Determination of critical micelle concentration of several nonionic surfactants by azo-hydrazone tautomerism of anionic dye

Abstract

The absorbance of the azo-hydrazone tautomer of sodium 4-phenylazo-l-naphtol-2,3′-disulfonate (PNDS) dissolved in aqueous solution of several nonionic and anionic surfactants was measured at the concentration ranges below and above each critical micelle concentration (CMC) of those surfactants. Below the CMC the absorbance of PNDS does not show any change upon addition of surfactants. In contrast, above the CMC the absorption band corresponding to the azo-form of PNDS appeared at 410 nm and increased gradually, whereas the absorption band of the hydrazone form at 490 nm decreased markedly. The changes at the CMC in absorbance for these nonionic surfactants were distinctly observed by plotting the absorbance at 410 and 490 nm as a function of the concentration of surfactants. The changes at the CMC were very noticeable with nonionic surfactants, whereas change was not observed in aqueous anionic surfactants (sodium dodecyl sulfate). The concentration corresponding to breakpoints for nonionic surfactants was in fairly good agreement with the published data of the CMC of each surfactant. This method of azo-hydrazone tautomerism using PNDS is applicable to the determination of the CMC for most nonionic surfactants. Furthermore, the PNDS tautomer is considered to be more effective for the study of the properties of mixed micelles between a nonionic and an anionic surfactant since the azo-hydrazone tautomerism is not observed in aqueous anionic surfactant solution.