129 Xe and 131 Xe NMR of Xenon in Mesophases of Cetyltrimethylammonium Bromide in Formamide

Abstract

For the first time, noble gas NMR has been applied to a study of a lyotropic liquid crystal. ¹²⁹Xe and ¹³¹Xe NMR spectra of natural xenon gas dissolved in pure formamide and three cetyltrimethylammonium bromide (CTAB)/formamide (FA) mixtures were recorded over wide temperature ranges. The CTAB/FA system forms micelles at low concentrations of CTAB and mesophases at higher concentrations and elevated temperatures. The phase transitions are observed in the series of xenon spectra. In addition, the free xenon gas signal is seen in the spectra at low temperature, when the CTAB/FA system has a coexisting solid/liquid phase. The gas signal indicates that Xe is not soluble in the solid but accumulates in macroscopic bubbles. The intensity variation of the central peak of the ¹³¹Xe NMR spectra shows the presence of a static external electric field gradient (EFG). However, the spectral satellites are not observed and, therefore, the magnitude of the EFG cannot be determined. The obvious reason for the missing ¹³¹Xe satellites is the orientation distribution of the EFG tensor leading to broadening of the satellites.