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Estimation of Critical Micelle Concentration (CMC) Using Tetrakis(dimethylamino)Ethylene Chemiluminescence

Keith Hohn and Chien-Chang Huang. Chemical Engineering, Kansas State University, Manhattan, KS 66506

Reverse microemulsions (water-in-oil microemulsions) have been extensively applied in the synthesis of ultra small nanoparticles and as reaction /extraction media. However, the available information on the properties of reverse microemulsions is limited, especially compared with normal microemulsions (oil-in water microemulsions). The reason for this is that, unlike normal microemulsions, the physical properties (such as surface tension, specific heat, conductivity, osmotic pressure, and light scattering) of reverse microemulsions change little as the concentration of surfactant approaches the critical micelle concentration (CMC). In addition, the aggregation number of reverse micelles, and therefore the micelle size, is usually smaller than that of normal micelles. These features of reverse microemulsions not only lead to difficulty in the measurement of CMC, but also cause large variations in the reported values.

This work proposes that the chemiluminescent reaction of tetrakis(dimethylamino)ethylene (TDE) in the presence of oxygen could be used in detection of the CMC of reverse microemulsions. The emission of TDE is quenched by the products of the reaction, mainly tetramethylurea (TMU) and tetramethyloxamide (TMO). Interestingly, TDE is a non-polar molecule, while TMU and TMO are polar molecules. These properties could allow TDE chemiluminescence to be used as a probe reaction for characterizing the critical micellar concentration (CMC) and the viscosity of reversed microemulsion systems (water-in-oil microemulsion, W/O).

The determination of the CMC of reverse microemulsion by TDE chemiluminescence is based on the migration of the quenching species (TMU and TMU) between the bulk phase and the water pool inside the micelles of a W/O system. It was noted that in the presence of micelles, the emission intensity of TDE chemiluminescence decreased exponentially, while in their absence, a “bump” was noted in the intensity versus time curve (It curve). The CMC of a particular surfactant in a specific solvent, therefore, could be estimated as the concentration at which the bump was noted in the It curve of TDE chemiluminescence.

Using this method, the CMCs of doctyl sulfosuccinatesodium salt (AOT) were evaluated in alkanes (n-dodecane, n-decane, n-octane, n-heptane and iso-octane), cyclohexane and mineral oil (light), respectively, while the CMCs of sodium dodecylbenzenesulfonate (NaDDBs) and Triton X-100 (X-100) were detected in cyclohexane. The CMC values determined by TDE chemiluminescence are close to the available literature values and distributed in a narrow range from 0.61-2.10 mM. It was also found that TDE chemiluminescence is sensitivity to the viscosity of the reverse microemulsion. TDE chemiluminescence was used to probe the viscosity of a reverse microemulsion made up of AOT and cyclohexane. The result shows a linear relationship between ln(I) (I, emission intensity) and ln(ő) (ő, viscosity).