Cholic acid behavior in water and organic solvent: study of normal and inverted aggregates

Water-soluble fluorescent probes, including 1-pyrene butyric acid, rhodamine B, sodium salt 1,5diaminoanthraquinone-2,6-disulfonic acid, quinine hemisulfate and monosodium salt 8-amino1-naphthol-3,6-disulfonic acid were used to determine the critical micelle concentration (cmc) of inverted micelles formed by cholic acid in tetrahydrofuran. Another water-insoluble fluorescent probe, namely pyrene, was used to determine the cmc of normal micelles formed by sodium cholate in water. Relative scattered light intensity measurements give direct evidence of transition concentrations similar to cmc in surfactants. Our results indicate that in the inverted micelles, hydrophilic sites placed in the core are present in cholic acid aggregates above the cmc region at 5–7 mM. In normal micelles, hydrophobic sites in the core are present in sodium cholate aggregates above the cmc region at 15 mM. Another critical point in the aggregation of cholic acid and sodium cholate occurs at 1 mM. Normal phase liquid chromatography determination of the cmc reversed micelles has also been performed, and the results obtained agree well with the spectroscopic studies. © 1998 Elsevier Science B.V. All rights reserved.