Liquid Crystal Templating Effects on Silica Gels Synthesized using Quaternary Ammonium Surfactants
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T. DABADIE, A. AYRAL, C. GUIZARD, L. COT, Laboratoire des Mat6riaux et Proc~d6s Membranaires, CNRS UMR 9987, ENSCM, 8, rue de l'Ecole Normale,
34053 Montpellier c6dex 1, France; J.C. ROBERT, 0. PONCELET, Kodak European Research, Centre de Recherche et Technologies, 71104 Chalon sur Sa6ne, France.
ABSTRACT
The use of lyotropic liquid crystal phases is a promising tool in tailoring the porous structure of inorganic gels. In this work, we examine the effect of hexagonal liquid crystal phases produced with quaternary ammonium surfactants on the textural ordering of silica gels. The sol-gel polymerization of the silicon alkoxide precursor (tetramethoxysi lane) is followed by 29Si NMR and rheological measurements. The structural evolution of the material from the starting sol to the thermally treated gel is studied using low angle X-ray diffraction. The textural characteristics of the gels are correlated with the nature of the surfactant molecules. INTRODUCTION
The sol-gel process is a very efficient way to produce inorganic porous layers for specific applications like separative membranes. Mesoporous continuous films can be produced from colloidal solutions. Microporous layers exhibiting a pore size lower than 2 nm can be obtained by controlled alkoxide polymerization. To improve the layer properties like separative efficiency, it is now necessary to have a full control of the pore size in the range from the micropores to the mesopores and also of the monodispersity of the pore size distribution. The easiest way to obtain monodispersity and control of the pore size is to develop materials with ordered porous texture. It would normally seem difficult however to associate the sol-gel process with the production of ordered texture. Concerning the polymeric gels, the use of an ordered medium acting as a template for the growing inorganic network can be considered. Moreover this gelation medium must be eliminated easily without collapse of the solid part. We explored this route using lyotropic liquid crystals as gelation medium. Different kinds of amphiphilic systems were studied in our laboratory [1-31. Lamellar silica gels were recently prepared by using non ionic surfactants [31. This paper presents the first results concerning the use of hexagonal phases as gelation medium for silica microporous gels.
849 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
EXPERIMENTAL The cationic surfactants used were all alkyltrimethylammonium bromides with single-chain length from 8 to 14 carbons. The silica precursor was tetramethoxysi lane (TMOS). The various sols were prepared using the different surfactants but always the same weight percentages of surfactant, TMOS and water: 21, 12 and 67% respectively. This composition corresponds to a hydrolysis ratio of 47.5 for TMOS. We will later label the samples Cx where x is equal to 8, 10, 12 or 14 and is the number of carbons of the alkyl chain of the surfactant.The gelation time, tg, was estimated macroscopically at about 3 hours for the various solutions. An
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