Qiarge-transfer Adsorption in Aqueous Media

A brief review of charge-transfer (CT-) thromatography is presented. Hydrophilic gels such as Sephadex exhibit weak electron acceptor and donor prOperties due to the presence of matrix hydroxyls or matrix-bound water. Stronger O-adsorbents for chromatography in aqueous solutions are obtained by introduction of electron acceptor or donor substituents. The adsorption properties of such gels are discussed with reference to results obtained with simple model substances such as amino acids and peptides for acceptor adsorbents and nucleotides for donor adsorbents . tal chelate adsorption (ligand exchange) , is considered as a special kind of O-adsorption with great potential for group separation according to differential metal ion affinity of the solutes. INTRODUOION I think it is appropriate to present a brief historical account on the uses of active charcoal, since this common material is in fact a special kind of 'impure' charge-transfer (O-) adsorbent . Active charcoal was used independently by C.W. Scheele (1) and F. Fontana (2) to adsorb certain gases (in 1773 and 1777 , respectively) . A few years later J.T. Lowitz (3) used wood charcoal to remove colored contaminants from solutions, and in 1812 Derosne (4) used animal charcoal for decolorizing sugar beet extracts. It is therefore not surprising that charcoal was used as a general adsorbent during the early phases of the development of chromatographic methods. In the l94Oies Tiselius and his students used this adsorbent very often in their attempts to fractionate amino acids, peptides, carbohydrates and other substances. These studies were the foundation for the definition of displacement and frontal chromatography as well as gradient elution techniques (5). It gradually became evident, however, that adsorption phenomena on active charcoal were too complicated to permit its successful use as a chromatographic medium for the fractionation of peptides or proteins. This is one of the reasons for the introduction and wide acceptance of simpler adsorbents, such as polymeric ion exchangers, molecular sieves and, more recently, even simple gel polymers with mixed hydrophobic-hydrophilic properties for the chromatographic fractionation of a wide variety of biological substances. All such adsorbents exhibit one or more of the adsorption effects seen with charcoal, but with the significant difference that their sorption behaviour is to some extent predictable and comparatively easier to control. The fact that active charcoal shows a preference for aromatic substances is the main reason for its capacity to clarify colored extracts. This "aromatic adsorption" is determined most probably by a combination of hydrophobic interaction and electron-transfer leading to the formation of x,rr-types of charge-transfer adsorbate complexes. The adsorption sites in charcoal may thus serve as electron donors and/or acceptors. But it has so far been difficult to BelectiVely use only one or the other of the adsorption sites present on charcoal. It is both desirable and essential that such adsorption centers should be well-defined in order to permit study of the adsorption phenomena in extenso and thus to predict their effect in chromatographic experiments. This should be relatively easy to achieve if one could introduce well-defined groups into an inert polymer matrix to serve as the adsorption centers. The question is: can we prepare effective, non-ionic hydrophilic adsorbents that operate primarily by wit, nit or air interactions? But, before going deeper into this problem I would like to facilitate forthcoming discussions by classifying the various sorbents into the following main groups: Class A: Sorbents without adsorption centers. Ex. Ideal molecular sieves. Class B: Sorbents with unsupported adsorption centers. Ex. Inorganic salts, insoluble organic substances. Class C: Sorbents of class B mixed with or coated on class A sorbents or inert solid supports. Class D: Sorbents of class A with covalently attached substituents functioning as adsorption centers. Ex. Cross-linked polymer ion exchangers, bioaffinity adsorbents. Class E: Sorbents of class D charged with bior multifunctional adsorbate molecules