Phase Separation in Silica Sol-gel System Containing Anionic Surfactant
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1056-HH11-60
Phase Separation in Silica Sol-gel System Containing Anionic Surfactant Taisuke Matsui, Kazuki Nakanishi, Kazuyoshi Kanamori, and Teiichi Hanada Department of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa, Sakyoku, Kyoto, 606-8502, Japan
ABSTRACT By inducing phase separation parallel to the sol-gel transition of alkoxy-derived silica systems, silica monoliths with well-defined co-continuous macropores were obtained from the systems containing anionic surfactants. We adopted three kinds of anionic surfactants which differ from each other in the length of alkyl chain (CH3(CH2)17SO3Na, CH3(CH2)15SO3Na, CH3(CH2)13SO3Na). Mesopores were also found in the silica skeletons presumably by the supramolecular templating. Characterization of the dried or heat-treated samples was carried out by a scanning electron microscope (SEM) and nitrogen adsorption measurements. Experimental results showed that due to the moderate interaction between silica oligomers and surfactants, most of the surfactants are distributed to the solvent phase which determines the macropore volume. The median size and volume of the macropores could be controlled independently by the starting composition. In the absence of any additive to enhance templating by the surfactant, the samples exhibited only amorphous mesopores.
INTRODUCTION Porous materials are widely used as adsorbents, catalyst supports, and separation media. For the applications that require enhanced contact of external liquid with the solid surface, materials or devices designed to have hierarchical pore structure are highly demanded [1, 2]. Taking separation media packed with porous particles for example, the particle size is selected so as to allow an external liquid to flow through under appreciable pumping pressure, whereas the specific surface area and mesopore distribution of each particle are designed so as to maximize the molecular-level interaction on the surface sites. Our approach to synthesize materials with hierarchical pore structure is the sol-gel process accompanied by phase separation [3]. In a polymerizing metal oxide system, spinodal decomposition is induced parallel to the sol-gel transition in the presence of appropriate additives, and transient heterogeneous structure developed by the phase separation is frozen in the gelling network. As a result, well-defined continuous pores in the micrometer range are formed between the continuous gel skeletons that have controlled mesopores [4].
Various additives have been examined as a phase-separation inducer. It has been reported that water-soluble polymers and cationic or neutral surfactant can act as the phaseseparation inducer [3, 5]. However, the behavior of anionic surfactant as a phase-separation inducer as well as a micellar template has not been examined. In the present study, we adopted three kinds of anionic surfactants which differ from each other in the length of alkyl chain (CH3(CH2)17SO3Na, CH3(CH2)15SO3Na, CH3(CH2)13SO3Na). Phase separation behavior and mesopore structure
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