## 3 Results and Discussion
### Electrochemical Characterization
For comparison reasons, diffusion potential values for the NaCl solution (due to the different transport number of Na\({}^{+}\) and Cl\({}^{-}\) in solution (without NPAS), that is, \(\mathrm{t_{Na+}}^{\mathrm{o}}\) and \(\mathrm{t_{Cl-}}^{\mathrm{o}}\), respectively) and those for an ideal anion-exchange structure (positively charged, with total cation (co-ions) exclusion, \(\mathrm{t_{Na+}}=0\) and \(\mathrm{t_{Cl-}}=1\) into the pores) are also indicated in Figure 5a by a dashed line and a solid line, respectively. Values in Figure 5a clearly show differences in \(\mathrm{\Delta\Phi}\) values associated to both pore size and sample asymmetry. The effects of ions distribution in charged nanoporous structures as a function of pore size, porosity, or charge are schematically shown in Figure 5b,c. According to the values shown in Figure 5a, the NPAS-Sf sample behaves almost like an ideal anion-exchanger at NaCl concentrations lower than 0.01 M, while \(\mathrm{\Delta\Phi}\) values for NAPS-Ph and NPAS-And samples hardly differ with solution diffusion potentials, which is an indication of their almost neutral character; moreover, the similarity in the electrical potential values determined for these two latter samples seems to indicate the higher effect of larger pore size in these results. The effective fixed charge, \(\mathrm{X_{ef}}\), as well as anion and cation transport numbers (\(\mathrm{t_{Cl-}}\) and \(\mathrm{t_{Na+}}=1-\mathrm{t_{Cl-}}\)) across the studied NPASs were determined by fitting the values presented in Figure 5a to Equation (4) by means of a non-linear procedure [26; 28]. Ionic diffusion coefficients ratio (\(\mathrm{D_{-}/D_{+}=t_{-}/t_{+}}\)) and ionic permselectivity (PS) were also determined, and the obtained values for each sample are indicated in Table 4. Ionic permselectivity is defined as \(\mathrm{PS}=(\mathrm{t_{Cl-}}-\mathrm{t^{o}}\mathrm{_{Cl-}})/\mathrm{t^{o}} \mathrm{_{Na+}}\), where \(\mathrm{t_{Cl-}}\), \(\mathrm{t^{o}}\mathrm{_{Cl-}}\), and \(\mathrm{t^{o}}\mathrm{_{Na+}}\) are the counter-anion transport number into sample pore, and the solution anion and cation transport numbers, respectively [40]. Although all samples exhibit electropositive character, the higher value of the NPAS-Sf fixed charge significantly increased the anion transport number into the nanopores with respect to solution (\(\mathrm{t^{o}}\mathrm{_{Cl-}}=0.615\) and \(\mathrm{t^{o}}\mathrm{_{Na+}}=0.385\); \((\mathrm{D_{Na+}/D_{Cl-}})^{\mathrm{o}}=0.626\)[59]), but this effect was much more reduced in the case of the two samples with higher pore size/porosity; on the other hand,anionic permselectivity values permit a more clear distinction of nanosample structure on diffusive transport behavior. These results show the influence of the open surface for the asymmetric sample in the diffusive transport of ions. The obtained values agree with those previously determined for a NPAS with similar thickness but slightly different pore size (16 nm) and porosity (8%): Xe, =+1.0 \(\times\) 10\({}^{-2}\) M, \(\mathrm{t_{CL-}}\) = 0.737 and PSCL- = 40.0% [28]. Transport of electrolyte solutions through nanopores involves both electrical and frictional interactions, the first related to effective fixed charge and the latter to pore size. In a previous paper, analysis of diffusive transport with trtiated water through the NPAS-Sf sample was performed to obtain information on solute-pore wall interactions [45]. Figure S4 in the Supplementary Information shows a comparison of time evolution of radioactive water activity for NPAS-Sf and NPAS-And samples and the estimation of water permeability for both samples. Tritiated water permeability values were Pw(NPAS-Sf) = 3.6 \(\times\) 10\({}^{-6}\) m/s and Pw(NPAS-And) = 7.9 \(\times\) 10\({}^{-6}\) m/s, which proves the