In Fig. (c), we compare R for different ion types. It shows that at c s = 0.1 M, R ≈ 100% for the anions, inhibiting most of these ions from entering the permeate reservoir. In general, the rejection of anions decreases to 78% as the salt concentration increases to c s = 0.5 M, although most of the Na + ions can pass through the membrane at all salt concentrations. This phenomenon is natural because -N + (CH 3 ) 3 cannot accommodate all anions. Consequently, some anions will form stronger attractions to the Na + ions, keeping them confined to the feed reservoir.