Nuclear spin relaxation and dynamical behaviour of short-chain surfactants in micellar and liquid-crystalline aggregates

The dynamical behaviour and conformations of the octylphosphate anion in micellar solution and liquid-crystalline hexagonal (H1) and lamellar (Lα) phases formed from pyridinium octylphosphate (OCTP) have been investigated by 13C relaxation at several frequencies. In the HI and Lα phases the conformer populations, the rate of conformational changes and the reorientation correlation times were derived from the fits of the 13C–1H dipolar splittings and of the longitudinal relaxation rates. In isotropic micellar solution the same information was only available from the frequency dependence of these rates. The experimental data were interpreted by means of a rotational jump model corresponding to trans⇌gauche isomerizations about one bond at a time. In these phases, the aggregation results in a strong enhancement of the probabilities of the trans rotamer, generally found in the 0.8–0.95 range. The surfactant therefore behaves as a quite rigid entity undergoing a very anisotropic and quasi-axial reorientation, a behaviour expected from the computation of the inertial tensor averaged over all accessible conformers. Similar conclusions have been drawn from the deuteron splittings and longitudinal relaxation rates of an octanoate probe diluted in the OCTP–H2O mesophases. The deuteron longitudinal relaxation of this probe being the same at 13.4 and 46 MHz, the contribution of collective motions (ODF) appears to be negligible under our experimental conditions.