Preparation of Novel Hollow Fiber Silica Using an Organic Gel as a Template
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ABSTRACT A novel hollow fiber silica was created by the use of the fibrous structures of an organic gel as a template. Its inner diameter ranged from 10 to 200 nm and the outer diameter ranged from 50 to 300 nm. A dichloromethane solution of an organic gelator was mixed with the silicate solution, TEOS/water/catalyst (acetic acid or benzylamine)/solvent (acetic acid or alcohol), which was gelated by the organic gelator. Subsequently, the resultant solution was evaporated in vacuo until the dichloromethane was removed to give a turbid gel. The SEM images of the product showed well-grown fibrous structures. After calcination, it was clearly found by TEM images that these fibers have a tubular structure. The most favorable gelator to form hollow fiber silica is one that has a cationic charge within the molecule, for example, a quaternary ammonium halide salt or a crown ether including a metal cation. In this study, the gel-forming segment of the gelator was a cholesterol derivative. Since the propagation species of silicate in the solution of acetic acid or benzylamine are considered to be anionic, the electrostatic interactions may be important for adsorption of silica onto the template fibers.
INTRODUCTION Increasing attention has been paid to low molecular-mass compounds that can gelate various organic solvents efficiently [1-11]. These phenomena are interesting in that the fibrous aggregates formed by non-covalent interactions are responsible for the gelation. In particular, cholesterol-based gelators, which can form stable gels using only non-hydrogenbonding interactions, show an excellent gelation ability towards a wide variety of organic solvents at sufficiently low concentrations [5-9, 11]. In addition, the resulting gels have chirally oriented structures which are imparted from the characteristic cholesterol skeleton. Through this study we found that even liquid silanol derivatives can be gelated by some cholesterol-based gelators [7,12]. It thus occurred to us that if the sol-gel polymerization of the silanol derivatives proceeds in the organic gel state, the gelator fibrils should act as a template which eventually creates a void in the resultant silica. After trial-and-error, we have found that under certain reaction conditions the gelator fibrils survive in the sol-gel process and construct a tubular structure in the fibrous silica [ 15]. It was proposed that the unique structure is created by a template effect of the fibrous gelator aggregates which, after calcination, eventually constitute the hollow moiety. In order to estimate the role of the cationic charge, compound 3 bearing a benzo-18-crown-6 moiety was used as a template. We previously found that 3 also acts as an excellent gelator of organic solvents [7]. With this compound, a "neutral" gelator can be consecutively converted into a
77 Mat. Res. Soc. Symp. Proc. Vol. 576 ©1999 Materials Research Society
"cationic" gelator by the addition of alkali metal salts. We have found that only when the benzo-18-crown-6 moiety binds a significant amount
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