Eleanor J. Fendler

Association Behavior in Steroidal Systems — NMR Spectroscopic and Vapor Pressure Osmometric Studies of Bile Acid Derivatives

Nmr and vapor pressure osmometric (VPO) studies indicate that bile acid esters self-associate and form small aggregates in organic solvents. Intermolecular hydrogen bonding between hydroxyl groups on the steroid nucleus and possibly the carboxylate on the side chain is proposed to play a vital role in the formation of aggregates. The concentration dependence of the chemical shift of the hydroxyl protons confirms the existence of H-bonds in these systems. The decrease in 13C relaxation times of methyl cholate in CDCl3 with increasing concentration is consistent with the self-association in this system. Analysis of the VPO data for methyl cholate in chloroform indicates that the model of self-association is best described by a Type III indefinite sequential type of self-association rather than a monomer-n-mer one.

Applications of NMR to Micellar Interactions and Catalysis

A short introduction to surfactant and micellar systems is given. A discussion of selected examples of micellar solubilization and catalysis studies with emphasis placed on nmr spectroscopic results is presented. The utility of nmr spectroscopy as well as its current status in investigations of micellar solutions is critically described.

Association Behavior of Synthetic and Naturally Occurring Surfactants in Nonaqueous Solvents

The behavior of associating systems, especially surfactants in nonaqueous media have been investigated using spectroscopic techniques, principally 1H and 13C nuclear magnetic resonance (NMR) spectroscopy, as well as thermodynamic ones, primarily vapor pressure osmometry (VPO). Among the surfactants investigated are alkylammonium carboxylates, such as hexylammonium propionate (HAP) and dodecylammonium propionate (DAP), and steroidal compounds, such as the methyl and ethyl esters of the bile acids. The association phenomena of alkylammonium carboxylates in organic solvents are indicated by the concentration dependence of the proton chemical shifts. The results of VPO studies of DAP in cyclohexane and benzene clearly show association behavior, which confirms the conclusion from proton NMR work. Additionally, an indefinite sequential type 1 association model for DAP was found to produce the best fit of the experimental VPO data. The proton chemical shifts of HAP in benzene-dimethyl sulfoxide mixtures also have been extensively studied. The self-association behavior of bile acid esters in chloroform at 37°C has been eluciadated using VPO techniques since only one peak in the proton NMR spectra of methyl cholate in chloroform, for example, was found to be sensitive to concentration effects. This peak was assigned to a hydroxyl group involved in hydrogen bonding. However, 13C spin-lattice relaxation times (T1) of methyl cholate in CDCl3 decrease with increasing concentration, indicative of the self-association behavior found using VPO. Finally, the conclusions obtained by the combination of experimental approaches to investigations of self-associating systems are summarized, and models describing the association behavior in non-aqueous solvents are discussed and compared to those for aqueous systems.