Smectic monolayer

Aside from exhibiting a rich range of conventional smectic phases, members of the m.OnO.m family also exhibit novel modulated hexatic phases [155] in which the tilted hexatic smectic monolayers have a periodic modulation in their smectic structure, analogous to the Sm C ribbon phase. Two such phases have been identified and have been termed the Sml and Sm2 phases. The Sml phase is exhibited by members of the m.OnO.m family with... 

Depending on a material, single smectic monolayers can exist in two different modificati(His, liquid-like and hexatic like. Properties of these monolayers are shown in Table 5.1. Upon melting, a two-dimensional hexatic layer undergoes the transition into the liquid-like layer. It is spectacular that hexatic layers like liquid layers do not support the in-plane shear [14]. The layer can be sheared by as small force (stress) as is wished. 

There has been much activity in the study of monolayer phases via the new optical, microscopic, and diffraction techniques described in the previous section. These experimental methods have elucidated the unit cell structure, bond orientational order and tilt in monolayer phases. Many of the condensed phases have been classified as mesophases having long-range correlational order and short-range translational order. A useful analogy between monolayer mesophases and die smectic mesophases in bulk liquid crystals aids in their characterization (see [182]). 

L. The liquid-expanded, L phase is a two-dimensionally isotropic arrangement of amphiphiles. This is in the smectic A class of liquidlike in-plane structure. There is a continuing debate on how best to formulate an equation of state of the liquid-expanded monolayer. Such monolayers are fluid and coherent, yet the average intermolecular distance is much greater than for bulk liquids. A typical bulk liquid is perhaps 10% less dense than its corresponding solid state. 

Smith D P E, Hdrber H, Gerber Ch and Binnig G 1989 Smectic liquid crystal monolayers on graphite observed by scanning tunnelling microscopy Science 245 43... 

The nematic phase of all the compounds CBn is characterized by a coherence length of about 1.4 times the elongated structure of the molecule. Based on this behaviour local associations in form of dimers with cyano-phenyl interactions were postulated. For the smectic A phase a partial bilayer arrangement of the molecules (SAd) is most likely. But there are also example for the smectic A phase with a monolayer (Sai) or a bilayer (Sa2) arrangement of the molecules as well as a commensurate structure A large number of X-ray measurements were carried out in the liquid crystalline state to clear up the structural richness and variability (see Chap. 2, this Vol. [52]). 

The combination of polar and steric dipoles and multipoles, together with the conformational flexibility of the tail chains, leads to the large variety of smectic phases [11-13]. In addition to the monolayer smectic Aj phase, where the molecules are oriented randomly up and down within each layer, there are... 

Fig. 4. Schematic representation of the smectic layering along with their characteristic diffraction patterns for the monolayer (Ai), the partially bilayer (Aj), the bilayer (A2) and the two-dimensional (A) phases. The arrows indicate permanent dipoles, the solid points are Bragg reflections...

If Lp = Lh, the monolayer smectic Ai phase is formed. In the case that Lp > Lh, the layer spacing d exceeds the molecular length L, which corresponds to the formation of the smectic A phase (Fig. lla,b). In the opposite case, Lp < Lh, the layer spacing is less than L, and the flexible hydrocarbon chains just All the space which is determined by the fluorinated tails (Fig. 10c). 

Both compound I and a mixture of the two derivatives M70 (70% 1 30% II) display the usual monolayer smectic Aj phase on cooling down from the isotropic liquid. At lower temperatures the particular smectic phases (X and X ) showing polar behaviour have been observed [78]. The smectic X phase in... 

Thus the formation of tilted analogues of the smectic A phases, i.e. monolayer Cl and bilayer C2, is possible for mesogens with relatively large electric quadrupoles. In the case of strongly sterically asymmetric molecules (e.g., zigzag shaped or dumbell shaped molecules, Fig. 3b) these quadrupolar interactions may be steric in origin. From this point of view observation of molecular tilt in the molecular dynamics simulations for a one-layer film of DOBAMBC in the absence of electrostatic interactions is not so surprising [106]. 

We have little information on the way low molecular weight molecules and oligomers adsorb (19). Apparently below DP s of about 100 they lie flat on the surface for concentrations up to a monolayer of segments, then seem to form thicker islands of smectic or nematic structure. Ordered condensed mono, -di, -or multi-layers are primarily the arrangements of smaller, especially amphipa-tic molecules on liquid-liquid interfaces. Polymers are too large to adsorb, in the ordinary sense, on micelles but segments of linear polymers may act as nucleation centers for micelles of small molecules which probably is one of the mechanisms for the lipid-, or detergent-, polymer interaction. 

Smith, D.P.E. et al. Smectic Liquid Crystal Monolayers on Graphite Observed by Scanning Tunneling Microscopy/ Science, 43 (July 7, 1989)... 

If one makes use of the rather limited information available and given above one may infer that a tilt of between 20° and 30° is normal for straight chain azobenzene derivatives when deposited as LB films, even when a homeotropic phase exists. Such a structure can only be produced in a rather loosely packed film. At the moment it is an open question whether monolayers of these materials exist in the hexatic phase as is the case for fatty acids or whether the structure more nearly corresponds to the smectic-A phase. In the case of the birefringent phase described by Jones et al. [151] it was shown that, once this phase was established, further layers deposited by the LB technique go down in an epitaxial manner. 

At 77.67°C the two surface monolayers are in the smectic-I phase and the three interior layers are in the smectic-C phase. 

Rod-Like Liquid Crystals Combining RH- and RF-Chains Monolayer Smectic Phases... 

In the presence of a suitable substrate, the resonant conjugated molecules are prone to precipitate as liquid crystals with the structure of the smectic phase. They form monolayers with the molecules aligned side-by-side. 

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