Reactivity of organic compounds inside micelles embedded in sol-gel glass. Kinetics of isomerization of azobenzene inside CTAB and SDS micelles embedded in silica matrix

The sol-gel method allows preparation of composite materials consisting of surfactant micelles embedded in glasses. Moreover, this method allows doping of the embedded micelles with various compounds. In this study, the four dopants are pyrene, azobenzene, and 3,3'-dimethyl and 3,3'-diacetyl derivatives of azobenzene. The relative intensities of the fluorescence bands of pyrene proved that cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) form micelles inside silica glass and that SDS does not hydrolyze during its encapsulation within silica. We studied the interactions of silica with the positively charged CTAB micelles and with the negatively charged SDS micelles and the kinetics of thermal (cis-to-trans) isomerization of the three azo dopants within micelles embedded in glass. The three isomerization reactions within the CTAB micelles embedded in silica and with water as the external liquid are monophasic; their rate constants are similar to those determined for the micelles in the absence of silica. The isomerization reactions within the SIDS micelles embedded in silica and with water as the external liquid are multiphasic; they are faster than the corresponding reactions within the SDS micelles in the absence of silica. Azobenzene isomerization within the CTAB micelles dissolved in aqueous solution is pH-independent and monophasic in the entire range 6.9-2.3. Azobenzene isomerization within the SDS micelles dissolved in aqueous solution is pH-independent and monophasic in the interval 6.9-3.9, but it becomes pH-dependent and multiphasic below pH 3.9. Azobenzene isomerization within SIDS micelles embedded in silica, in solution buffered at pH 6.9, remains multiphasic and is faster than the isomerization within the SDS micelles dissolved in the same buffer but in the absence of silica. Azobenzene stayed within the CTAB/silica glass, but it leaked from the SDS/silica glass into the surrounding water. This leakage from the latter glass was suppressed by replacing water with a 1.50 M aqueous solution of NaCl. The isomerizations within SDS/silica glass then became a monophasic reaction, with rate constants similar to those within the micelles in the absence of silica, Because partitioning of the dopant between the micelle/silica glass and the external liquid influences the reactivity of dopant molecules, control of the partitioning is necessary for the design and successful application of chen-deal sensors, optical devices, and drug-delivery systems.