RESUMEN
Uniaxial order parameters of the nematic and columnar mesophases in the lyotropic chromonic liquid crystal Sunset Yellow FCF have been determined from deuteron nuclear magnetic resonance, where random confinement of the system by the dispersion of aerosil nanoparticles has been performed to help obtain the angular dependent spectra. The long-time evolution study of the order parameters shows that the system requires tens of hours to stabilize after a deep change in temperature, in contrast with the very fast assembly process of the aggregates. Finally, the degree of order of the water molecules, forced by the uniaxial environment, has been determined.
RESUMEN
Lyotropic chromonic liquid crystals (LCLCs) are formed by molecules with ionic groups at the periphery that associate into stacks through noncovalent self-assembly while in water. The very existence of the nematic (N) phase in the typical LCLC, the dye Sunset Yellow (SSY) is a puzzle, as the correlation length associated with the stacking, as measured in the X-ray experiments, is too short to explain the orientational order by the Onsager model. We propose that the aggregates can be more complex than simple rods and contain "stacking faults" such as junctions with a shift of neighboring molecules, 3-fold junctions, etc. We study how ionic additives, such as salts of different valency and pH-altering agents, alter the N phase of SSY purified by recrystallization. The additives induce two general trends: (a) stabilization of the N phase, caused by the mono and divalent salts (such as NaCl), and evidenced by the increase of the N-to-I transition temperature and the correlation length; (b) suppression of the N phase manifested in the decrease of the N-to-I transition temperature and in separation of the N phase into a more densely packed N phase or the columnar (C) phase, coexisting with a less condensed I phase. The scenario (b) can be triggered by simply increasing pH (adding NaOH). The effects produced by tetravalent spermine fall mostly into the category (b), but the detail depends on whether this additive is in its salt form or a free base form. The base form causes changes through changes in pH and possible excluded volume effects whereas the salt form might disrupt the structure of SSY aggregates.
RESUMEN
The effect of disorder in the behavior of liquid crystal (LC) is assessed and controlled by dispersing known amounts of silica aerosil in the liquid crystal material. Using deuteron nuclear magnetic resonance, the director configuration and the orientational order was determined for hydrophilic aerosil dispersions in octylcyanobiphenyl. The confined liquid crystal exhibits a well-defined alignment as the silica spheres stabilize the molecular configuration. At low silica densities, a silica network is eventually established, forming a soft gel. When the sample orientation in the magnetic field is changed, a few silica strands links are broken and a fraction of the LC molecules is realigned. The field anneals the random disorder introduced by the aerosil up to a certain density beyond which, in the so-called stiff-gel regime, disordering effects completely dominate. At a fixed temperature in the isotropic phase, there is surface-induced order that is linearly proportional to the silica density.
RESUMEN
Thin molecular depositions of 4(')-pentyl-4-cyonobiphenyl (5CB) mesogenic molecules are investigated via quadrupole-perturbed deuteron nuclear magnetic resonance (DNMR) spectroscopy. Uniform and controlled thickness molecular surface depositions are prepared on the inner cylindrical surfaces of Anopore membranes by the solvent-evaporation technique. As a result, 5CB molecules are found in two different configurations: a bulklike one with parallel axial arrangement, and a surface one with planar radial arrangement. If the 5CB surface coverage exceeds c approximately 0.35, only the bulk state is present. In the coverage range between 0.015 and 0.35, the bulklike state and the surface layer coexist, conforming to a typical dewetting scenario. Below c approximately 0.015, only the surface layer is present. The dilution of the surface deposition with decreasing coverage is manifested as an increase in the DNMR doublet frequency splitting. The surface orientational order parameter Q, the surface biaxiality eta, and the diffusion coefficient D(S) are determined from the DNMR spectral patterns obtained at different sample orientations in the external magnetic field. These angular patterns prove that in highly diluted surface depositions the molecules lie flat on the surface. However, they are not frozen and their molecular axes rapidly reorient on the DNMR measurement time scale, typically 10(-4) s, while remaining confined to the surface. Simultaneously, molecules diffuse over the surface with a surface diffusion constant on the order of 10(-11) m(2) s(-1). Such molecular diffusion is responsible for an effective biaxiality on the DNMR time scale. However, an inherent biaxiality cannot be completely ruled out and thus may play a minor role. The surface phase has a two-dimensional (2D) gas character with some (possible) indicators of 2D-liquid properties.