About the difference in the quadrupole splitting of water between cationic and anionic nematic lyotropic liquid crystals. H-2-NMR and molecular dynamics study

Indexación: ScieloDeuterium 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

Understanding the stereo- and regioselectivities of the polar Diels-Alder reactions between 2-acetyl-1,4-benzoquinone and methyl substituted 1,3-butadienes: a DFT study

The polar Diels-Alder (DA) reactions of 2-acetyl-1,4-benzoquinone (acBQ) with methyl substituted 1,3-butadienes have been studied using DFT methods at the B3LYP/6-31 G(d) level of theory. These reactions are characterized by a nucleophilic attack of the unsubstituted ends of the 1,3-dienes to the beta conjugated position of the acBQ followed by ring-closure. The reactions present a total regioselectivity and large endo selectivity. The analysis based on the global electrophilicity of the reagents at the ground state, and the natural bond orbital (NBO) population analysis at the transition states correctly explain the polar nature of these cycloadditions. The large electrophilic character of acBQ is responsible for the acceleration observed in these polar DA reactions

Counterion and composition effects on discotic nematic lyotropic liquid crystals - I. Size and order

Counterion and composition effects on the size and interface dynamics of discotic nematic lyotropic liquid crystals made of tetradecyltrimethylammonium halide (TTAX)-decanol (DeOH)-water-NaX, with X = Cl- and Br-, were investigated using NMR and fluorescence spectroscopies. The dynamics of the interface was examined by measuring deuterium quadrupole splittings from HDO (0.1% D2O in H2O) and 1,1-dideuterodecanol (20% 1,1-dideuterodecanol in DeOH) in 27 samples of each liquid crystal. Aggregation numbers, N-D, from 15 samples of each mesophase were obtained using the fluorescence of pyrene quenched by hexadecylpyridinium chloride. N-D of TTAB and TTAC are about 230 30 and 300 20, respectively. ND of TTAC increases with increasing concentration of all mesophase components, whereas TTAB shows no correlation between size and composition. The dimension of these aggregates prevents the occurrence of undulations, previously observed in lamellar phases. The quadrupole splitting of decanol-d(2) in TTAC is about 5 kHz smaller than in TTAB, and the splitting of HDO is observed only in TTAB. All results are consistent with a more dynamic TTAC interface. The TTAC aggregate should be more dissociated from counterions and the excess ammonium-ammonium electrostatic repulsions contribute to increase the mobility of the interface components

Characterisation of a new nematic lyotropic liquid crystal with natural lipids from soybean

<p>Searching for a lubricant to be used in articular bone joints, a new nematic liquid crystal (NLC) was characterised. It is formed by dissolving tetradecyltrimethyl ammonium bromide (TTAB), decanol (DeOH), sodium bromide (NaBr) and a natural mixture of lipids (PL, from soybean) in water. Small amounts of DO, and SDS-d were added as²H-NMR probes. The mesophases were characterised using²H-NMR spectroscopy, kinematic viscosity measurements, polarised light microscopy (PLM) and molecular dynamics (MD) simulations. The present system is suitable for lubrication applications, since the solution viscosity is tunable from 49 to 811 cp, more than twice the range previously obtained in a similar system. The solution contains an important proportion of a natural mixture of lipids extracted from soybean, enhancing biocompatibility. The wider range of viscosity displayed by this system, without losing the nematic phase, is attributed to the greater amount of amphiphiles supported by the solution and the size and elongation of the aggregates when the surface charge density is increased. This elongation represents a phase transition from a discotic monoaxial phase to biaxial phase and is responsible for the changes in quadrupolar splittings.</p