Mixed partition-adsorption system

Stability depends upon so many things that it is easy to alter its value. However, in most eases the general phenomenology versus formulation and eomposition is valid. The presenee of aleohol, partieularly an intermediate-solubility alcohol, such as rec-butanol or ter-pentanol, or a mixture of propanol and butanol, tends to reduce the interfacial adsorption of the surfactant, thus reducing all associated effects, in particular the repulsion that eontributes to stabilization. It is worth noting that the use of mixed surfae-tant systems, which is often advised in emulsion making manuals, can be detrimental in some eases in which a selective partitioning of surfactant species takes plaee (191, 192), and little surfactant is left at the interfaee. 

Sorption of Cu(tfac)2 on a column depends on the amount of the compound injected, the content of the liquid phase in the bed, the nature of the support and temperature. Substantial sorption of Cu(tfac)2 by glass tubing and glass-wool plugs was observed. It was also shown that sorption of the copper chelate by the bed is partialy reversible . The retention data for Cr(dik)3, Co(dik)3 and Al(dik)3 complexes were measured at various temperatures and various flow rates. The results enable one to select conditions for the GC separation of Cr, Al and Co S-diketonates. Retention of tfac and hfac of various metals on various supports were also studied and were widely used for the determination of the metals. Both adsorption and partition coefficients were found to be functions of the average thickness of the film of the stationary phase . Specific retention volumes, adsorption isotherms, molar heats and entropy of solution were determined from the GC data . The retention of metal chelates on various stationary phases is mainly due to adsorption at the gas-liquid interface. However, the classical equation which describes the retention when mixed mechanisms occur is inappropriate to represent the behavior of such systems. This failure occurs because both adsorption and partition coefficients are functions of the average thickness of the film of the stationary phase. It was pointed out that the main problem is lack of stability under GC conditions. Dissociation of the chelates results in a smaller peak and a build-up of reactive metal ions. An improvement of the method could be achieved by addition of tfaH to the carrier gas of the GC equipped with aTCD" orFID" . ... 

A useful classification of the various LC techniques is based on the type of distribution mechanism applied in the separation (see Table 1.2). In practice, most LC separations are the result of mixed mechanisms, e.g., in partition chromatography in most cases contributions due to adsorption/desorption effects are observed. Most LC applications are done with reversed-phase LC, i.e., a nonpolar stationary phase and a polar mobile phase. Reversed-phase LC is ideally suited for the analysis of polar and ionic analytes, which are not amenable to GC analysis. Important characteristics of LC phase systems are summarized in Table 1.3. 

The one-step procedures can be utilized for the separations of the sample components with similar dimensions but different adsorption characteristics in the given system gel-eluent. The non-adsorbed part of the sample is separated according to the steric exclusion mechanism, while the elution of the adsorbed part of the sample is retarded [26]. Another combined procedure utilizes the thermodynamic partition of the sample as the auxiliary separation mechanism the gel is either swollen by a solvent immiscible with eluent [27] or a mixed eluent is used, one component of which is preferentially sorbed within the gel matrix [28]. In both cases, the composition of the stagnant phase within the gel differs significantly from the composition of the mobile phase and thermodynamic partition takes place. The extent of the partition is controlled by the porous structure of the gel so that steric exclusion remains the principal separation mechanism. 

In order to readily control enthalpic interactions within the liquid chromatography coluimi, usually two-component mobile phases are utilized. One solvent suppresses and another one promotes enthapic interactions, either adsorption or enthalpic partition within the chromatographic systems. Mixing both solvents in... 

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