Exploring transport and selectivity in salt-rejecting membranes using transition-state theory

Salt-rejecting membranes have been widely implemented in water purification and desalination processes. Separation between species at the molecular level is achievable in these membranes due to complex and poorly understood set of transport mechanisms that have attracted the attention of researchers within and beyond the membrane community for many years. Minimizing existing knowledge gaps in transport through salt-rejecting membranes can improve the sustainability of current water-treatment processes and expand the use of these membranes to other applications that require high selectivity between species. Since its establishment in 1949, Eyring’s transition-state theory (TST) for transmembrane permeation has been applied in numerous studies to mechanistically explore molecular transport in dense membranes, such as nanofiltration (NF) and reverse osmosis (RO) membranes. In this presentation, I will first discuss the limited ability of commonly used transport models to mechanistically explain transport and selectivity trends observed in NF and RO membranes. Next, I will introduce the underlying principles and equations of TST and establish the connection to transmembrane permeation with a focus on molecular-level enthalpic and entropic barriers that govern water and solute transport under confinement. I will then highlight mechanistic insights into transport in NF and RO membranes that can be gained by analyzing enthalpic and entropic activation barriers that were measured under different conditions. I will also discuss major limitations of the experimental application of TST and propose specific solutions to minimize the uncertainties surrounding the current approach.