The resulting Poisson-Boltzmann (PB) equation reads (32) where Φ(r) = βeψ(r) is the reduced electrostatic potential and κ 2 = 8πλ B n res the Debye screening parameter of the reservoir. The PB equation is subject to the Neumann boundary conditions Φ ′ (0) = 0 and Φ ′ (R) = 0, which state that the electric field must vanish both at the cell center (because of symmetry) and at the cell edge (because of electroneutrality). Equation ( ) is a nonlinear ordinary differential equation that has no analytical solution and thus has to be solved numerically. In this study, we numerically solve the nonlinear PB equation using the MATLAB routine bvp4c. Once we obtain the electrostatic potential, replacing back in Eqs. ( )- (31) provides us with the different ionic species. From these profiles, we can obtain the system pH by integrating c H + (r), or the net charge as