ARTICLE NO. CS974804 NOTE Determination of Critical Micelle Concentration of Anionic Surfactants: Comparison of Internal and External Fluorescent Probes the study of LAS micellar systems and earlier we have shown by NMR studies that the phenyl ring resides at the palisade layer with the water Fluorescence spectroscopy has been utilized to determine the boundary between ortho and meta protons (to the sulfonate group) (3). In critical micelle concentrations (CMC) of anionic surfactants by the case of SAS micellar systems, we used externally added phenol as the external and internal probes and the results have been compared fluorescent probe which resided at the palisade layer of these micelles. In to those obtained by the surface tension method. The selection of both cases, pyrene is used as an external fluorescent probe located at the anionic surfactants gave us the flexibility of having the probe micellar core. In Fig. 1 we show the micelle structures of LAS and SAS within the surfactant molecule and having it reside at the palisade / Phenol systems. layer or in the micellar core. It is shown that the CMC value is not affected by the nature (whether part of the surfactant molecule MATERIALS AND METHODS or externally added) and location of the probe within the micelle. CMC values determined by surface tension and fluorescence tech-A sample of sodium salt of LAS was prepared in the laboratory by niques using external and internal probes agree quite well in linear neutralizing commercial linear alkylbenzene sulfonic acid (ex. Reliance alkylbenzene sulfonate (LAS) and secondary alkyl sulfonate industries, Bombay) with sodium hydroxide solution in the Z-blade (Sigma) (SAS) systems. แญง 1997 Academic Press mixture. The paste of the Na-LAS obtained was dried in an oven at around