Thin preparation

Intensity fluctuations are also clearly visible. However, the practiced eye will notice that only disclinations of whole number rank are present in the preparation. Focal conic zones remain unchanged, up to some small details. But where only one colour was observed previously, we now often find two colours separated by a thin wall. All these observations are compatible with the simple idea that molecules are tilted relative to the plane of the layer, as shown in Fig. 9.11a. When there is no external field, the tilt direction remains indeterminate, just as we found for the directions of optical axes in nematic phases, which gave rise to their threadlike texture. However, the absence of disclinations of whole number rank is characteristic of a layered structure. The two colours can be understood as being due to occurrence of positive and negative tilts in thin preparations (see Fig. 9.11b). These arguments are corroborated by crystallographic studies. We have thus discovered a second type of layered liquid crystal, called the smectic C phase, or Sc- Note that this tilting does not preclude a liquid-type order within layers (a kind of 2-dimensional nematic phase). 

It is generally accepted that higher resolution may be achieved on thin preparative layers (0.5-1 mm). The resolution is much more limited on a high capacity (1.5-2 mm) layer, because of the thickness of the stationary phase. The loading capacity of a preparative layer increases with the square root of the thickness, practically without loss of separating power. The loading capacity of a 0.5 mm layer is approximately half that of a plate with a layer thickness of 2 mm. 

Thin oxide films may be prepared by substrate oxidation or by vapour deposition onto a suitable substrate. An example of the fomrer method is the preparation of silicon oxide thin-films by oxidation of a silicon wafer. In general, however, the thickness and stoichiometry of a film prepared by this method are difficult to control. 

Ultramicrotomy is sometimes also used to produce thin samples of solid materials, such as metals [13] which are, however, preferentially prepared by chemical- or ion-etching (see [1]) and focused ion beam (FIB) teclmiques [14]. 

Nagashima H, Kato Y, Satoh H, Kamegashima N, Itoh K, 01 Kand Salto Y 1996 Thermal study of a silylmethylated fullerene leading to preparation of its vaouum deposited thin film Chem. Lett. 519-20... 

Girling I R and Milverton D R J 1984 A method for the preparation of an alternating multilayers film Thin Solid Films 115 85-8... 

Luk S Y, Mayers F R and Williams J O 1988 Preparation and oharaoterization of Langmuir-Blodgett films of mesoporphyrin-IX dimethylester indium ohioride Thin Solid Films 157 69-79... 

Ultra-high vacuum (UHV) surface science methods allow preparation and characterization of perfectly clean, well ordered surfaces of single crystalline materials. By preparing pairs of such surfaces it is possible to fonn interfaces under highly controlled conditions. Furthennore, thin films of adsorbed species can be produced and characterized using a wide variety of methods. Surface science methods have been coupled with UHV measurements of macroscopic friction forces. Such measurements have demonstrated that adsorbate film thicknesses of a few monolayers are sufficient to lubricate metal surfaces [12, 181. 

Figure C2.17.3. Close-packed array of sub-micrometre silica nanoparticles. Wlren nanoparticles are very monodisperse, they will spontaneously arrange into hexagonal close-packed stmcture. This scanning electron micrograph shows an example of this for very monodisperse silica nanoparticles of -250 nm diameter, prepared in a thin-film fonnat following the teclmiques outlined in [236].

In order to prepare the free base, place the remaining half of the crude hydrochloride in a 200 ml. beaker, add 20 ml. of water, and then stir the mixture with a glass rod until a thin paste of uniform consistency (quite free from lumps) is obtained. Now... 

The 70 per cent, sulphuric acid is prepared by adding 60 ml. of concentrated sulphuric acid cautiously and in a thin stream with stirring to 45 ml. of water. 

Make a thin paste of 21 5 g. of finely-powdered o-tolidine (a commercial product) with 300 ml. of water in a 1-litre beaker, add 25 g. (21 ml.) of concentrated hydrochloric acid, and warm until dissolved. Cool the solution to 10° with ice, stir mechanically, and add a further 25 g. (21 ml.) of concentrated hydrochloric acid (1) partial separation of o tolidine dihydrochloride will occur. Add a solution of 15 g, of sodium nitrite in 30 ml. of water as rapidly as possible, but keep the temperature below 15° a slight excess of nitrous acid is not harmful in this preparation. Add the clear, orange tetrazonium solution to 175 ml. of 30 per cent, hypophosphorous acid (2), and allow the mixture to stand, loosely stoppered, at room temperature for 16-18 hours. Transfer to a separatory funnel, and remove the upper red oily layer. Extract the aqueous layer with 50 ml, of benzene. Dry the combined upper layer and benzene extract with anhydrous magnesium sulphate, and remove the benzene by distillation (compare Fig. II, 13, 4) from a Widmer or similar flask (Figs. II, 24, 3-5) heat in an oil bath to 150° to ensure the removal of the last traces of benzene. Distil the residue at ca. 3 mm. pressure and a temperature of 155°. Collect the 3 3 -dimethyldiphenyl as a pale yellow liquid at 114-115°/3 mm. raise the bath temperature to about 170° when the temperature of the thermometer in the flask commences to fall. The yield is 14 g. 

To prepare crystalline monoperphthalic acid, place the thoroughly dry ethereal solution (4) in a distilling flask equipped with a capillary tube connected with a calcium chloride or cotton wool drying tube, and attach the flask to a water pump. Evaporate the ether without the application of heat (ice will form on the flask) to a thin syrup (about 150 ml.). Transfer the syrup to an evaporating dish, rinse the flask with a little anhydrous ether, and add the rinsings to the syrup. Evaporate the remainder of the ether in a vacuum desiccator over concentrated sulphuric acid about 30 g. of monoperphthalic acid, m.p. 110° (decomp.), is obtained. 

Dissolve 15-0 g. of A.R. barium nitrate and 130 g. of A.R. cupric nitrate trihydrate in 450 ml. of water at 80°. Prepare a solution of sodium chromate by dissolving 89 g. of recrystallised sodium dichromate dihydrate in 200 ml. of water and adding 112 5 ml. of cone, ammonia solution (sp. gr. 0-90). Add the warm solution (80°) of nitrates in a thin stream, with stirring, to the sodium chromate solution (at 25°). Collect the orange precipitate by suction Bltration, wash it with two 50 ml. portions of 5fiter, drain well, and dry at 75-80° for 12 hours powder finely. 

The nitration of the 2-anilino-4-phenylselenazole (103) is much more complicated. Even careful nitration using the nitrate-sulfuric acid method leads to the formation of a mixture of variously nitrated compounds in an almost violent reaction. By the use of column chromatography as well as thin-layer chromatography a separation could be made, and the compounds could be partly identified by an independent synthesis. Scheme 33 shows a general view of the substances prepared. Ring fission was not obser ed under mild conditions. 

Not all synthetic polymers are used as fibers Mylar for example is chemically the same as Dacron but IS prepared in the form of a thin film instead of a fiber Lexan is a polyester which because of its impact resistance is used as a shatterproof substitute for glass It IS a polycarbonate having the structure shown... 

Liquid samples are analyzed in one of two ways. For nonvolatile liquids a suitable sample can be prepared by placing a drop of the liquid between two NaCl plates, forming a thin film that typically is less than 0.01 mm thick. Volatile liquids must be placed in a sealed cell to prevent their evaporation. 

A recent innovation in IR sample preparation is the use of disposable sample cards made from thin sheets of either polyethylene (PE) or polytetrafluoroethylene (PTFE). 

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