Fig. 1: Overview of direct Donnan potential measurement using tender APXPS.

a A schematic of the experimental setup during data acquisition from the CR-61 membrane equilibrated in aqueous salt solution. b Equilibrium ion-partitioning between a membrane in contact with a monovalent electrolyte solution. At equilibrium, electroneutrality requires an equivalent number of counter-ions (which, for a monovalent system, is equal to the fixed charge concentration) to balance the fixed charge groups. The co-ions sorbing into the membrane are accompanied by an equivalent number of counter-ions, leading to significantly higher concentrations of counter-ions than co-ions in the membrane. c Illustration of the potential drop at the membrane/solution interface. d Corresponding binding energy (BE) shifts in the membrane related core levels. e Relationship between the measured BE of membrane related core levels and the Donnan potential as a function of external solution concentration.