Sorption symmetrical

Table IV shows X-ray data (55) on the homogeneity of Pd-Ag films prepared by simultaneous evaporation from separate sources, either in conventional vacuum or in UHV, with the substrate maintained at 0°C. The second group of films was prepared using a stainless steel system incorporating a large (100 1/sec) getter-ion pump, sorption trap, etc., but deposited inside a glass vessel. By the tests of homogeneity adopted, alloy films evaporated in conventional vacuum were not satisfactory, i.e., the lattice constants were generally outside the limits of the experimental error, 0.004 A, and the X-ray line profiles were not always symmetrical. In contrast, alloy films evaporated in UHV were satisfactorily homogeneous. Further, electron micrographs showed that these latter films were reasonably unsintered and thus, this method provides clean Pd-Ag alloy films with the required characteristics for surface studies.

In all late-time regimes notably those represented by Eqs. (60), (62), (64) and (66) ideal kinetics is obeyed 144.15°.15i.154.159.l6l) whereas this is not so in the short-time regimes presented by Eqs. (59), (63) and (65) 150 ,51 154,163,64) (which convey essentially the same information).151 The short time behaviour of lower permeation curves represented by Eq. (61) appears to occupy an intermediate position, in the sense that ideal kinetics appears to be followed only to a first approximation151. The relation between permeation and symmetrical sorption indicated by Eq. (70) is also notable. The respective kinetics become very similar at long times154 as indicated by the relevant relations151) D2M = Ds = D7 = D8 and... 

The detailed study of Ref.150,151 indicates that the information about H(y) conveyed by different kinetic regimes is in part similar and in part different. The similarity between late time permeation and symmetrical sorption kinetics has already been pointed out above. Symmetrical sorption kinetics at both short and long times are shown to reflect primarily the properties of d2H/dy2 whereas short time unsymmetrical sorption is mainly sensitive to dH/dy. For more detailed information the original papers150,15l) should be consulted. 

Since the fraction of empty sites in a zeolite channel determines the correlation factor (Section 5.2.2), as is well known from single-file diffusion in the pores of a membrane, the strong dependence of the diffusion coefficients on concentration can be understood. This is why a simple Nernst-Planck type coupling of the diffusive fluxes (see, for example, [H, Schmalzried (1981)]) is also not adequate. Therefore, we should not expect that sorption and desorption are symmetric processes having identical kinetics. Surveys on zeolite kinetics can be found in [A. Dyer (1988) J. Karger, D.M. Ruthven (1992)]. 

In many circumstances, however, the assumption of equilibrium sorption is inappropriate. In many laboratory and field studies, contaminant concentration vs time profiles (breakthrough curves) have been observed that exhibited asymmetry. While the assumption of equilibrium sorption results in a prediction of relatively symmetric breakthrough curves, the sharp initial breakthrough and late-tailing curves that are frequently observed are indicative of rate-limited (non-equilibrium) transport [9]. Rate-limited sorption is also a... 

Most separations are performed using columns of resin and an elution procedure. The sample is introduced as a small band at the top of the column from where the various components are moved down the column at a rate depending on their selectivity coefficients. Sorption isotherms are approximately linear in dilute solutions so that elution peaks are symmetrical. Tailing can be expected at high concentrations as the isotherms curve towards the mobile phase concentration axis (p. 77). 

After sorption of toluene on Rb-X, the bands attributed to the asymmetric and symmetric CHj-stretching vibrations were observed at 12 and 20 cm lower wavenumbers than after sorption of toluene on HZSM5. This is interpreted as a weakening of the C-H bonds due to strong interactions of the methyl group with the lattice oxygens of Rb-X. 

Figure 4. Normalized, kl-weighted As-EXAFS spectra (a) and uncorrected Fourier transforms (FTs) (b) of scorodite (a crystalline ferric arsenate), an x-ray amorphous analog, and As(V) sorbed to amorphous hydrous ferric oxide (HFO), The EXAFS spectra can clearly be used to distinguish the coordination environment of arsenic in each of the materials. The highly symmetric local environment of arsenic in scorodite is shown in (c) each arsenic is surrounded by 4 Fe neighbors at a distance of 3.34 A. (see Table 2 for details of precipitate fits, and Table 3 for details of sorption sample fit). Reprinted from Foster (1999). The arrow highlights the region of particular difference among the three spectra. Peak positions in FTs are not corrected for phase-shift effects, and are therefore approximately 0.5 A shorter than the true distance.

Because the activities of species in the exchanger phase are not well defined in equation 2, a simplified model—that of an ideal mixture—is usually employed to calculate these activities according to the approach introduced bv Vanselow (20). Because of the approximate nature of this assumption and the fact that the mechanisms involved in ion exchange are influenced by factors (such as specific sorption) not represented by an ideal mixture, ion-exchange constants are strongly dependent on solution- and solid-phase characteristics. Thus, they are actually conditional equilibrium constants, more commonly referred to as selectivity coefficients. Both mole and equivalent fractions of cations have been used to represent the activities of species in the exchanger phase. Townsend (21) demonstrated that both the mole and equivalent fraction conventions are thermodynamically valid and that their use leads to solid-phase activity coefficients that differ but are entirely symmetrical and complementary. 

Sorption is the general term that refers to the interaction of a solute with the stationary phase, whether that interaction involves adsorption, ion-exchange, or gel permeation (size exclusion). When the relative concentrations of solute in the stationary and mobile phases are the same, independent of concentration (as is normally expected by reference to the of the solute), the peak shape will remain basically the same and will be symmetrical. This relationship, which describes amount of solute sorbed relative to concentration in mobile phase (at constant temperature), is the sorption isotherm. 

However, not all systems exhibit a linear isotherm when relatively strong interactions exist between solute and stationary phase, and relatively weak interactions exist between solutes themselves, there will initially be rapid sorption of solute onto the stationary phase until the stationary phase is covered by solute, at which point the uptake of solute will decrease. This means that the ATd of the solute is not constant across the peak, at low concentrations, will be large and this results m a peak shape that is not symmetrical but that tails. ... 

In RPLC, the solute continuously partitions between the stationary phase and the mobile phase. The nature of the partitioning between the two phases is very similar to partitioning between two immiscible liquids. For example, the process is noncompetitive and the sorption isotherms are hnear. As a result, peaks are usually symmetrical, and the separations are very reproducible. RPLC is by far the most popular hquid chromatographic technique currently in use. Other separation modes are usually considered only after RPLC fails to deliver desirable results. 

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