With concentrated sulfuric acid treatment, the resulting nano fibrillar structure introduces void space within the film, where charge balanced anion-cation pairs (along with water) are able to reside. This likely gives rise to the observed non-zero cation concentration at lower PSS mol fraction. Further, ions entering the crystalline nano fibrillar structures may themselves be in thermodynamic equilibrium with the ions in the voids within the film (in addition to the external electrolyte). Also, ion trapping within the film may also occur to give an increased ionic content.