When used in contact with solutions (in the liquid phase) Nafion™ in the form used in Perma Pure dryers functions as an cationic exchange resin, passing not only water but also positively charged ions (cations) from the solution, while resisting the passage of negatively charged ions (anions).
When used in contact with the gas phase, Nafion™ is much more selective. Ionic compounds do not dissociate into positive and negative ions in the gas phase at the operating temperatures of the dryers, so free negative ions are not available to migrate across the Nafion™ membrane (tubing wall).
Migration occurs as the result of a different mechanism. Compounds that present an exposed hydroxyl group (-OH) are essentially the only compounds known to migrate through Nafion™ in the gas phase. This is apparently due to hydrogen bonding with the sulfonic acid groups that are surrounded by the fluorocarbon matrix within Nafion. Most hydroxides are high-boiling solids (sodium hydroxide, calcium hydroxide, etc.) that are not present as gases within the operating temperature range of the dryers. Only three compounds or classes of compounds are normally removed directly by Nafion™ dryers:
- Water (H-OH)
- Ammonia (when water is present, NH3 reacts to form NH3-OH)
- Alcohols (R-OH, where R is any organic group)
In addition to these three, certain other organic compounds may also be removed if they can be converted into alcohols. Aldehydes (R-H-C=O) and ketones (R1-R2-C=O) can both undergo a process called enolization (conversion to alcohol or “enol”). The carbonyl group within aldehydes and ketones can be acid catalyzed to react with water to form an alcohol in the following reversible reaction: C=O + H2OHO-C-OH.
Nafion™ is strongly acidic due to the presence of the sulfonic acid groups.