Adsorption critical point

The elution behavior of various polymers near their critical adsorption point with silica gel packings and various eluents has been studied (12). It was of interest to apply hybrid column systems composed of active ( critical ) packings (silica gels) in combination with nonactive (nonadsorptive) PS/DVB and DVB-based gels. Some PS/DVB and DVB gels exhibited rather strong... 

It is evident from these results that the interactive properties of the investigated SEC PS/DVB or DVB gels are very different. Because polar electroneutral macromolecules of PMMA were more retained from a nonpolar solvent (toluene) than from polar ones (THF, chloroform), we conclude that the dipol-dipol interactions were operative. Columns No. 1 and No. 2 were very interactive and can be applied successfully to LC techniques that combine exclusion and interaction (adsorption) mechanisms. These emerging techniques are LC at the critical adsorption point (18), the already mentioned LC under limiting conditions of adsorption (15,18), and LC under limiting conditions of desorption (16). In these cases, the adsorptivity of the SEC columns may even be advantageous. In most conventional SEC applications, however, the interactive properties of columns may cause important problems. In any case, interactive properties of SEC columns should be considered when applying the universal calibration, especially for medium polar and polar polymers. It is therefore advisable to check the elution properties of SEC columns before use with the... 

The principle of the liquid chromatography under critical conditions (LC CC) was elucidated in Section 16.3.3. The mutual compensation of the exclusion—entropy and the interaction—enthalpy-based retention of macromolecules can be attained when applying in the controlled way the interactions that lead to either adsorption or enthalpic partition. The resulting methods are called LC at the critical adsorption point (LC CAP) or LC at the critical partition point (LC CPP), respectively. The term LC at the point of exclusion-adsorption transition (LC PEAT) was also proposed for the procedures employing compensation of exclusion and adsorption [161]. It is anticipated that also other kinds of enthalpic interactions, for example the ion interactions between column packing and macromolecules can be utilized for the exclusion-interaction compensation. 

Kitayama, T., Janco, M., Ute, K., Niimi, R., Hafada, K., and Berek, D., Analysis of Poly(ethyl methacrylate)s by On-line Hyphenation of Liquid Chromatography at the Critical Adsorption Point and Nuclear Magnetic Resonance Spectroscopy, Anal Chem., 71, 1518, 2000. 

Berek, D. Janco, M. Meira, G.R. Liquid chromatography of macromolecules at the critical adsorption point, n. Role of column packing Bare silica gel. J. Polym. Sci. Part A Polym. Chem. 1998, 36 (9), 1363-1371. 

Adsorption of polymers on surfaces plays a key role in numerous technological applications and is also relevant to many biological processes. During the last three decades it has been constantly a focus of research interest. The theoretical studies of the behavior of polymers interacting with solid substrate have been based predominantly on both scaling analysis [49] as well as on the self-consistent field (SCF) approach [50]. The close relationship between theory and computer experiments in this field [27, 51] has proved especially fruitful. Most investigations focus on the determination of the critical adsorption point (CAP) location and on the scaling behavior of a variety of quantities below, above, and at the CAP. 

Kitayama, T. Janco, M. Ute, K. Niimi, R. Hatada, K. Analysis of poly(ethyl methacrylate)s by on-hne hyphenation of liquid chromatography at the critical adsorption point and nuclear magnetic resonance spectroscopy. Anal. Chem. 2000, 72, 1518-1522. 

This mode is called liquid adsorption chromatography at the critical adsorption point (LACCC). The polymeric nature of the sample (that is, the repeating units) does not contribute to the retention of the species. Only defects (such as end groups, comonomers, branching points, etc.) contribute to the separation of the molecules. Figure 3 illustrates this behavior and shows the dependence of retention volume on the molar mass for the different modes of chromatography. 

The requirement of critical conditions is that all adsorbed structural units are statistically independent of each other and their interaction can be described by a single, effective interaction energy. These conditions are not fulfilled for segmented copolymers, as the adsorbed monomers of one type provoke the adsorption of the neighboring monomer units of the same type. That is why segmented copolymers have no critical adsorption point. As a consequence, the retention of copolymers is influenced by microstructure. Therefore, copolymers of identical composition but different microstructures can be separated by GELC, with the more blocky structures eluting later than the random ones [30]. 

Liquid chromatography at the critical adsorption point (LC LCS) was investigated. The solubility of macromolecules, the sensitivity of LC LCS to such variables as the injection volume, concentration injected, mobile phase flow rate and mixing between the injection zone, were investigated, using polymethyl methacrylate (PMMA) and polyacrylamide standards. It is proposed that using tetrahydrofuran and n-hexane as eluents for PMMA, at the LC LCS, limited polymer insolubility or even local precipitation is combined with the size exclusion of macromolecules. 12 refs. 

The role of column packing pore size in liquid chromatography at the critical adsorption point was investigating using three silica gels and a series of polymethyl methacrylates (PMMA), using toluene, tetrahydrofuran (THF) or their mixtures as eluents. 

Toluene strongly promotes adsorption of PMMAs on the silica surface, whereas THF suppresses adsorption. It was established that the pore size of the column packings played an important role in liquid chromatography at the critical adsorption point. The column with the smallest pores largely determined the behaviour of the whole set. The feasibility of using silica gel as column packings for liquid chromatography at the critical adsorption point is questioned. 14 refs. 

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