The Cano Method

The pitch can be obtained by the CANO method or from the wavelength of the maximum of the selective reflection of circularly polarized light by... 

The helical pitch of the SmC phase and its lyotropic analog can be measured by several methods. In this thesis two of them were used to double-check the obtained results. The first applied technique is the so-called direct method [7], which requires an orientation of the layer normal k normal to the direction of observation ( planar alignment ). The second one is the Cano method [7, 8] in which the layer normal k has to be parallel to this direction ( homeotropic ). 

Usually the Cano method [8] is used for chiral nematic liquid crystals. It can also be applied to SmC phases, but then the demands on the orientation of the liquid crystal are more extensive as a nearly perfect orientation of the layer normal k as well as of the c-director are required. The presented method utilizes the different thicknesses which occur in a sample if a lens is placed on top of it. A sketch of this is shown in Fig. 4.6a. Due to the anchoring conditions, only helical structures with integer multiples N of the pitch p are allowed and regions with different values of N are separated by disclination lines [14]. Thus, a picture similar to the one shown in Fig. 4.6b occurs [15]. If the radius of curvature Rc of the lens is known, the value of the pitch p can be calculated according to [9]... 

To avoid the influence of surface interactions on the measurement of the helical pitch length, two alterations of the experiment are possible. The first one is to enlarge the thickness of the sample significantly. The second is to measure the helical pitch length under homeotropic boundary conditions which are known to not afifect its value [33]. Both methods, i.e. the direct method cf. Sect. 4.5.1) with samples of 250 pm thickness and the Cano method cf. Sect. 4.5.2), were applied to determine the influence of the solvent concentration on the helical pitch length. 

The inverse of the pitch which corresponds to the helical twist of the lamellas against each other, is plotted in Fig. 5.35 for different concentrations of formamide. The values shown in the upper part of Fig. 5.35 were determined with the Cano method, while the bottom part shows the results obtained by the direct method. The two plots in Fig. 5.35 basically show the same behavior. In both plots no clear temperature dependence of the helical twist can be found. Right after the phase transition into the lyotropic SmC analog phase, the helical structure is... 

Fig. 5.35 Helical twist p in the lyotropic SmC phase for different ratios of C50 and formamide, determined with the Cano method (top) and with the direct method bottom)...

The cholesteric screw sense was measured by the Grandjean-Cano method, which showed that M-cis-17a and P-trans-17b give cholesteric phases with opposite handedness. Observations of the switching behavior of 17 in a large number of LC materials,... 

The same method was used by Cano et al. for the determination of nicotine in air and in the urine of smokers. Air was pumped through 30 ml sulphuric acid 2N and 30 ml urine was... 

All spectroscopic methods for an evaluation of the pitch lead to the size of the repeat distance of the supermolecular LC structure and the handedness of the twisting, although in UV-VlS spectroscopy, circularly polarized light has to be used. With some skill, the Cano technique may also result in the twisting sense of the helicoidal structure. 

The Cano-wedge method is an experimental technique to measure the pitch of cholesteric Uquid crystals. It consists of a flat substrate and a hemisphere with a cholesteric Uquid crystal sandwiched between them as shown in Figure 1.22(a). At the cento, the spherical surface touches the flat surface. On both the flat and spherical surfaces there is a homogeneous alignment layer. The intrinsic pitch of the Uquid crystal is Pg. Because of the boundary... 

In the cholesteric phase, this local tendency of molecules to twist is satisfied in only one direction of space—let us say the z-axis as shown in Figure 2.7. The pitch p of such a natural helix is usually quite short in pure materials p 1 pm and cannot be deduced from direct observations in a polarizing microscope. In order to measure the pitch, the so-called Cano wedge method [2] can be used. It is very tricky and quite simple to realize. These are two reasons at least for which the Cano wedge experiment deserves to be presented here. 

The Cano wedge method is very convenient for measurements of variations of the cholesteric pitch in the presence of the magnetic field orthogonal to the helix (Figure 2.13). The idea of this experiment, due to de Gennes [3] and... 

Figure 2.15. Variation of the cholesteric pitch versus the magnetic field measured by the method of the Cano wedge by Durand et al. [5]. The liquid crystal used in this experiment was the mixture of PA A with cholesteryl chloride at concentration 0.02%. The continuous line corresponds to the theoretical prediction by de Gennes.

TABLE 35.2 Pitch Values, HTP, and Handedness of the Cholesteric Phases Formed by Doping Compound 26a in K, and ZLI-389, as Determined by the Cano-Grandjean Method... 

Arnao MB, Cano A and Acosta M. 1999. Methods to measure the antioxidant activity in plant material. A comparative discussion. Free Radic Res 31(SUPPL.) S89-S96. 

Cano A, Hemandez-Ruiz J, Garcia-Canovas F, Acosta M and Arnao MB. 1998. An end-point method for estimation of the total antioxidant activity in plant material. Phytochem Anal 9(4) 196-202. 

The method described by Cano et al was modified by Dumas et al., the length of the column was reduced from 1.5 m to 1 m. Two column temperatures were used, 120°C and 180°C, for the determination of nicotine and cotinine, respectively. For the extraction of the urine... 

As one of new methods, the fluorimetric determination of zinc in biological materials, beverages, and alloys has been described, using salicylaldehyde thiocarbazone as a complexing agent (Cano-Pavon et al. 1986). In particular, the new zinc-specific fluorophores have been developed to measure and visualize intracellular zinc (Coyle etal. 1998). 

The roots ofthe cultivated kava plant were traditionally chewed and expectorated, after which the diluted mash was mixed and filtered prior to drinking (Lebot 1991 Schleiffer 1979). This method is stiU practiced in the New Hebrides (Gajdusek 1967), whereas in Samoa and elsewhere the more modern method, sard to yield a less potent preparation, is to grind or pulverize the roots rather than chew them (Holmes 1967 Lebot 1991). In Lau, Fiji, labor strife among canoe builders may be... 

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