Understanding the mechanisms of reaction and release of acid–base indicators entrapped in hybrid gels

Abstract

Listen

Translate article

The reactivity and release of acid–base indicators entrapped in silica and hybrid silica gels were studied. Methyl orange (MO), methyl red (MR), and phenyl red (PR) were used as the indicators. Tetraethoxysilane (TEOS), n-propyltriethoxysilane (PTES), and bis(trimethoxysilyl)hexane (TSH) were used as gel precursors. HCl (0.01M) and NaOH (0.01M) solutions were used as the media for acid–base reaction and release in the experiments. The effects of organosilane addition were examined to understand the processes and mechanisms of reaction and release of the indicators entrapped in the gels. The experimental results suggest that adding the organosilanes lowers the pore size and pore volume of the gels, and therefore suppresses the indicator release, but does not present any obvious effects on the acid–base reactivity of the entrapped indicators. MR entrapped in the gels has no acid–base response, while MO and PR entrapped in the gels can present acid–base response without definite acid–base indicating pH ranges. Only the color change pH values can be determined, which present a ‘lag effect’ or ‘hysteresis’ compared with the cases of the free indicators in solutions. The reactivity of the entrapped indicators is suggested to be mainly determined by their chemical nature, the interactions between the indicators and the gel matrices, and the possible involvement of the indicators in the sol–gel process. In general more indicators were found to release in basic solutions than in acidic solutions. For all the entrapped indicators, in acidic solutions through the whole release process, and in basic solutions at the beginning of the release, the indicator release was found to follow the sequence of TEOS Gel>TEOS/PTES Gel>TEOS/TSH Gel. The overall release process is diffusion-controlled, and the release is mainly affected by the textural properties of the gels, the interactions between the gel matrices and the indicators, and the indicator solubility.