Metachromasy in clay mineral systems

Adsorption of the metachromic (colour depends on concentration) cationic dyes crystal violet (CV) and ethyl violet (EV) by Na-kaolinite and Na-montmorillonite has been studied by visible spectroscopy and titration calorimetry. In the kaolinite system, metachromasy stems from association of the dye cations on the external surface of the clay. Enthalpies of adsorption at small dye/clay ratios are –3.5 and –3.8 kJ/mol for CV and EV, respectively. In the montmorillonite system, which has been studied more thoroughly, metachromasy stems from the electron interactions between the dyes and the oxygen plane of the alumino-silicate. Enthalpies of adsorption (more exothermic than for kaolinite) depend on the dye/clay ratio in ways that are consistent with the spectra and the derived picture of dye/clay interactions.

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Zusammenfassung Mittels Spektroskopie im sichtbaren Bereich und Titrationskalorimetrie wurde die Adsorption der metachromen (Farbe ist konzentrations-abhängig) Kationenfarben Kristallviolett (CV) und Ethylviolett (EV) durch Na-Kaolinit und Na-Montmorillonit untersucht. In den Kaolinit-Systemen rührt die Metachromasie von der Assoziation der farbigen Kationen an der externen Oberfläche der Tonerde her. Die Adsorptionsenthalpien für kleine Farbstoff/Tonerde Quotienten betragen für CV und EV-3.5 bzw. -3.8 kJ/mol. In den umfassender untersuchten Montmorillonit-Systemen rührt die Metachromasie von den pi-Elektronenwechselwirkungen zwischen den Farbstoffen und der Sauerstoffebene des Aluminosilikates her. Die Adsorptionsenthalpien (exothermer als bei Kaolinit) hängen vom Farbstoff/Tonerde-Quotienten ab, wie es in Übereinstimmung mit den Spektren und den Vorstellungen über die Farbstoff-Tonerde-Wechselwirkungen steht.

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We are grateful to the Natural Sciences and Engineering Research Council of Canada for their support of this and other research on clay chemistry. In addition, we are all grateful to the Hebrew University of Jerusalem for awarding a sabbatical leave to S. Y. that permitted the collaboration that led to this paper.

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Dedicated to Professor Dr. H. J. Seifert on the occasion of his 60^th birthday     

A tunable narrowband optical filter via phase conjugation by nondegenerate four-wave mixing in a doppler-broadened resonant medium

The formulation of phase conjugation via nondegenerate four-wave mixing in a Doppler-broadened resonant medium is generalized to include nondegenerate pump frequencies. This process is shown capable of yielding a tunable optical band-pass filter whose central frequency can be tuned over the entire Doppler profile with only a small degradation of the filter efficiency while still maintaining a bandwidth limited by the homogeneous linewidth.     

X-ray study of the thermal intercalation of alkali halides into kaolinite

Intercalation complexes of kaolinite with a series of alkali halides (NaCl (trace amounts), KCl, RbCl, CsCl, NaBr, KBr, CsBr, Kl, Rbl and Csl) were obtained by a thermal solid state reaction between the kaolinite-dimethylsulfoxide intercalation complex and the appropriate alkali halide. The ground mixtures (11 weight ratio) were pressed into disks that were gradually heated up to 250 °C for different times. X-ray diffractograms of the disks were recorded after each thermal treatment. At the end of the thermal treatment the disks were ground and basal spacings of the powders obtained. As a result of thermal treatment, alkali halide ions diffuse into the interlayers, replacing the intercalated dimethylsulfoxide molecules. Such a replacement may take place only if the thermal diffusion of the penetrating species is faster than the evolution of the intercalated organic molecule. With increasing temperature the intercalated salt diffused outside the interlayer space or underwent a thermal hydrolysis which resulted in the evolution of hydrogen halides from the interlayer space. Consequently, the amounts of intercalation complexes decreased at elevated temperatures.     

Frustration of Bragg reflection by cooperative dual-mode interference: a new mode of optical propagation

A new optical mode of propagation is described, which is the natural eigenmode (supermode) of a fiber (or any optical waveguide) with two cospatial periodic gratings. The mode frustrates the backward Bragg scattering from the grating by destructive interference of its two constituent submodes (which are eigenmodes of a uniform waveguide). It can be used in a new type of spatial mode conversion in optical guides.     

Thermo-XRD investigation of monoionic montmorillonites mechanochemically treated with urea

Montmorillonite (M) saturated with H⁺,Li⁺,Na⁺,K⁺,Rb⁺,Cs⁺,NH₄ ⁺,Mg²⁺,Ca²⁺,Sr²⁺,Ba²⁺,Mn²⁺,Co²⁺,Cu²⁺,Al³⁺ and Fe³⁺ were dry-ground with urea (U) in mass ratios U/M between 0.1 and 2.0 in an agate mortar and diffracted by X-ray. Extensive swellings occurred with H-, Li-, Na-, di-and trivalent cation-clays, suggesting the formation of urea-montmorillonite intercalation complexes. Mechanochemically treated samples were heated at different temperatures up to 375°C. The rise in temperature was accompanied by a decrease in the basal spacing. There was a correlation between the results of the thermo-XRD-analysis and the fine structures of the urea-montmorillonite complexes described in the literature. Five stages in the basal spacing vs. temperature curves were identified. In the first stage (at 150°C) the decrease was due to dehydration. In the second stage (175°C) this dehydration was accompanied by some thermal intercalation of excess urea. The other stages (at 225, 325 and 375°C) were associated with the degradation of urea and the condensation of the degraded species to polymeric products. At 375°C Li-, Na-, K-NH₄-, Mh-, Co- and Cu-montmorillonite collapsed, indicating that urea was evolved. The other urea-clay complexes did not collapse due to intercalated polymers formed by the degradation products of urea.     

Scenery reconstruction on finite abelian groups

We consider the question of when a random walk on a finite abelian group with a given step distribution can be used to reconstruct a binary labeling of the elements of the group, up to a shift. Matzinger and Lember (2006) give a sufficient condition for reconstructability on cycles. While, as we show, this condition is not in general necessary, our main result is that it is necessary when the length of the cycle is prime and larger than 5, and the step distribution has only rational probabilities. We extend this result to other abelian groups.