Lyotropic nematic phases

3 Other Lyotropic Phases 1.3.1 Lyotropic Nematic Phases 

In addition to the above lamellar, columnar and optically isotropic phases, there are also lyotropic nematic phases, which usually involve mixtures of a charged amphiphilic, such as simple soap, with an alkanol (a weaker amphiphilic where the head group is an alcohol), together with water and a simple salt. They are termed nematic because, like thermotropic nematics, their optical axes are easily oriented by external magnetic fields. In contrast to thermotropic nematics, the basic units of lyotropic nematics are molecular aggregates with dimensions of about 2-10 nm. In lyotropics, the nematic phase is much less usual than in thermotropic liquid crystals. Lyotropic nematics 

Nematic states occur also in nonionic monomeric and polymeric surfactant solutions, living amoeboid cells i and rigid or semiflexible polymeric rods, such as fd virus in water solutions. - We note that colloidal suspension of semiflexible virus particles also form nematic phases. -  

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Fig. 2 (a) Transmission electron microscopy (TEM) image of a disc-like beidellite clay sample (sizes of the platelets range from 69 to 480 nm in diameter), and (b) lyotropic nematic phase of an aqueous suspension of the same beidellite sample at an ionic strength of 10 4 mol L-1 ( = 0.5%) observed between crossed polarizers [263], (Copyright 2009, American Chemical Society)... 

Note that this behavior is in marked contrast to that of the zeroth to the second generation of related PAMAM and PPI dendrimers (Nc = 3, Mb = 2) with 4 -cyanobiphenyl hydrogen glutarate that did not produce any mesomorphic behavior, except a lyotropic nematic phase in 80 wt % N,N -dimethylformamide (DMF) solutions containing lithium bromide [204,205]. 

However, more remarkable was the discovery that in alkane solvents, large, orrfiometallated macrocyclic complexes of palladium shown in Figure 92 would form lyotropic columnar phases [171], These remarkable materials have been shown to form columnar hexagonal phases and, in suitable solvents, lyotropic nematic phases derived from columnar organization. Further, in certain non-mesomorphic examples, mesophases can be induced by the addition of an electron acceptor such as trinitrofluorenone chiral phases are introduced when the acceptor is resolved 2 -(2,4,5,7-tetranitro-9-fluorenylideneaminoxy)propionic acid (know as TAPA). 

Fig. 3 Schematic representation of lyotropic nematic phases with disk-like (Nj,) and rodlike (Nc) micellar aggregates. Partially reproduced with permission from [9]. 1988 by Wiley-VCH...

In addition, these complexes, except 49a and 50a, form lyotropic columnar (oblique) and nematic phases when dissolved in linear, apolar organic solvents (alkanes) over wide temperature and concentration ranges. Interestingly, for some of them, 49b-c, an unexpected transition between two lyotropic nematic phases has been observed, for which a model has recently be proposed [93]. As for 48, formation of lyotropic nematic and columnar mesophases is also extended by n-n interactions with electron-acceptors, such as TNF, in apolar solvents (pentadecane). Induction of chiral nematic phases by charge transfer interactions, in a ternary mixture (49b/alkane/TAPA TAPA is 2-(2,4,5,7-tetranitro-9-fluorenylideneaminooxy)-propionic acid and is used (and is available commercially) enantiomerically pure), has recently been demonstrated for the first time [94], and opens new perspective for producing chiral nematic phase of disc-like compounds. 

The dipalladium organyls 51, derived from 49 (M = Pd, X = C1) by ligand exchange reaction between the bridging group and acetylacetonate, are not mesomorphic in their pure state, but form mesomorphic charge transfer systems with the electron acceptor TNF [95]. The identity of the induced mesophase is still unknown, but seems very likely to be columnar [96]. Furthermore, lyotropic nematic phases were obtained in the ternary mixture 51/TNF/linear alkane (the binary mixture 51/alkane did not yield mesomorp-hism). The nematic phase in this system is though to have a columnar nature, namely a nematic columnar phase. 

Lyotropic nematic phases are generally found for short chain surfactants, for both hydrocarbon or fluorocarbon derivatives [55, 56]. Two different micelle shapes can occur (Fig. 10) [57]. One type (N. ) is thought to be composed of small cylindrical micelles and is related to the hexagonal phase, while the other type of nematic (Nj) is composed of planar disc micelles and is related to the lamellar phase. Note that the disc micelles are likely to be matchbox or ruler shaped , rather than the circular discs. Hence the disc nematic phase can have the director along the long axis of ruler micelles or along the shortest micelle dimension, as with match box micelles, while with phases the director always lies... 

As with thermotropic nematics, the addition of optically active species to lyotropic nematic phases gives lyotropic cholesteric phases. Whilst details of their structures are not fully established they appear to follow the general pattern outlined above. The cholesteric twist would appear to derive from the packing of optically active mole-... 

Kim et al. have developed a solution-phase synthesis method of ultralarge GO sheets involving pre-exfoliation of graphite flakes. Such large GO sheets spontaneously form lyotropic nematic phase at a very low concentration in water [125]. They produced self-assembled brick-Uke GO nanostmctures by simple casting of GO dispersions and drying in ambient conditions. These free-standing GO papers... 

Lyotropic nematic phases were first reported by Lawson and Flautt (62) for mixtures of Cg and Cio alkyl sulfates, together with their corresponding alcohols in water. They are somewhat less common than the mesophases discussed so far. When they do form, they occur at the boundary between an isotropic micellar phase (LQ and the hexagonal phase (Li/HQ, or between Li and the lamellar phase (Li/L ). As their name implies, they have a similar micellar order to that of the molecules in a thermotropic nematic phase. This long-range micellar orientational and translational order is lower than in the other lyotropic phases described above. Like the thermotropic phases, they are of low viscosity and can be aligned in a magnetic field. It is possible to identify nematic phases optically because of their characteristic schlieren optical texture. 

A lyotropic nematic phase formed by disk-shaped micelles resulting from the addition of small amounts of lauric acid to a micellar solution of sodium dodecyl sulfate and cocamidopropyl betaine has been studied using Na NMR quadrupolar studies, H pulsed field gradient NMR and other... 

By Heck-type coupling liquid crystalline rigid-rod polymers containing [1,3-(diethynyl)cyclobutadiene] cyclopentadienyl moieties were prepared [202]. One example is the reaction of the diethynyl derivative 41 with a 2,5-diodothiophene 42 to the polymer 43 [equation (21)] which show lyotropic nematic phases. 

Hertel G, Hoffmann H (1988) Lyotropic nematic phases of double chain surfactants. Trends Colloid Interface Sci 4 123-131... 

Quist PO, Halle B, Euro I (1992) Micelle size and order in lyotropic nematic phases from nuclear spin relaxation. J Chem Phys 96 3875-3891... 

Hendrikx Y, Charvofin J, Rawiso M, Liebert L, Holmes M (1983) Anisotropic aggregates of amphiphilic molecules in lyotropic nematic phases. J Phys Chem 87 3991-3999... 

LUE Luehmaim, B., Finkelmann, H., and Rehage, G., Phase behavior and structure of polymer surfactants in aqueous solutioa The occurrence of lyotropic nematic phases, Makromol. Chem., 186,1059,1985. 

Chiral nonbonded interactions have only low energies. Thus, they are more dominating in systems with small forces between the molecules. Amphiphilic compounds have strong molecular interactions and chiral phases are less common. An exception is the lyotropic nematic phase. Here, micelles are acting like molecules to build up the mesophase. The interaction between the micelles is small and cholesteric phases can be formed easily. 

Lyotropic nematic phases have been discovered a long time after the finding of the lyotropic mesophases. In 1967 first evidence of such a phase was published [87] but it took more than 10 years to prove unambiguously the existence of these phases [88]. The first nematic... 

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