Teflon surfaces

Filtering the tube suspension through a 0.2 xm pore ceramic paper leaves a uniform black deposit on the paper and can produce aligned CNT films [30]. The deposited material was transferred on a thin Teflon sheet by pressing the tube-coated side of the filter on the plastic and then the filter was lifted off to expose the surface. Scanning electron microscopic study reveals that the tubes are highly aligned perpendicular to the Teflon surface [30]. 

For duPont s Teflon tube (Vr-in. diameter) heat exchangers (Figure 10-8) for condensing, heating, and cooling service, the U values range from 15-35. Little or no fouling occurs on the Teflon surface. 

The linking agent, A3 or A4, was mixed in various concentrations with the divinyl-PDMS, together with 5 ppm of a Pt catalyst (8). The mixture was then degassed and cast as a thin sheet on a Teflon surface. Complete reaction was found to occur on heating... 

The influence of adsorption on the structure of a -chymotrypsin is shown in Fig. 10, where the circular dichroism (CD) spectrum of the protein in solution is compared with that of the protein adsorbed on Teflon and silica. Because of absorbance in the far UV by the aromatic styrene, it is impossible to obtain reliable CD spectra of proteins adsorbed on PS and PS- (EO)8. The CD spectrum of a protein reflects its composition of secondary structural elements (a -helices, / -sheets). The spectrum of dissolved a-chymotrypsin is indicative of a low content of or-helices and a high content of //-sheets. After adsorption at the silica surface, the CD spectrum is shifted, but the shift is much more pronounced when the protein was adsorbed at the Teflon surface. The shifts are in opposite directions for the hydrophobic and hydrophilic surfaces, respectively. The spectrum of the protein on the hydrophilic surface of silica indicates a decrease in ordered secondary structure, i.e., the polypeptide chain in the protein has an increased random structure and, hence, a larger conformational entropy. Adsorption on the hydrophobic Teflon surface induces the formation of ordered structural elements, notably an increase in the content of O -helices (cfi, the discussion in Sect. 3.1.4). 

The first case is the most likely to be a problem with new plastic samplers. Although there is little in the literature to substantiate the belief, folklore has it that aging most plastic samplers in seawater markedly reduces the subsequent leaching of plasticisers. The second case is known to be a problem in fact, the effect is used in the various Teflon surface film samplers already mentioned. This problem alone would seem to militate against the use of Teflon for any sampling of organic materials, unless a solvent wash of the sampler in included routinely. With such a solvent wash, we introduce all of the problems of impurities in the reagents. 

The functions of these three parts are based on the same fundamental principle, i.e., the selective wetting of internal component surfaces by both organic and aqueous phases. In general, it is found that organic solvents wet Teflon surfaces whereas aqueous phases wet glass surfaces. 

Obviously, one requirement for an adhesive is that it flow easily to cover a surface. This is a more complex business than it first appears. One might naively think that the governing feature is whether we are dealing with a thin or a thick liquid, but this is not the case. If we put a drop of oil in an iron skillet, it spreads, but on a Teflon surface it beads up. The explanation revolves around surface energies, which are a measure of the relative strengths with which atoms on the surface of a material are attracted to atoms inside the bulk of the material. In a sense, this determines how much attraction these surface atoms can spare for other substances. In the case of Teflon, very little. Teflon is composed of long chains of carbon atoms, with each carbon also joined to two fluorine atoms. The fluorines, which stick out from the carbon skeleton, represent the exposed part of the molecules, the part that could potentially interact with other molecules. Fluorine, once it has bonded to carbon, is notoriously unreactive, and it is not interested in forging other... 

This means that for an apolar surface, all apolar or monopolar compounds should fit the same straight line. We have already demonstrated this for a teflon surface in Fig. 3.7 (note that the vdW parameter used in this graph is proportional to Pi, Eqs. 11-5 and 11-6). 

Fluorine is one of the smallest atoms, and nonpolar molecules made with fluorine atoms exhibit only very weak induced dipole—induced dipole attractions. This is the principle behind the Teflon nonstick surface. The Teflon molecule, part of which is shown in Figure 7.9, is a long chain of carbon atoms chemically bonded to fluorine atoms, and the fluorine atoms exert essentially no attractions on any material in contact with the Teflon surface—scrambled eggs in a frying pan, for instance. 

Pacification is a technique for removing organics and buffers from HPLC metal and Teflon surfaces and protecting them from salt corrosion with 6 N nitric acid (see Chapter 4). First, remove the HPLC column and replace it with a column bridge. Do not flush this wash into the mass spectrometer. Wash the system with water. Remove the column and replace it with a column blank. Flush with 6 N nitric acid for at least 30min, then overnight with water. Ensure the effluent pH is back to that of lab water. Replace the column and flush with mobile phase. This should be done at least once a month to clean check valves, line, and injectors. Under no circumstances should this wash be done with an HPLC column in place or into the mass spectrometer ... 

K.R. Czerwinski et al. [3] could not be reproduced by C.D. Kacher et al. [5] who reported that they observed that significant amounts of Hf (more than 50 % in some cases) stick to Teflon surfaces. (They actually conducted their subsequent experiments with polypropylene equipment because only negligible adsorption was observed with polypropylene surfaces.) The Hf results from the Czerwinski et al. experiments [3] were based on on-line data taken at the 88-Inch Cyclotron where the activity was collected on a teflon disc which according to [5] accounts for the seemingly low Hf extraction. Surprisingly, a similar loss of Rf due to adsorption in the Czerwinski et al. work [3] was not suspected by Kacher et al, and so the latter authors, based on their new Zr-, Hf-, and Ti-results and on the old [3] Rf results, suggested a revised, still questionable, sequence of extraction into TBP/benzene from around 8 M HC1 as Zr > Hf > Rf > Ti. In a parallel study of liquid-liquid extractions into TBP/benzene from HBr solutions, extraction of Rf was found to be low and was only increased for bromide concentrations beyond 9 M [5]. The extraction behavior of the group-4 elements into TBP from both HC1 and HBr solutions was primarily attributed to their different tendencies to hydrolyze [5]. 

Figure 7 shows adsorption isotherms for this protein on the different sorbents. The adsorption plateau-values at PS-(EO)8, approximately 2.5 mg m 2, is compatible with a complete monolayer of side-on adsorbed a-chymotrypsin molecules. Adsorption saturation at the PS and, even more so, the Teflon surfaces, is beyond monolayer coverage suggesting that on these hydrophobic surfaces the protein molecules are severely perturbed as to accommodate more protein mass in the adsorbed layers and/or adsorption of a second layer of protein molecules (possibly triggered by structurally altered molecules in the... 

Figure 34.27 Effect of surface modification of Teflon surface on bubble size, number in parenthesis indicate the contact angle of water.

The effect of NaCl on bubble nucleation in the presence of hydrophobic surfaces has also been examined. Excess nitrogen gas was dissolved in solution by equilibration under 25 atmospheres of pressure. Immediately following decompression the solution was supersaturated with nitrogen gas. In water and 0.02M NaCl, it was found that bubbles nucleated quickly (<25 sec) at a (hydrophobic) teflon surface. However, a 0.20M solution of NaCl was found to inhibit bubble formation. In ref>eat experiments, bubbles were found to form at the same sites on the hydrophobic surface. It would appear that the microstructure of the surface is important for the nucleation of bubbles. Microscopic surface cracks would present hydrophobic surfaces at very close separations, enabling nucleation to occur more readily. 

So far, essentially three different approaches have been reported for the preparation of zeolitic membranes [119]. Tsikoyiannis and Haag [120] reported the coating of a Teflon slab during a "regular" synthesis of ZSM-5 by a continuous uniform zeolite film. Permeability tests and catals ic experiments were carried out with such membranes after the mechanical separation of the coating from the Teflon surface [121]. Geus et al. [122] used porous, sintered stainless steel discs covered with a thin top layer of metal wool to crystallize continuous polycrystalline layers of ZSM-5. Macroporous ceramic clay-type supports were also applied [123]. 

A small drop of water assumes an almost spherical form on a Teflon surface. 

Once most of the solvent has evaporated, the sample will become opaque and may be gently loosened from the Teflon surface and placed into an oven at 40°Cto remove any residual dichloromethane. 

The composition of dew collected from a Teflon surface was compared to summer rainwater concentrations at a site in Warren, Michigan. This comparison showed that natural dew is similar to rainwater with the exception that dew has much higher concentrations of Ca and Cl and much lower acidity. Dry deposition rates of several species were measured to artificially-generated dew and a dry surface. It was found that deposition rates were 2 to 20 times greater to the artificial dew than to the dry surface indicating that the presence of dew enhances both the retention of dry deposited particles and the absorption of water soluble gases. Measurement of the atmospheric concentrations of the depositing species permitted the calculation of deposition velocities for particulate... 

Dew Collection. In our previous work, natural dew was collected from a Teflon surface. The Teflon collector consisted of a sheet of aluminum backed FEP Teflon bonded to a 1 m copper plate mounted on a plywood base. The collector was tilted 30° from horizontal with the centerpoint 1 m above the ground. 

In these studies the rate of the mass and contact diameter of water and -octane drops placed on glass and Teflon surfaces were investigated. It was found that the evaporation occurred with a constant spherical cap geometry of the liquid drop. The experimental data supporting this were obtained by direct measurement of the variation of the mass of droplets with time, as well as by the observation of contact angles. A model based an the diffusion of vapor across the boundary of a spherical drop has been considered to explain the data. Further studies were reported, where the contact angle of the system was 9 < 99°. In these systems, the evaporation rates were found to be linear and the contact radius constant. In the latter case, with 9 > 99°, the evaporation rate was nonlinear, the contact radius decreased and the contact angle remained constant. 

FIG. 7 Feedback current-distance curves for a gallium SECM tip of 22-/xin diameter operating in amperometric mode in 0.1 M KC1 and 5 mM Ru(NH3)6C13 (a) Pt surface (b) Teflon surface. Pluses represent experimental data, while the solid lines are theoretical data. The tip potential was held at —0.6 V vs. Ag/AgCl reference electrode. (From Ref. 79.)... 

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