Ion Specific Effects in Charged Polymers for Electromembrane Applications

Providing sustainable supplies of purified water and energy is a critical global challenge for the future, and polymer membranes will play a key role in addressing these clear and pressing global needs for water and energy. Polymer membrane-based processes dominate the desalination market, and polymer membranes are crucial components in several rapidly developing power generation and storage applications that rely on membranes to control rates of water and/or ion transport. Much remains unknown about the influence of polymer structure on intrinsic water and ion transport properties, and these relationships must be developed to design next generation polymers for electromembrane applications.

In many emerging energy applications, membranes are exposed to ions that are very different from sodium and chloride, which have been used to characterize membranes for decades. Accordingly, ion specific effects have been observed in charged polymers that are of interest for electromembrane processes. These effects must be understood to prepare charged polymers with ion-specific selectivity properties to enable emerging advanced energy technologies.

The fundamental underpinnings of these ion specific effects extend beyond traditional ion exclusion and transport theories. Measured ion sorption and electrochemical permselectivity properties of charged cation exchange membranes suggest that thermodynamic factors, such as dispersion energy contributions, influence ion specific sorption properties in a manner that extends beyond the effects of simple electrostatic forces. Additionally, diffusion properties suggest that factors other than ion size influence ion mobility within the polymer. Together, these sorption and diffusion properties contribute to ion specific electrochemical permselectivity properties. This presentation will discuss these properties in terms of both experimental data and theory. Understanding the fundamental ion and polymer properties that contribute to ion specific effects will enable the development of advanced polymer membranes with selectivity that can be optimized for specific ions that are important for emerging electromembrane water purification and energy technologies.