156 A.S. Grandison by desorption, in which the separated species are recovered back into solution in a much purified form The ion-exchange solids bear fixed ions which are covalently attached to a solid matrix. There are two basic types of ion-exchanger (1) Cation exchangers(sometimes called 'anionic exchangers)which bear fixed nega tive charges and are therefore able to retain cations, and (2) Anion exchangers(sometimes called 'cationic exchangers) which bear fixed posi tive charges Ion-exchangers can be used to retain simple ionised species, but may also be used in the separation of polyelectrolytes which possess both positive and negative charges (i.e amphoteric molecules such as proteins) as long as the overall charge on the polyelectrolyte is opposite to the fixed charges on the ion-exchanger. This overall charge depends on the isoelectric point of the polyelectrolyte and the ph of the solution. At pH values lower than the isoelectric point the net overall charge will be positive and vice versa. In some circumstances it is even possible for ion-exchangers to retain macro- nolecules of like charge, presumably if a portion of the molecule carries a sufficient opposite charge(Peterson, 1970). The main interaction is via electrostatic forces, and in the case of polyelectrolytes the affinity is governed by the number of electrostatic bonds etween the solute molecule and the ion-exchanger. However, particularly with large molecules such as proteins, multiple interactions may occur involving steric effects. Size and geometric properties, and the degree of hydration of the ions may affect these interactions, and hence the selectivity of the ion-exchanger for different ions. Charge density may be more important than overall charge in determining the relative selectivity Figure 6. 1 is a schematic diagram showing a generalised anion exchanger -1e bearing fixed positive charges. To maintain electrical neutrality these fixed ions must be balanced by an equal number of mobile ions of the opposite charge (i.e. anions) which re held by electrostatic forces. These mobile ions can move in and out of the porous molecular framework of the solid matrix and may be exchanged stoichiometrically with other dissolved of the same charge, and are termed counterions. Ion-exchange systems can be considered to consist of two aqueous liquid phases-one confined within Matrix Fig. 6. 1 Schematic diagram of a generalised anion exchang156 A. S. Grandison by desorption, in which the separated species are recovered back into solution in a much purified form. The ion-exchange solids bear fixed ions which are covalently attached to a solid matrix. There are two basic types of ion-exchanger: (1) (2) Cation exchangers (sometimes called ‘anionic exchangers’) which bear fixed nega￾tive charges and are therefore able to retain cations, and Anion exchangers (sometimes called ‘cationic exchangers’) which bear fixed posi￾tive charges. Ion-exchangers can be used to retain simple ionised species, but may also be used in the separation of polyelectrolytes which possess both positive and negative charges (i.e. amphoteric molecules such as proteins) as long as the overall charge on the polyelectrolyte is opposite to the fixed charges on the ion-exchanger. This overall charge depends on the isoelectric point of the polyelectrolyte and the pH of the solution. At pH values lower than the isoelectric point the net overall charge will be positive and vice versa. In some circumstances it is even possible for ion-exchangers to retain macro￾molecules of like charge, presumably if a portion of the molecule carries a sufficient opposite charge (Peterson, 1970). The main interaction is via electrostatic forces, and in the case of polyelectrolytes the affinity is governed by the number of electrostatic bonds between the solute molecule and the ion-exchanger. However, particularly with large molecules such as proteins, multiple interactions may occur involving steric effects. Size and geometric properties, and the degree of hydration of the ions may affect these interactions, and hence the selectivity of the ion-exchanger for different ions. Charge density may be more important than overall charge in determining the relative selectivity. Figure 6.1 is a schematic diagram showing a generalised anion exchanger - i.e. bearing fixed positive charges. To maintain electrical neutrality these fixed ions must be balanced by an equal number of mobile ions of the opposite charge (Le. anions) which are held by electrostatic forces. These mobile ions can move in and out of the porous molecular framework of the solid matrix and may be exchanged stoichiometrically with other dissolved ions of the same charge, and are termed counterions. Ion-exchange systems can be considered to consist of two aqueous liquid phases - one confined within Counter-ions Imbibed solvent Fig. 6.1. Schematic diagram of a generalised anion exchanger