Fundamental water and salt transport properties of polymeric materials

Fundamental water and salt transport properties of polymers are critical for applications such as reverse osmosis (RO), nanofiltration (NF), forward osmosis (FO), pressure-retarded osmosis (PRO), and membrane capacitive deionization (MCDI) that require controlled water and salt transport. Key developments in the field of water and salt transport in polymer membranes are reviewed, and a survey of polymers considered for such applications is provided. Many polymers considered for such applications contain charged functional groups, such as sulfonate groups, that can dissociate in the presence of water. Water and ion transport data from the literature are reviewed to highlight the similarities and differences between charged and uncharged polymers. Additionally, the influence of other polymer structure characteristics, such as cross-linking and morphology in phase separated systems, on water and salt transport properties is discussed. The role of free volume on water and salt transport properties is discussed. The solution-diffusion model, which describes the transport of water and ions in nonporous polymers, is used as a framework for discussing structure/property relations in polymers related to water and salt transport properties. Areas where current knowledge is limited and opportunities for further research are also noted.