Columnar systems

Semicontinuous and continuous systems are, with few exceptions, practiced in columns. Most columnar systems are semicontinuous since flow of the stream being processed must be intermpted for regeneration. Columnar installations almost always involve the process stream flowing down through a resin bed. Those that are upflow use a flow rate that either partially fluidizes the bed, or forms a packed bed against an upper porous barrier or distributor for process streams. 

In addition to vertical bundles of myelinated axons, the cerebral cortex of monkeys (e.g., DeFelipe et al., 1990) and of humans (e.g., del Rio and DeFelipe, 1995) also contains vertically oriented bundles of unmyelinated axons that are referred to as horsetails. These horsetails are the axonal plexuses of the inhibitory double bouquet cells and can be demonstrated in monkey neocortex by immunolabeling with antibodies to calbindin and tachykinin. As shown by DeFelipe et al. (1990), in the monkey these axonal bundles are widespread and form a regular columnar system descending from layer 2 to layers 3-5. The bundles are most evident in tangential sections taken at the level of layer 3, where they can be seen to have a center-to-center spacing of 15-30 fim. In a later study of the calbindin labeled double bouquet cells in monkey striate cortex, Peters and Sethares (1997) showed that there is one double bouquet cell, and therefore one vertically oriented double bouquet cell axonal plexus, or horsetail, per pyramidal cell module (Fig. 7). Within layer 2/3 the double bouquet axons run alongside the apical dendritic clusters, while in layer 4C they are closely associated with the vertical myelinated axonal bundles. DeFelipe et al. (1989 1990) proposed that the axon terminals of the double bouquet cell synapse with the shafts and spines of basal dendrites and oblique shafts of apical dendrites of pyramidal cells, but the exact role of these vertical bundles of inhibitory axons is not known. It is likely that they constitute a vertical inhibitory system that acts upon pyramidal cells within the minicolumns. 

Discotic liquid crystals came to prominence in the late 1970s when Chandrasekhar, Sadashiva, and Suresh reported the discovery of this new class of liquid-crystalline molecules, which were found to form columnar phases. The first of these, a hexaalkanoate of benzene, is shown in Figure 9. There then followed a rather unfortunate confusion of nomenclature in which the phases formed by discotic molecules were themselves referred to as discotic, carrying the abbreviation D. A liquid-crystal phase must be characterized by its symmetry and organization and not the shape of the molecules of which it is composed this is particularly important in columnar systems as many non-discotic molecules exhibit columnar phases. Indeed, columnar mesophases have been recognized for many years and studies date back to at least the 1960s with the work of Skoulios with various metal soaps. " Therefore, columnar phases take the abbreviation Col followed by some descriptor that describes the symmetry of the phase. 

When paramagnetic complexes are condensed into a solid, magnetic interactions of several types may occur. The simplest interaction is that within isolated dimers in an insulator. In such cases a singlet-triplet behavior may be observed as for example in [NEt4][Pt(S2C2(CN)2)2], Fig. 18. Insulating columnar systems composed of paramagnetic complexes can have intrachain... 

As discussed above in the section on columnar systems, strong stabilization of the SmA phase was achieved in binary mixtures of copper " and palladium complexes ((210) m = 10, M = Cu, Pd, R = Ci4H29 M = Cu, R = OCi4H2g) with the electron acceptor 2,4,7-trinitro-9-fluorenone (TNF). Indeed, for the three complexes, the mesophase was stabilized by nearly 60 °C, and this tendency was confirmed upon increasing TNF contents in the mixtures. 

Just as in the smectic phases and in solids, both edge and screw dislocations can occur. Three typical examples are illustrated in Fig. 9.20. In fact, big deformations do not involve dislocations (or only slightly), but simply the column curvature, which costs far less in energy terms. This feature is easy to understand by considering an example. Let us try to construct a columnar system from a cylinder of revolution of radius Tq with the boundary condition that the columns should be orthogonal to the surface of the cylinder where they meet it. Requiring the columns to be straight, either the space will not... 

Prominent examples of such an approach in the supramolecular field involve the combination of X-ray scattering, spectroscopy and computer simulation to elucidate the packing in newly synthesized columnar systems [168]. The versatility of magnetic resonance techniques, in particular solid-state-type NMR, in elucidating the interplay between structure and dynamics in these systems is evident from the examples provided below. Yet none of the results in the examples are based on a single technique (Fig. 47). 

Discotic liquid crystals are a prototypical self-assembled columnar system [9-13]. This class of liquid-crystalline compounds is relatively new, discovered in 1977 by Chandrasekar and coworkers [14,15]. The assembly motif, shown in Figure 2, for this class of compounds has an aromatic core that is surrounded by hydrocarbon chains. These disk-shaped molecules then stack to form columns. These one-dimensional stacks aggregate to form arrayed columns. When the columns have a circular cross-section they typically stack into a hexagonal arrangement as shown in Figure 2(a). Some of the original discotics were hexa-substituted phthalocyanines (1), benzenes (2), and triphenylenes (3), shown in Figme 2(b). The self-assembly of classical discotics will not be presented in depth here because it has been a focus of several comprehensive reviews [9-15]. 

Recently, ferroelectric properties have been found in chiral columnar systems [27] and also discoid cholesteric and discoid-blue phases have been found [17]. H. Bock describes chiral ferroelectric systems in Chapter 10. 

Whilst tilted smectic phases of chiral materials are in general electro-optically switchable, many chiral tilted columnar systems show no optical effect whatsoever when an applied voltage is reversed (Figure 11.12). This resilience to the reorientation of dipoles by an electrical field may in some cases be due to the more crystalline than liquid crystalline nature of the systems, but even in the quite conventional high-temperature tilted phase of the first chiral material (triphenylene-2,3,6,7,10,ll-hexayl ( S )-3-... 

These results indicate that in any realistic theory of the statistical mechanics of columnar systems the molecules cannot be treated as rigid disks, but the conformational degrees of freedom of the hydrocarbon chains must also be taken into account. 

NMR spectroscopy is a particularly versatile experimental method for study of columnar systems. Investigations of columnar phases require the same type of NMR equipment as used in studies of the solid state. Deuterium NMR has over the years been the predominant method, employed in many investigations of the molecular structure, order, and dynamics in columnar mesophases. An attractive alternative to the technique is NMR spectroscopy. In this chapter, we have described applications to columnar phases of modern and advanced NMR methods for measuring the heteronuclear dipolar couplings. The major advantage of... 

Dipolar effects on GB discs have been studied for the case of axial dipoles [58], and coherent dipolar domains in the columns have been found, even if these mono-oriented domains do not comprise the whole colunm. This does not allow the formation of ferroelectric columnar systems [46], that indeed were not observed. 

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