Isodesmic self-assembly in lyotropic chromonic systems

We have developed simple models of chromonic molecules and by carrying out Monte Carlo simulation in a binary mixture of model chromonic and water molecules, have studied the effect of concentration and molecular shape on the pattern of molecular aggregation. We have also computed the free energy change associated with the formation of chromonic columnar aggregates by umbrella sampling. This helps us to verify the isodesmic behaviour which is characteristic of chromonic systems.     

The distinction between chromonic and amphiphilic lyotropic mesophases

Optical microscopy, X-ray diffraction and NMR spectroscopy have been used to examine whether the hexagonal phases of representative chromonic and amphiphilic mesogens are miscible. The systems studied were octaoxyethylene-glycol dodecylether with either disodium cromoglycate or 5-n-hexyl-7-(5-methyl-sulphonimidoyl) xanthone-2-carboxylic acid. The results clearly demonstrate that the hexagonal phases of these two systems are not miscible, although miscibility does occur in the isotropic solution. These observations suggest that chromonic mesophases are a new breed of lyotropic liquid crystals.     

Oriented monolayers prepared from lyotropic chromonic liquid crystal

We use a layer-by layer electrostatic self-assembly technique to obtain in-plane oriented aggregates of mesogenic dye molecules cast from lyotropic chromonic liquid crystals (LCLCs) on mica substrates. The aqueous solutions of dye used for deposition are in the nematic phase. Atomic force microscopy and X-ray photoelectron spectroscopy of the dried film reveal that the LCLC molecules adsorb at the charged substrate preserving ordered aggregates of elongated shape characteristic of the nematic phase in the aqueous solution. These elongated aggregates of LCLC molecules form films with in-plane orientational order and are compositionally distinct from the substrate.     

The intercalation of ethidium bromide in the chromonic lyotropic phases of drugs and nucleic acids

Chromonic liquid crystalline phases are formed by a variety of drug and dye/water systems. In contrast to conventional lyotropic phases (where micelle formation underlies the mesogenic properties), in chromonic systems the molecules stack in columns. The different chromonic phases are different arrangements of these columns. We have examined the solution of ethidium bromide (EB) in the well-documented chromonic Intal/water system. EB is a widely used nucleic acid stain which changes colour when intercalated into DNA and which becomes fluorescent.

We have charted the changes in the temperature/composition phase diagram of the Intal/water system caused by adding EB. Although there are changes in the position of the phase boundaries, the overall pattern remains qualitatively the same—implying that the host phase is accepting EB as a similar chromonic molecule. The intercalation of EB molecules in the chromonic host phase results in optical effects—a metachromic colour change and fluorescence, similar to those occurring when the dye stains DNA.

These observations strengthen our belief that the central stack of bases in DNA can be regarded as being chromonic in nature.     

Influence of saddle-splay deformation on lyotropic chromonic liquid crystals confined between two coaxial cylinders

ABSTRACT

A striking feature of lyotropic chromonic liquid crystals confined in cylinder model exhibit double-twist director configurations. Evidence suggests that saddle-splay deformation is among the most important factors for the distortions of director. Previous researches limit the director to distort at a fixed plane (r-? plane) by using specific boundary conditions such as degenerate planar anchoring condition. In this work, we consider lyotropic chromonic liquid crystals confined between two coaxial cylinders with free-surface boundary conditions and Rapini-Papoular-type anchoring conditions. By using finite-difference iterative method to solve the numerical solution of Euler equation, we find that saddle-splay deformation leads to double-twist director configurations under free-surface boundary conditions, which consist of the result under degenerate planar anchoring conditions. Furthermore, at Rapini-Papoular-type anchoring conditions, saddle-splay deformation has a great influence on the director in the radial direction (r direction) and the director distorts in three-dimensional space. Remarkably, our method provides a more accurate theory basis for the measured values of saddle-splay elastic constant K_24。     

Planar and twisted lyotropic chromonic liquid crystal cells as optical compensators for twisted nematic displays

We have explored the use of lyotropic chromonic liquid crystals (LCLCs) as promising materials for optical compensators for liquid crystal displays. We used aqueous solutions of disodium cromoglycate in the range of concentrations that yield a chromonic N phase. The N phase is aligned by rubbed polyimide layers giving planar orientation. Two different types of LCLC-based elements are described: (a) a uniformly aligned planar N phase that forms an optically uniaxial plate with a negative birefringence; (b) twisted cells of LCLC obtained by doping the N phase with chiral amino acids. Both planar and twisted N* phase cells were used for compensation of the positive birefringence of twisted nematic (TN) displays; the TN devices were appreciably improved. We achieved achromatic dark state and contrast ratio up to 50:1 at all directions within a 40° cone of viewing angle.     

Planar and twisted lyotropic chromonic liquid crystal cells as optical compensators for twisted nematic displays

We have explored the use of lyotropic chromonic liquid crystals (LCLCs) as promising materials for optical compensators for liquid crystal displays. We used aqueous solutions of disodium cromoglycate in the range of concentrations that yield a chromonic N phase. The N phase is aligned by rubbed polyimide layers giving planar orientation. Two different types of LCLC-based elements are described: (a) a uniformly aligned planar N phase that forms an optically uniaxial plate with a negative birefringence; (b) twisted cells of LCLC obtained by doping the N phase with chiral amino acids. Both planar and twisted N* phase cells were used for compensation of the positive birefringence of twisted nematic (TN) displays; the TN devices were appreciably improved. We achieved achromatic dark state and contrast ratio up to 50:1 at all directions within a 40° cone of viewing angle.     

The self-assembly and lyotropic mesomorphism of riboguanylic acids (GMP)

The three isomeric guanylic acids (5'-GMP, 3'-GMP and 2'-GMP) and the 3':5'-cyclic derivative (cGMP) form, in water solution, left-handed columnar aggregates which, at higher concentration, pack to give left-handed cholesteric mesophases. The self-assembly process and liquid crystal formation are discussed in relation to the behaviour of the 2'-deoxy analogues.     

Condensation of self-assembled lyotropic chromonic liquid crystal sunset yellow in aqueous solutions crowded with polyethylene glycol and doped with salt

We use optical and fluorescence microscopy, densitometry, cryo-transmission electron microscopy (cryo-TEM), spectroscopy, and synchrotron X-ray scattering to study the phase behavior of the reversible self-assembled chromonic aggregates of an anionic dye Sunset Yellow (SSY) in aqueous solutions crowded with an electrically neutral polymer polyethylene glycol (PEG) and doped with the salt NaCl. PEG causes the isotropic SSY solutions to condense into a liquid-crystalline region with a high concentration of SSY aggregates, coexisting with a PEG-rich isotropic (I) region. PEG added to the homogeneous nematic (N) phase causes separation into the coexisting N and I domains; the SSY concentration in the N domains is higher than the original concentration of PEG-free N phase. Finally, addition of PEG to the highly concentrated homogeneous N phase causes separation into the coexisting columnar hexagonal (C) phase and I phase. This behavior can be qualitatively explained by the depletion (excluded volume) effects that act at two different levels: at the level of aggregate assembly from monomers and short aggregates and at the level of interaggregate packing. We also show a strong effect of a monovalent salt NaCl on phase diagrams that is different for high and low concentrations of SSY. Upon the addition of salt, dilute I solutions of SSY show appearance of the condensed N domains, but the highly concentrated C phase transforms into a coexisting I and N domains. We suggest that the salt-induced screening of electric charges at the surface of chromonic aggregates leads to two different effects: (a) increase of the scission energy and the contour length of aggregates and (b) decrease of the persistence length of SSY aggregates.     

Enzyme-triggered model self-assembly in surfactant-cyclodextrin systems

We present here a host-guest approach to construct enzyme-triggered assembly systems on the basis of surfactant-cyclodextrin complexes and α-amylase. We realized enzyme-responsive model self-assembly systems including monolayers, micelles, and vesicles. The host-guest approach is expected to be extended to more complicated assembly systems with widespread applications.     

Theoretical prediction of the cholesteric pitch in thermotropic and lyotropic systems

The expression for the cholesteric pitch given by the theory of Lin-Liu et al. is extended by explicit consideration of the orientational entropy terms. The temperature dependence, as well as the dependence on the mole fraction in non-mesogenic solutions and on the degree of polymerization in polymer cholesterics can be obtained. In some cases helical inversion can be induced by variation of concentration as well as temperature.     

Highly ordered patterns of parabolic focal conics in lamellar lyotropic systems

In this paper, we report the experimental observation of the formation of highly ordered parabolic focal conical patterns in lamellar surfactant solutions. Predominantly, we investigated mixtures of sodium dodecyl sulfate, water, hexanol, and decane, located in the immediate vicinity of the region of the L3 and Lalpha phase coexistence. The experimental studies of the formation of the patterns and of their temporal development are described. We give a simple model picture for the underlying structure, corroborated by the experimental results. There appears to be only one independent length scale that controls the appearance of the whole pattern.     

Dispersion of thiol stabilized gold nanoparticles in lyotropic liquid crystalline systems

A new method of forming stable dispersions of alkanethiol and aromatic thiol stabilized gold nanoparticles in two different lyotropic liquid crystalline mediums, namely, a columnar hexagonal phase made up of a Triton X-100/water system and an inverse columnar hexagonal phase made up of pure AOT, are presented. The dispersions have been characterized using small-angle X-ray scattering (SAXS) and polarizing optical microscopy. Our studies show that the gold nanoparticles are distributed outside the columns formed by both the surfactants. Such dispersions can find applications in the study of nanoparticles as well as in the development of devices based on some unique properties of metal nanoparticles.     

Effect of solvent polarity on the self-assembly and dielectric dynamics of non-aqueous lyotropic liquid crystalline phases

Binary mixtures comprising cetylpyridinium chloride and non-aqueous solvents of varying polarity [ethylene glycol (ε ~ 37.2) and formamide (ε ~ 109)] were studied via X-ray diffraction, polarisation optical microscopy, differential scanning calorimetry and dielectric spectroscopy. Layered lamellar mesophase was observed in both mixtures. Formamide-based mesophase was found more ordered and stable up to higher temperature (140°C). Ordering and stability of the mesophase was explained considering the higher polarity and dipolar–dipolar interactions of formamide. Dielectric spectroscopy demonstrates the large magnitudes of capacitance and permittivity (Cp ≈ 9 µF and ε ≈ 2428) for mesophase derived from formamide. Dynamical relaxation parameters of both mixtures were discussed and correlated with their structural aspects.     

π-共轭体系超分子自组装

π-共轭体系因其分子结构的可设计性以及优异的光电性质得到了广泛的研究,其超分子自组装在制备结构复杂、规则的功能纳米材料方面表现出了显著的优势,且是调控材料宏观性质的一种有效的方法.因此,π-共轭体系超分子自组装已经成为近年来信息、材料、生物等前沿领域的研究热点.本文综述了π共轭体系超分子自组装的机理、外界环境的导向作用、自组装形态以及其在光电器件、生物传感等方面的应用研究,进一步提出了该领域尚待解决的问题并对其应用前景进行了展望.     

The chromonic phases of dyes

Abstract

It has been shown that the lyotropic liquid-crystalline phases formed by certain dyes are structurally analogous to the chromonic N and M liquid-crystalline phases previously thought to be unique to certain anti-asthmatic/anti-allergic drugs. We suspect that these two groups of compounds will prove to be representatives of a large new class of mesogenic materials.     

The chromonic phases of dyes

It has been shown that the lyotropic liquid-crystalline phases formed by certain dyes are structurally analogous to the chromonic N and M liquid-crystalline phases previously thought to be unique to certain anti-asthmatic/anti-allergic drugs. We suspect that these two groups of compounds will prove to be representatives of a large new class of mesogenic materials.