Laponite Dispersions Research Articles

Ionic liquid induced surface exclusion and anomalous first-order phase transition in Laponite dispersions

Spontaneous liquid–liquid phase separation occurring in Laponite dispersions of ionic liquid (IL) solutions was observed at room temperature driven by surface exclusion of clay platelets at the water–air interface. The water–air interface dependent demixing behavior followed anomalous first-order phase transition kinetics described by a partition reaction CSK↔S+C with a demixing partition coefficient K=[S]/[C] where the initial homogeneous dispersion [CS] phase separated into a Laponite-poor [S] and Laponite-rich [C] state. The colloidal aggregates were found to be fractal structures with a mass fractal dimension 1.35<df<1.75. The demixing process could be described by a droplet splitting model. The settling dynamics was induced by ionic liquids due to their charge specific interaction with clay platelets, and the surface exclusion of clay particles at the liquid–air interface governed the rate of demixing. Interestingly, the time-constant of first-order dynamics was found to be invariant of IL concentration.

Shaken but not stirred: The formation of reversible particle – polymer gels under shear

Abstract Some dispersions of clay and silica particles in water in the presence of relatively high molecular weight polyethyleneoxide (PEO), which are fluid when at rest, become solid-like after a quick shake to such an extent that they can be held in the hand. On leaving the dispersions for a certain period of time, minutes to days depending on the polymer molecular weight and concentration, the dispersions become liquid-like again. These dispersions have been called “shake gels”, and a number of physical variables are determinant for producing the gel and controlling its behavior. In this work, we have studied the effect of the shape and size of the particles and of the PEO molecular weights. To that aim, we have mapped the “phase” behavior of silica (Ludox TM-50), montmorillonite and laponite dispersions in presence of PEO of different molecular weight. Shake gels are formed under certain concentrations of particles and PEO. The necessary degree of particle coverage for shake gel formation seems to depend on the particle shape. Whereas in the case of disc-shaped particles this limit is around the saturation concentration, in the case of spherical particles the limit is around 2/3 of the particle surface saturation. On the other hand, we observe some differences between montmorillonite (micrometer-size particles) and laponite (nanometric particles) dispersions. When we shake the former we find in some cases an important and irreversible phase separation; on the contrary, in the case of the laponite dispersions, the phase separation is far less frequent and extensive. Finally, we have found that the nature of the applied shear field has a profound effect on the sample behavior. When we place the dispersions in a conventional rheometer and shear them at moderate shear rate, for many minutes the gel is not formed. However, shaking it by hand or extruding it through a syringe brings about this effect in a matter of seconds.

Surface chemistry and rheology of Laponite dispersions — Zeta potential, yield stress, ageing, fractal dimension and pyrophosphate

The ageing behaviour of Laponite gels at the phase boundary of attractive gel and flocculated state (ionic strength~0.01M 1:1 electrolyte) was investigated due to a lack of study in this region. Zeta potential and yield stress measurement revealed that freshly prepared Laponite dispersion took time to reach the surface chemical equilibrium (SCE) state. The higher the ionic strength, the shorter was the time needed. This well-defined structural state was employed at the commencement of ageing study. The ageing behaviour was characterised by a rapidly increasing yield stress region followed by a gradually increasing region and then by a plateau region. Both the initial and fully aged yield stress of Laponite gels increased with ionic strength and solid loading. Both the Leong and the two-parameter logarithmic time models described the ageing behaviour quite well. The relationship between the yield stress at the SCE state and at the fully recovered or rejuvenated state, and volume fraction obeyed a power law model. The fractal dimension of the two states was the same 2.0. This study also investigated the yield stress-pH behaviour of Laponite gel with and without pyrophosphate additive. Pure Laponite dispersion displayed a maximum yield stress at high pH. A drop in yield stress occurred at low pH region and eventually approaching zero yield stress. A totally opposite trend was observed with pyrophosphate additive. No yield stress was detected at high pH region and a maximum yield stress was located at pH5 followed by a drop in yield stress until pH2. This drop in the yield stress regardless of the presence of pyrophosphate, was due to the particle agglomeration promoted by low pH.

Preparation and mechanical properties of a transparent ionic nanocomposite hydrogel

The poor stability of clay dispersion in the presence of ionic species presents a challenge for preparing a transparent ionic nanocomposite (NC) hydrogel with the desired strength. A transparent ionic NC hydrogel was successfully prepared by in situ copolymerization of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) and acrylamide (AAm) in the Laponite dispersion. AMPS played a dominant role in the dispersion of Laponite. Mechanical analyses demonstrated that this type of hydrogel has a combination of high tensile strength and excellent stretch ability. An increase in AMPS content made a big contribution to the tensibility, while an increase in clay content improved the elastic modulus. The ionic NC hydrogel exhibited a self-healing property, which occurred autonomously, without any outside intervention. The results showed that physical interaction among the NC hydrogels can be recovered. This work is a continuation of our previous efforts on the preparation and application of ionic NC hydrogels, for which there is a range of potential applications in soft and smart machines in the future.

Improving Stability and Sizing Performance of Alkenylsuccinic Anhydride (ASA) Emulsion by Using Melamine-Modified Laponite Particles as Emulsion Stabilizer

Alkenylsuccinic anhydride (ASA) is commonly applied as oil-in-water (o/w) emulsions in the papermaking industry. Herein Laponite mineral nanoparticles were employed as a stabilizer of the ASA emulsions after being modified with melamine just before emulsion preparation. The emulsion was prepared by homogenizing the mixture of ASA and melamine-modified Laponite aqueous dispersion. The modification of melamine on the Laponite was characterized by infrared spectroscopy and X-ray diffraction, whereas the impacts of the modification on the morphology, wettability and ζ-potential of the Laponite, as well as the interfacial tension between ASA and Laponite aqueous dispersion, were also analyzed. It is found that the adsorption of melamine on Laponite particles neither causes the aggregation nor significantly changes the charge properties of the Laponite particles. However, the adsorption of melamine can significantly increase the wettability of Laponite by the ASA liquid, and adequately lower the apparent interfacial tension between ASA and Laponite aqueous dispersion when the melamine-to-Laponite mass ratio is less than 3%. This results in an improvement in emulsion stability, a reduction in emulsion droplet size and an enhancement in the sizing performance of the ASA emulsion when the emulsion is stabilized by melamine-modified Laponite particles. The ASA emulsion with the smallest droplet size and best sizing performance is produced at a melamine-to-Laponite mass ratio of 3%. By monitoring the variations of the emulsion with time, it is discovered that the modification of Laponite with melamine can restrain the growth of emulsion droplets and the hydrolytic action of ASA substantially, thus decreasing the loss in sizing performance of the ASA emulsion with time. This is particularly important for the wide application of ASA emulsions in the papermaking industry.

ASA-in-water emulsions stabilized by laponite nanoparticles modified with tetramethylammonium chloride

Alkenyl succinic anhydride (ASA) is a widely used paper sizing agent that is applied in the form of oil-in-water (o/w) emulsions in order to impart a water-resistant character to the resulting paper. To obtain stable o/w emulsions of ASA, laponite, a highly hydrophilic synthetic clay, was selected as the stabilizer after it had been modified with tetramethylammonium chloride (TMAC), a quaternary ammonium salt with the shortest possible hydrocarbon groups. It was found that the TMAC moderately neutralized the negative charges of laponite particles, lowered the apparent viscosity, but enhanced the turbidity of laponite aqueous dispersion by enhancing the hydrophobicity of the laponite particles, favoring adsorption of laponite particles on the ASA–water interface. Meanwhile, the TMAC significantly decreased the interfacial tension between ASA and water/aqueous laponite dispersion, promoting the formation of an emulsion with small droplets. When the added amount of TMAC reached 1wt% based on laponite, the as-prepared ASA emulsion had small droplet size, low viscosity and uniform droplet size distribution, and exhibited good creaming/coalescence stability. By using TMAC to modify laponite nanoparticles, the hydrolysis stability and sizing performance of ASA emulsion were also improved.

Coexistence of iso-nonergodic laponite gel and glass in 1-methyl-3-octylimidazolium chloride ionic liquid solutions.

We report unique colloidal gel-glass coexistence in aqueous laponite dispersion (2% w/v) in the presence of 1-methyl-3-octylimidazolium chloride ionic liquid (IL, [C8mim][Cl], concentration 0.01 to 0.05% w/v), where both of the phases had identical nonergodicity and were dynamically interactive. With aging, the nascent heterogeneous dispersion exhibited spontaneous two-phase separation, and the time-dependent relative viscosity followed: η(r) = |ε|(-k) where ε = (t - t(g))/t(g) and t(g) is the time required for the system to get arrested, with k decreasing from 3.13 to 2.54 as the IL concentration was increased from 0 to 0.03% (w/v), implying slowing down of the arrest kinetics. This time was measured from viscosity and rheology studies, revealing the formation of IL-mediated finite size colloidal networks on a time scale of ~4 × 10(3) s, whereas the dispersion developed a large viscosity one decade in time later (~4 × 10(4) s). Homogeneous transparent upper phase was an entropic glass and exhibited substantial storage modulus gain (300-3000 Pa) with an increase in IL concentration (0 to 0.05% (w/v)). The translucent lower gel phase had a much higher storage modulus. Dynamic light scattering measured bimodal relaxation time of concentration fluctuations. The degree of nonergodicity in the two phases was approximately the same, implying laponite-IL cluster exchange across the interface (identical slow-mode diffusivity). In summary, IL-induced first-order phase separation in laponite dispersion produced a homogeneous colloidal gel coexisting with a glass not commonly observed in soft matter systems. This implied that the two phases were dynamically coupled on long time scales, whereas their short-time behavior was distinctively different.

Time-dependent rheology of clay particle-stabilised emulsions

We have studied the thixotropic behaviour of clay particle-stabilised emulsions using a combination of rheology and confocal fluorescence microscopy. Oil drops were stabilised by laponite particles attached to their surfaces and incorporated into a three dimensional super-aggregate structure of laponite particles that formed in the water. The self-supporting, three-dimensional network of drops and laponite particles gives the emulsions pronounced gel-like properties. The transient flow behaviour of the emulsions depends on the mechanical properties of the laponite dispersions and the oil drop volume fraction.

Ergodic-to-nonergodic phase inversion and reentrant ergodicity transition in DNA–nanoclay dispersions

We have observed DNA concentration and hydration dependent inversion from ergodic to non-ergodic phase followed by reentry into the ergodic phase in DNA-nanoclay (laponite) dispersions at room temperature (25 °C), using results obtained from dynamic light scattering (DLS) and rheology data. The interaction between the DNA strand and the anisotropically charged discotic platelets of laponite (L) was found to be strongly hierarchical in DNA concentration. For a fixed laponite concentration (CL = 1% (w/v)) and varying DNA concentration (CDNA) from 0.3-2.3% (w/v), we observed three distinct phase regions characterized by the following: region (i): CDNA < 1.0% (w/v), ergodic region with weak DNA-L attractive interaction, region (ii): 1.0% < CDNA < 1.6% (w/v), non-ergodic regime having strong DNA-L associative interaction and region (iii): CDNA > 1.6% (w/v), showing phase reentry into the ergodic regime due to repulsion between DNA strands. Hydration study in these three regions revealed that a loss in the abundance of amorphous water, signified by Raman frequency 3460 cm(-1), caused the ergodic to nonergodic phase transition. In summary, it is shown that maximum stability and interaction between DNA and nanoclay platelets occurred at an intermediate concentration of DNA where the hydration was at its minimum. The present system is qualitatively different from the hard-sphere/polymer systems for which reentrant phase transition has been reported in the literature. However, some similarity between the two classes of systems is not ruled out.

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).

Dual aging behaviour in a clay-polymer dispersion.

Clay-polymer compounds have recently attracted increasing attention due to their intriguing physical properties in colloidal science and their rheological non-trivial behaviour in technological applications. Aqueous solutions of Laponite clay spontaneously age from a liquid up to an arrested state of different nature (gel or glass) depending on the colloidal volume fraction and ionic strength. We have investigated, through dynamic light scattering, how the aging dynamics of Laponite dispersions at fixed clay concentration (Cw = 2.0%) is modified by the addition of various amounts of poly(ethylene oxide) (PEO) (CPEO = (0.05 ÷ 0.50) %) at two different molecular weights (Mw = 100 kg mol(-1) and Mw = 200 kg mol(-1)). A surprising and intriguing phenomenon has been observed: the existence of a critical polymer concentration C that discriminates between two different aging dynamics. With respect to pure Laponite systems the aging will be assisted (faster) or hindered (slower) for PEO concentrations respectively lower (CPEO < C) or higher (CPEO > C) than the critical concentration. In this way a control on the aging dynamics of PEO-Laponite systems is obtained. A possible explanation based on the balance of competitive mechanisms related to the progressive saturation of the clay surface by polymers is proposed. This study shows how a real control on the aging speed of the PEO-Laponite system is at hand and renders possible a real control of the complex interparticle interaction potential.

Effects of ultrasound on colloidal organization at nanometer length scale during cross-flow ultrafiltration probed by in-situ SAXS

Effects of ultrasound (US) on the structural organization within concentrated particle layers during cross-flow ultrafiltration of Laponite dispersions have been characterized for the first time by in-situ time-resolved small-angle X-ray scattering (SAXS). A novel ‘SAXS Cross-Flow US-coupled Filtration Cell’ has been developed to, on one hand, apply ultrasonic waves close to the flat membrane by embedding in the feed compartment a thin titanium vibrating blade connected to a 20kHz ultrasonic generator and on the other hand, to monitor in-situ the colloidal organization of the concentrated layer by SAXS. Thanks to this cell, concentration profiles have been measured as a function of the distance z from the membrane surface with a 20µm accuracy and simultaneously linked to the permeate flux, cross-flux and transmembrane pressure. In-situ ultrasonication leads to a significant increase of permeate flux arising from the break-up of the concentrated layer. Results also suggest that ultrasonication could be considered as an additional force of an effective range on the order of micrometers or smaller. It is capable to completely remove the particles from the membrane surface when the feed dispersions are dense and aggregated, and is more efficient than classical procedures based on an increase of the cross-flow flux.

Cross-linkage effect of cellulose/laponite hybrids in aqueous dispersions and solid films

Homogenous cellulose/laponite aqueous dispersions and composite films were respectively prepared from the pre-cooling NaOH/urea aqueous systems. Rheological measurements of aqueous dispersions demonstrated a sol-to-gel transition triggered by loading of laponite, reflecting a cross-linkage effect of cellulose/laponite hybrids. Similarly, based on scanning electron microscopy (SEM), Fourier-transform infrared (FTIR) spectroscopy, and X-ray diffraction (XRD) characterizations, as well as mechanical and thermal measurements, the cross-linkage effect of cellulose/laponite hybrids was also found in solid films, which played an important role in improving the tensile strength (σb) of composite films. For instance, the σb exhibited a largest enhancement up to 75.7% at a critical laponite content of 0.100wt%, indicating that the property of composite film was closely related with the dispersion and interaction state of laponite, i.e. its content in cellulose matrix. These results were expected to provide significant information for fabrication and utility of cellulose-based materials.

Kinetics of anisotropic ordering in Laponite dispersions induced by a water-air interface

In this work, we report the kinetics of ordering occurring at the water-air interface of Laponite dispersions. Propagation of such ordering into the bulk and its relaxation dynamics were systematically studied through light scattering measurements. Depolarization ratio D(p), which accounted for the optical anisotropy, was measured as a function of depth from the interface and aging of the samples. The extent of spatial ordering was found to be several decades larger than the typical particle size. Spatial ordering originated from the interface and percolated into the bulk with aging time t(w). Growth in D(p) with waiting time was found to follow power-law behavior given as D(p)~t(w)(n), with n increasing from 0.1 to 4 as one moved away from the interface into the bulk. D(p) decreased exponentially with depth h given as D(p)~e(-(h/h(0))), where h(0) is the decay length, increasing from 0.4 to 0.75 mm with aging time. Dynamic structure factor measurements performed on the samples at various aging times, depths, and temperatures yielded two distinct relaxation times: one fast mode followed by a slow mode. The fast mode remained invariant while slow mode relaxation time followed an exponential decay with depth. This study indicated that the arrested phase nucleated from the interface and propagated into the bulk, which was not observed when the surface was insulated with a layer of hydrophobic liquid. Dilution of the concentrated samples destroyed the aforesaid ordering and made the dispersion homogeneous implying the ordered state was a glass.

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.

Adsorption of a jet fuel on a model organic–clay soil: Application of small angle neutron scattering

Small angle neutron scattering (SANS) data are reported from a system that models the contamination of a clay–organic matter soil from a fuel spillage. The soil was represented as an aqueous dispersion of the synthetic clay mineral Laponite coated with lysine, and the contaminant was a representative jet fuel, quadricyclane, mixed with the detergent cetyltrimethylammonium bromide (CTAB). The adsorbed surface coverage on the clay was estimated. It is shown that the presence of adsorbed lysine considerably enhances the subsequent adsorption of both CTAB and quadricyclane. It is demonstrated that the SANS technique can contribute to the general problem of environmental remediation and retention by probing the interactions of pollutants and clay surfaces.

Restricted diffusion of small probe particles in a laponite dispersion

Evanescent wave microscopy is used to study the dynamics of probe particles inside a laponite suspension, when the size of the latex probes is of the order of the diameter of the laponite disks. A correlation procedure is introduced that allows us to study quantitatively the diffusion of small probes. For all studied sizes, the motion exhibits two modes: a fast relaxation mode and a slow relaxation mode. In the fast relaxation mode, the probes diffuse in a viscous medium, whose viscosity does not depend on the diameter of the probes and is slightly larger than the viscosity of water. Then, the diffusion of the particles is restricted over distances larger than their diameters, which increase when the particle diameter decreases. In this regime, the probe particles experience the elasticity of the solution and the apparent elastic modulus increases when the diameter of the probe particle increases, whereas for large enough particles, the macroscopic behavior is recovered, in which the diffusing particles experience a homogeneous medium, and the macroscopic elastic modulus is recovered.

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.

Polymer-Mediated Clustering of Charged Anisotropic Colloids

Formation of stable, dense nanoparticle clusters is interesting due to both the underlying physics and use of nanoclusters in applications such as digital printing, imaging and biosensing, and energy storage. Here, we explore formation of nanoparticle clusters in dispersions of the model disk-shaped colloid Laponite. Under basic conditions, the model disk-shaped colloid Laponite forms a repulsive glass in water due to strong electrostatic interactions. Addition of a nonadsorbing polymer, the sodium salt of poly(acrylic acid) (PAA), induces a depletion attraction between particles. Through dynamic light scattering (DLS) and rheology, we see that the polymer initially causes a transition from the glassy phase to an ergodic fluid. Samples at higher particle concentration age to a weak nonergodic state, while samples at lower Laponite remain as fluids. As the strength of attraction between particles is increased, we find an increase in the fast relaxation time measured via dynamic light scattering (e.g., slowing of the short-time diffusion of a single particle). While this may in part be attributed to an increase in the ionic strength, the aging behavior and glass-fluid transition we observe appear to be unique to the presence of polymer, suggesting that depletion plays an important role. DLS data on the fluid samples were consistent with two widely spaced diffusive relaxation modes, corresponding to motion of single particles and motion of large clusters, although other slow dynamic processes may be present. On the basis of the estimated volume fraction and depletion attraction, we believe the Laponite-PAA suspensions to be either fluids of stable clusters or glasses of clusters, although it is possible that the nonergodic state we observe is instead a gel of clusters. Additionally, the cluster size was found to be stable for at least 120 days and was directly related to the polymer concentration. This may serve as an important means of tuning cluster size in products and processes based on dense nanoparticle assemblies.

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