Juraj Bujdák

Comparative FT-IR study of structural modifications during acid treatment of dioctahedral smectites and hectorite

Abstract Seven dioctahedral smectites of various composition and one trioctahedral smectite (hectorite) were leached in the HCl under different treatment conditions. Hectorite is much more susceptible to acid attack than the dioctahedral smectites. The rate of smectite dissolution increases with increasing octahedral Mg or Fe content as well as with the treatment temperature and acid concentration. The mechanism of smectite dissolution in HCl is independent of layer composition. The final reaction product obtained from acid dissolution of both smectite types is a hydrous amorphous silica phase. FT-IR spectroscopy is a sensitive method for characterization of the starting mineral, identification of some admixtures, such as quartz or kaolinite, and can be useful to assist in controling the extent of acid dissolution.

Effect of non-swelling layers on the dissolution of reduced-charge montmorillonite in hydrochloric acid

Abstract A series of reduced-charge montmorillonites (RCM), prepared from the same parent Li-montmorillonite (Jelšový Potok, Slovakia) by heating at various temperatures (105-210°C) for 24 h, was treated with 6 M HCl at 95°C for periods up to 30 h. Reaction solutions obtained were analysed for Al, Fe, Mg and Li and the solid reaction products were investigated by FTIR spectroscopy. Both analyses provided evidence that the extent of dissolution decreased with increased amounts of Li fixed within the montmorillonite structure, i.e. with increased heating temperature. Differences in the acid dissolution process were reflected in the structural changes which occurred within the RCM samples, due presumably to different positions of fixed Li. The ethylene glycol monoethyl ether (EGME) surface areas, and XRD and HRTEM analyses of the RCM series revealed an increased amount of non-swelling layers in the samples prepared at higher temperatures, which caused a substantially slower decomposition of M7 and M8 in HCl. The [calculated XRD patterns of M6 and M7 confirmed the presence of 20% and 45% pyrophyllite-like layers, respectively, in these samples. Mixed-layer pyrophyllite-like-smectite and pyrophyllite-like crystals, containing only non-swelling layers, were found in sample MB. The results confirmed that the amount of swelling layers in RCM significantly affects their dissolution rate in HCl.

The aggregation of methylene blue in montmorillonite dispersions

Abstract Theories concerning the optical properties of cationic dyes adsorbed on clay surfaces are analysed in detail. An investigation of the aggregation of methylene blue (MB) in montmorillonite dispersions is conducted using visible (VIS) spectroscopy. The effects of the dye/ clay ratio and of the swelling properties of the montmorillonite substrate on dye aggregation are compared in terms of the effect of clay layer charge. The observed influence on dye aggregation was almost negligible for both swelling and dye loading. The layer charge of the silicate determines the extent and the type of dye aggregation in freshly prepared MB/montmorillonite dispersions. Observed spectral changes with time indicate a rearrangement and redistribution of dye H-aggregates (band close to 570 nm) to monomers (660 nm), dimers (605 nm) and J-aggregates (760 nm). Dye aggregates are probably already formed during dye cation migration in the vicinity of clay colloid particles. The extent and the type of initially formed species are probably affected by the electric double layer of clay layers. After reaching the clay surface, dye cation assemblies are rearranged and decompose as described above. Reaching chemical equilibrium, dye cations adjust the distribution of the layer charge, in order that each cation could balance the charge due to one unequivalent substitution.

PREPARATION AND INFRARED SPECTROSCOPIC CHARACTERIZATION OF REDUCED-CHARGE MONTMORILLONITE WITH VARIOUS LI CONTENTS

Abstract A series of reduced-charge montmorillonites (RCMs) was prepared from Limontmorillonite from Ješový Potok (Slovakia) by heating at various temperatures (105-210° for 24 h. The amount of fixed Li, 0.09-0.67 per O20(OH)4, increased with increasing temperature, confirming preparation of a set of samples of variable layer charge from the same parent Limontmorillonite by varying only the preparation temperature. Infrared spectroscopy revealed that Li was trapped in the hexagonal cavities of the tetrahedral sheet at all temperatures. Partial deprotonation of the samples, reflected in the decrease of the intensities of the OH-bending bands, was observed after treatments above 120°C. Analysis of the OH-stretching region showed Li in the previously vacant octahedra in the samples heated above 150°C. Weak inflections near 660 and 720 cm-1 confirmed development of local trioctahedral character of octahedral cations coordinated with OH groups in the sample heated at 210°C. Gradual decrease of the layer charge due to Li fixation led to a shift of the Si-O stretching band to higher frequencies and to the appearance of new, pyrophyllite-like bands at 1120 and 419 cm-1

Preparation and properties of reduced-charge smectites : A review

Layer charge reduction of selected homoionic swelling clay minerals upon heating is reviewed. This phenomenon is known for Li+-montmorillonites as the Hofmann-Klemen effect. Aspects covered in the review include: mechanism of the charge reduction caused by the irreversible migration of small cations into the mineral layers; final sites of the fixed cations in the octahedral and/or tetrahedral sheets, as deduced on the basis of structural and spectroscopic data obtained in several studies; effects of octahedral and tetrahedral components of the layer charge; properties of the exchangeable cations; and the chemistry and structure of the mineral on charge reduction. Current knowledge has been summarized on the changes of various mineral properties connected with charge reduction, including the loss of swelling and the decrease in the cation exchange capacity, the most important change upon cation fixation. Also discussed are the preparation, properties, and advantages of uses of series of reduced-charge montmorillonites (RCMs) in research; interactions of RCMs with alkylammonium cations and organic cationic dyes, with some examples on the interactions with organic polymers and other organic compounds. Properties of organo-complexes and composite materials prepared from RCMs are also included in this review.

METHYLENE BLUE INTERACTIONS WITH REDUCED-CHARGE SMECTITES

The objective of this work was to prepare series of reduced-charge materials from different parent Li-saturated dioctahedral smectites, to investigate the effects of temperature, chemical composition and charge location in smectites on the charge reduction and to characterize reduced-charge smectites (RCSs) using methylene blue (MB) adsorption. The layer charge decrease, induced by Li fixation, is correlated with the trends in the spectra of MB-RCS dispersions in the visible region (VIS) spectra. Distribution of the negative surface charge of the clay minerals controls the distance between the adsorbed MB cations and thus affects the formation of MB dimers and higher agglomerates. Because each form of MB (monomer, dimer, higher agglomerate, J-aggregates) absorbs light at a different wavelength, the VIS spectra of MB depend sensitively on the charge density at the clay surface. Both cation exchange capacity (CEC) values and spectra of MB-clay dispersions clearly detect extensive reduction of the layer-charge density in reduced-charge montmorillonites (RCMs) upon Li-thermal treatment. The extent of charge reduction depends on the temperature of the thermal treatment, as well as on the octahedral charge of the montmorillonite. Reduction of the layer charge proceeds to a much lesser extent for smectites with mainly tetrahedral charge and high Fe content (Fe-rich beidellite and ferruginous smectite). Both CEC data and MB spectra detect only a slight decrease of the layer charge density, which relates to the low octahedral charge of these minerals. Following heating at higher temperatures (120-160°C), slightly higher Li fixation is indicated by CEC values; however, no charge reduction is confirmed by MB spectra. Release of protons accompanying Li+ fixation in Fe-rich smectites heated at 180 or 200°C was detected in the spectra of MB-clay dispersions and confirmed by Potentiometrie titrations.

Silica, Alumina and Clay Catalyzed Peptide Bond Formation: Enhanced Efficiency of Alumina Catalyst

Catalytic efficiencies of clay (hectorite), silica and alumina were tested in peptide bond formation reactions of glycine (Gly), alanine (Ala), proline (Pro), valine (Val) and leucine (Leu). The reactions were performed as drying/wetting (hectorite) and temperature fluctuation (silica and alumina) experiments at 85 °C. The reactivity of amino acids decreased in order Gly > Ala > Pro ≈ Val ≈ Leu. The highest catalytic efficiency was observed for alumina, the only catalyst producing oligopeptides in all investigated reaction systems. The peptide bond formation on alumina is probably catalyzed by the same sites and via similar reaction mechanisms as some alumina-catalyzed dehydration reactions used in industrial chemistry.

The Combination of Salt Induced Peptide Formation Reaction and Clay Catalysis: A Way to Higher Peptides under Primitive Earth Conditions

Two reactions with suggested prebiotic relevance for peptide evolution, the saltinduced peptide formation reaction and the peptide chain elongation/stabilization on clay minerals have been combined in experimental series starting from dipeptides and dipeptide/amino acid mixtures. The results show that both reactions can take place simultaneously in the same reaction environment and that the presence of mineral catalysts favours the formation of higher oligopeptides. These findings lend further support to the relevance of these reactions for peptide evolution on the primitive earth. The detailed effects of the specific clay mineral depend both on the nature of the mineral and the reactants in solution.

Influence of the layer charge density of smectites on the interaction with methylene blue

Six Na-saturated smectites were isolated from bentonites of various deposits. Their H-forms were prepared using H–OH–H resins. The interaction of methylene blue (MB) (0.05 mmol g-1) with Na-saturated untreated and H-autotransformed clay samples was investigated by the VIS spectroscopy of their dispersions. The spectra provided information about the formation of protonated dye cations, dimers, and higher agglomerates on the clay surface. The amounts of various dye forms were compared with the distributions of negative layer charge of the used clays, obtained by the alkylammonium method. Formation of MB dimers and higher agglomerates reflected sensitively the layer charge density of the smectites. MB cations formed predominantly higher agglomerates at the surface of those smectites with high charge density. Suppressed dye agglomeration was observed for the samples of lower layer charge density and was caused by greater distances between the adsorbed MB cations. Due to the high sensitivity of MB agglomeration on the layer charge and the simplicity of the method, MB–clay interaction can potentially be utilized for the layer charge characterization of smectites.

Molecular arrangement of rhodamine 6G cations in the films of layered silicates: the effect of the layer charge

Montmorillonite nanocor saturated with Li+ cations was used for the synthesis of reduced charge montmorillonites (RCMs). The films of RCMs with intercalated rhodamine 6G (R6G) were prepared by adsorption of the dye on montmorillonite film deposited on a quartz slide. Penetration of the dye into the interlayer spaces of silicates was monitored by elemental analysis and X-ray diffraction measurements of the prepared R6G/silicate films. Polarized UV-Vis spectroscopy was used for the characterization of the molecular orientation of dye cations on silicate surface. There were several species of R6G cations in the interlayer spaces of the inorganic host, which was easily resolvable by the polarized spectra. The cations forming H-aggregates and absorbing light at low wavelengths were oriented in nearly perpendicular fashion, which was observed as a strongly positive dichroism, i.e., the increase of the dichroic ratio with film tilting. Adsorbed isolated dye cations were inclined at low angles with respect to the plane of the silicate surface. The layer charge of the inorganic host affected only partially the orientation of dye cations in the H-aggregates, but rather it controlled the extent of aggregation and in this way the number of nearly perpendicularly oriented cations.