Retention adsorption-based

Note that the initial slope of the adsorption isotherm can be easily obtained from the knowledge of the retention time associated to a small injection performed on a column, as this retention time is given by tR = t0 ji + 1 - -k where /0 = V/Q is tiie zero-retention time based on the external bed porosity e (commonly, e is about 0.36-0.4). 

An alternative (or just different) description of HPLC retention is based on consideration of the adsorption process instead of partitioning. Adsorption is a process of the analyte concentrational variation (positive or negative) at the interface as a result of the influence of the surface forces. Physical interface between contacting phases (solid adsorbent and liquid mobile phase) is not the same as its mathematical interpretation. The physical interface has certain thickness because the variation of the chemical potential can have very sharp change, but it could not have a break in its derivative at the transition point through the interface. The interface could be considered to have a thickness of one or two monomolecular layers, and in RPLC with chemically modified adsorbents the bonded layer is a monomolecular layer that is more correctly... 

Yet another adsorption-based retention model similar to that of Snyder was proposed by Soczewinski [6] to describe the retention in NPC. It assumes that retention in NPC is the product of competitive adsorption between solute and solvent molecules for active sites on the stationary phase surface. The stationary-phase surface consists of a layer of solute and/or solvent molecules, but, unhke the former, the latter model assumes an energetically heterogeneous surface where adsorption occurs entirely at the high-energy active sites, leading to discrete, one-to-one complexes of the form... 

However, in normal phase adsorption systems (or liquid-solid chromatography) the interaction of the mobile phase solvent with the solute is often less Important than the competing Interactions of the mobile phase solvent and the solute with the stationary phase adsorption sites. Solute retention is based upon a displacement mechanism. Multicomponent mobile phases and their combination to optimize separations in liquid-solid chromatography have been studied in detail (31-35). Here, solvents are classified as to their interaction with the adsorption surface (Reference 32, in particular) ... 

The role of mobile phase in the adsorption and enthalpic partition processes is substantially different. As explained, solvent quality toward macromolecules only marginally affects their adsorption on a given surface. Solvent strength controls the adsorption based retention of polymer species in the particular column pack-... 

The general characteristics of liquid-solid adsorption based elution chromatography developed in Sections 7.1.5.1.1 and 7.1.5.1.4 are also valid here. For example, the retention volume Vr, for species i in a column having a stationary phase volume Vs is related to the mobile-phase volume within the column, Vm, and the solute partition coefBdent k,i by relation (7.1.99h) ... 

The technique of IGC may be employed to obtain acid-base information, as suggested by Schultz and Lavielle [99], by using acid and base probe gases on a solid for which the alkane line has already been obtained. If acid-base interaction is involved in the adsorption, the retention volume should be greater than that corresponding to the dispersion force interaction alone, which should be the same as that of the equivalent alkane , i.e. the hypothetical alkane for which the value... 

Studies on mechanisms are described by Balzer [192]. In the case of anionics the residual oil in the injection zone is removed via displacement into the adjacent reservoirs ether carboxylates show their good adaptation to differences in temperature and salinity. Further it was found from interfacial tension measurements, adsorption and retention studies, and flooding tests that use of surfactant blends based on ether carboxylates and alkylbenzensulfonates resulted... 

NR, styrene-butadiene mbber (SBR), polybutadiene rubber, nitrile mbber, acrylic copolymer, ethylene-vinyl acetate (EVA) copolymer, and A-B-A type block copolymer with conjugated dienes have been used to prepare pressure-sensitive adhesives by EB radiation [116-126]. It is not necessary to heat up the sample to join the elastomeric joints. This has only been possible due to cross-linking procedure by EB irradiation [127]. Polyfunctional acrylates, tackifier resin, and other additives have also been used to improve adhesive properties. Sasaki et al. [128] have studied the EB radiation-curable pressure-sensitive adhesives from dimer acid-based polyester urethane diacrylate with various methacrylate monomers. Acrylamide has been polymerized in the intercalation space of montmorillonite using an EB. The polymerization condition has been studied using a statistical method. The product shows a good water adsorption and retention capacity [129]. 

The competition model and solvent interaction model were at one time heatedly debated but current thinking maintains that under defined r iitions the two theories are equivalent, however, it is impossible to distinguish between then on the basis of experimental retention data alone [231,249]. Based on the measurement of solute and solvent activity coefficients it was concluded that both models operate alternately. At higher solvent B concentrations, the competition effect diminishes, since under these conditions the solute molecule can enter the Interfacial layer without displacing solvent molecules. The competition model, in its expanded form, is more general, and can be used to derive the principal results of the solvent interaction model as a special case. In essence, it seems that the end result is the same, only the tenet that surface adsorption or solvent association are the dominant retention interactions remain at variance. 

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