NMR studies of counterion binding in the water-sodium octanoate-pentanol system

The nuclear magnetic relaxation rate and quadrupole splitting of 23Na+ counterions have been studied as a function of temperature and composition for all the different phases of the three-component system sodium octanoate pentanol-water. For isotropic aqueous sodium octanoate solutions the effect on 23Na relaxation of adding decanol, N,N-dimethylaniline, p-xylene, cyclohexane, and sodium chloride was investigated. The results are used to obtain information on the ionic interactions in the solutions. For aqueous sodium octanoate solutions a strengthened interaction of the micellarly bound counterions is observed as the amphiphile concentration is increased and addition of sodium chloride is found to lead in general to a change, not only in the fraction of bound counterions, but also in the interactions of these ions. Solubilization of pentanol and decanol in octanoate micelles leads to an enforced counterion relaxation. For the sodium octanoate-pentanol-water system, counterion binding appears to be mainly determined by the ratio of octanoate to water while the effects of phase structure (normal and reversed micellar solutions and hexagonal and lamellar liquid crystals) as well as of pentanol concentration are small. Very rapid 23Na relaxation is observed for the highest alcohol contents of the alcohol-rich solution phases of the sodium octanoate-decanol-water and sodium octanoate-pentanol-water systems. This is ascribed to a partial dehydration of the sodium ions due to a dissociation of the reversed micelles and a formation of ion-pairs. The activation energies of the relaxation process support the conclusions drawn from the relaxation rates as do the quadrupole splittings. The latter permit some insight into the geometrical arrangement of counterions and amphiphile polar end-groups at the micellar surface. This is achieved by using the temperature dependence to deduce the sign of the order parameter. Both the relaxation rates and the quadrupole splittings indicate a considerable specificity of the ionic interactions and, over a wide concentration region in the lamellar phase, the quadrupole splittings suggest some penetration of the counterions between the polar end-groups. At the highest and lowest water contents the counterions appear to take up positions in which the ion-ion vectors are more nearly perpendicular to the lamellae. © 1976.