Anion Exchange of Methyl Orange into Zn−Al Synthetic Hydrotalcite and Photophysical Characterization of the Intercalates Obtained

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Abstract

The intercalation via ion exchange of the azoic dye methyl orange (MO-) into the hydrotalcite-like compound Zn0.67Al0.33(OH)2Cl0.33·0.6H2O has been investigated. X-ray diffraction patterns of samples with increasing dye loading showed that Cl-/MO- exchange occurs with a first-order phase transition from the Cl- phase (interlayer distance 7.74 Å) to the MO phase with an interlayer distance of 24.2 Å. A sample of composition [Zn0.67Al0.33(OH)2][MO0.31Cl0.02]·0.85H2O has been studied by thermogravimetric analysis and by X-ray powder diffraction at different temperatures. The loss of hydration water between 80 and 120 °C causes a decrease of the interlayer distance from 24.2 to 21.5 Å. Computer models and calculations based on the structure of the host showed that MO anions are arranged in the interlayer space as a monolayer of species with the main axis perpendicular to the layer plane. Emission fluorescence spectra of the dye only exchanged on the external surface of the host or intercalated, at different loading, were compared with the fluorescence spectra of MO as microcrystals or dissolved in ethanol. By changing the experimental conditions, MO fluorescence emission can cover the whole visible wavelength range. The spectrum in ethanolic solution, λmax = 480 nm, is at the highest energies, while that of microcrystals is shifted toward the red (λmax = 690 nm). The fluorescence of MO-intercalated samples is near that of microcrystalline methyl orange but shifted at higher energies. A further shift is observed for the sample containing only surface-exchanged MO. The energy difference between the fluorescence spectra of the MO in different environments has been attributed to the change of the emitting state energy caused by interactions of the excited species with neighboring unexcited species. The fluorescence measurements can thus be considered a valuable tool for studying the microenvironment of the dye.

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