Fluorescence probes for critical micelle concentration determination.

The utility of pyrene and pyrene-3-carboxaldehyde as fluorescence probes for determining the critical micelle concentration (cmc) of nonionic (Tergitol25-L-9, Tergitol15-5-40, Neodol91-8), anionic (sodium decyl sulfate), and cationic (cetyltrimethylammonium bromide) surfactanb was investigated. The probes were dissolved in trace amounts (<lo4 kmol/m3) in a series of aqueous solutions of these surfactants. Results obtained indicated that the surfactant concentration at which a marked decrease in the A,, parameter of pyrene-3-carboxaldehyde occurs corresponds to the cmc determined by surface tension. In contrast, the customarily used parameter, 11/13, of pyrene undergoes a reduction at concentrations close to, but consistently lower than, the accepted cmc values of the surfactant. Thus, it appears that pyrene-3carboxaldehyde is the more reliable probe for cmc determination. Surface tension results show, furthermore, that the probe molecules at the levels used to determine the cmc by fluorescence do not noticeably affect the surfactant properties, viz., surface tension, nor the surface tension derived cmc value.