The mineralogical evolution of di- and trioctahedral smectites (i.e. montmorillonite and saponite) exposed to high-pH environments has been studied to determine the influence of compositional differences on clay dissolution and the formation of new phases. The present study helps to gauge the effects of highly alkaline solutions on the swelling capacity of smectitic clays and experimental results are extrapolated to predict the behavior of smectite-rich soils in various technical applications such as nuclear-waste storage and architectural conservation. The present study revealed extensive dissolution of montmorillonite in 5 M NaOH or 5 M KOH solutions and the neoformation of various zeolites, thereby reducing the clay’s swelling capacity significantly. Saponite, in contrast, experienced less pronounced changes, including transformation into a randomly interstratified saponite-chlorite and a Si-rich amorphous phase. These changes only provoked a partial reduction in swelling capacity. The results imply that under repository conditions (e.g. alkaline environment caused by hyperalkaline fluids released during concrete leaching), the slow and limited transformation of saponite into corrensite-type minerals would be beneficial for preserving the clay’s swelling capacity and, therefore, its effectiveness as a sealing material. Conversely, the loss of swelling capacity as a result of zeolite formation in montmorillonite observed in the present experiments would limit the clay’s effectiveness as a sealing material in waste repositories. In the case of earthen architecture conservation, alkaline consolidation treatments aimed at reducing the soils’ swelling capacity and, thereby, improving water resistance, would only be effective for treating earthen structures made of soils rich in dioctahedral smectites. Soils containing trioctahedral smectites, in contrast, are not likely to improve their water resistance because the swelling capacity will only be partially reduced.

The main objective of the present work was to functionalize nanoclays with organosilanes and surfactant in order to facilitate the dispersion of the nanofillers in the host fluoroelastomer (FKM) polymer matrix. Better dispersion was achieved by improving interaction between the clay polymer nanocomposite (CPN) constituents. The first part of this study investigated modification of montmorillonite (Mnt) using different saturated and unsaturated alkyl silanes and an alkyl hydrocarbon ammonium quaternary surfactant. Silicon magic angle spinning nuclear magnetic resonance spectroscopy, thermal gravimetric analysis (TGA), elemental analysis, X-ray diffraction (XRD), and Fourier transform infrared spectroscopy were used to characterize the silane-grafted clays. Results indicated that the amount of silane grafted depended on the specific structure of the silane. Silane-grafted Mnt was also modified with ionic surfactants intercalated between the clay layers. A 169% increase in the clay basal spacing (from initial spacing of 10.0 Å to 26.9 Å) was achieved. The second part of the study successfully synthesized FKM nanocomposites containing custom-functionalized Mnt, with the aim of producing reinforced high-performance materials. The effects of clay modification on the morphology and thermal properties of the CPN were studied using XRD, TGA, scanning electron microscopy, and transmission electron microscopy. The CPN made with the modified clay exhibited greater thermal stability than the CPN of the commercially available modified Mnt, with a degradation onset point ~ 40°C higher.

Transformation of kaolinite to dickite is a common diagenetic reaction. The present report is part of a wider study to investigate the pathways of this polytype change. Fourier-transform infrared spectroscopy (FTIR) was used to attempt quantification of the relative proportions of kaolinite and dickite, validated by X-ray diffraction (XRD) results, in order to link mineral and structural features during the mineralogical changes. A group of kaolinite and dickite samples was investigated: 13 samples from the Frøy and Rind oil fields (North Sea), three kaolinite specimens with different crystal order and particle size (KGa-2, kaolinite API 17, Keokuk kaolinite), and two dickite-rich samples (Natural History Museum collection). Six FTIR spectral features were analyzed: (1) intensity ratio of the minima at 3675 and 3635 cm−1; (2) position of the band at ~1115 cm−1; (3) difference between the frequency of the bands at ~1030 and ~1000 cm−1; (4) intensity ratio of the bands generating shoulders at ~922 and ~900 cm−1; (5) position of the band at ~370 cm−1; and (6) intensity of the band at ~268 cm−1. Correlation of the features above with polytype relative proportions derived from XRD showed non-linear behavior, with maximum curvature at the dickite end, which precludes kaolinite-dickite quantification. Increasing kaolin particle size is known to cause decreased intensity of the FTIR spectra. A model was developed to test whether this effect is consistent with the non-linear progression of the IR features. The relative intensity of kaolinite and dickite IR features were calculated in a series of kaolinite-to-dickite transformations, where the size of particles increases with dickite proportion, and where dickite-dominated particles reach a larger size than kaolinite-dominated particles. The results indicated that the differential particle size increase is possibly the cause of the lack of linearity between IR- and XRD-measured dickite proportions.

Because of their geochemical properties, the Çakmaktepe (Uşak) kaolin deposits have been considered as primary. New sedimentological, mineralogical, and geochemical data suggest that the Çakmaktepe kaolins are secondary deposits of sedimentary processes after hydrothermal alteration of the source rocks. The kaolins in the Çakmaktepe deposit were formed from the hydrothermal alteration of calcalkaline Karaboldere volcanics (KBV). The kaolinized materials were then reworked and accumulated in a lacustrine basin. The argillic alteration zones were associated with faults, and lateral zonation of minerals was observed in the KBV. Smectite was the major phyllosilicate in the ‘outer zone’. The alteration mineralogy of the ‘inner zone’ was similar to that of the Çakmaktepe kaolins and consisted mainly of kaolinite with minor amounts of smectite and alunite. The trace-element abundances in the kaolinized volcanics and the Çakmaktepe kaolins indicated hypogene conditions. The δ18O values of the Çakmaktepe kaolins ranged from 0.2 to 5.92%, which indicated that the Çakmaktepe kaolinites were formed at temperatures between 92 and 156°C, and the δD values ranged from −91.68 to −109.45‰. The irregular edge-to-face morphology, the variation in grain-size, a few broken crystals of kaolinite, the deficiency of dissolution-replacement and crystallization mechanisms, and the the low sphericity, very angular, and poorly sorted quartz crystals in the kaolins all result from transport processes. The sedimentary structures, including trough cross-lamination, tool marks, and load casts, indicate transportation by turbulent waters and deposition of kaolin layers in a shallow lake.

The objective of the study was to contribute to the understanding of the influence of the structure and the 2:1 layer dimension of smectites on cation exchange capacity (CEC) reduction and the hydration behavior of Li-saturated smectites after heating. Five montmorillonites extracted from bentonites of different provenance were saturated with Li+ and heated to 300°C. Initial montmorillonites and montmorillonites with reduced layer charge (RCM) were characterized by comprehensive mineralogical analysis supplemented by CEC measurements, surface-area measurements by Ar adsorption, and 7Li, 27Al, and 29Si magic-angle spinning nuclear magnetic resonance spectroscopy (MAS NMR). The CEC of the initial montmorillonites varied between 89 and 130 cmol(+)/kg while the CEC of the RCM prepared at 300°C varied between 8 and 25 cmol(+)/kg. The lateral dimension of the 2:1 layers varied between 70 and 200 nm. The greatest decrease in CEC was observed for the montmorillonite with the largest diameter of the 2:1 layers and the smallest decrease was observed for the montmorillonite with the smallest diameter of the 2:1 layers. 7Li MAS NMR revealed an axially symmetric chemical environment of the hydrated interlayer Li+ with ηΔ = 0 for the chemical shift anisotropy tensor for unheated montmorillonites with >33% tetrahedral layer charge (ξ). The chemical environment is typical of innersphere hydration complexes of interlayer Li+. An axially non-symmetric chemical environment of the interlayer Li+ with ηCS of close to one was observed for all RCM. While the remaining CEC of RCM prepared at 300°C reflected the variable CEC at the edges, and thus the lateral size or aspect ratio of the 2:1 layers, the hydration complex of interlayer Li+ was strongly determined by the isomorphic substitutions in the dioctahedral 2:1 layers.

Bentonites are commonly used as chemical containment barriers to minimize liquid flow and contaminant transport. However, chemicals can adversely affect bentonite performance to the extent that modified bentonites have been developed to improve chemical resistance relative to traditional (unmodified) bentonites. The present study focused on the diffusion of potassium chloride (KCl) through a bentonite-polymer composite, or BPC, that was known to behave as a semipermeable membrane. Specifically, the effective diffusion coefficients, D*, for chloride (Cl−) and potassium (K+) were measured and correlated with previously measured membrane efficiency coefficients, ω, for the BPC. The values of D* at steady-state for chloride ($D_{ss,C{l}^{-}}^{*}$\$\end{document}) and potassium ($D_{ss,K^{+}}^{*}$\$\end{document}) decreased as the ω values increased. The decrease in $D_{ss,C{l}^{-}}^{*}$\$\end{document} and $D_{ss,K^{+}}^{*}$\$\end{document} was approximately a linear function of (1 − ω), which is consistent with previous research performed on unmodified Na-bentonite contained within a geosynthetic clay liner (GCL). In contrast to the previous GCL tests, however, $D_{ss,C{l}^{-}}^{*}$\$\end{document} values for the BPC generally were greater than the $D_{ss,K^{+}}^{*}$\$\end{document} values, and the differences between $D_{ss,C{l}^{-}}^{*}$\$\end{document} and $D_{ss,K^{+}}^{*}$\$\end{document} decreased as KCl concentration increased. The apparent discrepancy between $D_{ss,C{l}^{-}}^{*}$\$\end{document} and $D_{ss,K^{+}}^{*}$\$\end{document} is consistent with excess sodium (Na+) in the BPC prior to testing and the requirement for electroneutrality during testing. Also, despite an apparent linear trend in diffusive mass flux for K+, lack of agreement between the ratio of the diffusive mass flux of K+ relative to that for Cl− as required on the basis of electroneutrality at steady state suggested that steadystate diffusive mass flux for K+ had probably not been achieved due to continual K+-for-Na+ cation exchange. Nonetheless, the excess Na+ and bentonite modification did not affect the fundamental correlation between D* and ω, which requires that D* approaches zero as ω approaches unity (D* → 0 as ω → 1).

Widespread lateritized ultramafic rocks in the southern part of the Muratdağı region of Turkey constitute a significant source of Ni-Cr-bearing ore with economic potential. However, no mineralogical or geochemical characterizations of these important materials have been performed previously. The aim of the present study was to describe the mineralogy, geochemistry, and genesis of Ni-Cr-bearing smectite in garnierite and ferruginous saprolite associated with the lateritized ophiolite-related ultramafic rocks. The lateritic zones are well developed over serpentinized harzburgitic mantle peridotites. The lateritized units and related bedrocks were examined using polarized-light microscopy, X-ray diffraction, scanning and transmission electron microscopies, and chemical and isotopic methods. The garnierite-containing saprolites are enriched in smectite, Fe-(oxyhydr)oxide phases, and opal-CT. Micromorphological images revealed that flaky smectite and, locally, Fe-rich particles, alunite, gypsum, gibbsite, and sulfur crystals developed along the fractures and dissolution voids. The development of saprolite demonstrates chemical weathering. The presence of silicified and Fe-(oxyhydr)oxide phases associated with gypsum, alunite, and local native sulfur in vertical and/or subvertical fractures and fault infillings are indicative of hydrothermal processes along the extensional, tectonically related fault systems. Chemical weathering and hydrothermal processes, which probably started during the Oligocene and Miocene, led to the formation of nontronite, Fe-bearing montmorillonite, and local Fe-rich kaolinite. Nickel and Cr are concentrated significantly in the saprolite zone and are positively correlated with Fe2O3 content, which is controlled by the formation of nontronite, montmorillonite, and Fe-(oxyhydr)oxide phases. Nickel-Cr-bearing nontronite and montmorillonite precipitated from alkaline water as a result of the increasing (Fe2O3+Al2O3+Cr2O5+Ni+Co)/(MgO+SiO2) ratio under the control of both chemical weathering and hydrothermal processes. The Fe and Mg (associated with Ni and Cr) required for the formation of smectite were supplied by solutions from both chemical weathering and hydrothermal alteration of Ni-Cr-bearing olivine and pyroxene in the harzburgitic bedrock; the Al was supplied by schists, granite, and volcanic units.

During recent decades, the search for possible repositories for high-level nuclear waste has yielded large amounts of sorption data for actinides on different minerals. Clays and clay minerals are of special interest as potential host-rock formations and backfill materials, by virtue of their good retardation properties. Neptunium (Np) is one of the actinides which is considered in long-term scenarios due to its long-lived nuclide 237Np (t1/2 = 2.1 × 106 y). Because neptunium sorption is heavily dependent on the experimental conditions, comparison of sorption data from different experiments is challenging. Normalizing reported data with respect to the surface area of the sorbent enables conversion of conventional distribution coefficients (Kd) to normalized (Ka) values, which improves comparability among the results of different experiments. The present review gives a detailed summary of sorption data of Np on clays and clay minerals and examines critically the applicability of the Ka approach.

Chlorites are petrogenetically important minerals, exercise controls on petroleum reservoir qualities, are common in alteration zones during hydrothermal ore mineralization, and may form during carbon sequestration in sedimentary formations. Chlorite thermochemistry and structure have been investigated, in the present study, to facilitate an improved understanding of chlorite equilibria.

Three natural IIb chlorites were studied by powder diffraction and calorimetric methods (low-temperature relaxation calorimetry using a Physical Properties Measurement System [PPMS] and differential scanning calorimetry [DSC]). The samples include a low-Fe clinochlore [Mg-Chl] and two Fe-rich chlorites from Windsor [Fe-Chl(W)] and Michigan [Fe-Chl(M)]. The structure of each chlorite was refined in the ideal C2/m symmetry using Rietveld techniques. Lattice parameters for the Windsor chlorite are a = 5.3786(6) Å, b = 9.3176(9) Å, c = 14.2187(6) Å, β = 96.98(10)°. The Michigan chlorite returned a = 5.3897(3) Å, b = 9.3300(3) Å, c = 14.2376(2) Å, β = 97.043(5)° whereas the low-Fe clinochlore yielded a = 5.3301(3) Å, b = 9.2231(8) Å, c = 14.2912(4) Å, β= 97.03(10)°.

Heat capacities (Cp) for the three natural chlorites were measured using both PPMS (2–303 K) and DSC (282–564 K). Employing a combination of Debye-Einstein-Schottky functions, the lattice dynamics component of the Cp at lower temperature was evaluated allowing a separation of the magnetic spin ordering component of Cp from the lattice vibrational part. For Mg-Chl, Fe-Chl(W), and Fe-Chl(M), the polynomials defining the temperature dependencies of the heat capacities between 280 and 570 K are:

Cp = 1185.44(±68.93) − 9753.21(±186.85)T−0.5 − 1.9094(±1.0288)·107T−2 + 3.3013(±1.5363)·109T−3

Cp = 1006.06(±48.46) − 4134.83 (±1515.16)T−0.5 − 40.0949(±6.9413)·106T−2 + 5.9386(±1.0287)·109T−3

and

Cp = 1268.60(±67.16) − 11983.09(±2107.07)T−0.5 − 7.6037(±9.6417)·106T−2 + 1.5398(±1.4187)·109T−3, respectively.

Standard state molar thermodynamic functions, CP, ST, (HT−H0)/T, and φ were evaluated for the samples. S298.15 for Fe-Chl(W), Mg-Chl, and Fe-Chl(M) were found to be 499.14 ± 3.40, 437.81 ± 3.00 and 515.06 ± 3.60 J mol-1K-1, respectively, whereas S° for Fe-Chl(W) and Mg-Chl were determined to be 578.24 ± 3.76 and 503.21 ± 3.60 J mol−1K−1, −1

Clay swelling, an important phenomenon in natural systems, can dramatically affect the properties of soils and sediments. Of particular interest in low-salinity, saturated systems are osmotic hydrates, forms of smectite in which the layer separation greatly exceeds the thickness of a single smectite layer due to the intercalation of water. In situ X-ray diffraction (XRD) studies have shown a strong link between ionic strength and average interlayer spacing in osmotic hydrates but also indicate the presence of structural disorder that has not been fully described. In the present study the structural state of expanded smectite in sodium chloride solutions was investigated by combining very low electron dose, high-resolution cryogenic-transmission electron microscopy observations with XRD experiments. Wyoming smectite (SWy-2) was embedded in vitreous ice to evaluate clay structure in aqua. Lattice-fringe images showed that smectite equilibrated in aqueous, low-ionic-strength solutions, exists as individual smectite layers, osmotic hydrates composed of parallel layers, as well as disordered layer conformations. No evidence was found here for edge-to-sheet attractions, but significant variability in interlayer spacing was observed. Whether this variation could be explained by a dependence of the magnitude of long-range cohesive (van der Waals) forces on the number of layers in a smectite particle was investigated here. Calculations of the Hamaker constant for layer-layer interactions showed that van der Waals forces may span at least five layers plus the intervening water and confirmed that forces vary with layer number. Drying of the disordered osmotic hydrates induced re-aggregation of the smectite to form particles that exhibited coherent scattering domains. Clay disaggregation and restacking may be considered as an example of oriented attachment, with the unusual distinction that it may be cycled repeatedly by changing solution conditions.

An overlap of bands produced by the O−H stretching vibrations of H2O (O–Hw) and structural OH (O−Hs) in smectite hampers the study by infrared spectroscopy (IR) of both their layer and interlayer structure. The present study re-evaluated the D2O saturation of smectite as a tool to enable separation of the overlapping bands at ambient conditions. Real-time monitoring by Attenuated Total Reflectance infrared spectroscopy (ATR-IR) was employed during in situ sample drying and H2O or D2O saturation at ambient temperature. Six dioctahedral and one trioctahedral pure smectites in Ca2+-, Na+-, and Cs+-cationic forms were studied to explore variability in total layer charge, charge location, and interlayer cation. The IR data showed the interlayer O−Dw signature at 2700–2200 cm−1 as a proxy for the O−Hw signature in the 3700–3000 cm−1 region. In addition to the expected liquid-like bands of D2O in the interlayer, these O−Dw spectra exhibited an additional sharp stretching feature in the 2695–2680 cm−1 range. No significant cation dependence of the sharp band position was observed between pairs of Ca- and Na-smectites for relative humidity (RH) between 60 and 80%, despite the large difference in the ionic potential between these interlayer cations. The intensity of the sharp band was found to be almost insensitive to changes in water content within the range 60–80% RH. The sharp band frequency decreased linearly with increasing total charge of the 2:1 layer (and can be used as a proxy for it), but no effect of charge location could be discerned. In agreement with early studies, this band was attributed to D2O located on the surface of the interlayer, pointing one O−D group toward the siloxane surface. Based on its high frequency, this band was indicative of free O−D oscillators, with very little or no involvement in hydrogen bonding (“dangling OD”). By analogy to the spectra of D2O-smectites, the spectrum of H2O-smectites also involves a sharp O−Hw analog at ~3630 cm−1 overlapping with typical OHs bands (e.g. Al2OH). As a result of this overlap, the sharp 3630 cm−1 O−Hw contribution was often missed or attributed solely to O−Hs.

The pyroclastic sediments studied here contained varied amounts of zeolite and were formed in the saline alkaline Tuzgölü Basin following the alteration of dacitic volcanic materials during the Early to Late Miocene. The present study focused on the geological-geochemical properties of the zeolites and describes their formation. Mineralogical and chemical compositions were determined by X-ray diffraction, scanning electron microscopy, optical microscopy, and inductively coupled plasma mass spectrometry. Results indicated that the zeolitic tuffs consisted mainly of heulandite/clinoptilolite (Hul/Cpt), chabazite, erionite, and analcime associated with smectite. Smectite, calcite, and dolomite are abundant in the clay and carbonate layers which alternate with the zeolitic tuffs. K-feldspar, gypsum, and hexahydrite (MgSO4·6H2O) were also found in some altered tuffs and clay-marl layers as accessory minerals. The zeolite and other authigenic minerals showed weak stratigraphic zonation. Some vitric tuff layers contained no zeolite minerals and others were found to consist of almost pure Hul/Cpt and chabazite layers with economic potential. The rare earth elements (REE), large ion lithophile elements (LILE), and high-field strength elements (HFSE) in the Hul/Cpt-rich tuffs and vitric tuffs were enriched or depleted relative to the precursor rock, while many major elements were slightly or significantly depleted in all zeolitic tuffs. The amounts of REE in the chabazite- and erionite-rich tuffs were generally smaller than those in the precursor rock. The middle and heavy REE (MREE and HREE, respectively) were abundant in the Hul/Cpt-rich tuffs, tuffs, and smectitic bentonites. Chondrite-normalized REE values of the sample groups are characterized by sub-parallel patterns with enrichment in LREE relative to HREE. The mineral assemblages and geological setting indicated that zeolite diagenesis occurred in a saline-alkaline basin. The δ18O and δD compositions of the Hul/Cpt, chabazite, and smectite indicated that the minerals formed at low to moderate temperatures and that some of the zeolitization occurred due to diagenetic alteration under closed-system conditions that varied according to the nature of the basin and with the composition and physical properties of the volcanic materials.

Atomistic simulations of 2:1 clay minerals based on parameterized forcefields have been applied successfully to provide a detailed description of the interfacial structure and dynamics of basal planes and interlayers, but have made limited progress in exploring the edge surfaces of these ubiquitous layer-type aluminosilicates. In the present study, molecular dynamics simulations and energy-minimization calculations of the edge surfaces using the fully flexible CLAYFF forcefield are reported. Pyrophyllite provides an ideal prototype for the 2:1 clay-mineral edge surface because it possesses no structural charge, thus rendering the basal planes inert, while crystal-growth theory can be applied to identify two major candidates for the structure of the edge surfaces. Models based on these candidate structures reproduced bulk crystal bond distances accurately when compared to X-ray data and ab initio molecular simulations, and the predicted edge surface bond distances were in agreement with those determined via ab initio simulation. The calculated surface free energy and surface stress led to an accurate prediction of pyrophyllite nanoparticle morphology, while surface excess energies calculated for the edge surfaces were always negative. These results are consistent with the observed pyrophyllite nanoparticle morphology, with the concept of negative interfacial energies, and conditions that may give rise to them including a role in the stabilization of layer-type nanoparticulate minerals. Molecular dynamics simulations of hydrated nanoparticle edge surfaces indicated five reactive surface oxygen sites on the dominant candidate edge, in agreement with a recent model of proton titration data for 2:1 clay minerals. These promising results illustrate the potential for classical mechanical atomistic simulations that explore edge surface phenomena at much greater length- and times-scales than are currently possible with computationally expensive ab initio methods.

Montmorillonite-rich clays are important in many engineering applications. The compressibility of such plastic clays at high consolidation pressures is important for predicting routine settlement and for applications in nuclear-waste repositories. Laboratory measurement of compressibility data at high consolidation pressures is not only time consuming but very expensive also. Theoretical predictions can help to determine the compressibility of plastic clays at high consolidation pressures. A linear relationship between e/eNvs. 1/√P (eN is the normalization void ratio at normalization pressure N and P is the consolidation pressure) was derived using diffuse double-layer theory. The compressibility data of several plastic clays in published studies were found to support the derived relationship. A generalized theoretical equation was proposed to predict the compressibility data over a wide range of consolidation pressures using an experimentally measured void ratio at low consolidation pressure. The compressibility data for different plastic clays were predicted accurately up to maximum consolidation pressures that ranged from 0.7 to 30 MPa using an experimentally measured void ratio near the pre-consolidation pressure. The preconsolidation pressures for different clays considered here ranged from 25 to 133 kPa. The proposed predictive model is supported by experimental data, is simple, and does not require knowledge of clay-surface and pore-fluid parameters.

Catanionic surfactant systems are used as drug-delivery vehicles and as nanocompartments in the formation of biomaterials and nanosized particles. Clay minerals are compatible with organic tissues and also have biomedical applications. The aim of the present study was to combine the properties of catanionic surfactants and clay minerals to obtain new materials with potential uses in medicine, waste-water treatment, and antibacterial applications. The surfactants chosen to make the catanionic surfactant were cetylpyridinium (CP) and lauroyl sarcosinate (SR), which interact strongly in aqueous media and cause specific aggregations such as ion-pair amphiphiles and needle- and leaf-like structures. Aside from the aqueous solution, new ternary systems are formed with different structures and properties through the addition of montmorillonite (Mnt). The surface and interlayer structures of the different Mnt-CP-SR samples prepared by using CP and SR in amounts equal to various ratios of cationic exchange capacity of the clay mineral were studied. They were also compared with the structured surfactant aggregates formed in aqueous media. The Mnt-CP-SR samples were subjected to X-ray diffraction (XRD), thermogravimetric analyses, and zeta-potential measurements to elucidate the interlayer- and external-surface structures. The XRD analyses showed the formation of a compact structure in the interlayer region resulting from the interaction between randomly oriented pyridinium and negatively charged SR head groups. The triple interactions among the Mnt surface, CP, and SR were more complex than the double interactions between the Mnt and cationic surfactant, and the CP played a dominant role in the formation of external and interlayer surface structures regardless of the amount and order of the addition of SR. The new findings support new applications for organoclays in the fields of biomedicine, remediation of polluted water, and nanocomposite materials.

Natural mineral materials such as tabular and spheroidal halloysites have recently been suggested as candidates for intercalating metal ions or organic molecules. Their potential use as nanoadsorbents is related to their porous structure and water content. Although the two morphologies can coexist in natural deposits, spheroidal halloysites remain poorly characterized whereas much literature exists on tubular halloysites. The present study investigates the native morphology, internal porous structure, and behavior upon dehydration of spheroidal halloysite from Opotiki (New Zealand). This mineral was characterized in its natural hydrated state using a transmission electron microscope equipped with an environmental cell (EC-TEM). The sample was placed in a sealed block in which water vapor-saturated air circulated at a pressure of 30 Torr. The observed particles consisted of almost complete spheroids displaying polyhedral external surfaces. 1:1 layers stack concentrically as a pore-free, onion-like structure. The dynamic processes of dehydration created by slow depressurization of the cell resulted in a decrease in the layer-to-layer distance (d001) from ~10 Å to ~7 Å due to the loss of interlayer water molecules. Irreversible formation of spurious ‘internal pores’ was recorded during this process. These pores were not indigenous to the hydrated 10 Å halloysite and resulted from the collapse of the native layers. They cannot account for the physical chemical properties of spheroidal halloysite. Spheroidal halloysites would have a lower propensity for intercalating ions or molecules than tubular halloysites. Isolated facets were also observed in high-resolution-TEM and displayed a pseudo-hexagonal morphology. The three-dimensional microstructure of the spheroid appeared bent along the three pseudo equivalent yi directions of the kaolinite-like single layers. An analogy with polyhedral serpentine has allowed the proposal of a formation process of hydrated spheroidal halloysite triggered by enrichment in divalent ions in the growth system.

Organic dyes such as methylene blue (MB) are often used in the characterization of clays and related minerals, but details of the adsorption mechanisms of such dyes are only partially understood from spectroscopic data, which indicate the presence of monomers, dimers, and higher aggregates for varying mineral surfaces. A combination of quantum (density functional theory) and classical molecular simulation methods was used to provide molecular detail of such adsorption processes, specifically the adsorption of MB onto kaolinite basal surfaces. Slab models with vacuum-terminated surfaces were used to obtain detailed structural properties and binding energies at both levels of theory, while classical molecular dynamics simulations of aqueous pores were used to characterize MB adsorption at infinite dilution and at higher concentration in which MB dimers and one-dimensional chains formed. Results for the neutral MB molecules are compared with those for the corresponding cation. Simulations of the aqueous pore indicate preferred adsorption on the hydrophobic siloxane surface, while charge-balancing chloride ions adsorb at the aluminol surface. At infinite dilution and in the gas-phase models, MB adsorbs with its primary molecular plane parallel to the siloxane surface to enhance hydrophobic interactions. Sandwiched dimers and chains are oriented perpendicular to the surface to facilitate the strong hydrophobic intermolecular interactions. Compared with quantum results, the hybrid force field predicts a weaker MB adsorption energy but a stronger dimerization energy. The structure and energetics of adsorbed MB at infinite dilution are consistent with the gas-phase binding results, which indicate that monomer adsorption is driven by strong interfacial forces rather than by the hydration properties of the dye. These results inform spectroscopic studies of MB adsorption on mineral surfaces while also revealing critical areas for development of improved hybrid force fields.

Layer charge (LC) is a fundamental property of smectite but its measurement remains challenging and tedious to apply on a high-throughput basis. The present study demonstrates that the position of a sharp, high-energy O—D stretching band of adsorbed D2O (νO—D, at ~2686–2700 cm−1), determined by infrared spectroscopy, correlates with LC and provides a simple method for its measurement. Twenty nine natural dioctahedral smectites and 14 reduced-charge montmorillonites with LC determined previously by different methodologies were saturated with D2O and examined by attenuated total reflectance infrared spectroscopy (ATR-IR). The samples included smectites in Mg, Ca, Na, Li, K, and Cs forms and covered the full range of the smectite LC (0.2 to 0.6 e per formula unit). Statistically significant correlations were found between νO—D and LC values determined with each of the two main methods of LC determination: the structural formula method (R2 = 0.96, σ = 0.02, ~0.2 < LC < 0.6) and the alkylammonium method (R2 = 0.92, σ = 0.01, 0.27 < LC < 0.37). These correlations were based on Li- and Na-saturated smectites, respectively, but other cationic forms can be employed provided that the exchangeable cations are of sufficiently high hydration enthalpy (e.g. Mg2+ or Ca2+, but not K+ or Cs+). The new method is fast, low-cost, implemented easily in laboratories equipped with ATR-FTIR, and applicable to samples as small as ~5 mg.

The swelling properties of smectite-type clay particles (including montmorillonite) are of interest in various industries. A fundamental understanding of the surface properties of smectite particles at the sub-micron level would facilitate investigation of the effect of distributed properties such as charge and elemental composition. Swelling and delamination of SWy-2 Na-montmorillonite (Na-Mnt) nano-clay particles were studied here using size distributions obtained by sedimentation field-flow fractionation (SdFFF). Fractions were examined by electron microscopy and inductively-coupled optical emission spectroscopy (ICP-OES). Two distinct populations were observed in the size distribution of SWy-2 Na-Mnt particles (bimodal size distribution), with mean equivalent spherical diameters of ~60 nm and 250 nm, respectively. In contrast, the size distribution of STx-1 Ca-montmorillonite (Ca-Mnt) particles showed only one peak with a mean equivalent spherical diameter of ~410 nm, which changed to 440 nm after 4 days of hydration. Analyses of the fractions by ICP-OES obtained along the size distribution of Na-Mnt showed an abundance of Ca and Mg in the fractions below 250 nm, and confirmed the presence of Fe and Mg as isomorphous substituents. Electron micrographs of the fractions obtained from Na-Mnt size distributions were used to calculate the thickness of the clay particles. Bridging forces between pure orMgsubstituted montmorillonite and either Ca2+ or Na+ were calculated using semi-empirical methods. The results demonstrated that swelling and delamination of Na-Mnt clay particles are dictated by properties such as elemental composition and surface charge which are distributed along the size distribution.

When clay minerals, notably smectites, intercalate organic cations, their interlayer surfaces change from hydrophilic to hydrophobic. The resultant intercalates, known as organo-clays (OCs), have a large affinity for hydrophobic organic contaminants (HOCs). Organo-clays are used as sorbents of HOCs in wastewater treatment and as sorptive barriers in landfill liners. The structural and sorptive characteristics of OCs with respect to HOCs have been studied extensively, and a large volume of literature has accumulated over the past few decades. The interactions of OCs with HOCs and the various approaches to improving the sorption capacity of OCs are reviewed here, with particular reference to the application of novel analytical techniques, such as molecular modeling, to characterizing the OC—HOC interaction.