A microstructural investigation of nanostructured boehmite films prepared by the sol-gel route

The microstructure of dispersed aqueous boebmite systems during the preparation of xerogel membranes was investigated in relation to the textural properties of the final film. When the boehmite sols are dried at room temperature and atmospheric pressure, a transparent film is obtained. During the solvent evaporation process, the sols undergo a macroscopic change, namely a sol-gel transition, leading finaly to the formation of a porous glass. As shown by X-ray diffraction, the air-dried films are well-structured xerogels resulting from an uniplanar arrangement of the primary plate-like crystallises with the (020) axis normal to the film layer. Assessment of the microstructural evolution using small-angle X-ray scattering experiments demonstrated that a reorganisation of the particles occurs at the gel point. This phenomenon, which can be explained by an excluded volume effect when interacting particles approach in solution during the solvent evaporation, leads to a birefringent gel phase where primary colloidal particles adopt a cooperative face-face preferential state extending over macroscopic scales (>1 micron). This particular structure persists on substantial concentration of the gel phase and clearly shows that the crystallite organisation found in the film exists before the xerogel formation. It is suggested that the gelation mechanism is responsible for the structure and porous texture of the films. (C) 1999 Elsevier Science B.V. All rights reserved.