Desalination is vital for more than 300 million people around the world and that number is likely to grow as fresh water supplies dry up in many regions through desertification, urbanization, and climate change. Writing in the journal Science at team from the University of Connecticut has developed a new type of membrane for desalination. They used an additive manufacturing approach that involves electrospraying to fabricate an ultra-thin, ultra-smooth polyamide membrane. The new material is less prone to fouling and should use less energy to push water through in reverse osmosis.

"Today's membranes for reverse osmosis are not made in a way that allows their properties to be controlled," explains UConn's Jeffrey McCutcheon. "Our approach uses an 'additive' technique that allows for control of a membrane's fundamental properties such as thickness and roughness, which is currently impossible using conventional methods." Conventional reverse osmosis membranes are made by interfacial polymerization, which relies on a self-terminating reaction between an aqueous phase amine and an organic phase acid chloride monomer. While this allows exceedingly thin polyamide films to be produced that are highly selective and permeable to water molecules, there has for at least forty years remained a need to make better membranes. The new approach offers control over thickness and roughness that is not possible with the conventional fabrication methods.

The standard polyamide membranes are between 100 and 200 nanometers thick. The new electrospray method allows a membrane just 15 nm to be produced and to make membranes thicker in 4 nm increments. The roughness of the new membranes can be as low as 2 nm, compared with 80 nm for conventional reverse osmosis membranes.

"Our printing approach to making polyamide membranes has the additional benefit of being scalable," McCutcheon adds. "Much like electrospinning has seen dramatic improvements in roll-to-roll processing, electrospraying can be scaled with relative ease." The researchers suggest that this approach could reduce the amount of chemical resources needed for production as traditional chemical baths are no longer necessary in this approach.

"In the lab, we use 95 percent less chemical volume making membranes by printing when compared to conventional interfacial polymerization," explains McCutcheon. "These benefits would be magnified in large-scale membrane manufacturing and make the process more 'green' than it has been for the past 40 years." The team adds that it is not only desalination technology that might benefit from this new approach to membrane fabrication. The same approach could be used for other separation membranes used in the chemical industry and elsewhere. [Chowdhury, M.R., et al. Science (2018) 361(6403): 682-686 DOI: 10.1126/science.aar2122].

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase. His popular science book Deceived Wisdom is now available.