Preparation of Methylene Blue-Silica Composite Microcapsules

Abstract

Silica microcapsules comprising methylene blue (MB-SiO₂) of mean diameter 4.0 μm were prepared by sol-gel and water-in-oil emulsion techniques where water pools are used as microreactors in which methylene blue is dissolved at a high water to surfactant molar ratio. It is confirmed that the cationic methylene blue is incorporated in the silica microcapsule wall during hydrolysis and condensation of tetraethoxysilane. Fixation of methylene blue in the silica wall is examined by passing water or acetic acid aq. soln. (pH 2–4) through a cartridge in which MB-SiO₂ is packed and then measuring the fractions using UV-VIS analysis. The elution behavior is affected drastically by the pH value of the eluent. Methylene blue tends to stay in the wall when the pH value of eluent is above the isoelectoric point (IEP) of the silica microcapsules. However, release of methylene blue cation occurs when it is below the IEP. These results reveal that coulomb interaction has an important role to fix methylene blue in the silica microcapsule.