Selecting the right Amberlite XAD adsorbent

Selecting the Right Adsorbent

The following is a brief description of a suggested methodology to determine which type of polymeric adsorbent should be used in a particular application. It should serve as a useful guide to the initial experimental work that should be performed in selecting the correct adsorbent resins for your application. The decision steps examined are given below:

Nature of the solvent
Functionality of solute
Polarisation
Size of solute.

NATURE OF THE SOLVENT
Since the majority of the known applications for polymeric adsorbents are in the aqueous hase, this will be taken as the example. Hence the first decision, that of the nature of the solvent, is to move towards polar solvent on the diagram. The next decision is related to the bulk properties of the solute itself. For this, the solutes can be divided into two arbitrary classes "I" and "II".

"Class I" solute can be thought of as having one or more of the following characteristics:

Aromatic groups such as : Examples include benzene, toluene, xylene and molecules such as carotene.
High local electron density due to the presence of atoms within the molecular structure such as nitrogen, sulphur, chloride or bromide. If these atoms are oxidised to any degree then the solute could also be recovered by means of ion exchange. There are many examples of this type of molecule but perhaps one of better known is that of Cephalosporin C.

"Class II" solutes can be regarded as those whose structure contains predominately saturated C-C bonds with few areas of high electron density. Examples of these types of solutes include proteins/peptides, aminoacids, vitamins and hydrocarbons such as cyclohexane.

POLARISATION
A further subdivision can be made by determining whether the solute has groups that can be easily polarised or not. This is an arbitrary decision but can be related to the dipole moment or dielectric constant of a particular species.

For example, - CO₂H and - NH₂ groups can be regarded as easily polarisable and capable of hydrogen bonding, whereas groups such as or ethers, whilst they can be polarised under extreme conditions, are not normally so. This type of classification is intended only as a guide to the points to note in certain applications and not as a definitive approach.

SIZE OF SOLUTE
The next important parameter that then needs to be considered is that of the size of the target molecule. One useful definition is that small molecules are defined as having a molecular weight < 1000 D whilst large molecules have a molecular weight > 10,000 D.

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