Chapter 14
Review of Critical Conditions of Adsorption and Limiting Conditions of Solubility in the Liquid Chromatography of Macromolecules Downloaded by UNIV OF TENNESSEE KNOXVILLE on November 13, 2016 | http://pubs.acs.org Publication Date: May 30, 1996 | doi: 10.1021/bk-1996-0635.ch014
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D. Hunkeler , M . Janco , and D. Berek 1
Department of Chemical Engineering, Vanderbilt University, Nashville, TN 37235 Polymer Institute, Slovak Academy of Sciences, 84 236 Bratislava, Slovakia 2
The development of critical conditions of adsorption and limiting conditions of solubility of macromolecules are reviewed. These two related methodologies, which both involve the combination of entropic and enthalpic polymer-sorbent-mobile phase interactions, are contrasted. The unique separation mechanisms and area of applicability are also discussed. A summary of the systems for which critical conditions and limiting conditions have been observed is presented along with examples of some historical and key results. The limitations of these methodologies is also discussed. This brief review chapter concludes with a list of some unresolved problems and some possible future research directions. This chapter presents an introduction and concise review of two similar liquid chromatographic techniques which combine interactive and size exclusion separations of macromolecules. Critical conditions of adsorption (LCCC), as so named by Belenkii in his pioneering works in this area [1,2], involve the combination of exclusion (SEC) and adsorption (LAC) mechanisms. By appropriately balancing the adsorption of macromolecules on the column packing and their exclusion from the packing pores the critical conditions are reached at which the retention volume is independent of the molar mass of solutes, i.e. macromolecules of different molar masses elute at the same retention volume roughly equal to the volume of eluent within the column. The extent of adsorption of macromolecules can be controlled by the composition of appropriate mixed mobile phases and alternatively by temperature or even pressure within the liquid chromatographic system. For critical conditions of adsorption the interactions between the polymer chain with the mobile and stationary phases control the elution. Therefore, the type of adsorbent utilized and its surface characteristics have significant effects on the separation characteristics. As we shall discuss, this permits the separation of macromolecules according to non-steric properties. The method has so far been limited to molar masses of approximately 1 0 0 , 0 0 0 daltons.
0097-6156/%/0635-0250$15.00/0 © 1996 American Chemical Society
Potschka and Dubin; Strategies in Size Exclusion Chromatography ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
Downloaded by UNIV OF TENNESSEE KNOXVILLE on November 13, 2016 | http://pubs.acs.org Publication Date: May 30, 1996 | doi: 10.1021/bk-1996-0635.ch014
14. HUNKELER ET AL.
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Hunkeler, Macko and Berek [3] attempted to modify the critical condition approach and sought a binary mobile phase where small changes in the composition had a significant effect on the thermodynamic quality of the solvent. Initially, the toluene-methanol system was investigated since the Mark-Houwink exponent is very sensitive to the level of methanol for polystyrene-toluene. The aim of the limiting condition work was to attempt to extend the molecular weight range where retention was independent of the size of macromolecules, particularly to high polymers so that the method could be applied to commercially relevant systems. The limiting condition approach of Hunkeler, Macko, Berek and later Janco [4,5] involved the exclusion of a polymer in a mobile phase which was indeed a non-solvent for the polymer probe. To accomplish this the polymer needed to be injected in a thermodynamically good solvent. For example, polystyrene was injected in toluene for the methanol-toluene system. The resulting mechanism, which will be detailed later in this chapter, is the combination of size exclusion, precipitation and possibly also liquid adsorption chromatography, with the result that the polymer elutes just on the 'limit of its solubility. These so named limiting conditions of solubility are not as sensitive to the mobile phase composition as they are for critical conditions of adsorption. They have, however, been identified for a variety of macromolecules for molar masses up to several hundred thousand daltons. For limiting conditions, the interaction between the polymer chain and molecules of mobile phase influences the retention of the solute. However, the chemical nature of the column packing surface does not seem to play an important role in the mechanism. It needs to be mentioned that the liquid chromatography under limiting conditions of solubility is a fairly recent development. Therefore, this review, while comprehensive, is somewhat limited in scope. Furthermore, extensive reviews on critical conditions are under preparation [6] and this chapter is not intended to duplicate such efforts. Rather, it is presented as an introduction to provoke some further basic and applied research in the combination of size exclusion and interactive mechanisms of chromatography. Both the critical and limiting conditions are potentially very powerful tools. If either method is to proceed significantly it will require the cooperation of synthesis and application chemists with those involved in the analytical methods development. The prospects for such techniques could be enormous since they basically involve the application of an inexpensive, and isocratic, liquid chromatography methodology to characterizations which would normally be limited to much more elaborate equipment. Alternatively, they have the potential to provide data that cannot be obtained by conventional methods, at all. One of the ultimate goals of either of these methods is the application to various types of polymer blends or copolymers. The evaluation of such potential is one of the likely directions for further research in the immediate future. 1
Discussion L i q u i d Chromatography of Macromolecules under Critical Conditions of Adsorption. Belenkii and Gankina [1,2] first observed critical conditions by varying the eluent composition in a mixture of three solvents. Their experiments involved the size exclusion chromatography of polystyrene on silica gel in a thin layer chromatography arrangement. Through small adjustments to the eluent composition, a mobile phase was identified where the retention volume of a polymer was independent of its molar mass. Indeed, the polymeric retention volumes were as high as observed for inert low molecular weight substances. Belenkii and Gankina called these conditions 'critical' and postulated that under such an arrangement the macromolecules were 'invisible' to the gel. In this way the gel seemed to be totally permeable, independent of its pore size. Later, Tennikov found a similar effect in a column liquid chromatography arrangement [7] and noted that the critical conditions were highly temperature sensitive. Nefedov and Zhmakina have added that the critical composition of the eluent, for a given polymer and column packing, may also depend on the pressure within the system [8]. By comparison, Hunkeler, Macko and Berek [4]
Potschka and Dubin; Strategies in Size Exclusion Chromatography ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
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showed that limiting conditions were unaffected by large changes in the operating pressure of a liquid chromatograph. Belenkii and Gankina [1,2] proposed that at the critical eluent composition the total free energy of the process which takes places around the gel is zero due to a balancing of entropie (exclusion) and energetic (adsorption) effects. This is somewhat unexpected since both the entropie and enthalpic terms change with the molecular weight of the polymer in a different manner, as is illustrated by the schematic in Figure 1. To achieve a molar mass independent elution therefore requires the two effects to have an identical molecular weight dependence. This would certainly not be expected based on a priori knowledge, however, the elucidation of critical conditions is well documented in several polymer-binary eluent-stationary phase systems (Table I). This includes the separation of linear and cyclic oligomers and polymers with molar masses up to 100,000 daltons. The upper molecular weight limit may well be caused by the rapid increase in the extent of adsorption with chain length. Applications of Critical Conditions of Adsorption. Liquid chromatography under critical conditions of adsorption have been used for determination of functionality type distribution of oligomers [9], for separation of oligomers [10] and polymers [11] according to their topology, for separation of polymer blends [12] and our latest results indicate that L C C C can be used for polymer separation according to their tacticity [13]. Critical conditions are attractive for the chromatography of copolymers. If the stationary phase does not 'see' one part of the copolymer molecule the second Visible' portion can be separated as if it were in isolation and its molar mass (M) and molar mass distribution (MWD) can be determined [14-17]. Skvortsov and Gorbunov [18] have noted that it is necessary for the 'invisible' portion of the chain to be a free end. This statement has not been disproved experimentally and thus far critical conditions have been limited to the characterization of the central block of triblock copolymers. It is generally believed that the critical condition concept cannot be used for the central block of a triblock copolymer or for random copolymers, although further investigations are proceeding. Liquid Chromatography of Macromolecules under Limiting Conditions of Solubility. The so-named 'limiting conditions of solubility are accomplished by employing an eluent which is a weak non-solvent, for the polymer probe, while the sample is dissolved in a thermodynamically good solvent. The mobile phases so far employed have been binary or ternary mixtures which, at the temperatures used for the measurement, consist of a thermodynamically good solvent and a non-solvent. In this case, homopolymers with different molecular weights elute from the chromatographic column at the same retention volume which is roughly equal to the volume of liquid in column. That is, under limiting conditions of solubility there is no separation according to molar mass, as is also the case for critical conditions of adsorption. This permits the method to be applied to separations based on other properties of the given polymer, as will be discussed later. The following mechanism is believed to cause the limiting condition phenomena: at low levels of non-solvent, such as methanol in toluene or water in tetrahydrofuran for the polystyrene or polymethylmethacrylate systems, the calibration curves shift slightly to lower retention volumes due to the suppression of adsorption and a reduced effective pore size of the column packing. The accumulation of nonsolvent in the sorbent pores may lead partitioning effects which reduce the retention volume in the pores of the column packing [19]. At higher quantities of non-solvent, in the vicinity of the theta-composition (e.g. 76.9 vol% of toluene for polystyrene in toluene-methanol at 25 °C [20], or 35.7 vol% of toluene for polymethylmethacrylate in toluene-methanol at 26.2 °C [21]) the thermodynamic quality of the solvent is strongly reduced. Mixtures containing more methanol are non-solvents for polystyrenes. If such a mixture is used as an SEC eluent and the injected polymer is dissolved in a good 1
Potschka and Dubin; Strategies in Size Exclusion Chromatography ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
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