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Swelling clays are known for their ability to intercalate organic and inorganic species in their interlamellar space. For example, Montmorillonite and Vermiculite interact strongly with organic and inorganic guests, which provoke dimensional changes upon intercalation. Unlike other minerals like zeolites, clays present geometrical changes at the molecular scale by penetration of polar species of suitable size into the layers. The anionic charge of swelling clays attract positive ions which remain fixed on the flakes surface until other ions exchange. The intermediate charge densities, i.e. 57 - 100 square angstroms of interlayer surface per unit charge, which are typical of vermiculites and smectites that swell easily, are different to the high surface charges of about 12 - 30 square angstroms per unit charge that are typical for non swelling micas like Muscovite. In Ca- Monmorillonite, the basal spacing d(100) expands proportionally to the carbon number of monohydric alcohols. One and two layer complexes having the organic molecules parallel to the basal surfaces were reported for the polyethyleneglicol ester of the oleic acids in Na-, Ca- and Mg-montmorillonite. This has proved that intercalation of high molecular weight materials can readily occur from aqueous solutions. In addition, smectites form colored complexes with a number of bases, i.e. Pyridine, which are adsorbed into well outgassed natural montmorillonites, giving rise to blue complexes which become gray upon humidification with water steam. Also, glycine and its peptides, as well as a variety of other aminoacids are sorbed by Na-, Ca- and H-Montmorillonites from aqueous solutions. Also, photochromic molecules were reported to intercalate readily in montmorillonite type clays. On the other hand, spiropyrans are molecules with long alkyl chains and their use as 3-D memory devices has been proposed. The molecules are formed by two (pi) -moieties which are set orthogonally to each other, exhibiting the individual absorption spectra rather than the conjugated spectrum. In this view, swelling clays may be used to host reactive molecules for optical, thermal or electrical recording in paper. Most of the clays that are introduced already into the composition of paper, i.e. kaolin paper clays, can host molecules externally and are potentially useful for the design of interactive papers. In this work, the interaction of dyes and clays was investigated, based on the reactive forms of spiropyrans, which are sensitive materials for recording optical and electrical signals.
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