Association Behavior of Synthetic and Naturally Occurring Surfactants in Nonaqueous Solvents

Abstract

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.