Capillary thin film

Liquid and gas samples do not need much preparation, but special cells to contain the samples are often necessary. The simplest method to prepare a liquid sample is to make a capillary thin film of the liquid. The capillary thin film is made by placing a drop of liquid on a KBr plate and sandwiching it with another KBr plate. This method, however, is not suitable for volatile liquids. Liquid cells can be used for volatile liquid and toxic liquid samples, particularly for quantitative analysis. The spacing between the bottom and the top of liquid cell is typically from 1 to 100 /u.m. The cell is made of an infrared-transparent material. Typically, KBr is used however, KBr should not be selected as the material for holding samples containing water because water dissolves KBr. Instead, ZeSe or AgCl should be used because they are infrared-transparent but not water soluble. Cells for gas samples are structurally similar to cells for liquid but the dimension is much larger. 

Possibly the best way to obtain transmission spectra from hydrogels is to prepare a capillary thin film onto a water-insoluble and IR-transparent window. The preparation of capillary films is simple a drop or smear of the hydrogel sample is placed onto the window and a second window is placed on the top of the first. The resulting sandwich is placed in the IR beam. 

The number of scans is perhaps the most important user-adjustable scanning parameter in FTIR, which begs the question, How many scans should I use Ultimately, it depends on your sample. For routine samples that are analyzed in the FTIR sample compartment using sample preparation techniques such as ATR, KBr pellets, and capillary thin films (more on these in Chapter 4), anywhere between 10 and 100 scans will typically yield a spectrum with a good SNR. For example, if 16 scans give a poor spectrum, by all means more scans should be tried. On the other... 

A quality spectrum obtained using the capillary thin hhn method and the holder shown in Figure 4.19 is shown in Figure 4.20. Note how the spectrum in Figure 4.20 has a flat baseline, low noise, well-resolved peaks that are on scale, and negligible offset. Cleanup with the capillary thin film method involves rinsing the windows with a solvent that will dissolve the sample but not the windows. For example, acetone, ethanol, methylene chloride, and tetrahydrofuran have been used. 

In addition to being useful with liquids, the capillary thin film technique can be used with soft solids or viscous liquids using the elegantly named smear technique. 

FIG U RE 4.19 A sample holder for transmission analysis shown installed in the sample slide mount of an FTIR. The protruding metal arms can support the KBr windows, KBr pellets, mulls, cast films, and capillary thin films. 

FIGURE 4.20 The infrared spectrum of henzonitrile obtained using the capillary thin film method. 

FIGURE 4.21 The capillary thin film spectrum of a smear of chunky peanut butter. 

The capillary thin film method offers many advantages. The ease of forming a liquid into a thin film means there is less trial and error than with preparing solids for analysis. The technique is also versatile since via the smear technique it can be used to obtain spectra of soft solids and viscous liquids. The quality of the spectra included in this section shows that this technique is capable of delivering spectra with flat baselines and good SNRs. 

The Advantages and Disadvantages of the Capillary Thin Film Method... 

Sealed liquid cells have a gasket that seals the liquid in the cell, thus preventing evaporation and counteracting the evaporation problem snffered by the capillary thin film method. It also means that volatile, smelly, and toxic liqnids can be analyzed because they will not escape and cause harm. A schematic diagram of how a sealed liquid cell works is shown in Figure 4.22. 

Liquids Capillary thin films, sealed liquid cells, ATR... 

Capillary Thin Film A transmission sampling technique used to obtain spectra of liquids. Typically, a drop of liquid is placed between two infrared transparent windows, which are then placed directly into the infrared beam. The capillary action of the liquid holds the two windows together. 

One of the most important advances in column construction has been the development of open tubular, or capillary columns that contain no packing material (dp = 0). Instead, the interior wall of a capillary column is coated with a thin film of the stationary phase. The absence of packing material means that the mobile phase... 

Another approach to improving resolution is to use thin films of stationary phase. Capillary columns used in gas chromatography and the bonded phases commonly used in HPLC provide a significant decrease in plate height due to the reduction of the Hs term in equation 12.27. 

To minimize the multiple path and mass transfer contributions to plate height (equations 12.23 and 12.26), the packing material should be of as small a diameter as is practical and loaded with a thin film of stationary phase (equation 12.25). Compared with capillary columns, which are discussed in the next section, packed columns can handle larger amounts of sample. Samples of 0.1-10 )J,L are routinely analyzed with a packed column. Column efficiencies are typically several hundred to 2000 plates/m, providing columns with 3000-10,000 theoretical plates. Assuming Wiax/Wiin is approximately 50, a packed column with 10,000 theoretical plates has a peak capacity (equation 12.18) of... 

The flows of gas and liquid need not be concentric for aerosol formation and, indeed, the two flows could meet at any angle. In the cross-flow nebulizers, the flows of gas and sample solution are approximately at right angles to each other. In the simplest arrangement (Figure 19.11), a vertical capillary tube carries the sample solution. A stream of gas from a second capillary is blown across this vertical tube and creates a partial vacuum, so some sample solution lifts above the top of the capillary. There, the fast-flowing gas stream breaks down the thin film of sample... 

Solid bed dehydration systems work on the principle of adsorption. Adsorption involves a form of adhesion between the surface of the solid desiccant and the water vapor in the gas. The water forms an extremely thin film that is held to the desiccant surface by forces of attraction, but there is no chemical reaction. The desiccant is a solid, granulated drying or dehydrating medium with an extremely large effective surface area per unit weight because of a multitude of microscopic pores and capillary... 

In Fig. 15 we show similar results, but for = 10. Part (a) displays some examples of the adsorption isotherms at three temperatures. The highest temperature, T = 1.27, is the critical temperature for this system. At any T > 0.7 the layering transition is not observed, always the condensation in the pore is via an instantaneous filling of the entire pore. Part (b) shows the density profiles at T = 1. The transition from gas to hquid occurs at p/, = 0.004 15. Before the capillary condensation point, only a thin film adjacent to a pore wall is formed. The capillary condensation is now competing with wetting. 

D. J. Srolowitz, S. A. Safran. Capillary instabilities in thin films. J Appl Phys 60 247-260, 1986. 

It can be shown, (Gibbs, Scientific Papers, I. J. J. Thomson, Applications of Dynamics to Physics and Chemistry), that a chemical equilibrium can be modified by the action of capillary forces. Thus, a state of equilibrium in solution may conceivably be modified if the latter is in the form of thin films, such as soap bubbles. Since, according to Freundlich (Kapillarchemie, 116), there is at present no direct evidence of the existence of such modification (which would no doubt be exceedingly, though possibly measurably, small) we shall not enter any further into the matter here. 

PicArsn FRL Res Engr Lgbk 761-146 (1973) 56) S.I. Morrow, Microscopical Combustion Studies of Nitrocellulose Thin Films in Pres surized Capillary Tubes , Microscope 22, 229— 241 (1973) 57) Anon, Element Analysis of... 

The surface forces apparatus (SEA) can measure the interaction forces between two surfaces through a liquid [10,11]. The SEA consists of two curved, molecularly smooth mica surfaces made from sheets with a thickness of a few micrometers. These sheets are glued to quartz cylindrical lenses ( 10-mm radius of curvature) and mounted with then-axes perpendicular to each other. The distance is measured by a Fabry-Perot optical technique using multiple beam interference fringes. The distance resolution is 1-2 A and the force sensitivity is about 10 nN. With the SEA many fundamental interactions between surfaces in aqueous solutions and nonaqueous liquids have been identified and quantified. These include the van der Waals and electrostatic double-layer forces, oscillatory forces, repulsive hydration forces, attractive hydrophobic forces, steric interactions involving polymeric systems, and capillary and adhesion forces. Although cleaved mica is the most commonly used substrate material in the SEA, it can also be coated with thin films of materials with different chemical and physical properties [12]. 

For materials which are available not in the form of substantial individual crystals but as powders, the technique pioneered by Debye and Scherrer is employed (Moore, 1972). The powder is placed into a thin-walled glass capillary or deposited as a thin film, and the sample is placed in the X-ray beam. Within the powder there are a very large number of small crystals of the substance under examination, and therefore all possible crystal orientations occur at random. Hence for each value of d some of the crystallites are correctly oriented to fulfil the Bragg condition. The reflections are recorded as lines by means of a film or detector from their positions, the d values are obtained (Mackay Mackay, 1972). 

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