Partitioning mechanism

From the description of the kinetic partitioning mechanism (KPM) given above it follows that generically the time dependence of the fraction of molecules that have not folded at time t, is given by... 

Otherwise, prediction of partition coefficient from the known physical properties of the solute and solvents have been developed by Taft, Abraham, Kamlet, and coworkers [207-210]. They have introduced solvatochromic parameters to assess the intermolecu-lar forces governing the partition mechanisms of neutral organic solutes nonspecific... 

The retention mechanism of organic solutes by porous polymer beads remains ambiguous [478]. At low temperatures adso tion dominates but at higher temperatures the polymer beads could behave as a highly extended liquid with solvation interactions. The evidence for a partition mechanism is not very strong and its importance, at present, remains speculative. Like other adsorbents it has proven possible to control retention and enhance efficiency by diluting porous polymers with an inert support material (479). 

Solute diffusion through hydrogels has been described in terms of two basic mechanisms, the partition and pore mechanisms. The partition mechanism... 

Oligosaccharides and related materials can be separated by a partition mechanism on ion-exchange columns where water/alcohol mobile... 

The C-Type isotherm indicates partitioning mechanisms whereby adsorptive ions or molecules are distributed or partitioned between the interfacial phase and the bulk solution phase without any specific bonding between the adsorbent and the adsorbate. 

Thus, we now focus on the partitioning mechanism in terms of both thermodynamic and modeling approaches as follows. 

The evidence presented in the literature on the dominance of a partition mechanism in the process of adsorption of a nonionic organic pollutant onto SOM does not mean, for instance, that the physical adsorption model based on weak chemical forces of interaction can be ignored or excluded [82,99,107,109, 114,115,183,192,204-218]. The following summary is a critical evaluation for reconsidering the universal applicability of the partitioning model to various nonionic compounds onto SP0M [82,84,92,103,113,130,182,184,185,187,193, 219,220,222-226] ... 

If sorption and partitioning mechanisms dominate the fate of PAHs in soils, then the PAHs remaining in SOM should be primarily parent compounds which are sorbed to organic surfaces. Slow rates of desorption become the primary limitation for biodegradation however, the presence of adapted PAH-minerali-zing communities in contaminated soils suggests that PAH desorption occurs at sufficient rates over time to establish and maintain adapted microbial communities [36,264,356]. PAH biodegradation appears to proceed, albeit at much slower rates than predicted or desired [264,278,279]. 

Two types of system are used for ion-pair liquid chromatography. When polar stationary phase materials, such as silica gel, are used an ion-pair partition mechanism is applied. When non-polar stationary phase materials, such as octadecyl-bonded silica gel and polystyrene gel, are employed a paired-ion adsorption mechanism is involved. The former is called normal-phase ion-pair partition liquid chromatography, and the latter is called reversed-phase ion-pair liquid chromatography. 

CSPs has, overall, a hydrophobic character (very similar to RP phases with C4-C8 ligands) which stems from contributions of the chiral selectors itself and (capped) linker groups (only a portion of the linkers are utilized for selector attachment) which constitutes a kind of hydrophobic basic layer on the support surface. Hence under typical RP-conditions, hydrophobic interactions between lipophilic residues of the solute and hydrophobic patches of the sorbent may be active and thus a reversed-phase like partition mechanism may be superimposed upon the primary ion-exchange process k = A rp -I- A ix). This A Rp-retention contribution may be especially important for eluents with high aqueous content. 

Li, L., Wang, L., Han, S., and Zhang. Z. Comparison of four methods of predicting newly measured octanol/water coefficients (log Ko ) for heterocyclic nitrogen compounds and the partition mechanism. Environ. Toxicol. Chem., 18(12) 2723-2728. 1999. 

This definition erases some of the differences between the interpretation ul experimental data by different workers. Fur instance, I lie conclusions of Scott and Kucera (2/0), which apparently support a partitioning mechanism, are also consistent with the adsorption model in the view of the above definition. 

The effect of halide, cyanate, cyanide, and thiocyanate ions on the partitioning of Hg in [BMIM][PF6]/aqueous systems (Figure 3.3-2) has been studied [8]. The results indicate that the metal ion transfer to the IL phase depends on the relative hydrophobicity of the metal complex. Hg-I complexes have the highest formation constants, decreasing to those of Hg-F [42]. Results from pseudohalides, however, suggest a more complex partitioning mechanism, since Hg-CN complexes have even higher formation constants [42], but display the lowest distribution ratios. 

As will be shown later, the former three mechanisms mentioned above are applicable to TLC separation of polymers. From the standpoint of TLC applied to polymer separation, the partition mechanism may be better expressed by phase-separation or precipitation mechanism, as will be explained in Section II.3. It should be noted that all these mechanisms are generally present during a chromatographic separation. Therefore, one mechanism should be made to be predominant for a given separation aim. This can, in principle, be done by properly selecting the developer and adsorbent. However, such a selection is the major problem in application of TLC, especially, to polymer separation, and the following three sections will be devoted to describing the rules that have been established to solve this problem. 

The other is the aggregate-partitioning mechanism [2,15], where it is considered that the SS molecules work as J-aggregate partitioners, which decrease the size of the aggregates. This causes an increase in the fluorescence lifetime and/or an increase in the electron injection rate, and hence an increase in the quantum yield of the electron injection [2], In the present work the sensitization and supersensitization processes are described below by the hole-trapping supersensitization. 

The activity of the silica is an important parameter. When silica is wet by several layers of water, pores are filled and solutes cannot undergo adsorption. As a consequence these are retained only slightly. They can even undergo partition mechanism. Figure 7 demonstrates the dramatic influence of water on retention. Reproducible retention can be achieved by Engelhardt s method (17) One tank is filled with dry solvent and another tank is filled with fully wet solvent. A proportioning valve allows for the exact mixing, and the water content is controlled. A moisture... 

The retention mechanism is not yet fully understood. The solvophobic theory does not account for any interaction in the stationary phase, which plays a passive role. The partition mechanism as described by Dill and Dorsey (27) is generally accepted. The most relevant feature is the linear plot of In k versus carbon number in a homologous series (Fig. 10), which is similar to what is observed in isothermal gas-liquid chromatography. Retention is governed mainly by hydro-... 

There are two ways to classify liquid chromatographic methods. The first and more common classification is based on the mechanism of retention, and from this the chromatographic modes discussed in Chapter 2 are derived. For example, the normal-phase mode can be performed by taking advantage of either the adsorption mechanism or the partition mechanism. The gel-filtration mode is performed using the mechanism of size exclusion. The second classification discussed below is based on the separation principle and is found mostly in the literature published before the 1990s. 

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