Liquid crystals lyotropics studies

Recent solid state NMR studies of liquid crystalline materials are surveyed. The review deals first with some background information in order to facilitate discussions on various NMR (13C, ll, 21 , I9F etc.) works to be followed. This includes the following spin Hamiltonians, spin relaxation theory, and a survey of recent solid state NMR methods (mainly 13C) for liquid crystals on the one hand, while on the other hand molecular ordering of mesogens and motional models for liquid crystals. NMR studies done since 1997 on both solutes and solvent molecules are discussed. For the latter, thermotropic and lyotropic liquid crystals are included with an emphasis on newly discovered liquid crystalline materials. For the solute studies, both small molecules and weakly ordered biomolecules are briefly surveyed. 

Bonazzi, S., Capocianco, M., De Morais, M. M., Garbesi, A., Gottarelli, G., Mariani, P, Grazia M., Bossi, P, Spada, G. P, Tondelli, L. (1991). Four-stranded aggregates of oligodeoxyguanylates forming lyotropic liquid crystals A study by circular dichro-ism, optical microscopy, and x-ray diffraction, J. Am. Chem. Soc., 113 5809. 

A. C. Tracey, P. Diehl Lyotropic Liquid Crystals 18 figures, 3 tables. IV, 85 pages. 1975 Contents Nuclear Magnetic Resonance Studies in Lyotropic Liquid Crystals Introduction.— Studies of Lyotropic Liquid Crystals. — Studies of Molecular and Ionic Species Dissolved in the Nematic Phase of Lyotropic Liquid Crystals. 

Zhao Tongyang, and Wang Xiaogong. Solvent effect on phase transition of lyotropic rigid-chain liquid crystal polymer studied by dissipative particle dynamics. J. Chem. Phys. 138 (2013) 024910. 

Lukaschek M, Grabowsld D, Schmidt C (1995) Shear-induced aligmnent of a hexagonal lyotropic liquid crystal as studied by rheo-nmr. Langmuir 11(9) 3590-3594... 

Muller S, Fischer P, Schmidt C (1997) Solid-like ditector reorientation in sheared hexagonal lyotropic liquid crystals as studied by nuclear magnetic resonance. J Phys n France 7 421-432... 

To conclude this section on 2D NMR of liquid crystals, some studies of more exotic liquid crystalline systems are pointed out. Polymer dispersed nematic liquid crystals have attracted much attention because of their applications as optical display panels. Deuteron 2D quadrupole echo experiments have been reported [9.28] in the isotropic and nematic phases of / -deuterated 5CB dispersed in polymers. A similar technique was used [9.29] to study two model bilayer membranes. Both studies allow determination of the lineshape F(u ) due to quadrupolar interactions and the homogeneous linewidth L(u ) of the individual lines [9.28]. The 2D quadrupole echo experiment has also been used [9.30] to separate chemical shift and quadrupolar splitting information of a perdeuterated solute dissolved in a lyotropic liquid crystal. The method was compared with the multiple-quantum spectroscopy that is based on the observation of double-quantum coherence whose evolution depends on the chemical shift but not on the quadrupolar splitting. The multiple-quantum method was found to give a substantial chemical shift resolution. The pulse sequences for these methods and their treatment using density matrix formalism were summarized [9.30] for a spin 1=1 system with non-zero chemical shift. Finally, 2D deuteron exchange NMR was used [9.31] to study ring inversion of solutes in liquid crystalline solvents. 

Similar behavior can occur when a crystalline network is disassembled by adding a solvent rather than by heating. These mesogens are called lyotropic liquid crystals and the mesophase formation shows temperature and concentration dependence. They are very important in biological systems, but have been much less studied in materials science. 

The role of various surfactant association structures such as micelles and lyotropic liquid crystals (372), adsorption-desorption kinetics at liquid-gas interfaces (373) and interfacial rheology (373) and capillary pressure (374) on foam lamellae stability has been studied. Microvisual studies in model porous media indicate... 

DNAs are soluble only in aqueous solutions and their fibrous crystals can be prepared by slow evaporation from the aqueous solution. Duplex structures in the fibers have been studied by X-ray diffraction [2,3] and sohd state NMR [4-6]. Orientation of DNA strands by using hydrodynamic flow gradients in the dilute aqueous solution [7,8] and lyotropic liquid crystal... 

Advances in the chemistry of [M(CN)5L]" complexes, for M = Fe, Ru, and Os, have been reviewed.There has been rather little activity in the preparation of novel complexes, but considerable activity in studying the properties, especially solvatochromism and various aspects of kinetics of substitution, of known complexes. However there has been an attempted preparation of [Fe(CN)5(Ci2H25NH2)], in the hope of generating micelles or lyotropic liquid crystals. This preparation appeared to yield [Fe(CN)4(H20)(Ci2H25NH2)], whose alkali metal salts gave a hexagonal mesophase in water, but were also readily hydrolyzed to [Fe(CN)4(H20)2] . Heterobinuclear complexes of the form [(NC)5FeL ML 5] " " have been much studied, especially in relation to intramolecular electron transfer (see Section 5.4.2.2.5). 

Alkyl chain(s) with carbohydrates at both termini (bolaamphiphiles) have also been reported since they are potential building blocks for the construction of membrane mimetics with a single monolayer [78]. Bisgluconamide and lactobionamides 6b (sug=Glc-A or Lac-A, X=NH) were studied for their crystalline properties and their arrangements in water [39, 40]. Alkyl-a,cc)-dimannitol 6b (sug=Man-ol, n=16-22) [66] or bolaamphiphiles with identical or different carbohydrates at both ends of the alkyl chain 6a (sug=D-Glc/, D-Galp, DL-Xyl-oI) were found to form micelles and lyotropic liquid crystals as well [41]. 

The brief data presented in this chapter concerning the initial steps of structure formation in LC polymer solutions, are significant from two viewpoints. On the one hand, the study of these processes provides quantitative information about the molecular parameters and IMM of LC polymers, which is the basis for the understanding and prediction of physico-chemical behaviour of polymeric liquid crystals in bulk. On the other hand, understanding of the features of intramolecular structure formation in dilute solution, reveals broad prospects for the investigation of the formation of lyotropic LC systems of polymers with mesogenic side groups, which is in its infancy 195). 

The potential for novel phase behaviour in rod-coil block copolymers is illustrated by the recent work of Thomas and co-workers on poly(hexyl iso-cyanate)(PHIC)-PS rod-coil diblock copolymers (Chen etal. 1996). PHIC, which adopts a helical conformation in the solid state, has a long persistence length (50-60 A) (Bur and Fetters 1976) and can form lyotropic liquid crystal phases in solution (Aharoni 1980). The polymer studied by Thomas and co-workers has a short PS block attached to a long PHIC block. A number of morphologies were reported—wavy lamellar, zigzag and arrowhead structures—where the rod block is tilted with respect to the layers, and there are different alternations of tilt between domains (Chen et al. 1996) (Fig. 2.37). These structures are analogous to tilted smectic thermotropic liquid crystalline phases (Chen et al. 1996). 

The widespread interest in transport across membranes of living cells has led to studies of diffusion in lyotropic liquid crystals. Biological membranes are generally thought to contain single bimolecular leaflets of phospholipid material, leaflets which are like the large, flat micelles of lamellar liquid crystals. No effort is made here to review the literature on transport either across actual cell membranes or across single bimolecular leaflets (black lipid films) which have often been used recently as model systems for membrane studies. Instead, experiments where lamellar liquid crystals have been used as model systems are discussed. 

Interest in thermotropic liquid crystals has focussed mainly on macroscopic properties studies relating these properties to the microscopic molecular order are new. Lyotropic liquid crystals, e.g. lipid-water systems, however, are better known from a microscopic point of view. We detail the descriptions of chain flexibility that were obtained from recent DMR experiments on deuterated soap molecules. Models were developed, and most chain deformations appear to result from intramolecular isomeric rotations that are compatible with intermodular steric hindrance. The characteristic times of chain motions can be estimated from earlier proton resonance experiments. There is a possibility of collective motions in the bilayer. The biological relevance of these findings is considered briefly. Recent similar DMR studies of thermotropic liquid crystals also suggest some molecular flexibility. 

We note that earlier research focused on the similarities of defect interaction and their motion in block copolymers and thermotropic nematics or smectics [181, 182], Thermotropic liquid crystals, however, are one-component homogeneous systems and are characterized by a non-conserved orientational order parameter. In contrast, in block copolymers the local concentration difference between two components is essentially conserved. In this respect, the microphase-separated structures in block copolymers are anticipated to have close similarities to lyotropic systems, which are composed of a polar medium (water) and a non-polar medium (surfactant structure). The phases of the lyotropic systems (such as lamella, cylinder, or micellar phases) are determined by the surfactant concentration. Similarly to lyotropic phases, the morphology in block copolymers is ascertained by the volume fraction of the components and their interaction. Therefore, in lyotropic systems and in block copolymers, the dynamics and annihilation of structural defects require a change in the local concentration difference between components as well as a change in the orientational order. Consequently, if single defect transformations could be monitored in real time and space, block copolymers could be considered as suitable model systems for studying transport mechanisms and phase transitions in 2D fluid materials such as membranes [183], lyotropic liquid crystals [184], and microemulsions [185],... 

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