¹⁹F Multipulse NMR Study of Some Amphiphilic Liquid Crystals

Abstract

Multipulse NMR techniques have been used to investigate the dynamic properties of cesium perfluoro-octanoate (CsPFO) and ammonium perfluoro-octanoate (APFO) + water systems, which are liquid crystals over a wide range of temperature and concentration.

Axially symmetric fluorine chemical shift tensors have been measured for the CF₂ and CF₃ groups, with respect to a C₆F₆ internal reference, by performing a rotation study of an aligned sample (50% CsPFO: 50% D₂O) at room temperature. By fitting experimental spectra the order parameter S_r relative to 50% CsPFO: 50% D₂O at room temperature, has been determined for 70% APFO: 30% D₂O and 72.2% CsPFO: 27.8% D₂O in the lamellar mesophase over a temperature range 20°C-85°C. A model is assumed in which wagging motion of the molecular chain leads to scaling of the shift anisotropy.

The order parameter S has been estimated using known values of the ¹⁹F chemical shift tensor elements for CF₂ groups in teflon at liquid nitrogen temperature.