Gel filtration chromatographic study on the self-association of surfactants and related compounds

After a brief survey of gel filtration chromatography (GFC) and the self-association equilibrium, it is shown that GFC is a unique tool for investigating the self-association of many substances, such as surfactants, chlorpromazine hydrochloride (CPZ), Methylene Blue (MB) and a sulfobetaine derivative (CHAPS) of cholic acid. The hydrodynamic radius of an aggregate can be estimated with the gel of large pore size, such as Sephadex G-200. This method is utilized for the study of the sphere-rod transition of surfactant micelles. A precise value of monomer concentration can be determined from the centroid elution volume of a frontal chromatogram on Sephadex G-10. This important parameter in self-associating systems allows us to determine the micellar aggregation number as a function of the total surfactant concentration. The derivative chromatogram can be used to detect slight changes in self-association. From these results, it is found that nonionic surfactants form premicelles including dimer and that multiple equilibrium model for micelle formation is more appropriate than mass-action model. The Tanford theory with some modifications appears to fit the concentration dependence of the aggregation properties of surfactants forming small globular micelles and allows us to estimate the stepwise aggregation constant and the micelle size distribution function. The pronounced cooperativity in the self-association of surfactants forms the basis of mass action model and the concept of the cmc, though they are actually approximations even for surfactants. They would be more inadequate, though often used, for the self-association of drugs and bile salts. The stepwise aggregation constant of MB is almost independent of aggregation numbers, which may be expected from the stacking mode of aggregation. The main aggregates of CPZ in 154 mM sodium chloride solution are dimer and 38-mer and its dimerization constant is smaller than that of MB and larger than those of surfactants. The stepwise aggregation constant of CHAPS has the maximum at hexamer and oscillates, depending on odd or even aggregation numbers. Thus, the aggregation pattern of a substance is closely related with the chemical structure of the hydrophobic group of its molecule. The definition of cmc is discussed together with its significance.