## 3 Results and Discussion
### Electrochemical Characterizations
Because the electrical charge exhibited by most membranes in contact with aqueous solutions (polar medium) corresponds to the adsorption of charged species from solution on the pore walls and/or dissociation of specific ionic groups, the similarity obtained for surface charge density values seems to be in concordance with the similitude in membrane surface material determined from XPS analysis. Figure 4: Membrane potential as a function of variable solution C\({}_{\rm v}\). (**a**) Sf-NPAM/TiO\({}_{2}\) membrane (\(\square\)) experimental values and (**b**) Ox-NPAM/TiO\({}_{2}\) membrane (\(\lozenge\)) experimental values, fitted values (solid line), diffusion potential contribution (dashed line) and Donnan potential contribution (dashed dot line). Fitted values: (**a**) X\({}_{\rm ef}\) = +0.015 M, t\({}_{\rm Cl}-\)/t\({}_{\rm Na}\)+ = 3.74; (**b**) X\({}_{\rm ef}\) = +0.012 M, t\({}_{\rm Cl}-\)/t\({}_{\rm Na}\)+ = 2.47. Another electrochemical technique used for characterization of membranes in working conditions is electrochemical impedance spectroscopy (EIS). This technique gives quantitative and/or qualitative information related to charge movement or charge adsorption by means of the electrical resistance or capacitance (equivalent capacitance in case of non-homogeneous systems [39]) respectively, which can be significantly affected by membrane structure and material characteristics [50; 51]. Qualitative information is obtained by comparing impedance diagrams for membranes from the same material but different geometry, while characteristic parameter values are estimated by the fitting of experimental data to adequate equivalent circuits [39; 40]. A comparison of impedance diagrams for clean Sf-NPAM/TiO\({}_{2}\) and Ox-NPAM/TiO\({}_{2}\) membranes measured in the system: electrode/electrolyte solution (C)/membrane/electrolyte solution (C)//electrode, with C = 0.002 M NaCl, is shown in Figure 5. The Nyquist plot (\(-Z_{\text{img}}\) vs. \(Z_{\text{real}}\)) shows significant differences between both membranes, because the impedance diagram for Sf-NPAM/TiO\({}_{2}\) sample exhibits two differentiated relaxations that correspond to the separated contributions of membrane (m) and electrolyte solution (e) (see insert in Figure 5 where this part of the diagram is enlarged), while a unique relaxation process is observed in the diagram for the Ox-NPAM/TiO\({}_{2}\) sample, which corresponds to the \"membrane plus electrolyte\" contribution. As already reported, differences in impedance diagrams can depend on both membrane structure/geometry and material hydrophilic/hydrophobic character, because membrane and electrolyte separated impedance contributions have been obtained for composite reverse osmosis and even nanofiltration membranes with a two-layer structure (a denser thin active layer and a porous support) [48; 52; 53], while only a relaxation process was observed for wider pore size ultrafiltration/microfiltration membranes [54; 55] and also for nanoporous regenerated cellulose membranes (2 kDa cut off, which should correspond to a pore size \(\sim\) 3.0 nm) associated with high water (aqueous solution) adsorption due to the hydrophilic character of this membrane material [48]. On the other hand, the analysis of impedance diagrams allows us to obtain the following values for the electrical resistance of Sf-NPAM/TiO\({}_{2}\) and Ox-NPAM/TiO\({}_{2}\) membranes: R\({}^{\text{Sf-NPAM/TiO2}}\) = 4510 ohm and R\({}^{\text{Ox-NPAM/TiO2}}\) = 1665 ohm, although this latter value was determined by subtraction of electrolyte electrical resistance (R\({}_{\text{e}}\)) from that determined for the Ox-NPAM/TiO\({}_{2}\) membrane plus electrolyte system (R\({}_{\text{me}}\)); hence: (R\({}_{\text{m}}\) = R\({}_{\text{me}}\) - R\({}_{\text{e}}\)), that is, R\({}_{\text{m}}\) was obtained from two different measurements that may slightly affect the real value.