Critical condition of adsorption

LC LC as rule produces narrow, focused polymer peaks because solutes accumulate at the edge of barrier. It is supposed that here again the critical conditions of adsorption and enthalpic partition represent the limits of the barrier composition, which hinders the fast progression of polymer species. 

The primary differences between limiting conditions of solubility and Belenkii s critical condition of adsorption approach 4, 5) are the use of a thermodynamically poor (bad) eluent, or even an eluent that is a nonsolvent for the polymer, whereas the polymer is dissolved and injected in a thermodynamically good solvent. The limiting condition... 

Whereas SEC, well-known for the determination of molar mass distribution, has been used since 1959, LACCC was first mentioned by Belenkii et al. in 1976 and then by Entelis et al. in 1986. The theory and experimental details for the determination of critical conditions have been described by Pasch. " The separation of polymers under critical conditions of adsorption allows, for example, the elution of homopolymers with the same repeat unit irrespective of their molar mass, according to the number and nature of functional groups. 

Figure 31.4a. Determination of critical conditions of adsorption of PDMS.

LC under critical conditions of adsorption (LACCC) and SEC methods with matrix-assisted laser desorption/ioniza-tion-time-of-flight spectrometry (MALDI-TOF) and postcolumn derivatization. The critical conditions of polymer adsorption in the LC and an optimal matrix system for MALDI-TOF are reported. The changes of molar mass distribution and chemical heterogeneity are said to be due to the simultaneous processes of degradation and recombination. ... 

Radke W, Rode K, Gorshkov AV, Biela T. Chromatographic behavior of functionalized star-shaped poly(lactide)s under critical conditions of adsorption. Comparison of theory and experiment. Polymer 2006 46 5456-65. 

Falkenhagen J, Weidner S. Determination of critical conditions of adsorption for chromatography of polymers. Anal Chem 2009 81 282-7. 

Poly(propylene oxide)s were analysed efficiently by coupled liquid chromatography and MALDI-TOF mass spectrometry. The techniques were coupled via a robotic interface where the matrix was coaxially added to the eluate and spotted dropwise onto the MALDl target. Size exclusion chromatography and liquid chromatography at critical conditions of adsorption coupled to MALDI-TOF... 

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... 

As has been pointed out, both entropic and enthalpic interactions affect the chromatographic behavior of macromolecules. They are adjusted to the required type of separation by selecting appropriate stationary and mobile phases. In a third mode of liquid chromatography of polymers, liquid chromatography at the critical condition (LCCC) (Entelis etal., 1985,1986 Pasch, 1997), the adsorptive interactions are fully compensated by entropic interactions. This mode is also referred to as liquid chromatography at the critical point of adsorption. Hence, TAS is equal to AH and therefore, AG becomes zero. K is 1 irrespective of molar mass and, consequently, homopolymer molecules of different molar masses coelute in one chromatographic... 

In many physically important cases of localized adsorption, each adatom of the compact monolayer covers effectively n > 1 adsorption sites [3.87-3.89, 3.98, 3.122, 3.191, 3.214, 3.261]. Such a multisite or 1/n adsorption can be caused by a crystallographic Me-S misfit, i.e., the adatom diameter exceeds the distance between two neighboring adsorption sites, and/or by a partial charge of adatoms (A < 1 in eq. (3.2)), i.e., a partly ionic character of the Meads-S bond. The theoretical treatment of a /n adsorption differs from the description of the 1/1 adsorption by a simple Ising model. It implies the so-called hard-core lattice gas models with different approximations [3.214, 3.262-3.266]. Generally, these theoretical approaches can only be applied far away from the critical conditions for a first order phase transition. In addition, Monte Carlo simulations are a reliable tool for obtaining valuable information on both the shape of isotherms and the critical conditions of a 1/n adsorption [3.214, 3.265-3.267]. 

According to Gorbunov and Skvortsov [18], triblock copolymers of the ABA type may be analyzed by liquid chromatography at the critical point of adsorption similar to the analysis of diblock copolymers. The two possible cases for this type of investigation, i.e. (a) the analysis with respect to the inner block B using the critical conditions of the outer block A, and (b) the analysis of the outer block A using the critical conditions of the inner block B, will be discussed briefly. 

To explain the separation procedure of polymer blends using chromatography at the critical point of adsorption, the behavior of blends of polystyrene (PS) and polymethyl methacrylate (PMMA) in different chromatographic modes is shown in Fig. 24. With silica gel Si-100 as the stationary phase, the mobile phase comprised mixtures of MEK and cyclohexane. In pure MEK a size exclusion mode was operating for both components. Under these conditions PS and... 

The principle of liquid chromatography under critical conditions of enthalpic interactions (LC CC) was elucidated in section 11.5.2.3. Mutiral compensation of the exclusion - entropy, and the interaction - enthalpy based retention of mac-romolecitles (see Figitre 3(d)) can be attained when the interactions that lead to either adsorption or enthalpic partition of sample are in the controlled way added... 

Poly(methyl methacrylate) and polytetrahydrofuran polymers were studied at the critical point of adsorption. This critical point of adsorption occurs where the retention of a given polymer is governed strictly hy the number and types of functional groups on the polymer [858]. The authors show plots of log MW vs. retention time for various mobile phase compositions on a given column. The critical point is reached when the retention time becomes independent of the molecular weight of the polymer. For poly(methyl methacrylate) that point was reached on a silica column (RI detector) with a 73/27 methyl ethyl ketone/cyclohexane mobile phase. For polytetrahydrofuran, the silica column and a 95/5 acetone/hexane mobile phase created the critical conditions. This approach has enabled the individual blocks within the co-polymer to be studied (i.e., the portion of the polymer that can make contact with the support surface). 

The molecular-statistical theory of polymer solutions in confined media was also applied to the conventional chromatographic theory of gradient elution [ 163]. This approach leads to the prediction of the special mode of interactive polymer chromatography gradient elution at critical point of adsorption. It was demonstrated theoretically and experimentally that under appropriate conditions elution of each compositionally homogeneous fraction of copolymer occurs at the critical mobile phase composition. This critical mobile phase composition depends only on the local structure of the copolymer chain and is independent of its molecular weight. As a consequence, gradient elution produces the chemical com-... 

The chromatographic separation of polymers by liquid chromatography under critical conditions (LCCC), also referred to as liquid chromatography (LC) at the critical point of adsorption, LC in the critical range or LC at the point of exclusion-adsorption transition, has attracted significant attention within polymer community. Russian scientists using TLC [1-3] and later LC [4,5] have been the first experimentally identify critical conditions. At the critical conditions polymers of a given kind are eluted independently from their molar mass (for example. Fig. 1 [6]). 

Pig. 3. Gradient elution of polystyrene samples from Figure 1. The same chromatographic system and conditions as in Figure 1, except eluent gradient 100% ACN to 100% THF over 10 min (dashed line). Numbers on the graph molecular weights of individual polystyrenes. Broken line critical point of adsorption (48% THF). 

An additional technique that has been found useful in analysis of the composition of polymers and blends is liquid chromatography. Pasch and Rode used the critical point of adsorption of the least polar component of a blend to determine the liquid chromatographic conditions for separating blends of polymethacrylates into components (50). High pressure liquid chromatography (HPLC) in combination with mass spectroscopy was used to analyze the components of an epoxy resin (51). HPLC has also been used with a precipitation-redissolution technique to separate polymer molecular weights for several polymers as a shorter technique compared to SEC (52). Reverse-phase liquid chromatography with UV detection was useful in qualitative determination of brominated flame retardants in polymeric waste materials (53). 

If inhibiting anions I are present as well, they compete with the aggressive anions A for the adsorption sites according to the same type of adsorption isotherm as given in Eq. (33) with their specific AG°j values and bulk concentrations. Combining the equations for the inhibiting and aggressive anions leads to the critical conditions of inhibition of localized corrosion. The detailed discussion [2] yields for the inhibition potential... 

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