Media attributes

Numerous application examples are related to gel filtration (e.g., see Hagel, 1989 Hagel and Janson, 1992 Pharmacia, 1991). A selected number of applications are discussed with respect to their goals and to types of methods and SEC media attributes, which impact the selection and or construction of a suitable SEC column. Specific examples of these various applications types are given later under Sections II,C and III. The optimization of running conditions to achieve the desired results are discussed in Section VI. 

The older literature on the electrochemistry of dioxygen in acidic media attributes the difference between the thermodynamic potential for the four-electron reduction (02/H20, +1.23 V vs. NHE Table 9.3) and the observed value at a freshly activated platinum electrode [+0.67 V vs. NHE, Eq. (9.13)] to overvoltage (or kinetic inhibition). Likewise, the difference between the thermodynamic potential for the two-electron reduction (02/H00H, +0.70 V vs. NHE, Table 9.3) and the observed value at passivated electrodes [+0.05 V vs. NHE, Eq. (6.12)] was believed to be due to the kinetic inhibition of the two-electron process. 

Salomon and Clark (1977) called many studies of media research with media, meaning that some other issue was really at the heart of any differences observed between groups of students. According to Clark, some studies used the medium as a conveyance of some other treatment, and sometimes the difference in medium was mistakenly given credit for any benefits to students. Media attributes, rather than media per se, were dealt with, and a strong need for conceptualizing these attributes in terms of their effects and functions was suggested, wrote Salomon and Clark (1977, p. 116). 

Clark (1987) and Salomon (1979) continued to recommend that researchers study attributes of media and how these attributes affect the processing of information in learning. Clark described the promise of an approach based on media attributes based on three expectations ... 

According to Clark, only the second expectation was fulfilled, implying that studies of media attributes may contribute to instructional design but not to theory development. 

Though by no means a complete theory, this is at least a reasonable explanation of the Knudsen minimum, and it then remains to explain why the minimum is not observed for flow through porous media. Pollard and Present attributed this to the limited length/diameter ratio of the channels in a typical porous medium and gave a plausible argument in favor of this view. 

The catalytic effect on unimolecular reactions can be attributed exclusively to the local medium effect. For more complicated bimolecular or higher-order reactions, the rate of the reaction is affected by an additional parameter the local concentration of the reacting species in or at the micelle. Also for higher-order reactions the pseudophase model is usually adopted (Figure 5.2). However, in these systems the dependence of the rate on the concentration of surfactant does not allow direct estimation of all of the rate constants and partition coefficients involved. Generally independent assessment of at least one of the partition coefficients is required before the other relevant parameters can be accessed. 

Phenylboronic acid. The orientation of nitration in phenylboronic acid is very susceptible to changes in the medium (table 5.8). The high proportion of o-substitution in acetic anhydride is not attributable to a specific o-reaction, for the nt -ratios of the last tabulated pair of results are not constant. The marked change in the ratio was considered to be due to the formation in acetic anhydride of a complex, as illustrated below, which is 0 -orienting and activated as a result of the -t-1 effect. This species need only be formed in a small concentration to overwhelm... 

The effects of uv radiation on V/-nitroso compounds depend on the pH and the medium. Under neutral conditions and ia the absence of radical scavengers, these compounds often appear chemically stable, although the E—Z equiUbrium, with respect to rotation around the N—N bond, can be affected (70). This apparent stabiUty is due to rapid recombination of aminyl radicals and nitric oxide [10102-43-9] formed duting photolysis. In the presence of radical scavengers nitrosamines decay rapidly (71). At lower pH, a variety of photoproducts are formed, including compounds attributed to photoelimination, photoreduction, and photo-oxidation (69). Low concentrations of most nitrosamines, even at neutral pH, can be eliminated by prolonged kradiation at 366 nm. This technique is used ki the identification of /V-nitrosamines that are present ki low concentrations ki complex mixtures (72). 

Mechanical and Chemical Properties. Colorants, especially pigments, can affect the tensile, compressive, elongation, stress, and impact properties of a polymer (5). The colorants can act as an interstitial medium and cause microcracks to form in the polymer colorant matrix. This then leads to degradation of the physical properties of the system. Certain chemicals can attack colorants and there can be a loss of physical properties as well as a loss of the chromatic attributes of the colorant. Colorants should always be evaluated in the resin in which they will be used to check for loss of properties that ate needed for the particular appHcations. 

For filter-medium filtration, attributes 3, 4, 5, 8, and 9 of the preceding list apply and must have added to them (o) ability to retain the solids required, (b) freedom from discharge of lint or other adulterant into the filtrate, and (c) ability to plug slowly (i.e., long life). 

Thioketals are readily formed by acid-catalyzed reaction with ethane-dithiol. Selective thioketal formation is achieved at C-3 in the presence of a 6-ketone by carrying out the boron trifluoride catalyzed reaction in diluted medium. Selective protection of the 3-carbonyl group as a thioketal has been effected in high yield with A" -3,17-diketones, A" -3,20-diketones and A" -3,l 1,17-triones in acetic acid at room temperature in the presence of p-toluenesulfonic acid. In the case of thioketals the double bond remains in the 4,5-position. This result is attributed to the greater nucleophilicity of sulfur as compared to oxygen, which promotes closure of intermediate (66) to the protonated cyclic mercaptal (67) rather than elimination to the 3,5-diene [cf. ketal (70) via intermediates (68) and (69)]." " ... 

The resolution required in any analytical SEC procedure, e.g., to detect sample impurities, is primarily based on the nature of the sample components with respect to their shape, the relative size differences of species contained in the sample, and the minimal size difference to be resolved. These sample attributes, in addition to the range of sizes to be examined, determine the required selectivity. Earlier work has shown that the limit of resolvability in SEC of molecules [i.e., the ability to completely resolve solutes of different sizes as a function of (1) plate number, (2) different solute shapes, and (3) media pore volumes] ranges from close to 20% for the molecular mass difference required to resolve spherical solutes down to near a 10% difference in molecular mass required for the separation of rod-shaped molecules (Hagel, 1993). To approach these limits, a SEC medium and a system with appropriate selectivity and efficiency must be employed. 

A theoretical model whereby maximum peak capacity could be achieved by the use of 3-D planar chromatographic separation was proposed by Guiochon and coworkers (23-27). Unfortunately, until now, because of technical problems, this idea could not be realized in practice. Very recently, however, a special stationary phase, namely Empore silica TLC sheets, has now become available for realization of 3-D PC. This stationary phase, developed as a new separation medium for planar chromatography, contains silica entrapped in an inert matrix of polytetrafluoroethy-lene (PTFE) microfibrils. It has been established that the separating power is only ca. 60% of that of conventional TLC (28) this has been attributed to the very slow solvent migration velocity resulting from capillary action. 

Figures 6 and 8 show, generally that the conversion first increases fast then slowly with the dose. This is in agreement with the findings of Azzam [48] and Siyam [61]. The significant increase in the conversion percentage is attributed to the gel-effect [51,65]. In the gel-state, as the conversion percentage increases the viscosity of the medium is highly increased and the growing poly-...

The increase in the temperature reduces the viscosity of the polymerization medium which increases the termination reactions. This is attributed to an increase in chain transfer reactions higher than that of propagation reactions [16,51]. Consequently, the weight-average molecular weight of the formed polymer decreases. 

The grafting in aqueous medium is higher than in case of the 97% methanol medium. This can be attributed to the lower dissolution of the initiator in the solvent medium. On the other hand, grafting in methanol-water medium (8 22) produces higher grafting than in water medium. 

The positive bromination of aromatics ethers was first studied by Bradfield et al.193 and by Branch and Jones194. The reaction of hypobromous acid in 75 % aqueous acetic acid with benzyl 4-nitrophenyl ether and 4-nitrophenetole at 20 °C was very rapid and approximately second-order193. The value of k2/[H+] remained constant in the [H+] range 0.005-0.090 M for the effect of added mineral acids on the bromination of 4-nitroanisole and 4-nitrophenetole (at 19.8 °C)194. The variation in reaction rate with the percentage of acetic acid in the medium was also studied and showed a large increase in the 0-10 % range with a levelling off at approximately 25 % acetic acid (Table 52) this was attributed... 

In contrast to these results, Stanley and Shorter207 found the catalytic effect of perchloric acid to be large in 40 % aqueous acetic acid, the difference in behaviour from 75 % acid being attributed to the presence of a much higher concentration of hypochlorous acid in the more aqueous medium. 

Extension of these studies to formic acid media (containing 4 vol. % ethylene glycol and 1.3 vol. % water) showed that for protodeboronation of 4-methoxy-benzeneboronic acid at 25 °C) rates were invariant of a tenfold variation in acidity produced by adding sodium formate (0.05-0.20 M) to the medium (Table 194), and in this range the concentration of molecular formic acid is essentially constant. This was, therefore, assumed to be the reactive species. At higher acidities the rate increased, which was attributed to the increase in concentration of hydronium ions and protonated formic acid ions which bring about reaction more readily625. 

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