Thisreviewdiscusses preparationanduseofthesyntheticlayerednanoparticlesinpolymermatrices, i.e., in the polymeric nanocomposites (PNCs). Several types of synthetic or semi-synthetic layered materials are considered, namely the phyllosilicates (clays), silicic acid (magadiite), layered double hydroxides (LDHs), zirconium phosphates (ZrPs), and di-chalcogenides. The main advantage of synthetic clays is their chemical purity (e.g. absence of amorphous and gritty contaminants, as well as arsenic, iron, and other heavy metals), white to transparent color that assures reproducibly of brightly colored products, as well as a wide range of aspect ratios, p ¼ 20 to £6000. Several large scale production facilities have been established. The synthetic clay and LDH industries are oriented toward big volume markets: catalysis, foodstuff, cosmetics, pharmaceuticals, toiletry, etc. The use of these materials in PNCs is limited to synthetic clays and LDHs, mainly for reinforcement, permeability control, reduction of flammability, and stabilization, e.g. during dehydrohalogenation of chlorinated macromolecules. The use of lamellar ZrPs and di-chalcogenides is at the laboratory stage of functional polymeric systems development, e.g. for electrically conductive materials, catalysts or support for catalysts, in photochemistry, molecular and chiral recognition, or in fuel cell technologies, etc. Copyright # 2007 John Wiley & Sons, Ltd.

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Synthetic, layered nanoparticles for polymeric nanocomposites (PNCs)

Synthesis of clay minerals

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Electron transfer at the mineral/water interface: Selenium reduction by ferrous iron sorbed on clay

The broad charge heterogeneity typical of nearly all smectites is not necessarily characteristic of vermiculites. In addition to vermiculites with pronounced heterogeneity, minerals with no or only limited charge heterogeneities are known. Layer charge and charge heterogeneity of 25 vermiculites were determined by alkylammonium ion exchange. The comparison of experimental basal spacings with dL/n-plots provided a simple determination of the average charge density. The spacings of high-charged vermiculites (≥0.8 eq/(Si,Al)4O10) with paraffin-type interlayers follow a straight line in the dL/n-plots. Lower-charged vermiculites were recognized by stepwise increasing spacings due to mono-, two-, or three-layer chain packings. Charge heterogeneity produced a superposition of the dL/n-curves for different charges, and the basal reflections of some of the alkylammonium derivatives became nonintegral.

Layer charge heterogeneity in vermiculites

Binding of antibiotics to clay minerals can decrease both their physical and biological availability in soils. To elucidate the binding mechanisms of tetracycline antibiotics on smectite clays as a function of pH, we probed the interactions of oxytetracycline (OTC) with Na-montmorillonite (MONT) using X-ray diffraction (XRD), infrared (IR), and solid-state nuclear magnetic resonance (NMR) spectroscopies, and Monte Carlo molecular simulations. The XRD patterns demonstrate the presence of OTC in the MONT interlayer space at acidic pH whereas complexation of OTC by external basal and edge sites seems to prevail at pH 8. At both pH, the (1)H-(13)C NMR profile indicates restricted mobility of the adsorbed OTC species; and, -CH(3) deformation and C-N stretching IR vibration bands confirm a binding mechanism involving the protonated dimethylamino group of OTC. Changes in the (23)Na NMR environments are consistent with cation-exchange and cation complexation reactions at the different sites of adsorption. Molecular simulations indicate that MONT interlayer spacing and structural charge localization dictate favorable binding conformations of the intercalated OTC, facilitating multiple interactions in agreement with the spectroscopic data. Our results present complementary insights into the mechanisms of adsorption of TETs on smectites important for their retention in natural and engineered soil environments.

Interactions of oxytetracycline with a smectite clay: a spectroscopic study with molecular simulations.

The fluorine route is thoroughly investigated for the hydrothermal synthesis of montmorillonite in the Na2O-MgO-Al2O3-SiO2-H2O system. Using the optimal conditions suggested by Reinholdt et al . (2001) for the crystallization of pure montmorillonites with the formula Na2 x (Al2(1− x )Mg2 x □)Si4O10(OH)2, several parameters ( x , Mg content, duration of crystallization, F/Si atomic ratio, pH, nature of counterbalance cation) are varied independently from their ideal values. The products are analysed by various techniques (X-ray diffraction, thermogravimetric analysis-differential thermal analysis, 29Si, 27Al and 19F magic angle spinning-nuclear magnetic resonance). It appears that a pure montmorillonite can only be obtained within a narrow x range (0.10 ≤ x ≤ 0.20). The presence of F in the starting hydrogel and the crystallization time also have significant effects on the purity of the final products. It is shown that a small amount of fluorine is needed for the crystallization of pure montmorillonite phyllosilicates.

Synthesis and characterization of montmorillonite-type phyllosilicates in a fluoride medium

K + ⇄ Ca 2+ exchange was performed on a mixed-layer (phlogopite-vermiculite) sample from Palabora, Republic of South Africa. Exchange isotherms, X-ray diffraction patterns and far infrared (FIR) spectra of the mineral were obtained. Different mineralogical phases were identified during the exchange process: K-vermiculite, Ca-vermiculite and a regular mixed-layer (V K -V Ca ) phase. Two types of exchange sites for K + were identified by FIR during the exchange process. The first type, dominant in the K-vermiculite, gives an absorption band at 76 cm -1 . The other, which was the most pronounced site in the mixed-layer phase, gives an absorption band at 72 cm -1 . The mixed-layer phase was formed only when oxidation of Fe 2+ resulted in the internal compensation of part of the negative charge of vermiculite. This study shows that the high selectivity displayed by vermiculite for K + is enhanced by the formation of the mixed-layer phase.

A far infrared study of K+ ions during K+ ⇋ Ca2+ exchange in vermiculite

All the exchange isotherms of Na + or Ca 2+ in vermiculite for K + , Rb + or Cs + show two kinds of exchanges sites: the first corresponds to sites located in phlogopite-like interlamellar spaces and the second to sites located in smectite-like interlamellar spaces. During the exchange, three mineralogical phases are formed: Na- or Ca-vermiculite; K-, Rb- or Cs-vermiculite and a mixed-layer phase. Far infrared absorption bands of K + , Rb + and Cs + located in the phlogopite-like interlamellar spaces are at 72, 59 and 53 cm -1 , respectively. These frequencies are characteristic of a trioctahedral mixed-layer phase. Selectivity is greater for cations with a low hydration energy (K + , Rb + , Cs + ) than for cations with a high hydration energy (Na + , Ca 2+ ). On the other hand, for cations with a low hydration energy, structural parameters such as the tetrahedral rotation angle, α, have to be taken into account.

R. Le Dred

Papers

Synthesis and characterization of montmorillonite-type phyllosilicates in a fluoride medium

A far infrared study of K+ ions during K+ ⇋ Ca2+ exchange in vermiculite

Far infrared study of K+, Rb+ and Cs+ during their exchange with Na+ and Ca2+ in vermiculite

Contribution a l'etude des interactions vermiculite-colorants cationiques

Formation et applications de complexes mica-vermiculite-chlorure de sodium