Bulk mobility

Attainment of a maximum double bond conversion is typical in multifunctional monomer polymerizations and results from the severe restriction on bulk mobility of reacting species in highly crosslinked networks [26]. In particular, radicals become trapped or shielded within densely crosslinked regions known as microgels, and the rate of polymerization becomes diffusion limited. Further double bond conversion is almost impossible at this point, and the polymerization stops prior to 100% functional group conversion. In polymeric dental composites, which use multifunctional methacrylate monomers, final double bond conversions have been reported ranging anywhere from 55-75% [22,27-29]. 

In this context the lipase was immobilized on a support which also adsorbed water and propionic acid. During the reaction, the water caused a decrease of the reaction rate. While the water adsorption on the catalyst results in a reversible decrease of the enzyme activity, an excessive accumulation of water in the bulk mobile phase resulted in rapid irreversible deactivation of the enzyme. 

Peptides larger than 10 to 20 residues adopt conformations in solution through the interplay of hydrogen bonding, electrostatic and hydrophobic interactions, positioning of polar residues on the solvated surface of the polypeptide, and sequestering of hydrophobic residues in the nonpolar interior. Protein shape is dynamic, changing continuously in response to the solvent environment. The retention process in RPLC is initiated as the protein approaches the stationary-phase surface. Structured water associated at the phase surface and adjacent to hydrophobic contact surfaces on the polypeptide is released into the bulk mobile... 

FIGURE 5.24 (See color insert following page 280.) Schematic comparison of the adsorption mechanisms of a solute from aqueous solutions of (a) methanol and (b) acetonitrile onto a RPLC material. Three different phases (bulk mobile phase, the adsorbed mono- or multilayer of organic modifier molecules, and the C is phase) are involved in the chromatographic system. The solute is represented by small ovals. (Reproduced from Gritti, F. and Guiochon, G, Anal. Chem., 77, 4257, 2005. With permission.)... 

It has been considered that HILIC stationary phases absorb or imbibe water and that partition of analytes occurs between this layer of water and water in the bulk mobile phase. This mechanism occurs alongside ionic retention, as many of the commonly used HILIC stationary phases (as is the case with bare silica) have ion-exchange properties [34]. Partitioning into the water layer may explain the retention... 

The dependence of slip on the interaction strength of the surface and liquid was studied early on by Tolstoi [3], later revisited by Blake [12], a model that is linked to interfacial viscosity. Tolstoi modeled the surface using Frenkel s model for the bulk mobility of a liquid molecule [38],... 

Two limiting mechanisms for solute retention can be imagined to occur in RPC binding to the stationary phase surface or partitioning into a liquid layer at the surface. In the previous treatment we assumed that retention is caused by eluite interaction with the hydrocarbonaceous surface, i.e., the first type of mechanism prevails. When the eluent is a mixed solvent, however, the less polar solvent component could accumulate near the apolar surface of the stationary phase. In the extreme case, an essentially stagnant layer of the mobile phase rich in the less polar solvent could exist at the surface. As a result eluites could partition between this layer and the bulk mobile phase without interacting directly with the stationary phase proper. 

Several mass balance equations are written for the kinetics of each step as the analyte is passing through the porous stationary phase. For the bulk mobile phase in the interstitial volume, the following differential mass balance equation is written... 

For rapid confirmation of the molecular mass of a recovered peptide following RPC purification, either ESI-MS or MALDI-MS procedures are now indispensable. Studies on the interaction of peptides with solvent molecules, ions, and free or immobilized ligands in chromatographic or electrophoretic environments have, however, been largely confined to methods that examine global properties of different peptides in the bulk mobile phase. In... 

Sorption of the IL cation anion partners also modifies the stationary phase which can introduce an ion-exchange type of retenhon. Further, either of the IL partners in the bulk mobile phase can serve as an ion-pairing agent for ionized analytes [44]. The extent to which any of these roles contribute to overall retention likely depends on the structure of the analytes as well as the lipophilicity of the cahon, charge diffusivity of the anion, and concentration of the IL in the mobile phase. 

For unsupported catalysts, where particle sizes are typically an order of magnitude larger than those for supported catalysts, the mobility of various species in the bulk structure may be of interest when considering how the bulk structure and composition are reflected in the surface properties of the particle. In addition, bulk mobility is an important consideration in the understanding of solid state reactions and phenomena such as sintering. 

In the case of gel permeation or size-exclusion HPLC (HP-SEC), selectivity arises from differential migration of the biomolecules as they permeate by diffusion from the bulk mobile phase to within the pore chambers of the stationary phase. Ideally, the stationary phase in HP-SEC has been so prepared that the surface itself has no chemical interaction with the biosolutes, with the extent of retardation simply mediated by the physical nature of the pores, their connectivity, and their tortuosity. In this regard, HP-SEC contrasts with the other modes of HPLC, where the surfaces of the stationary phase have been deliberately modified by chemical procedures by (usually) low molecular weight compounds to enable selective retardation of the biosolutes by adsorptive processes. Ideally, the surface of an interactive HPLC sorbent enables separation to occur by only one retention process, i.e., the stationary phase functions as a monomodal sorbent. In practice with porous materials, this is rarely achieved with the consequence that most adsorption HPLC sorbents exhibit multimodal characteristics. The retention behavior and selectivity of the chromatographic system will thus depend on the nature and magnitude of the complex interplay of intermolecular forces... 

The separation of anions by the use of a cationic micellar mobile phase results in a high degree of flexibility not available from other methods of ion chromatography. The importance of micelles in the mobile phase lies in their ability to participate in the partitioning mechanism. The three equilibria involved in micellar chromatography are schematically represented in Figure 1. The elution behaviour of the anionic solute depends on three partition coefficients K p, the partition coefficient between the bulk mobile phase and and the micelle K nn the partition coefficient between the bonded phase and the micelle and K mpi the partition coefficient between the bonded phase and the bulk mobile phase. 

The use of cyclodextrins as the mobile phase components which impart stereoselectivity to reversed phase high performance liquid chromatography (RP-HPLC) systems are surveyed. The exemplary separations of structural and geometrical isomers are presented as well as the resolution of some enantiomeric compounds. A simplified scheme of the separation process occurring in RP-HPLC system modified by cyclodextrin is discussed and equations which relate the capacity factors of solutes to cyclodextrin concentration are given. The results are considered in the light of two phenomena influencing separation processes adsorption of inclusion complexes on stationary phase and complexation of solutes in the bulk mobile phase solution. 

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