Synthesis of Novel Inorganic/Organic

Host-Guest Composites

    Layered silicates such as hectorite and montmorillonite possess unique structural and chemical properties that facilitate novel reactions with organic guests such as benzene, aniline, pyrrole, thiophene, and even certain amino acids and nucleotides. In many of these organic-inorganic hybrids, the organic compound provides the desired functional character while the inorganic component provides the stable host framework to adapt the material to different forms and applications. In the smectite clay mineral hectorite, individual layers are formed by sandwiching a sheet of octahedrally coordinated Mg2+ ions between two silica sheets (Fig. 1). Substitution of Li+ for Mg2+ in the octahedral sheet gives each 3-sheet layer an overall negative charge (in the range 0.2 - 0.6 per formula unit). The final structure is formed by alternating these 3-sheet layers with "gallery" layers containing water and exchangeable metal counterions. The gallery regions contain positive charge and thus serve to hold the individual negatively charged clay layers together through electrostatic forces.  Faults, edges and pores in the macroscopic clay fabric make these gallery regions accessible to external chemical agents. Even under standard atmospheric conditions, organic monomers such as benzene, aniline, thiophene and pyrrole may intercalate into these regions and react with the gallery cations. Strong local electric fields in the interlayer regions enhance the oxidation potential of the transition metal ions and drive the reactions, such as the oxidation of benzene by Cu(II), which does not normally occur under ambient conditions.

  Fig1

Aniline Intercalation into Cu(II)-Exchanged Hectorite

    Exposing thin films of Cu(II)-exchanged hectorite to aniline vapor results in rapid intercalation of the monomer into the clay intergallery regions.  Powder X-ray diffraction (XRD) measurements of this intercalated hybrid show an intergallery spacing increase of about 3 Å during this process.  Electron spin resonance (ESR) spectroscopy also shows full reduction of all intergallery Cu(II) cations, indicating spontaneous, in-situ polymerization of the aniline monomer is taking place in the gallery regions.  Using scanning force microscopy (SFM) in non-contact mode with phase-contrast imaging, we have imaged regions of this intergallery polyaniline in cleaved samples.  In Fig. 2 below, a 3 micron square region is shown in topographical mode (a) and in phase-contrast mode (b).  The darker area on Fig. 2b corresponds to the intergallery polyaniline, while the lighter areas indicate the hectorite host.

   Fig. 2a

   Fig. 2b

Intercalation of Styrene into Cu(II)-Exchanged Hectorite Hosts

    The intercalation of styrene monomer into Cu(II)-exchanged hectorite is markedly different than for molecules such as aniline.   During vapor-phase exposure, no indication of any reaction of the non-polar styrene with the hectorite host is observed.  Direct exposure of hectorite films to styrene in solution (for a period of one week), however, does result in intercalation.   Intercalated styrene is found in only about 20% of the intergallery regions, though.  In Fig. 4 below, non-contact topographical and phase-contrast images of the hectorite/styrene composite intergallery regions are shown.  (a) SFM topological scan of an interclay region exposed using the lift-off technique. The dimensions of this image are 4 microns square.  The height of the polymer regions above the surrounding clay layers ranges from 10 Å to about 100 Å. For the sample as a whole, the polystyrene material occupies 20% of the exposed "surface" area.  (b) SFM phase-contrast image of the same region. We can see from this image that the interclay material is composed of two phases, possibly polystyrene similar to that of bulk polystyrene, along with a more rigid polystyrene form.

   Fig. 3a

   Fig. 3b