A vanadium-51 nuclear magnetic resonance investigation of vanadate oxyanions in a lyotropic liquid crystalline bilayer system

Abstract

The vanadate ion VO4H2− and two of its polymeric forms contained in mixed lyotropic mesophase prepared from potassium dodecanoate/n-alkyltrimethylammonium bromide in water have been investigated by 51V nuclear magnetic resonance spectroscopy. The results indicate that the tetrahedral ion interacts with the micellar surface through the hydrating water associated with this ion. The ion undergoes strong interactions with the carboxylate head group of the dodecanoate as well as with the headgroup of the alkyltrimethylammonium detergent, which of course is of opposite charge. The other two species investigated have previously been assigned by various workers to V2O74− and its cyclic dimer V4O124−. The results obtained here are not consistent with those assignments but rather with more symmetrical compounds such as the tetrahedral uncharged molecule V4O10 and a higher analogue of this, respectively. The nmr spectra from these latter species show that they undergo, at best, extremely weak interactions with the cationic surface but strong interactions with an anionic one. The behaviour of the quadrupole splittings observed is consistent with specific interactions to the carboxylate moiety. The observation of an induced chemical shift separation between vanadium resonances from the same highly symmetric species confirms that binding of this compound is very strong.