The Determination of the CMC of Surfactants in Some Organic Solvents

Extra absorption spectra appeared when 7,7,8,8-tetracyanoquinodimethane (TCNQ) was solubilized into surfactant organic solvent solutions above the critical micelle concentration (CMC). It was found also that extra absorption spectra appeared when iodine was solubilized into surfactant organic solvent solutions above the CMC. In the former case, the extra absorption spectra are attributed to TCNQ anion radical salts formed between TCNQ and the surfactants. In the latter, the extra absorption spectra are due to charge-transfer interactions of iodine with the surfactants. The intensity of the absorption bands increased with the concentrations of the surfactant. The values of the CMC of the surfactants were determined from this spectral change. The CMC values obtained by the TCNQ solubilization method were larger than those obtained by the iodine solubilization technique. The difference in the CMC between the two methods was interpreted in terms of the difference in solubility and chargetransfer power to the organic solvent between iodine and TCNQ, and the difference in type of electron transition, that is, n(D)→σ(A) or n(D)→π(A).