged disks, one possible model for Laponite, a synthetic clay deeply investigated in the last few years by means of various experimental techniques. At variance with previous numerical investigations, mainly focusing on static structure and equilibrium dynamics, the authors explore the out-of-equilibrium aging dynamics. They analyze the wave vector and waiting time dependence of the dynamics, focusing on the single-particle and collective density fluctuations (intermediate scattering functions), the mean-squared displacement, and the rotational dynamics. Their findings confirm the complexity of the out-of-equilibrium dynamical behavior of this class of colloidal suspensions and suggest that an arrested disordered state driven by a repulsive Yukawa potential, i.e., a Wigner glass, can be observed in this model.

Adsorption and radical stabilization of humic-Acid analogues and Pb2+ on restricted phyllomorphous clay

Humic acids have stable radicals that are indigenous to their structure. Hydroxybenzoic acid derivatives such as gallic acid (GA) and protocatechuic acid are appropriate models for the radical properties of humic acids. Here we show that the adsorption or intercalation of gallic acid in Laponite clay results in a significant thermodynamic stabilization of gallic acid radicals. Moreover, the formed organoclay shows enhanced stability against acid dissolution. The structural details of the association of gallic acid with Laponite depend on the GA/Laponite loading. At low GA/Laponite ratios (approximately 10(-6) M of gallic acid per gram of clay), gallic acid is adsorbed at the variable charge sites of Laponite. This adsorption can be adequately described by surface complexation modeling. At higher GA/Laponite ratios (approximately 10(-3) M of gallic acid per gram of clay), X-ray diffraction data show that gallic acid is intercalated at the interlamellar sites of Laponite. In the presence of Pb2+ ions, the formed GA/Pb complex is associated with Laponite in an analogous structural manner, that is, adsorption at variable charge sites or intercalation at the interlamellar sites of Laponite, depending on the loading. Laponite stabilizes the GA/Pb radicals. At prolonged exposure to ambient O2, Laponite promotes the formation of stable oligomeric GA/Pb radical species, which are intercalated into interlamellar sites.

Electrostatic stability of a large collection of charged anisotropic nanoparticles: a monte carlo study exploiting a hierarchical approach

We have performed Monte Carlo simulations of the self-organization of large collections of charged disks of various electric charges to probe the influence of electrostatic coupling on the structure and the mechanical stability of aqueous dispersions of charged anisotropic nanoparticles in the presence of salt. A hierarchical approach of the long-range Coulombic potential was used to perform such Monte Carlo simulations of a large number of charged species. By analyzing the influence of the net electric charge of the disks on their self-organization, we detected a negative contribution to the mean force potential resulting from their electrostatic coupling. In addition, it has been shown that the same electrostatic coupling restricts the spatial extend of locally ordered microdomains within dilute dispersions of charged anisotropic colloids.

Relating clay yield stress to colloidal parameters

The ability to predict rheological behavior of clay dispersions would be useful in formulating systems rheologically modified by clays as well as designing equipment to handle clay based materials. Deriving predictive equations for these materials has been hindered due to the various mechanisms by which clay systems develop microstructure. This work seeks to observe the relationship between the yield stress, tauy, and the zeta potential, zeta, and determine the nature of the particle interactions for dispersions of laponite (a synthetic clay) and dispersions of a mixture of naturally occurring kaolinites and bentonites. It is found that the relationship of tauy to zeta2 for clay suspensions is opposite to that found for homogeneously charged spheroidal colloids. This result can be traced to the type of particle interactions occurring, which for the systems studied appear to be edge-to-face attractions.

High-salt stabilization of Laponite clay particles

Colloidal radioactive transuranic wastes are currently buried in large tanks in the form of dense colloids in high salt, high pH aqueous media. These facilities are beginning to fail, so it is necessary to transport and "package" them for more permanent disposal, processes requiring understanding of the microstructure that develops under such conditions. Laponite RD clay is believed to be a good simulant for the colloids in the waste tanks, and the present study addresses their behavior under high salt conditions, where previous studies have frequently observed the phenomenon of "restabilization," i.e., the attainment of aggregation stability at high electrolyte conditions. Specifically, the aggregation kinetics and the resulting cluster structure (fractal dimension) of Laponite RD clay colloids at high concentrations of BaCl2 (an electrolyte previously shown to lead to restabilization) are investigated. At low-to-intermediate electrolyte concentrations, the clay is found to behave in accord with DLVO theory, i.e., low salt conditions yield slow aggregation into densely-packed aggregates, whereas intermediate salt concentrations, sufficient to cause double layer collapse, produce rapid aggregation into open aggregates. High salt concentrations, however, show slow rates of aggregation. The aggregate structure under these conditions is found to mimic that found for very low electrolyte concentrations, i.e., high fractal dimension. Further experiments show that a sudden increase in salt concentration in a system containing young open aggregates produced under intermediate salt concentrations causes them to reform into more compact structures.

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.