Hectorite Suspensions Research Articles

Direct Observation of the Relationship between Thixotropic Behavior and Shear-Induced Orientation of Clay Particles in Synesthetic Hectorite Suspensions.

A thixotropic characteristics of aqueous gels containing smectite clay minerals were used in various industrial applications such as paint additives, which have been affected by the clay types and clay particle sizes. A model called a house-of-card arrangement of clay particles and anisotropic arrangement in aqueous gels has been proposed. We prepared different sizes of synthetic hectorite and studied them by scanning electron-assisted dielectric microscopy (SE-ADM) and simultaneous small-angle neutron scattering and rheological measurements (Rheo-SANS). The Rheo-SANS results indicated that the clay particles with the cross-sectional radius of 30 nm were clearly oriented in the direction of shear-flow (1 × 103 s-1) direction, but the anisotropic change was not observed for an aqueous gel with clays whose average radius was 19.5 nm. The present study suggested the thixotropic characteristics of aqueous gels depend on the hectorite particle size and aggregation structure under shear conditions.

Surface chemistry, rheology and microstructure of purified natural and synthetic hectorite suspensions.

Natural (N-) and synthetic (S-) hectorite suspensions were found to display significant time-dependent rheology or ageing behaviour and shear thinning flow behaviour. The ageing behaviour was characterised by an increasing yield stress with rest time. The yield stress continued to increase even after a week of rest, a reflection of a long process. An open sponge-like cellular microstructure formed by platelet particles interacting attractively in the overlapping edge-face configuration was captured by cryo-SEM of gel samples prepared at high pressure (∼2000 bar) and subjected to rapid cryo-freezing, for both N- and S-hectorite gels. Even nano-discotic S-hectorite particles formed platelet particles hundreds of nanometres in length in the overlapping coin configuration. This structure, displaying a cell size ranging from tens to several hundred nanometres, is formed by strong attractive and repulsive forces. The platelets showed deformations such as bending and curling of the edges in response to these forces. The S-hectorite platelets are smaller and more rigid. During ageing the particles in the structure experience a net force. These particles will move in response causing force imbalance to be experienced by neighbouring particles and they will move in response. This action and reaction percolate through the network structure causing a high concentration of particles to respond. As a consequence the ageing process takes a long time to reach equilibrium. Various ageing models were used to fit the ageing data. The N-hectorite gels displayed a maximum yield stress at pH ∼ 8 and a particle zeta potential of -35 mV. This suggests the presence of critical positive and negative charge density is needed to form a structure with maximum strength. The zeta potential is negative and quite insensitive to pH from pH 4 to 12.

Rheological inversion of the universal aging dynamics of hectorite clay suspensions

Abstract Aging of salt-free suspensions of hectorite clay was investigated with rheology of multi-wave small amplitude oscillatory shear. The frequency-dependent storage and loss moduli at different waiting times and temperatures were scaled into a master curve, conforming the time-temperature-aging time superposition. In other words, thermorheological simplicity was maintained during aging. While the relaxation time of the system grew monotonically with increasing waiting time, and showed non-monotonic dependence on temperature, consistent with our previous results. The relaxation spectra converted from the master curves of different clay concentrations were described by a power-law distribution with a stretched exponential cutoff. The dynamics were slower for high clay concentration. A rheological inverse from gel-like to glass-like distribution was observed with the increase in clay concentration.

Temperature dependence of aging kinetics of hectorite clay suspensions

The aging of salt-free hectorite suspensions with different concentrations (cL=2.9, 3.2 and 3.5wt%) stored for 2days or 4days was studied by rheology at different temperatures. The evolution of storage and loss moduli G′ and G″ during aging followed aging time–temperature superposition. The temperature dependence of the shift factor aT, which reflected the aging kinetics, was interpreted by the reaction-limited colloidal aggregation (RLCA) mechanism with counterion condensation in calculating the double-layer interaction of the charged clay particles. Temperature dependence of the plateau modulus and yield stress of the suspension aged for 800s was modeled with the soft glassy rheology (SGR) theory. The estimated noise temperature x indicated that the sample aged at higher temperature corresponded to a deeper quench in the nonergodic state. Under larger amplitude of oscillatory shear, the suspension exhibited a strain rate-frequency superposition (SRFS). The shearing eliminated the effects of aging and heating.

Viscosity change in aqueous hectorite suspension activated by DC electric field

The change in the viscosity of deionized aqueous hectorite suspensions by applying an electric field was investigated. The deionized suspensions had low viscosity, which was almost the same as that of its solvent, water. Upon applying a DC electric field of the order of a few volt per millimeter to the deionized suspension, the viscosity started to increase gradually and reached a constant value that was high enough relative to the original value. Regarding the mechanism of the electrically induced stress increase, it is highly plausible that a three-dimensional network structure formed under the electric field due to a deformation of the electrical double layer.

Scaling of the dynamic response of hectorite clay suspensions containing poly(ethylene glycol) along the universal route of aging

The temporal growth of dynamic response during the aging of hectorite clay suspensions containing NaCl with different poly(ethylene glycol) (PEG) concentrations was measured. For each PEG concentration, the dynamic responses at different waiting times were shifted into a master curve against the rescaled frequency, which indicated that the dynamic spectrum was preserved during the aging process. At each fixed waiting time, the dynamic responses of suspensions with different PEG concentrations were also shifted into a master curve against the rescaled frequency, which revealed that the modification of inter-particle interaction by the PEG adsorption did not change the relaxation during the aging route. These two superposition approaches led to a universal master curve revealing the equivalent effect of PEG concentration and waiting time on the dynamic response. Finally, the relative mean relaxation time extracted from the master curves of waiting time and PEG concentration reflecting the slowing down of the dynamics during aging was superimposed to a master curve, implying the existence of a similar dynamic response. The present results suggested that the universal route of aging was preserved for the clay suspensions and a similar dynamic response was found from the mechanical spectra at different aging stages.

An aqueous pathway to polymeric foaming with nanoclay

The property of osmotic swelling of a hectorite clay suspension in water is exploited for dispersing it in a polymeric foam matrix. This green route eludes the surface modification of clay as well as the mediation by organic solvents for clay dispersion during foaming. The route follows the initial swelling and partial delamination of clay galleries by water molecules, followed by further dispersion in an oligomeric isocyanate foaming component, before conducting reactive foaming. A nominal mass fraction of clay provides an effective barrier, restricting the gas phase mass transport in the foam. Electron microscopic investigations show that clay particles are preferentially located on the lamellar part of the microscopic foam cell, rather than on plateau borders.

Dispersion state and rheology of hectorite particles in water over a broad range of salt and particle concentrations

The relationship between the rheological properties of deionized aqueous suspensions of hectorite particles and the dispersion states of the particles has been studied with a broad range of salt and particle concentrations. The shear viscosity of the hectorite suspensions decreases drastically after exhaustively deionizing the suspensions with ion-exchange resins. By means of DLS measurements, it is clarified that the average size of the flocs of hectorite particles decreases and reaches the Stokes diameter of the individual particle as the degree of deionization advances. This fact strongly supports the idea that the electrical double layer around the hectorite particles expands significantly in the exhaustively deionized state and the particles are well-dispersed individually and do not form a three-dimensional network structure composed of particles, whereas such a network structure forms in the presence of a large amount of salt. In the case of exhaustively deionized state, the suspension forms a glassy state, at high particle fractions. The results show the importance of the electrical double layer that causes a strong repulsive force among the particles on the particle dispersion state, especially in the exhaustive deionization area below 10 − 4 M, and on the rheological properties; the hectorite suspension can be considered a Newtonian liquid in the deionized state, but it becomes elastic-solid in the presence of salt above a certain concentration confirmed by normal stress measurements.

Determination of lipid peroxidation and cytotoxicity in calcium, magnesium, titanium and hectorite (SHCa-1) suspensions

This paper reports data on the relative ability of CaO, CaCl 2, MgO, MgCl 2, TiO 2, and hectorite (SHCa-1) to induce oxidative stress (as determined by lipid peroxidation, LP) in biological matrices. The effectiveness of structural (oxide form) versus soluble Ca and Mg to induce LP is compared. An assessment on cytotoxicity as affected by soluble and structural Ca, Mg, TiO 2 and SHCa-1 is also addressed. LP was screened and monitored using the Thiobarbituric Acid Reactive Substances (TBARS). The extent of TBARS production was found to vary with the type and initial concentration of the soluble or structural cation, Ca or Mg respectively. Obtained results showed higher magnitude values for the latter set of experiments. In the presence of TiO 2 no significant TBARS production was detected pointing out a negligible effect of TiO 2 on LP. At solid concentrations ca. 100 ppm, CaO appears to be more effective than SHCa-1 to induce LP. By contrast at ca. 25 ppm, MgO appears to be more effective than the clay mineral. The SHCa-1 LP-inducing activity has been proven to closely relate to structural Ca. The prevalence of mechanisms that may induce LP but not cytotoxicity (as determined by cell growth inhibition) was also addressed. Results on cell growth inhibition as affected by soluble and structural Ca, Mg, TiO 2 and hectorite provide evidence to support that structural Ca or Mg brings about significantly higher variations than soluble Ca.

Polarized EXAFS characterization of the sorption mechanism of yttrium on hectorite

AbstractYttrium uptake in 2 g/L hectorite suspensions at high ionic strength (0.5 M NaCl) as a function of pH (6.5 and 7) and total Y concentration (10 and 50 μM) was investigated by solution chemistry and Polarized EXAFS (P-EXAFS) spectroscopy. At pH 6.5, Y uptake readily occurred, but equilibrium was not attained within 96 h of reaction. A greater fraction of Y was sorbed at pH 7, and near-equilibrium conditions were attained after 48 h of reaction.Polarized EXAFS data indicated that the environments of Y sorbed at the two pH values were structurally similar. Yttrium was surrounded by a split first coordination sphere of four O atoms at 2.27−2.28 Å and four O at 2.43 Å. The averaged(Y-O) distance agrees with eight-fold coordination of Y. Neighboring cationic shells of Mg atoms at 3.16−3.18 Å oriented at the magic angle and out-of-plane Si atoms at 3.48−3.51 Å were also detected. These distances and orientations are consistent with Y forming inner-sphere surface complexes at layer edges of hectorite by sharing edges with Mg octahedra and corner with Si tetrahedra. The observed angular dependence further suggests that Y is tilted away from the Mg-containing median planes of hectorite platelets.

Particle tracking to reveal gelation of hectorite dispersions

Passive microrheological techniques using particle tracking have been developed for the study of the gelation of hectorite suspensions. By following the Brownian motion of the particles, it is possible to determine the increasing caging of the particles with time, as the system gels. Since only the Brownian motion is followed, the gelation process itself should not be affected by the measurement. As gelation proceeds the increasing heterogeneity of the particle environments can be monitored by a variety of measures, including kurtosis. An effective viscosity can be extracted from the measurements and used to indicate the gelation process.

Reactivity of Fe(II) species associated with clay minerals.

Mineral-bound Fe(II) species represent important natural reductants of pollutants in the anaerobic subsurface. At clay minerals, three types of Fe(II) species in fundamentally different chemical environments may be present simultaneously, i.e., structural Fe(II), Fe(II) complexed by surface hydroxyl groups, and Fe(II) bound by ion exchange. We investigated the accessibility and reactivity of these three types of Fe(II) species in suspensions of two different clay minerals containing either ferrous iron-bearing nontronite or iron-free hectorite. Nitroaromatic compounds (NACs) exhibiting different sorption behavior on clays were used to probe the reactivity of the various types of reduced iron species. The clay treatment allowed for a preparation of nontronite and hectorite surfaces with Fe(II) adsorbed by surface hydroxyl groups at the edge surfaces. Furthermore, hectorite suspensions with additional Fe(II) bound to the ion exchange sites at the basal siloxane surfaces were set up. We found that both structural Fe(II) and Fe(II) complexed by surface hydroxyl groups of nontronite reduced the NACs to anilines. An electron balance revealed that more than 10% of the total iron in nontronite was reactive Fe(II). Fe(II) bound by ion exchange did not contribute to the observed reduction of NACs. Reversible adsorption of the NACs at the basal siloxane surface of the clays strongly retarded NAC reduction, even in the presence of high concentrations of Fe(II) bound by ion exchange to the basal siloxane surfaces. Our work shows that in natural systems a fraction of the total Fe(II) present on clays may contribute to the pool of highly reactive Fe(II) species in the subsurface. Furthermore, this work may help to distinguish between Fe(II) species of different reactivity regarding pollutant reduction. Although structural iron in clays represents only a small fraction of the total iron pool in soils and aquifers, reactive Fe(II) species originating from the reduction of structural Fe(III) in clays may contribute significantly to the biogeochemical cycling of electrons in the subsurface since it is not subject to depletion by reductive dissolution.

Adsorption mechanisms of Zn on hectorite as a function of time, pH, and ionic strength

The mechanisms of Zn uptake in dilute suspensions ( ∼2 g L−-1) of hectorite were investigated by kinetics experiments and extended X-ray absorption .ne structure (EXAFS) spectroscopy on wet pastes and self-supporting films of Zn-sorbed hectorite. Kinetics experiments were performed at pH 4 and 6.5, in 0.3 and 0.01 M NaNO3, and with a total Zn concentration of 100 μM. In 0.01 M NaNO3, signi.cant amounts of Zn were sorbed within the first 5 min of reaction at both pHs. This rapid uptake is consistent with Zn adsorption on exchange sites located on hectorite basal planes. After this fast sorption, the amount of sorbed Zn slowly increased at pH 6.5 but decreased at pH 4. In 0.3 M NaNO3, Zn uptake was less rapid within the .rst 5min, and the amount of sorbed Zn subsequently increased at both pHs, with more Zn sorbing at the higher pH. This behavior is consistent with Zn adsorption on pH-dependent sites. The dissolution of hectorite was monitored during Zn sorption. In 0.3 M NaNO3 at pH 6.5, initial Zn uptake was correlated with an excess release of Mg as compared to a hectorite suspension without added Zn. In contrast, Si release was inhibited initially by Zn addition. At pH 4, Zn addition did not affect the dissolution rate of hectorite. At pH 6.5, polarized-EXAFS (P-EXAFS) spectra obtained on self-supporting films for the two ionic strengths at reaction times between 6 and 120 h show similar crystallochemical environments for Zn. Four atomic shells were identi.ed: a nearest O shell at an interatomic distance RZn-O = 2.06 ± 0.01 A, a Mg shell at RZn-Mg = 3.06-3.09 ± 0.03 A, a Si shell at RZn-Si = 3.23-3.26 ± 0.03 A, and a next-nearest O shell at RZn-O = 3.69-3.74 ± 0.06 A. RZn-Mg and RZn-Si values are characteristic of edge-shared Zn and Mg octahedra and of corner-shared Zn octahedra and Si tetrahedra, respectively. The angular dependencies of the Zn-Mg and Zn-Si contributions indicated that Zn-Mg pairs were oriented parallel to the film plane, whereas Zn-Si pairs were not. These results indicate that inner-sphere (IS) Zn surface complexes formed at layer edges of hectorite platelets, in continuity to octahedral sheets. Magic-angle EXAFS spectra of wet pastes and self-supporting films obtained at pH 6.5 in 0.3 M NaNO3 are similar, con.rming the IS uptake mechanism operated under fully wet conditions at high ionic strength. In 0.01 M NaNO3, a continuous evolution with increasing reaction time from predominantly outer-sphere (OS) to predominantly IS complexes was observed for wet pastes, suggesting that Zn initially sorbed as exchangeable OS complexes on interlayer sites, and then migrated to layer edges to form IS surface complexes. Based on previous work, Zn has a higher af.nity than Co for the hectorite surface, in keeping with the higher stability of Zn phyllosilicates. SYMBOLS EXAFS: extended X-ray absorption fine structure P-EXAFS: polarized extended X-ray absorption fine structure FT: Fourier transform of the EXAFS spectrum RSF: radial structure function α: angle between the X-ray polarization vector and the phyllosilicate plane βj: angle between the c* axis of the phyllosilicate layer and the vectors connecting the X-ray absorbing atom to backscattering atoms in the j shell k: modulus of the wavevector in EXAFS spectroscopy χ(k): EXAFS function χjα: EXAFS contribution of a j shell at the α angle χjiso: isotropic EXAFS contribution of a j shell at the magic angle Njα: apparent number of backscatterers in the j shell at the α angle Nj35°: structural number of backscatterers in the j shell RZn − jEXAFS: EXAFS-derived interatomic distance between Zn absorber and backscattering atoms in the j shell σ: Debye-Waller term in EXAFS spectroscopy S02: amplitude reduction factor in EXAFS spectroscopy RP: reliability factor used to adjust model to sample EXAFS spectra ZnKer: Zn-containing kerolite ZnKer300: Zn-kerolite Zn3Si4O10(OH)2·nH2O ZnKer070: Zn-containing kerolite Zn0.7Mg2.3Si4O10(OH)2·nH2O ZnKer003: Zn-containing kerolite Zn0.03Mg2.97Si4O10(OH)2·nH2O OS: outer sphere IS: inner sphere E linkage: edge-sharing linkage C linkage: corner-sharing linkage I: ionic strength Zn(aq)2+: fully solvated Zn2+ cation [Zn]aq: concentration of Zn in the supernatant [Si]aq: concentration of Si in the supernatant [Mg]aq: concentration of Mg in the supernatant ZnT: total Zn concentration in the suspension CoT: total Co concentration in the suspension

Formation of layer-by-layer composite films by the Langmuir-Blodgett technique : Smectite-amphiphilic ruthenium(II) complex systems

Organized organic-inorganic ultrathin films have been constructed from the fabrication of smectite unit layers and an amphiphilic ruthenium (II) complex monolayer using the conventional Langmuir-Blodgett (LB) technique. According to the method, a chloroform solution of an amphiphilic ruthenium (II) complex, [Ru(phen) 2 (dC18bpy)] 2- (phen = 1.10-phenanthroline, dC18bpy = 4,4'-dioctadecyl-2,2'-bipyridyl as shown in Figure. 1), was spread onto an aqueous subphase containing either exfoliated saponite or hectorite suspension. Brewster angle microscopy (BAM) observation showed that Ru(II) complex molecules formed a self-assembled monolayer on a water surface and that clay particles were adsorbed by the monolayer from an aqueous subphase. A composite film of a Ru(II) complex and a clay particle thus formed was transferred onto a hydrophilic glass plate by the vertical dipping method. The surface structure of a deposited film was studied with an atomic force microscope (AFM). As a results, a glass substrate was covered with the single sheet of a clay on which amphiphilic metal complexes were attached.

Sorption and Redox Activity of Cobalt Corrinoids on Hectorite

AbstractHalogenated synthetic organic compounds are widespread contaminants of the environment; however, little is known concerning their potential dehalogenation by extracellular corrinoids. Our primary objective was to determine if corrinoids sorbed to soil colloids are redox active and thus viable electron transfer mediators of contaminant dehalogenation in reduced soils and sediments. Dicyanocobínamide, cyanocobalamin (vitamin B12), and aquocobalamin were sorbed onto Ca2+‐, K+‐, and Na+‐hectorite and sorption isotherms determined. Additional assessment of sorption reactions was performed using x‐ray diffraction (XRD) and infrared (IR) spectroscopy. Redox states of the sorbed corrinoids in hectorite suspensions were monitored using ultraviolet and visible (UV‐VIS) spectrophotometry. Corrinoid molecular size and charge, as well as the saturating cation, controlled corrinoid sorption. The XRD results indicated that Co corrinoids intercalated hectorite, expanding the basal spacing from 1.45 to 2.21 nm for Ca2+‐, 1.17 to 2.70 nm for K+‐, and 1.31 to 2.81 nm for Na+‐hectorite. The IR spectra of bound corrinoids yielded little information on binding mechanisms, but indicated that axial ligands remained intact. The central metal of bound corrinoids was reduced to Co(I) by Ti(III) and reoxidized to Co(III) by introduction of O2, thus demonstrating that redox activity of sorbed corrinoids was maintained and reversible. Microbially produced extracellular corrinoids may act as potential electron transfer mediators when in association with clays, possibly participating in the reductive dehalogenation of organic contaminants.

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.

Deuterium and oxygen-17 nuclear magnetic resonance of aqueous clay suspensions

Properties of interfacial water have been studied in saponite, hectorite and montmorillonite suspensions. These data, together with our previous results are explained by the presence of two water interfacial sites or by a fast exchange between them.

Dielectric properties of clay suspensions in MHz to GHz range

Dielectric response of various clay suspensions were measured in the range 0.5 MHz to 1.3 GHz. A swelling clay, Na +-montmorillonite, shows large values of dielectric constants (∼200, at 0.5 MHz for 1.5% wt/wt suspension). The dielectric constant shows a monotonic increase with particle concentration, but the conductivity over concentration does not decrease monotonically. The dielectric constant of Na + montmorillonite first decreases and then increases with salinity. This behavior comes from the combination of a decrease in double-layer effect with an increase due to geometrical effect with increasing salinity. We find that the current theoretical models cannot explain quantitatively the frequency-dependent dielectric constant for low salinity suspensions or suspensions in distilled water. When the salinity of the suspension is increased, the dielectric constant agrees well with the geometrical model, without any adjustable parameters. The dielectric constant of homoionic montmorillonites with same particle sizes and same concentrations, varies with the mobility of the exchangeable rations—Li +, K +, Na +, Rb +, Cs +, NH 4 +. This implies a large electrochemical effect even in this frequency range. The dielectric constant is reduced substantially upon exchanging Na + ions with Ca 2+, or with relatively large, less mobile, organic cations, such as tetramethylammonium. X-Ray diffraction measurements showed a reduction in aspect ratio, even though these clays were suspended in water. The dielectric constant of Na +-hectorite suspension was nearly 60 dielectric units lower than that of Na +-montmorillonite. This difference is attributed to the difference in their effective aspect ratio. A dilute suspension of kaolinite, a nonswelling clay, shows no dielectric enhancement, which is expected from its low cation exchange capacity (CEC) and nonplatey morphology. But, at high concentration, remnants of the electrochemical effects are observed. Thus both size and the charge effects are important in this frequency range.

Hydrolysis and Dehydration Reactions of Exchangeable Cu2+ on Hectorite

AbstractElectron spin resonance (ESR) analysis of Cu2+-hectorite suspensions provides evidence for the surface-induced hydrolysis of Cu(H2O)62+ at low pH and surface-inhibited hydrolysis (or precipitation) at high pH. Dehydration of the hectorite by heating to 110°C appears to promote hydrolysis in high pH clays further. Heating to even higher temperatures removes ligand water from Cu2+, allowing the metal ion to coordinate with silicate oxygen atoms. The planar Cu(H2O)42+ ion predominates in the interlamellar regions of hectorite that has been air dried or heated to temperatures of 110°C or lower, but more extreme thermal treatment changes the apparent orientation of the Cu2+-ligand axes as some or all of the four water ligands are removed, A loss in ESR signal intensity upon heating Cu2+-hectorite above 110°C is evidence for lowered symmetry of the dehydrated, surface-coordinated Cu2+ ion.

Cationic Spin Probes on Hectorite Surfaces: Demixing and Mobility as a Function of Adsorption Level

AbstractThe electron spin resonance (ESR) technique has been used to study the motion and segregation of an organic spin probe cation (4-amino-2,2,6,6-tetramethylpiperidine N-oxide) on K+-hectorite as a function of average surface concentration. The organic cation tends to concentrate in certain interlayers of aqueous hectorite suspensions even when it occupies a small fraction of the cation-exchange sites. This demixing effect is not evident in methanol-solvated hectorite. The average mobility of the probe increases at higher adsorption levels as a result of the shift of the equilibrium in favor of the solution state. Calculated time-averaged orientations of the probe on the clay surfaces are quite different for methanol- and water-solvated systems, emphasizing the importance of the solvent in modifying the surface-cation interaction.