Reliable methods to determine experimental energy barriers for transport in salt-rejecting membranes

Abstract

Understanding the transport mechanisms in salt-rejecting membranes is critical for improving their separation efficiency and selectivity. Examining transmembrane permeation in terms of energy barriers using the Arrhenius or Eyring approach provides valuable insights into molecular transport within the membrane and at the solution-membrane interfaces. Although useful insights have been gained using the energy barriers framework, which is based on measuring permeability at different temperatures, the method can sometimes show counterintuitive and inconsistent results. In this study, we examine methods to improve the reliability of experimentally obtained energy barriers for transport in salt-rejecting membranes. We first compile energy barrier results for the transport of various solutes in loose and tight salt-rejecting membranes, observing data variability across studies and a weak correlation between energy barriers and membrane type. Next, we demonstrate the importance of thermally stabilizing membranes prior to experimentally evaluating energy barriers, showing that membranes equilibrated at high temperatures and tested with descending temperature produce more stable and reliable trends. In addition to thermal stabilization, we identify that comparing energy barrier values based on a similar concentration polarization modulus is critical when analyzing trends between different solutes and membranes. Following these recommendations, we obtain energy barriers for ion permeation that align with the performance of loose and tight salt-rejecting membranes. We conclude by demonstrating consistent and rational energy barrier measurements in two independent laboratories using the principles discussed. Overall, this study provides important guidelines for the experimental quantification of energy barriers for transport in salt-rejecting membranes.