Acid Autocatalysis and Front Propagation in Water-in-Oil Microemulsions

Abstract

Experimental results are presented of an acid autocatalytic reaction (the bromate-sulfite clock reaction) performed in water-in-oil (w/o) microemulsions with neutral (Triton X-100, TX) or cationic (CTAB) surfactants. The characteristics of the pH-sensitive reaction in the stirred system are found to depend on the molar ratio of water to surfactant (ω0) and the nature of the surfactant. The well-stirred reaction (clock) time is faster in the TX w/o microemulsion and slower in the CTAB w/o microemulsion compared to the aqueous phase clock. The pH change is reduced in the TX system and the initial and final pH are shifted to higher values in the CTAB system. The unstirred water-in-oil microemulsions support propagating acid reaction fronts with speeds up to a factor of 10 lower than the aqueous phase fronts. The results are explained through consideration of the effect of the confinement of water in the nanosized droplets on rates and equilibria and assuming front propagation is driven by the diffusion of hydrated reverse micelles.