Detection of pH in Microemulsions, without a Probe Molecule, Using Magnetic Resonance

Proton NMR relaxation times have been used to probe the pH of water inside reverse micelles of a cetyltrimethylammonium bromide (CTAB)-hexanol-aq microemulsion. T-2 relaxation times were found to change with pH; however, T-1 relaxation times remained unaffected. This behavior was attributed to acid-catalyzed exchange between protons of water and hydroxyl protons of the cosurfactant hexanol. The rate of exchange and its influence on the T., relaxation time of water, inside the reverse micelle, were investigated as a function of water-to-surfactant ratio (omega(0)) and ionic strength. This exchange behavior and pH-dependent T-2 relaxation time were also observed for two other microemulsions-CTAB-hexane-pentanol-aq and Triton X-100-cyclohexane-hexanol-aq. Using this pH-dependent T-2 relaxation time, it was possible to monitor pH changes in the bromate-sullite reaction inside a CTAB-hexanol-aq microemulsion.