Synthetic Hectorite Research Articles

Influence of Polyelectrolyte Charge Density on the Formation of Multilayers of Strong Polyelectrolytes at Low Ionic Strength

The influence of the charge density of polyelectrolytes on the growth of polyelectrolyte multilayers via layer-by-layer self-assembly from pure aqueous solutions was studied. Multilayers were built from strong polyanions, namely poly(styrenesulfonate) and an exfoliated synthetic hectorite, and cationic copolymers of diallyldimethylammonium chloride (DADMAC) with N-methyl-N-vinylformamide (NMVF) for which the composition and thus the charge density was varied systematically. The analysis of the system {cationic copolymer/poly(styrenesulfonate)} reveals that a critical linear charge density λc of 0.036 elementary charge/Å of contour length is necessary to obtain stable multilayer growth in pure water. Above λc, the increment of thickness/deposition cycle varies with the linear charge density of the cationic copolymers, in good agreement with current theories of polyelectrolyte solutions. As linear charge density increases, the system passes successively through a charge-dependent “Debye−Hückel” regime and then through a charge-independent “strong-screening” regime where counterion condensation dominates the behavior. Analogous results were obtained for the variation of the basal spacing of internally structured hybrid multilayers {cationic copolymer/hectorite}. However, by contrast with the first system, no critical linear charge density was found for the hybrid system. This is explained by additional, nonelectrostatic interactions between the clay platelets and the formamide fragment.

Adsorption Study of Cationic Dyes Having a Trimethylammonium Anchor Group on Hectorite Using Electrooptic and Spectroscopic Methods

Clay minerals are natural or synthetic material of colloidal dimensions. Due to the sheetlike structure clay minerals offer a huge specific surface area and hence optimal properties for modification through adsorption. The current work studies the adsorption of five cationic dyes on the synthetic clay mineral hectorite. All dyes have a trimethylammonium anchoring group in common. The adsorbed dye molecules are characterized by means of pulsed electric linear dichroism and UV-VIS spectroscopy. With increasing dye loading a continuous shift in the absorption spectra is observed. But there is no occurrence of a new absorption band. Therefore we conclude that the dyes preferentially adsorb as amorphous aggregates on the clay surface. At low dye loadings the dye molecules lie flat on the clay mineral surface. Increasing dye concentration leads to a continuous increase in average tilt angle. However the orientation of the dye molecules is very sensitive to functional groups. The introduction of a nitro group to a particular dye increases significantly the tendency to lie flat on the surface whereas the introduction of a methoxy group at the same position has the opposite effect.

An NMR Investigation of Water Self-Diffusion and Relaxation Rates in Controlled Ionic Strength Laponite Sols and Gels

1H relaxation and pulsed-gradient NMR techniques have been used to probe solvent mobility and binding in aqueous suspensions of Laponite, a synthetic Hectorite clay. The ionic strength and the particle concentration were varied in wide ranges in order to explore the phase diagram. Increasing either one of these parameters is known to induce a sol–soft solid rheological transition. However, neither the proton relaxation rates, R1 and R2, nor the water self-diffusion coefficient, D, were found to be affected by this transition. Only a continuous increase of R1 and R2 and a continuous decrease of D were observed as the particle concentration was increased, whatever the ionic strength. The results are discussed in terms of obstruction to diffusion and surface hydration, in the fast exchange regime. Specific effects due to the particle anisotropy and surface heterogeneity are evidenced.

Ordered Polyelectrolyte “Multilayers”. 3. Complexing Clay Platelets with Polycations of Varying Structure

Different poly(ammonium) salts with various chemical structures were self-assembled by the layer-by-layer method with a synthetic hectorite, Laponite. Using X-ray reflectometry, we show that internal order is present in all multilayers. The structure of the polycation slightly modulates the spatial extent of order in the film as well as the value of the repeat period which increases with the volume per charge of macromolecule. By combining XRR, XPS, and TGA results, we demonstrate that the internal order in multilayers is due to the regular intercalation of organic polymer between individual platelets of Laponite and not due to the presence of nonexfoliated particules. This conclusion is corroborated by a structural analysis of the bulk complex between Laponite and a poly(diallylammonium) salt.

Intercalation of various oxide species in-between Laponite layers studied by spectroscopic methods

Synthetic hectorite (laponite) was prepared and pillared by partially hydrolyzed Al, Zr and Cr salts. The synthesis of the host material as well as its pillaring was successful. The samples were treated in various ways and were characterized by X-ray diffractometry, Fourier-transform infrared spectroscopy and BET surface area measurements. It was found that the pillared structure survived calcination at 773 K making these materials potential shape selective catalysts up to this temperature.

Colloidal properties of synthetic hectorite clay dispersion measured by dynamic light scattering and small angle X-ray scattering

Abstract Clay suspensions are composed of anisotropic disk-like charged particles and are of great importance in many industrial applications because of their gelation. However, the origin of the gelation is still under debate. The synthetic hectorite-type clays such as Laponite XLG form clear dispersions due to their high purity and very small crystallite size, so that they are particularly suited to light scattering studies. In the present work, we reported the structure of Laponite XLG clay suspension and the gelation process by using both dynamic light scattering (DLS) and small-angle X-ray scattering methods. The sol-gel transition (phase transition) was observed at a clay concentration of about 0.01 g/ml and the phase transition phenomena depends on neither the types nor the concentration of salt when below a certain salt concentration. Above a certain concentration of salt, flocculation (phase separation) was observed. The dependency of the value of this concentration on the salt type was the same as the empirical Schulze Hardy law for the critical coagulation concentration of isotropic colloidal suspensions. At low clay concentrations (below 0.01 g/ml), clay particles did not interact strongly; however, at higher concentrations (above 0.02 g/ml) where gelation and elasticity occurred, there were clear indications of inter particle interactions and random short-range ordering. DLS is thus one suitable method of analysis for the sol—gel transition without destroying the sample.

Experimental mixtures of smectite and rectorite: Re-investigation of “fundamental particles” and “interparticle diffraction”

To re-investigate “fundamental particles” and “interparticle diffraction” proposed by Nadeau et al. (1984a, 1984b, 1984c), we made physical mixtures of smectite and rectorite and examined them by X-ray diffraction analysis and transmission electron microscopy. Smectite and rectorite consisting of at least 10 and 23 silicate layers in their parent rocks, respectively, reduce their thickness by dispersion in water and a fractionation procedure that releases thin silicate layers from the original crystals. The <0.1 μm fractions of the smectites are composed of mostly two to three silicate layers that are not fundamental particles. The mixtures of smectite and rectorite of 0.5-2 μm fractions are mere physical mixtures and do not show interparticle diffraction. In contrast, the mixtures of smectite and rectorite of <0.1 μm fractions have interstratified structures of a segregation type and show interparticle diffraction. However, interparticle diffraction does not occur for synthetic hectorite with aspect ratios smaller than those of the above smectites of the <0.1 μm fractions. These results strongly suggest that interparticle diffraction occurs, and that interstratified structures are formed by mixing rectorite and smectite physically and preparing the oriented specimens when the particle size in the a-b plane and the particle thickness are appropriate. However, such mixtures do not reflect the actual characteristics of natural smectite and illite. A rectorite particle of about 30 nm thickness consists of sub-particles of two or more rectorite units, or four or more 2:1 layers that are nearly perfectly coherent with one another, and the sub-particles are turbostratically stacked. Our results suggest that natural, interstratified illite/smectite are not accumulations of fundamental particles but represent their structural sequences when formed.

Effect of Additive NaCl on Rheological Characteristics of Synthetic Hectorite Aqueous Suspensions.

The rheological characteristics of synthetic hectorite/water suspensions have been measured by a cone-plate rheometer as a function of additive NaCl concentration (CNaCl). The hectorite particles, dialyzed preliminarily to remove electrolytic impurities involved, were utilized for preparing a 2.0 wt% aqueous suspension. The apparent viscosities of the suspensions decreased monotonically with increasing CNaCl; these changes in the viscosities came from the primary electroviscous effect. In dynamic viscoelastic measurements of the suspensions, we found that the critical strain also decreased with increasing CNaCl. By the use of a Cryo-SEM, we observed that different types of the particle-coagulated structures in the various suspensions with respective NaCl concentrations. These findings can be explained by the different interaction modes originated from the electrostatic charge near the surface of hectorite particles, as in the following situations : (1) in the low CNaCl region, the electrostatic repulsion among the particles leads effectively to form an expanded network structure in the system; (2) in the high CNaCl region, the van der Waals attraction force among the particles governs the interaction mode in the system, thus resulting to form a dense-packed coagulation structures.

Effect of pH on Rheological Properties of Synthetic. Hectorite/Water Suspensions.

The rheological properties of synthetic hectorite/water suspensions have been measured by a cone-plate rheometer as a function of their pH values. Synthetic hectorite was dialyzed to remove electrolyte impurities involved, and then utilized for preparing suspensions. When HNO3 and NaOH were added to the aqueous suspensions to adjust pH values, the yield value and shear modulus of the suspensions decreased both in acidic and alkali regions. It was found by the use of a Cryo-SEM observation that different types of coagulated structures existed for different pH regions. These results were probably caused a change in the charge at the edge of hectorite particles as follows: (1) in the acid region, the electrostatic attraction between negatively charged faces and positively charged edges of the particle are dominant and a compact card-house structure is formed; (2) in the alkali regions, the edges of the particle are negatively charged and resulting electrostatic repulsion leads to dispersed state of the particle; (3) in the neutral region, around the isoelectric point of the particle's edge an extended network structure is formed due to edge-to-edge association.

Smectites as colloidal stabilizers of emulsions: II. Rheological properties of smectite-laden emulsions

Oil-in-water emulsions were prepared by dispersing aqueous dispersions of several bentonites, montmorillonites, and hectorites (2% solid content) in paraffin oil which contained 100 mg/25 ml of a coemulsifier (alkyl polyglucoside, glycerol monostearate, lecithin, tetra- and deca(ethylene oxide) hexadecyl ethers). The colloidal stabilizers were sodium montmorillonite from Wyoming (USA), sodium bentonite from Kimolos (Greece), soda-activated bentonite from Milos (Greece) and Kimolos, also after decomposition of the carbonate. Three samples of synthetic hectorites (as synthesized, after washing, and after impregnation with sodium polyphosphate) were also used as colloidal stabilizers. The processing steps had strong effects on the rheological properties of the clay mineral dispersions. The rheological behavior of emulsions reflected these properties of the dispersions. Due to the presence of the dispersed oil droplets, the shear stresses of the emulsions were distinctly higher than the corresponding values of the dispersions and increased with the oil volume fraction. Their behavior changed from antithixotropic to thixotropic with increasing oil volume fraction. The storage modulus G′ of the emulsions with alkyl polyglucoside and glycerol monostearate showed no linear viscoelastic regions indicating the destructive character of the oscillation experiments even at the smallest deformation and the lowest frequencies. The structure of the emulsion was more stable in the presence of lecithin and ethylene oxide ethers and resisted to the shearing forces during the oscillation experiments as indicated by the observation of linear viscoelastic regions.

Morphology and toughness/stiffness balance of nanocomposites based upon anhydride-cured epoxy resins and layered silicates

Basic correlations between polymer morphology, silicate superstructures, glass temperature, stiffness and toughness of thermoset nanocomposites were investigated as a function of layered silicate type and content. The nanocomposites were based upon hexahydrophthalic anhydride-cured bisphenol A diglycidyl ether and layered silicates such as synthetic fluoromica (Fmica), purified sodium bentonite and synthetic hectorite, all of which were rendered organophilic by means of ion-exchange with various mono- and difunctional alkyl ammonium ions. Enhanced toughness was associated with the formation of dispersed anisotropic laminated nanoparticles consisting of intercalated layered silicates. Nanocomposite superstructures were imaged by means of transmission electron microscopy (TEM) and atomic force microscopy (AFM).

Smectites as colloidal stabilizers of emulsions: I. Preparation and properties of emulsions with smectites and nonionic surfactants

Bentonites, montmorillonites, and hectorites were used as colloidal stabilizers of oil-in-water (O/W) emulsions. The enrichment of the solid particles on the oil–water interface was attained by the addition of nonionic coemulsifiers (glycerol monostearate (GMS), deca(ethylene glycol) hexadecyl ether, alkyl polyglucoside, and lecithin). The clay mineral content of the aqueous dispersion was 2% (w/w). Stable emulsions required amounts of 0.5–1.5 g coemulsifier per 100 ml aqueous dispersion. Oil volume fraction was varied between φ=0.17 and φ=0.50. At φ>0.50 the O/W emulsions changed into water-in-oil (W/O) emulsions. The number average diameter of the droplets was about 25 nm. The volume average diameter (50–100 nm) more strongly depended on the clay mineral/coemulsifier combinations. Wyoming bentonite and the corresponding delaminated sodium montmorillonite were useful stabilizers; technical, soda-activated bentonites yielded unstable emulsions, or emulsification was not successful. A synthetic hectorite which caused pronounced thickening of the coherent phase was an effective stabilizer. Creaming was often observed because of the buoyancy of the large droplets. Most of the creamed emulsions were stable over long periods and did not separate an oil phase. The resistance against creaming increased with the oil volume fraction. An increase of the solid content had to be accompanied by an increase of the coemulsifier concentration to reduce the rate of creaming.

Li ion conductors based on laponite/poly(ethylene oxide) composites

Synthesis and characterization of single-ion conducting poly(ethylene oxide) (PEO)/Li-laponite nanocomposites are reported. The amount of PEO that can be intercalated into laponite, a synthetic hectorite with high surface area, ranges from about 0.7 g/g Li-laponite when the polymer average molecular weight is 1000 or above, to about 1 g/g for oligomers of average molecular weight 500. The interlayer spacing increases from about 10 Å in the dry clay to 20–24 Å in the nanocomposites, depending upon polymer molecular weight, and the average particle size increases proportionally, but is still in the sub-micron range. AC impedance measurements on the clear, slightly brittle, self-supporting films indicate that the nanocomposite conductivity is greatly enhanced over that of the dry clay. A maximum of about 10 −6 S/cm at 80°C is obtained for materials containing a slight excess of polymer, and conductivities of nanocomposites containing PEO were generally higher than that of those containing oxymethylene-linked polyethylene glycols (amorphous PEOs). Suggestions for further improving conductivity and mechanical properties of these novel materials are presented.

Stereochemical Effects on Monolayer Formation of a Chiral Amphiphilic Ruthenium(III) Complex

A chiral amphiphilic ruthenium(III) complex, [Ru(acac)2(C12-acac)] (acac = 2,4-pentanedionato, C12-acac = 3-dodecyl-2,4-pentanedionato), has been synthesized. The complex was resolved into pure optical isomers by being eluted with methanol on a column packed with an ion-exchange adduct of Δ-[Ru(phen) 3 ] 2+ (phen = 1,10-phenanthroline) and a synthetic hectorite clay. The surface pressure-molecular area (π-A) curves of the pure enantiomer and the racemic mixture were obtained in the temperature range 10-30 °C. The π-A curve of the pure enantiomer was little affected by the change of temperature, while that of the racemic mixture was displaced toward the larger molecular area with the increase of temperature. The pure enantiomer was concluded to form a more packed monolayer than the racemic mixture. When the monolayer of either the A- or A-enantiomer was formed on a subphase containing Δ-[Ru(phen) 3 ]Cl 2 , the molecular area ofthe monolayer was expanded due to the binding of Δ-[Ru(phen) 3 ] 2+ in a stereoselective way. The AFM images of the chiral and racemic monolayers deposited on mica at 20 mN m -1 showed that the chiral molecules formed an oblique lattice (a = 0.59 nm, b = 0.55 nm, and φ = 63 °C), while the racemic mixture exhibited no two-dimensional surface order.

Studies of Gadolinium(III)-Modified Hectorite Clays as Potential Oral MRI Contrast Agents

Hectorite type clays modified with gadolinium(III) were prepared by ion-exchange of natural (SHCa) hectorite and synthetic (SLH) hectorite. Results show that Gd(III) content in the clay increases with increasing gadolinium(III) in the exchange solution within the range between 0 and 25% of the Gd/clay weight ratio. Studies relevant to the in vivo stability and efficacy of Gd(III) ion-exchanged hectorite as an oral MRI contrast agent are detailed. The stability of Gd(III) ion-exchanged hectorite at low pH as well as in the presence of sodium ion at concentrations relevant to the gastrointestinal tract has been evaluated. The room-temperature relaxivities (1/T1 vs [Gd]) of aqueous Gd(III)-exchanged hectorite suspensions at different stirring time, concentration, and pH were measured at 40 MHz and room-temperature. The type of clay and the gadolinium loading as well as the amount of clay in suspension and the stirring time were found to have a profound effect on the relaxivity.

Intercalation of catalytically active metal complexes in phyllosilicates and their application as propene polymerisation catalysts

The cationic Ziegler–Natta polymerisation catalyst [Zr(η-C5H5Me(thf)]+, is intercalated in a synthetic fluorinated mica-type silicate (NaMg2.5Si4O10F2) and the modified, swelling synthetic hectorite [LaponiteRD, Na0.46Mg5.42Li0.46Si8(OH)4O20], by ion-exchange of [Zr(η-C5H5Me(thf)]+ ions with the interlamellar cations of the layered silicates; in the presence of methylaumoxane {MAO, (MeAlO)n} these organometallic intercalation compounds are active catalysts for the polymerisation of propone giving low molecular mass atactic polypropylene with a narrow molecular mass distribution.

The thermal reactions of synthetic hectorite studied by 29Si, 25Mg and 7Li magic angle spinning nuclear magnetic resonance

The thermal decomposition sequence of synthetic hectorite (Laponite CP) is shown by a combination of thermal analysis, X-ray powder diffraction and solid-state MAS-NMR to be very similar to that of the structurally related mineral talc, in transforming smoothly into the high-temperature product without the intervention of any crystallographically distinguishable intermediates. Loss of interlayer water at about 200°C causes little change in the hectorite basal spacing or in the 29Si, 25Mg or 7Li MAS-NMR spectra; small but significant changes in the latter after heating to 600–650°C may however be related to the movement of interlayer Na + closer to the tetrahedral sheets, influencing the octahedral Li (but not the octahedral Mg). Dehydroxylation above 650°C disrupts the phyllosilicate structure into pyroxene units (MgSiC 3) and amorphous silica with distortion of the octahedral Mg sites, as evidenced by the broadening and loss of intensity of the 25Mg resonance. The 7Li MAS-NMR spectra suggest that the Li becomes mobile just prior to dehydroxylation, and may eventually be incorporated, together with the interlayer Na, in the siliceous phase.

Pillaring of synthetic hectorite by mixed [Al13-xFex] Pillars

AbstractThe pillaring of a Laponite by alumina pillars, iron pillars and mixed Al-Fe pillars was obtained by a simple in situ method starting from AlCl3 and/or FeCl3 solutions with different molar ratio Fe/(Fe + Al), and from NaOH solution added simultaneously to the clay suspension. The Al13 pillars were evidenced by XRD and NMR spectra. The density and the charge of these pillars were calculated from the chemical analysis. The Fe pillars (in samples with Fe content >26%) are larger than Al13 pillars and slowly reducible by hydrogen temperature programmed reduction (H2TPR), but we do not have any information on their structure. There is also an excess of non-pillar Fe retained by the pillared clays probably as oxyhydroxides. The isomorphic substitution in the mixed (Al-Fe)13 pillars (in samples with an Fe content >26%) was evidenced by H2TPR results compared to the chemical analyses and by the interpretation of the 27A1 peaks in the NMR spectra.

Microdynamics of exchangeable lithium and sodium ions in laponite gels

Hectorites are natural minerals of the mica family, within the manifold of clay rocks. Organometallic complexes can be inserted within their lamellar structure with interesting catalytic and stereochemical consequences ( Z ). Laponites are synthetic hectorites. They belong to the smectite, i.e., swelling family of clays. Their structure consists of an octahedral layer sandwiched between two tetrahedral layers. In the octahedral layer, all the sites are occupied either by a divalent magnesium ion (trioctahedral clay) or by a lithium ion (one octahedral site in eight). In the tetrahedral layers, there is occasional substitution of a silanol O$iOH group by the fluorinated counterpart O$iF. The counterions responsible for electroneutrality are mostly sodium ions. Laponites exist as platelets about 1000 A in length and about 10 A thick. Their cation-exchange capacity is 50100 meq/ 100 g, about half that of a montmorillonite clay. We have studied laponite B (Laporte Inorganics, Widnes, Cheshire, United Kingdom), whose BET surface area is 330 m*/g and whose cation-exchange capacity is 85 meq / 100 g. Its composition is (percentage per 100 g of clay) SiOz ,55.08; A1203, 0.05; Fe203, 0.06; CaO, 0.20; MgO, 24.82; Na20, 2.55; K20, 0.05; Li20, 1.74; F, 5.6. The interest of laponite for NMR study is twofold: it possesses a rather simple structure, essentially a magnesiosilicate, with an extremely low level of paramagnetic iron impurities, and the water suspensions form gels at high ionic content, due to the binding of the negative silicate planar sheet to the positively charged edges (“house of cards” structure). We have obtained (Bruker AM 300WB and 400 spectrometers) 7Li and 23Na spectra. Longitudinal relaxation times were measured with the inversion-recovery sequence 180”--t-90” followed by a nonlinear regression on the exponential curve, using between 13 and 16 data points. Standard deconvolution procedures were applied to all lineshapes for these Z = 3 nuclei. Theoretical treatments have been provided by Hubbard (2) and Bull (3). Samples consisted of 75 mg of laponite B exchanged with lithium or sodium ions suspended in 4.5 ml of D20. In heavy water, lithium-7 relaxation is quadrupolar (4, 5). The ionic strength was increased by adding the same amount of exchanged clay to aqueous solutions of LiCl or NaCl. The suspension was stirred overnight. Since lithium ions in laponites are either structural or exchangeable, we checked that the high-resolution NMR is opaque to the former and monitors only the latter:

New Method for Separation and Purification of Polyunsaturated Fatty Acids Using a Silver Ion-exchanged Spherical Clay Mineral

New method was developed for separation and purification of polyunsaturated fatty acid ethyl esters using a silver ion-exchanged spherical clay mineral [SC (Ag)]. The SC (Ag) was prepared by ion-exchanging silver ion into synthetic hectorite that had been spray-dried to form a spherical shape. Ethyl icosapentaenoate and ethyl docosahexaenoate were separated and purified from the mixture of fatty acid ethyl esters originated from several natural lipids, using SC (Ag) preparative columns under the step gradient condition of organic solvent.