Aqueous Laponite Dispersions Research Articles

Aqueous Laponite® dispersions are attractive gels, not repulsive Wigner glasses: A critical commentary

An aqueous dispersion of Laponite® has been studied in the literature for over the past three decades. Typically, the aqueous dispersion of Laponite® undergoes incessant evolution of its microstructure, wherein its elastic modulus and the mean relaxation time show a continuous increase as a function of time. A considerable amount of discussion has revolved around the nature of this dispersion, specifically whether it can be classified as a repulsive Wigner glass state, characterized by disconnected Laponite® particles stabilized by electrostatic repulsions, or an attractive gel state, in which the particles form a percolated space-spanning network. The proponents of the Wigner glass state also conjecture that this system experiences a glass–glass transition after a period of 2 days has elapsed since its preparation. In this Commentary, we explore this topic from a rheological point of view, analyzing the published literature and performing new experiments. Aided by additional evidence from the literature, we propose that rheological behavior overwhelmingly suggests that an aqueous dispersion of Laponite® undergoes a sol–attractive gel transition and remains in the attractive gel state over at least up to 7 days without undergoing any additional transition. Importantly, rheology, despite being a macroscopic tool governed by principles of mechanics, offers profound insight into the microstructure of this particular system. The corresponding analysis conclusively determines the state of an aqueous dispersion of Laponite® to be an attractive gel.

Structural evolution of salt-free aqueous Laponite dispersions: A study based on low-field NMR relaxometry and rheological investigations

The combination of Low-Field NMR relaxometry (LF-NMR) and rheological analyses was used to investigate the kinetics of aging and structural evolution of salt-free aqueous Laponite dispersions with concentrations ranging from 0.33 to 2.00 wt%. The examination of LF-NMR results reveals that the rate of nanoclay delamination increases with increasing Laponite concentration while its degree decreases. Furthermore, the LF-NMR results were interpreted together with rheological results to assess the kinetics of processes occurring on different length scales reflecting on the time evolution of the three-dimensional structure. The structural evolution is described by the definition and comparison of three different characteristic times. The LF-NMR decay time (tr) designates the time required for Laponite delamination. The induction time (ti) indicates when interparticle edge-face interactions become effective enough for incipient networking. The gelation time (tg) corresponds to sufficiently high degree of interparticle connectivity for the development of a macroscopic gel behavior. The comparison of ti and tg, on one hand, and tr, on the other hand, allows to conclude that for low concentration Laponite dispersions nanoclay delamination is a prerequisite for the formation of the new dispersion structure with a macroscopic gel behavior. Interestingly, the development of the new structure, based on orientation of clay disks into edge-to-face pattern and mutual interparticle attractive interactions, starts when the delamination process is still running.

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.

In situ ion induced gelation of colloidal dispersion of Laponite: Relating microscopic interactions to macroscopic behavior

Aqueous dispersion of Laponite, when exposed to CO2 environment leads to in situ inducement of magnesium and lithium ions, which is, however absent when dispersion is exposed to air. Consequently, in the rheological experiments, Laponite dispersion preserved under CO2 shows more spectacular enhancement in the elastic and viscous moduli as a function of time compared to that exposed to air. By measuring concentration of all the ions present in a dispersion as well as change in pH, the evolving inter-particle interactions among the Laponite particles is estimated. DLVO analysis of a limiting case is performed, wherein two particles approach each other in a parallel fashion – a situation with maximum repulsive interactions. Interestingly it is observed that DLVO analysis explains the qualitative details of an evolution of elastic and viscous moduli remarkably well thereby successfully relating the macroscopic phenomena to the microscopic interactions.

Nanocomposite DNA hydrogels with temperature sensitivity

DNA hydrogels are promising soft materials because of the distinctive properties of DNA such as biocompatibility, coil-globule transition, molecular recognition, and selective binding. However, such hydrogels synthesized so far suffer from poor mechanical properties and low dimensional stability, which limit their stress-bearing technological applications. Here, we introduce a novel concept for the preparation of high-strength double-stranded (ds) DNA hydrogels with temperature sensitivity. The hydrogels were synthesized by free-radical polymerization of N-isopropylacrylamide in aqueous Laponite dispersions containing ds-DNA in a highly entangled state. The nanoparticles of synthetic Laponite clay act as a dynamic cross-linker to form a three-dimensional physical network due to strong clay-polymer interactions. We observed that ds-DNA maintains its native structure and function during the formation of nanocomposite hydrogels. Cyclic heating/cooling experiments conducted between below and above the melting temperature of ds-DNA reveal a drastic increase in the elastic modulus of nanocomposite hydrogels due to the thermally induced denaturation and renaturation of DNA within the physical network. The hydrogels prepared at various Laponite contents display the characteristics of both poly(N-isopropylacrylamide) and ds-DNA e.g., temperature sensitivity, denaturation and renaturation of DNA strands, and they exhibit a high tensile fracture stress (up to 53 kPa) and elongation to break (up to 544%).

Evolution of Self-organization and gelation in Laponite Ionogels

We report formation of colloidal gel network in aqueous laponite dispersions containing ionic liquid (IL). With the passage of time, these flowing dispersions reorganized and attained a solid-like rigid structure. Structural reorganization of clay platelets in the IL environment caused reduction in the inter platelet repulsion due the IL-double layer screening and facilitated formation of stronger ionogels for samples with [IL] 0.03% (w/v)). The structure factor comprised two relaxation modes diffusion fast mode followed by an anomalous slow mode. The slow mode was frozen at all IL concentrations whereas the fast mode contained two diffusion coefficients both strongly dependent on IL concentration. In summary, laponite gels could be systematically customized in IL solutions to generate a range of soft materials with properties solely dependent on IL concentration which may not exceed 0.05% (w/v).

Stabilization of ASA-in-water emulsions by Laponite modified with alanine

Alkenyl succinic anhydride (ASA) is an oily paper sizing agent that is generally introduced into the paper manufacturing system as an ASA-in-water emulsion. In this paper, ASA-in-water emulsions are prepared by homogenizing the mixture of ASA and aqueous dispersion of Laponite modified with alanine. The modification of Laponite with alanine reduces the emulsion droplet size, and enhances the emulsion stability against coalescence due to lowering the interfacial tension between ASA and water containing Laponite and improving the affinity of Laponite particles toward ASA. Infrared spectrum analysis indicates that the adsorption of alanine on the Laponite particles retards the hydrolysis of ASA when the alanine-modified particles are employed to stabilize ASA emulsions. Meanwhile, the sizing efficiency of the as-prepared ASA emulsion is also improved by the modification of Laponite with alanine, and more dependent on emulsion droplet size than on the hydrolysis of ASA.

Laponite and PAS costabilized ASA emulsion with high hydrolysis resistance and sizing efficiency

AbstractASA emulsions costabilized by Laponite and polyaluminum sulfate (PAS) were prepared by homogenizing the mixture of ASA and aqueous dispersion of Laponite and PAS. The properties, sizing performance and hydrolysis resistance of the prepared ASA emulsions, were investigated in this article. The results show that the introduction of PAS significantly improves the stability, hydrolysis resistance, and sizing performance of ASA emulsions. The ASA emulsion stabilized by 0.5% of Laponite (based on total mass of emulsion) and 2% of PAS (based on the mass of Laponite) shows high hydrolysis stability and high sizing efficiency on bleached chemi‐thermomechanical pulp. At the addition level of 0.3% based on bleached chemi‐thermomechanical pulp, the sizing degree of ASA‐sized paper can be as high as 120 s. Even when the storage time of the ASA emulsion exceeds 3 h, its sizing performance is hardly changed. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Magnetorheology in an aging, yield stress matrix fluid

Field-induced static and dynamic yield stresses are explored for magnetorheological (MR) suspensions in an aging, yield stress matrix fluid composed of an aqueous dispersion of Laponite® clay. Using a custom-built magnetorheometry fixture, the MR response is studied for magnetic field strengths up to 1 T and magnetic particle concentrations up to 30 v%. The yield stress of the matrix fluid, which serves to inhibit sedimentation of dispersed carbonyl iron magnetic microparticles, is found to have a negligible effect on the field-induced static yield stress for sufficient applied fields, and good agreement is observed between field-induced static and dynamic yield stresses for all but the lowest field strengths and particle concentrations. These results, which generally imply a dominance of inter-particle dipolar interactions over the matrix fluid yield stress, are analyzed by considering a dimensionless magnetic yield parameter that quantifies the balance of stresses on particles. By characterizing the applied magnetic field in terms of the average particle magnetization, a rheological master curve is generated for the field-induced static yield stress that indicates a concentration–magnetization superposition. The results presented herein will provide guidance to formulators of MR fluids and designers of MR devices who require a field-induced static yield stress and a dispersion that is essentially indefinitely stable to sedimentation.

Double Inversion of Emulsions Induced by Salt Concentration

The effects of salt on emulsions containing sorbitan oleate (Span 80) and Laponite particles were investigated. Surprisingly, a novel double phase inversion was induced by simply changing the salt concentration. At fixed concentration of Laponite particles in the aqueous phase and surfactant in paraffin oil, emulsions are oil in water (o/w) when the concentration of NaCl is lower than 5 mM. Emulsions of water in oil (w/o) are obtained when the NaCl concentration is between 5 and 20 mM. Then the emulsions invert to o/w when the salt concentration is higher than 50 mM. In this process, different emulsifiers dominate the composition of the interfacial layer, and the emulsion type is correspondingly controlled. When the salt concentration is low in the aqueous dispersion of Laponite, the particles are discrete and can move to the interface freely. Therefore, the emulsions are stabilized by particles and surfactant, and the type is o/w as particles are in domination. At intermediate salt concentrations, the aqueous dispersions of Laponite are gel-like, the viscosity is high, and the transition of the particles from the aqueous phase to the interface is inhibited. The emulsions are stabilized mainly by lipophilic surfactant, and w/o emulsions are obtained. For high salt concentration, flocculation occurs and the viscosity of the dispersion is reduced; thus, the adsorption of particles is promoted and the type of emulsions inverts to o/w. Laser-induced fluorescent confocal micrographs and cryo transmission electron microscopy clearly confirm the adsorption of Laponite particles on the surface of o/w emulsion droplets, whereas the accumulation of particles at the w/o emulsion droplet surfaces was not observed. This mechanism is also supported by the results of rheology and interfacial tension measurements.

Size dependence of microprobe dynamics during gelation of a discotic colloidal clay

Soft materials, such as gels and colloidal glasses, often exhibit different rheological properties at bulk and microscopic scales as a result of their complex microstructure. This phenomenon has recently been demonstrated for a gel-forming aqueous dispersion of Laponite® clay [Oppong et al. Phys. Rev. E 78, 021405 (2008)]. For this material, microrheology reveals a significantly weaker gel and a longer gelation time than bulk measurements. By performing multiple particle tracking microrheology experiments with different probe sizes, we show that length-scale–dependent rheology is a general feature of Laponite® gels. Small changes in probe size are accompanied by order of magnitude differences in the observed rheological properties and gelation time. The probe dynamics also exhibit size-dependent spatial heterogeneities that help to elucidate a microstructural length scale in the system. Through analytical theory and Brownian dynamics simulations, we find that the correlations described by previous authors between successive displacements of individual probes are more directly a result of material elasticity than of microstructural confinement. The apparent gelation times of dispersions with different Laponite® concentrations exhibit a self-similar dependence on probe size, suggesting a superposition of Laponite® concentration and probe size. From these observations, we propose an accordant description of the microstructural evolution of the gel.

Temperature-Triggered Gelation of Aqueous Laponite Dispersions Containing a Cationic Poly(N-isopropyl acrylamide) Graft Copolymer

In this work, temperature-triggered gelation of aqueous laponite dispersions containing a cationic poly(N-isopropylacrylamide) (PNIPAm) graft copolymer was investigated. The copolymer used was PDMA(+)(30)-g-(PNIPAm(210))(14) [Liu et al. Langmuir 2008, 24, 7099]. DMA(+) is quarternarized N,N-dimethylaminoethyl methacrylate. The presence of small concentrations of laponite enabled temperature-triggered gel formation to occur at low copolymer concentrations (e.g., 1 wt %). Dynamic rheological measurements of the gels showed that they had storage modulus values of up to 400 Pa when the total solid volume fraction (polymer and laponite) was only about 0.02. The storage modulus was dependent on both the temperature and the composition of the dispersion used for preparation. The key component that provided the temperature-triggered gels with their elasticity was found to be self-assembled nanocomposite (NC) sheets. These NC sheets spontaneously formed at room temperature upon addition of laponite to the copolymer solution. The NC sheets had lateral dimensions on the order of hundreds of micrometers and a thickness of a few micrometers. The NC sheets were present within the temperature-triggered gels and formed elastically effective chains. The NC sheets exhibited temperature-triggered contraction with a contraction onset temperature of 27 degrees C. A conceptual model is proposed to qualitatively explain the relationship between gel elasticity and dispersion composition.

Effect of aging time on the rheology of Laponite dispersions

The effect of aging time on aqueous Laponite dispersions containing sodium polyacrylate was analyzed by rheometric measurements. The rheological behavior of these dispersions was determined by oscillatory and flow experiments. Addition of sodium polyacrylate modifies the interactions between Laponite particles and, therefore, the physical state of the dispersion. The phase diagram of Laponite dispersions as a function of sodium polyacrylate concentration shows different sol–gel transitions for a specific Laponite concentration. The transition line goes to lower Laponite concentrations with aging time. A structural model permitted the characterization of the thixotropy of Laponite dispersions by means of the alignment of the internal structure to the applied shear rate and by the thixotropic times of the net breakdown and buildup processes. The thixotropy of dispersions in presence of sodium polyacrylate was less dependent on aging time, indicating that sodium polyacrylate reduces the aggregation rate of the formation of a three-dimensional network and permits better control of rheology, at least for 12 days after the preparation of dispersion.

On the refractive index of ageing dispersions of Laponite

Aqueous dispersion of Laponite at low ionic concentration is of interest since it undergoes structural evolution with respect to time, which is usually termed as ageing. In this work we study the refractive index behavior as a function of ageing time, concentration and temperature. We observed that the extended Lorenz–Lorentz equation fitted the refractive index dependence on concentration and temperature very well. The refractive index did not show any dependence on ageing time. However, the dependence of refractive index on concentration showed a marked change as the system underwent transition from an isotropic to a biphasic state. The slope of the refractive index–density data is remarkably close to that of water at all Laponite concentrations. In the context of transport phenomena, optical measurements such as interferometry can exploit the water-like behavior of Laponite dispersions.

Photochemical production or depletion of hydrogen peroxide controlled by different electron transfer pathways in methyl viologen intercalated clays

The composites prepared by intercalating methyl viologen (MV 2+) into clay lamella could lead to either production or depletion of hydrogen peroxide (H 2O 2) under UV light irradiation. H 2O 2 was found to be generated in aerated aqueous dispersion of Laponite intercalated with MV 2+ (MV 2+–Lap). In contrast, MV 2+ intercalated Nontronite (MV 2+–Non), promoted the decomposition of H 2O 2. In both cases, the intercalated MV 2+ was excited upon UV irradiation and then captured an electron from clay hosts, through which methyl viologen cation radical (MV + ) was formed. In iron free Lap, MV + reduced the dissolved oxygen to superoxide/hydroperoxie radicals (O 2 − /HO 2 ) and H 2O 2 was produced via self-disproportionation. When iron species were present in clay host, however, MV + reduced structural Fe(III) to Fe(II) in clay lattice, leading to the decomposition of H 2O 2.

Influence of sodium polyacrylate on the rheology of aqueous Laponite dispersions

Aqueous Laponite dispersions containing a sodium polyacrylate were analyzed, at fixed ionic strength and pH, by rheometric and electroacoustic (for zeta-potential determinations) techniques at 7 days after their preparation. The rheological behavior of these dispersions was determined by oscillatory and flow experiments. Addition of sodium polyacrylate modifies the interactions between Laponite particles and therefore the physical state of the dispersion. The phase diagram of Laponite dispersion as a function of sodium polyacrylate concentration shows different sol–gel transitions for a specific Laponite concentration as a function of the polyacrylate concentration. Under equilibrium flow conditions the Laponite dispersions fit the pseudoplastic Oswald–de Waele power law model. At the same time, these dispersions show thixotropy, which was analyzed using a second-order kinetic equation. The kinetic processes were characterized by breakdown and build-up parameters, which were found to depend on shear rate. This kinetic equation was modified by a power law exponent of viscosity with shear rate that takes into account the viscosity variations when the shear rates are suddenly changed, in order to fit the hysteresis loops.

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

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

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.