Adsorption solvent-free systems

In a competitive hydrogenation of two substrates in a solvent-free system the relative reactivity is again given by Eq. (8). On the other hand, however, the properties of the bulk phase vary with each change in the substrate, and this change may of course variously affect the rate constants and the adsorption coefficients. Moreover, the rate constant of A cannot be directly measured in the presence of an unsaturated compound B. As a consequence, substitution of /cah and knu into Eq. (8) instead of and k, yields the values Ka/Kb subjected to the same inaccuracy. In this case, therefore, the re-... 

Cerveny et al. (100 report an investigation of the hydrogenation of 12 olefinic substrates in the liquid state with 5% Pt on silica gel as catalyst under usual conditions and without solvents. The reaction rates related to 2,3-dimethyl-2-butene and the relative adsorption coefficients obtained in systems of various pairs of substrates and by recalculation using Eq. (25) to 2,3-dimethyl-2-butene are given in Table IV. Comparison between the measured reaction rates and the rates of hydrogenation in solvents (71 has revealed that relations existing between the rates of hydrogenation in a solvent-free system approximately correspond to those determined in... 

Relative Hydrogenation Rates and Adsorption Coefficients of Substrates on Pt in Solvent-Free Systems... 

The measured data also were used (700) in a quantitative representation of the effect of structure on the reactivity and adsorptivity of substrates by means of the Taft-Pavelich equation (22). The adsorption data suffered from a larger scatter than the rate data. No substrate or substituent could be detected that would fail to satisfy completely the correlation equations. In the correlation of the initial reaction rates and relative adsorption coefficients the parameter p was negative, while the parameter S was positive. In correlations of the reaction rates obtained by the hydrogenation of a similar series of substrates on the same catalyst in a number of solvents, the parameters p and had the same sign as in the hydrogenation in solvent-free systems, while in the correlation of the adsorption coefficients the signs of the parameters p and in systems with solvents were opposite to those in solvent-free systems. This clearly indicates that solvents considerably affect the influence of the structure of substrates on their reactivity. 

In the 1990s, Pawliszyn [3] developed a rapid, simple, and solvent-free extraction technique termed solid-phase microextraction. In this technique, a fused-silica fiber is coated with a polymer that allows for fast mass transfer—both in the adsorption and desorption of analytes. SPME coupled with GC/MS has been used to detect explosive residues in seawater and sediments from Hawaii [33]. Various fibers coated with carbowax/divinylbenzene, polydimethylsiloxane/divinylbenzene, and polyacrylate are used. The SPME devices are simply immersed into the water samples. The sediment samples are first sonicated with acetonitrile, evaporated, and reconstituted in water, and then sampled by SPME. The device is then inserted into the injection port of the GC/MS system and the analytes thermally desorbed from the fiber. Various... 

The comparison mentioned under 2. will result in knowledge on which method can reliably be used under which conditions. As other methods might be simpler (in terms of man-hours and equipment needed to perform the measurement) compared to the Guideline or produce online results on-site, we have named them shortcut methods . Examples of these methods are a) the solid phase adsorption (SPA) method developed by KTH (7], b) a number of solvent-free tar collection systems used by BTG, BEF, IGT [8-10] and c) the FID online tar analysing method under development at the University of Stuttgart (11,12]. Currently it is unknown under which conditions these methods give reliable results, for example it is unknown whether the SPA method can be used for updraft gasifier tars and at which conditions the solvent-free methods fail to collect ail tars, for instance as a result of aerosol formation. 

In headspace SPME, there are two processes involved the release of analytes from their matrix and the adsorption of analytes by the liber coating. The volatile organic analytes are extracted, concentrated in the coating and transferred to the analytical instrument for desorption and analysis. In comparison to well-established techniques, SPME is inexpensive, solvent free, and convenient. In addition, because relatively mild conditions can be used, i.e., systems at equilibrium and temperatures less than 50°C, SPME gives a better quantitative estimate of the flavor profile. ... 

Use of the canister as an adsorption vessel without local desorption is a "carbon shuttle." It s simply a method of transporting artivated carbon from a supplier to a customer, and returning it to the supplier after it has adsorbed solvent fumes to the extent possible. Essentially, it is a system for "toll processing" " of the activated carbon, and not the solvent — because an equivalent quantity of solvent-free activated carbon is returned to the customer, but no solvent is recovered or returned to the customer. 

Solid-phase microextraction (SPME) is a solvent-free sample preparation technique. The volume of the extraction phase is very small compared to the sample volume. The extraction is not exhaustive, but is based on equilibrium between the sample and the extraction phase, which is located on a fiber. SPME involves an adsorption step of the analyte, from a gas headspace or in a liquid sample (direct immersion), and a desorption step, which often is coupled directly with injection in the analytical system. Although SPME is mainly used in combination with GC, it has also been automated for HPLC. Eigure 9.10 shows a schematic representation of an SPME device. 

This work deals with the statistical description of poly(dimethylsiloxane) chains adsorbed upon the surface of silica particles within silica-siloxane mixtures. In these systems, the incorporation of fumed silica into the PDMS polymer melt is obtained by mechanical mixing. Subsequently, the adsorption which occurs through the formation of hydrogen bonds between the oxygen atoms on the polymer chains and the silanol groups located on the silica s surface is a solvent free or melt adsorption process. Consequently, the law of adsorption observed from these systems is specific to such a process. 

The law of adsorption observed from silica-PDMS mixtures is specific to the solvent free or melt saturated adsorption that occurs within these systems. The number of monomeric units of one chain fixed onto the silica surface is proportional to the square root of the number of monomers. As a direct consequence of these particular statistics and the saturation of the... 

Thus, the spacing of the chains relative to the neutral, free, swollen size of the buoy blocks is, for a given chemical system and temperature, a unique function of the solvent-enhanced size asymmetry of the diblock polymer and a weak function of the effective Hamaker constant for adsorption. The degree of crowding of the nonadsorbing blocks, measured by a decrease in the left-hand side of Eq. 28, increases with increasing asymmetry of the block copolymer. 

Alternatively, it has been found that the Galvani potential of zero charge, in the absence of specific adsorption, equals zero. This means that there is no specific orientation of the molecules of both solvents, and the dipolar part of the Galvani potential, Eq. (12), is zero [8,22,41]. The observed discrepancies between the results of various measurements in different ITIES systems have been mainly caused by the specific adsorption [8]. Recently, the analysis of thermodynamic and free charge potentials at ITIES was performed by Volkov [42]. 

Using this approach, a model can be developed by considering the chemical potentials of the individual surfactant components. Here, we consider only the region where the adsorbed monolayer is "saturated" with surfactant (for example, at or above the cmc) and where no "bulk-like" water is present at the interface. Under these conditions the sum of the surface mole fractions of surfactant is assumed to equal unity. This approach diverges from standard treatments of adsorption at interfaces (see ref 28) in that the solvent is not explicitly Included in the treatment. While the "residual" solvent at the interface can clearly effect the surface free energy of the system, we now consider these effects to be accounted for in the standard chemical potentials at the surface and in the nonideal net interaction parameter in the mixed pseudo-phase. 

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