Abstract
The absorption maximum of a drug in surfactant solution often shifts at surfactant concentrations above the critical micelle concentration (cmc). The first derivative absorption spectrum (FDAS) was used to evaluate the wavelength shift and to measure the cmc of surfactants.
The cmc's of heptaethylene glycol dodecyl ether (HED), octaethylene glycol dodecyl ether (OED) and sodium dodecyl sulfate (SDS) obtained by the FDAS method were 1.2×10-4, 1.4×10-4 and 7.6×10-3 M, respectively. These cmc values were higher than those obtained by the surface tension method (the Wilhelmy plate method) because of the difference of the positions where the values were estimated on the curve (see text). In the measurement of cmc by the FDAS method, less sample and a shorter time are needed, and the reproducibility of this method is satisfactorily high. Though the FDAS method can be utilized for ionic and nonionic surfactants, it is particularly effective for nonionic surfactants, while the previous methods for cmc measurement of nonionic surfactant are generally cumbersome and time-consuming.
It was found that the degree of shift-length increased with increasing alkyl chain length of alkylparabens (or alkyl gallates). Therefore, it seems that the use of alkylparabens (or alkyl gallates) of greater alkyl chain length is most convenient in the determination of cmc values by the FDAS method.
