About the difference in the quadrupole splitting of water between cationic and anionic nematic lyotropic liquid crystals. H-2-NMR and molecular dynamics study
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Deuterium quadrupole splittings, of deuterated water, Deltav, in anionic discotic nematic lyomesophases are always much larger than in cationic mesophases. To explore the possible origins of this difference, Deltav and T-1 relaxation times of HDO (H2O 0.2% D2O) and decanol (DeOH 14% alpha-d(2)), in solutions of cationic and anionic discotic lyotropic nernatic liquid crystals, were measured using H-2-NMR. The four component mesophases were prepared based on tetradecyltrimethylammonium bromide, (TTAB/DeOH/NaBr/H2O), and cesium N-dodecanoyl-L-alaninate, (CsDAla/DeOH/KCI/H2O), amphiphiles with cationic and anionic head-groups, respectively. For a better understanding of the experimental results, 15 ns molecular dynamics (MD) trajectories of both systems were calculated. The results suggest that the large difference observed in the quadrupole splittings of the solvent can be mainly attributed to a preferential orientation of the water molecules, induced by the strong electric field generated by the electrical bilayer formed at the interface of the anionic mesophase. Restrictions to solvent reorientational dynamics or differences in the thickness of the interface do not seem to play a significant role to explain the observed difference.