Transition from glass- to gel-like states in clay at a liquid interface.

Colloidal clay in water suspensions are known to exhibit a multitude of bulk phases depending on initial colloidal concentration and ionic strength, and examples of this include repulsive Wigner colloidal glasses at low ionic strength and attractive gels at higher ionic strength due to screened electrostatic forces by the electrolyte. From confocal Raman microscopy combined with elasticity measurements, we infer that clay trapped at quasi two-dimensional interfaces between oil and water also exhibit confined glass-like or gel-like states. The results can be important for the preparation of particles stabilized colloidal emulsions or colloidal capsules, and a better understanding of this phenomenon may lead to new emulsion or encapsulation technologies.

Intercalation and retention of carbon dioxide in a smectite clay promoted by interlayer cations.

A good material for CO2 capture should possess some specific properties: (i) a large effective surface area with good adsorption capacity, (ii) selectivity for CO2, (iii) regeneration capacity with minimum energy input, allowing reutilization of the material for CO2 adsorption, and (iv) low cost and high environmental friendliness. Smectite clays are layered nanoporous materials that may be good candidates in this context. Here we report experiments which show that gaseous CO2 intercalates into the interlayer nano-space of smectite clay (synthetic fluorohectorite) at conditions close to ambient. The rate of intercalation, as well as the retention ability of CO2 was found to be strongly dependent on the type of the interlayer cation, which in the present case is Li(+), Na(+) or Ni(2+). Interestingly, we observe that the smectite Li-fluorohectorite is able to retain CO2 up to a temperature of 35°C at ambient pressure, and that the captured CO2 can be released by heating above this temperature. Our estimates indicate that smectite clays, even with the standard cations analyzed here, can capture an amount of CO2 comparable to other materials studied in this context.

Swelling transition of a clay induced by heating.

Clays are of paramount importance for soil stability, but also in applications ranging from oil recovery to composites and hydrogels. Generically, clays are divided into two subclasses: macroscopically swelling, 'active' clays that have the capacity for taking up large amounts of water to form stable gels, and 'passive' or non-swelling clays; the former stabilize soils whereas the latter are known to lead to landslides. However, it has been unclear so far what mechanisms underlie clay swelling. Here, we report the first observation of a temperature-induced transition from a passive to an active, swelling clay. We propose a simple description of the swelling transition; while net attractive interactions are dominant at low temperatures so that the clay particles remain attached to each other in stacks, at higher temperatures it is energetically favourable for the clay to swell due to the entropy that is gained by counterions which are liberated during swelling.

Dipolar structuring of organically modified fluorohectorite clay particles.

This report focuses on both the characterization of organically modified fluorohectorite (Fh) clay particles and their electric-field-induced alignment when suspended in a non-polar liquid (silicone oil). Thermal decomposition temperatures of the surfactant molecules adsorbed on the clay surfaces and those being intercalated between clay crystalline layers were measured by thermal gravimetric analysis (TGA). Zeta potential measurements confirmed the successful modification of the clay surfaces. Optical microscopy observations showed that the sedimentation of modified particles was much slower compared to that of the non-modified system. It was shown that organic modification has a significant effect on colloidal stability of the system, preventing particles from forming large aggregates when suspended in a non-polar liquid. There are also signs of a slight increase in overall alignment of the clay particles when exposed to in an electric field, with the nematic order parameter (S(2)) being higher for the organically modified particles, compared to that of the non-modified counterparts. This behaviour is mainly a result of the formation of smaller and more uniform aggregates, in contrast to the large aggregate structures formed by non-modified clay particles.

Structuring from nanoparticles in oil-based ferrofluids.

The effect of magnetic field on the structure formation in an oil-based magnetic fluid with various concentrations of magnetite particles was studied. The evaluation of the experimental data obtained from small-angle X-ray scattering and ultrasonic attenuation indicates the formation of chain-like aggregates composed of magnetite particles. The experimental data obtained from ultrasonic spectroscopy fit well with the recent theoretical model by Shliomis, Mond and Morozov but only for a diluted magnetic fluid. In this model it is assumed that a dimer is the main building block of a B -field-induced chain-like structure, thus the estimation of the nematic order parameter does not depend on the actual length of the structure. The scattering method used reveals information about the aggregated structure size and relative changes in the degree of anisotropy in qualitative terms. The coupling constant [Formula: see text] , concentrations [Formula: see text] , average particle size d and its polydispersity [Formula: see text] were initially obtained using the vibrating sample magnetometry and these results were further confirmed by rheometry and scattering methods. Both the particles' orientational distribution and the nematic order parameter S were inferred from the ultrasonic measurements. The investigation of SAXS patterns reveals the orientation and sizes of aggregated structures under application of different magnetic-field strengths. In addition, the magnetic-field-dependent yield stress was measured, and a relationship between the yield stress and magnetic-field strength up to 0.5 T was established.

Nematic textures in colloidal dispersions of Na-fluorohectorite synthetic clay.

We have studied stable strata of gravity-induced phase separation in suspensions of synthetic Na-fluorohectorite clay in saline solutions. We have observed how the strata depend on clay concentration as well as on salt content. The mass distribution and density variation at the isotropic-nematic interface indicate that existing models and assumptions in existing simulations are able to relatively well account for the observed behavior. We suggest that discrepancies could be due to the high polydispersity and the irregular shape of our Na-fluorohectorite particles, as well as diffusive double-layer effects, which could result in a competition between nematic ordering and gelation. The dependence on ionic strength displays three main regimes irrespective of clay concentration. At low ionic strength (approximately 0.1-5 mM NaCl), the Debye screening length is longer than the van der Waals force range. In this regime, the particles repel each other electrostatically and entropy-driven Onsager-type nematic ordering may occur, although gelation effects could also play a role. For ionic strengths above about 5 mM, we believe that the van der Waals force comes into play and that particles attract each other locally according to the classical Derjaguin, Landau, Verwey, and Overbeek (DLVO) model of colloid interactions, resulting in a small-domain regime of attractive nematiclike ordering. In the third regime, for ionic strengths above approximately 10 mM, the clay particles aggregate into larger assemblies, due to the dominant van der Waals force, and the observed birefringency is reduced. We have studied the nematic phase in detail between crossed polarizers and have found textures showing nematic Schlieren patterns. By rotating the polarizers as well as the samples, we have observed examples of disclinations of strengths -1, -1/2, and +1.

X-ray studies of interlayer water absorption and mesoporous water transport in a weakly hydrated clay.

The swelling of layered smectite clay particles consists of a change in the interlayer repetition distance ( d -spacing) as a function of temperature and humidity. For the synthetic clay sodium fluorohectorite, hydrodynamically stable hydration states with zero, one and two intercalated monolayers of water have previously been reported, with discrete jumps in d -spacing at the transitions between the hydration states. Keeping the temperature fixed and varying the ambient relative humidity, we find small reproducible d -spacing changes also within the hydration states. These changes are monotonous as a function of relative humidity, and one order of magnitude smaller than the shift in d -spacing that is typical of the transition between two hydration states. The reproducibility and reliability of this relative humidity controlled d -shift enables us to use the interlayer repetition distance d as a measure of the local humidity surrounding the clay particles. We provide an example of application of this observation: imposing a humidity gradient over a quasi-one-dimensional temperature-controlled sample, and using x-ray diffraction to record the d -spacing, we are able to extract profiles of the relative humidity along the sample length. Their time evolution describes the transport of water through the mesoporous space inside the clay. An analysis of the measured humidity profiles based on the Boltzmann transformation, under certain simplifying assumptions, yields a diffusive behavior that is either normal or possibly weakly anomalous.

Electric field induced structuring in clay-oil suspensions: new insights from WAXS, SEM, leak current, dielectric permittivity, and rheometry.

The electric field induced structuring in clay-oil suspensions has been studied by means of wide angle x-ray scattering (WAXS), rheometry, scanning electron microscopy (SEM), as well as leak current density and dielectric constant measurements. The clay particles' orientation distribution was inferred from the azimuthal changes of the clay diffraction peak intensity. The angular width of that distribution was quantified through an orientational order parameter. Chain and column formation processes were distinguished by comparison of the time evolution of the diffraction peak amplitude with that of the current density. Leak current density was measured for different electric field strengths E and clay particle concentrations Φ. The following scaling relation was found: [Formula: see text]. In addition, the dependence of the yield stress on the electric field and on the particle concentration was measured and shown to scale as: [Formula: see text].

Phase diagram of polydisperse Na-fluorohectorite-water suspensions: a synchrotron small-angle x-ray scattering study.

Systems of platelet-shaped nanostacks of the synthetic clay Na-fluorohectorite, suspended in saline solutions of various salt concentrations, exhibit a rich phase behavior with up to four phases coexisting in a single sample tube. They are studied here using small-angle x-ray scattering: the anisotropy of the obtained images is quantified, and, together with x-ray absorption measurements, this provides a precise determination of the phase boundaries, as well as a measure of the orientational ordering of the clay colloids in the various gel phases. The coexistence of different phases results from a sedimentation-induced vertical gradient in particle fraction. Quantitative relation of the vertical coordinate to the clay particle fraction in these samples allows determination of a phase diagram for these Na-fluorohectorite systems, as a function of the particle fraction and salt concentration.

New advances in the 3D characterization of mineral coating layers on paper.

The surface characteristics of a large set of commercial lightweight coated paper grades are explored. The quantification of the 3D structure is revealed by atomic force microscopy, laser profilometry and X-ray microtomography. This comprehensive study demonstrates the suitability of different and modern methods for assessing critical coating layer properties, thus identifying the right tools for specific structural analyses. Based on the assessment of the top and bottom surfaces of 25 commercial lightweight coated samples, three main conclusions can be drawn: (1) the facet orientation polar angle is a function of roughness, (2) skewness did not describe the surface details affecting the gloss of the commercial lightweight coated samples assessed in this study and (3) surface roughness at wavelengths below approximately 1.0 microm does not affect the paper gloss significantly. This is important knowledge for the understanding of lightweight coated paper surface structure and its properties.

Electrorheological suspensions of laponite in oil: rheometry studies.

We have studied the effect of an external direct current (DC) electric field ( approximately 1 kV/mm) on the rheological properties of colloidal suspensions consisting of aggregates of laponite particles in a silicone oil. Microscopy observations show that, under application of an electric field greater than a triggering electric field Ec approximately 0.6 kV/mm, laponite aggregates assemble into chain- and/or columnlike structures in the oil. Without an applied electric field, the steady-state shear behavior of such suspensions is Newtonian-like. Under application of an electric field larger than Ec, it changes dramatically as a result of the changes in the microstructure: a significant yield stress is measured, and under continuous shear the fluid is shear-thinning. The rheological properties, in particular the dynamic and static shear stress, were studied as a function of particle volume fraction for various strengths (including null) of the applied electric field. The flow curves at constant shear rate can be scaled with respect to both the particle fraction and electric field strength onto a master curve. This scaling is consistent with simple scaling arguments. The shape of the master curve accounts for the system's complexity; it approaches a standard power-law model at high Mason numbers. Both dynamic and static yield stresses are observed to depend on the particle fraction Phi and electric field E as PhibetaEalpha, with alpha approximately 1.85 and beta approximately 1 and 1.70 for the dynamic and static yield stresses, respectively. The yield stress was also determined as the critical stress at which there occurs a bifurcation in the rheological behavior of suspensions that are submitted to a constant shear stress; a scaling law with alpha approximately 1.84 and beta approximately 1.70 was obtained. The effectiveness of the latter technique confirms that such electrorheological (ER) fluids can be studied in the framework of thixotropic fluids. The method is very reproducible; we suggest that it could be used routinely for studying ER fluids. The measured overall yield stress behavior of the suspensions may be explained in terms of standard conduction models for electrorheological systems. Interesting prospects include using such systems for guided self-assembly of clay nanoparticles.

Anisotropic water diffusion in nematic self-assemblies of clay nanoplatelets suspended in water.

Diffusion-weighted magnetic resonance imaging provides a vivid description of the little understood role played by interfacial interactions with macroscopic bodies in the cooperative self-assembly of clay nanoplatelets suspended in water. The interfacial interaction between hydrophilic glass walls and clay platelets in a Na-fluorhectorite gel can produce, for dilute gels, a face-to-wall anchoring of the platelets that leads to a uniaxial nematic order with platelet faces parallel to the walls but with randomly distributed normals of the faces. The application of a magnetic field perpendicular to the walls transforms this uniaxial order to an extended biaxial nematic order with orthogonal alignment between normals and the field. Moreover, for apolar walls, this face-to-wall anchoring is considerably hindered, and the uniaxial nematic order can be substantially disrupted.

Self-affine crossover length in a layered silicate deposit.

Self-affine dehydrated colloidal deposits on fresh mica surfaces of the synthetic layered silicate 2:1 smectite clay laponite have been studied by means of atomic force microscopy (AFM). AFM images of these prepared assemblies of sol and gel aggregates have been analyzed both by means of standard AFM Fourier software and a wavelet method. The deposited surfaces show a persistence to antipersistent crossover with a clay concentration dependent crossover length. It is concluded that the crossover length is associated with aggregate size, and further that the persistent roughness at small length scales signals near compact clusters of fractal dimension three, whereas the antipersistent roughness at large length scales signals a sedimentation process.

Synchrotron x-ray scattering studies of water intercalation in a layered synthetic silicate.

Synchrotron x-ray diffraction studies were performed on a synthetic layered silicate Fluorohectorite clay. Diffraction patterns along the stacking direction were obtained in surface reflection and bulk transmission geometries on bulk pressed samples under controlled temperature and relative humidity. One-dimensional structure factors modeling the positions of the intercalant atoms have been obtained for three stable hydration states. From the narrow (00l) peak widths we conclude that well-crystallized domains consist of stacks of about 100 platelets, forming crystallites of the order of 0.1 microm thick. These crystallites have an orientational angular distribution of about 24 degrees around the stacking direction and represent the solid framework for microporosity in these samples.

Orientational order in gravity dispersed clay colloids: a synchrotron x-ray scattering study of Na fluorohectorite suspensions.

Colloidal suspensions of clay particles in aqueous salt solutions make ideal model systems for the study of interactions between plate-shaped particles, due to the ease in tuning their electrostatic repulsion with the concentration of the salt. Numerous gel and sol structures are possible, including nematic liquid crystalline order, although only qualitative identification of the latter in clay colloids has been available so far. We present synchrotron x-ray diffraction from gravity dispersed solutions of Na fluorohectorite, a synthetic swelling clay, over a large NaCl concentration range. Our use of liquid scattering techniques allows us to identify regions in which particles reorient from horizontal to vertical alignments in strata coexisting at different heights within the sample. We identify two distinct gel regions characterized by differences in orientational anisotropy and domain size. Our results provide direct evidence for nematic order, as well as unique structural information regarding particle morphology and alignment within each of the colloid phases.