NMR investigations of micellar systems in nonaqueous media

Abstract

The behavior of self-associating bile acid esters 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). The surfactants used in these studies w ere steroidal compounds and include methyl and ethyl cholate, methyl deoxycholate, methyl and ethyl lithocholate, and sodium cholate. The self-association behavior of bile acid esters in chloroform at 37°C has been elucidated 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 hydro- 13 gen bonding. However, ¹³C spin-lattice relaxation times (T[subscript 1]) of methyl cholate in CDCl[subscript 3] decrease with increasing concentration, indicative of the self-association behavior found using VPO. A number of s e l f-association models have been tested. A monomer-n-mer equilibrium was found to be incompatible with the VPO results. Therefore the self-association of bile acid esters in organic solvents must proceed via a multiple type equilibrium. Analysis of the nmr and VPO data showed that the extent of aggregation was low and probably not much more than tetrameric. The advantages of using a combination of experimental approaches, such as nmr and VPO, for investigations of self-associating systems have also been demonstrated.