(601c) Small Molecule Deposition to Improve Membrane Fouling Resistance

Membrane fouling in water purification processes causes dramatic decreases in membrane performance.  Membrane surface modifications are effective at remediating fouling of many common contaminants.  Dopamine was recently found to non-selectively deposit from buffered alkaline solution onto virtually any surface, rendering the surface very hydrophilic after only a few minutes of contact with dopamine solution.  Polymeric ultrafiltration and microfiltration membranes of various polymer compositions and architectures showed reduced fouling while filtering a synthetic oil-water emulsion.  Fouling was observed in both constant-transmembrane pressure and constant-permeate flux operation and the results were compared.  Deposition conditions, including dopamine concentration, solution pH, and deposition time, were evaluated for their effects on the performance of modified membranes.  Dopamine coatings showed good resistance to many cleaning chemicals but deteriorated in the presence of hypochlorite. 

Because dopamine forms a non-selective coating on any surface tested to date, its deposition provides a route to universal membrane modification, eliminating the need to choose membrane-specific chemistries for further surface modification.  Dopamine coatings have previously been used as a platform for molecular conjugation in membrane biomaterials applications.  Poly(ethylene glycol) ad-layer formation on the dopamine-modified surface further improved membrane performance in oil-water emulsion filtration.  Due to the non-specific nature of dopamine deposition, entire membrane modules may be easily modified by flowing dopamine solution through the module, allowing coating of the membrane, spacers, and pressure vessel.  Dopamine-modified modules may be further modified with poly(ethylene glycol) by a similar technique.   

Despite recent interest in dopamine coatings in membrane, biomaterial, and other applications, the chemical nature of the dopamine coating has not been conclusively determined.  Using a variety of spectroscopic techniques, it was demonstrated that the dopamine coating is formed primarily through non-covalent bonding, likely via a combination of strong hydrogen bonding, charge transfer between the co-monomers, and π-stacking.  The proposed molecular structure may have important implications for understanding other melanins derived from similar catecholamines as well as for developing variations of this unique surface modification agent.  Molecules with similar functionalities to dopamine, such as hydroquinone and catechol, showed similar deposition behavior from alkaline solution and imparted fouling resistance on modified membranes.  Deposition of a variety of small molecules from alkaline solution, therefore, provides a facile means of modifying water purification membranes for improved fouling resistance.