Selectivity hexagons

Consider, for example, six different pairs of analytes, all containing an aromatic ring (bases, acids, isomers, etc.) which were separated under different conditions (methanol, methanol/water, mefhanol/buffer). The more symmetrical the hexagon, the more applicable is the corresponding stationary phase for the separation of aromatic substances in general. In addition, it is easy to decide which column is suitable, e.g., for the separation of quite strong aromatic acids (phthalic acid/terephthalic acid, Tere/Phthal ), which is best for weaker aromatic acids (3-hydroxy-/4 hydroxybenzoic acid, 3/4-OH ), and which is best for planar/nonplanar aromatic substances (triphenylene/o-terphenyl, Triph/ o-Ter ). 

Finally, the similarity of phases can be recognized at first glance because of the condensed information as with pictograms, e.g., for traffic signs, characteristic pictures are produced see below. 

In the following, three figures containing a lot of representative hexagons (columns) for different pairs of analytes and eluents are shown and brief comments are made with regard to the similarity of the columns. 

Note In some hexagons, certain a-values are missing. The reasons for this are various problems during the measurements (i.a. air bubbles, plugged injection needle). In view of the aim to compare the similarity of the columns under really identical conditions, repeat measurements were dispensed with. 

Consider Fig. 27. As mentioned above, the most important distinctive feature of stationary RP-phases is their capacity for polar/ionic interactions. If this ability is missing or not needed, as is frequently the case when separating neutral analytes in unbuffered eluents, then a lot of stationary phases show a similar behavior. Other properties then come to the fore, such as the degree of coverage. 

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The data obtained from such investigations can be depicted in various ways. Good visualization tools are selectivity maps, selectivity plots, selectivity hexagons, and dendrograms. In the following, the similarity of some commercially available phases will be discussed with the aid of some examples. 

Fig. 28. Selectivity hexagons neutral and acidic aromatic compounds in unbuflFered eluent/acidic phosphate buffer .

Fig. 30. Some selected hexagons from Fig. 29 for comments, see text.

TS-l/MCM-41 catalysts synttiesized by the dry gel conversion method are shown to have hexagonal mesopores. The catalytic activity of synthesized TS-l/MCM-41 catalysts was tested with qroxidation reaction of olefins to reved that both the conversion of olefins and selectivity to epoxide are higher than those of H-MCM-41,... 

Titanium containing hexagonal mesoporous materials were synthesized by the modified hydrothermal synthesis method. The synthesized Ti-MCM-41 has hi y ordered hexa rud structure. Ti-MCM-41 was transformed into TS-l/MCM-41 by using the dry gel conversion process. For the synthesis of Ti-MCM-41 with TS-1(TS-1/MCM-41) structure TPAOH was used as the template. The synthesized TS-l/MCM-41 has hexagonal mesopores when the DGC process was carried out for less than 3 6 h. The catalytic activity of synthesized TS-l/MCM-41 catalysts was measured by the epoxidation of 1-hexene and cyclohexene. For the comparison of the catalytic activity, TS-1 and Ti-MCM-41 samples were also applied to the epoxidation reaction under the same reaction conditions. Both the conversion of olefins and selectivity to epoxide over TS-l/MCM-41 are found hi er flian those of other catalysts. 

Nanowires of hexagonal cobalt can be grown by the selective adsorption of ligands on all crystal faces except the faces that become the sides of the wires. 

The selection of materials for high-temperature applications is discussed by Day (1979). At low temperatures, less than 10°C, metals that are normally ductile can fail in a brittle manner. Serious disasters have occurred through the failure of welded carbon steel vessels at low temperatures. The phenomenon of brittle failure is associated with the crystalline structure of metals. Metals with a body-centred-cubic (bcc) lattice are more liable to brittle failure than those with a face-centred-cubic (fee) or hexagonal lattice. For low-temperature equipment, such as cryogenic plant and liquefied-gas storages, austenitic stainless steel (fee) or aluminium alloys (hex) should be specified see Wigley (1978). 

We will then examine other flexible polymer crystallization instances which may be interpreted, at least qualitatively, in terms of the bundle model. We will concentrate on crystallization occurring through metastable mesophases which develop by quenching polymers like isotactic polypropylene, syndiotactic polypropylene etc. In principle also hexagonal crystallization of highly defective polymers, and order developing in some microphase-separated copolymer systems could be discussed in a similar perspective but these two areas will be treated in future work. A comparison between the bundle approach and pertinent results of selected molecular simulation approaches follows. 

Table 1 Thermal and geometrical data of selected semiflexible polymers giving rise to thermotropic hexagonal mesophases or main-chain disordered crystalline phases (adapted from [11])... 

The hexameric E3M3 clusters (M = Al, Ga E = P, As) 25a, 25c, and 25d comprise hexagonal prismatic E3M3 skeletons, as representatively shown for 25a in Fig. 25 (58). Thus, the cluster core resembles that of the Sn3P3 drums 19 (see Fig. 20), and the Al-As distances are only marginally longer than those in 27a. Apparently, the Al-H bonds are shielded by the bulky silyl groups, which hampered selective substitution reactions at the Al—H bond. 

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