Pressure-driven ionic transport through nanochannels with inhomogenous charge distributions.

The effect of spatially inhomogeneous fixed charge distributions on the pressure-driven transport of ions through cylindrical nanopores have been investigated theoretically by means of an approximate version of the Poisson-Nernst-Planck model that can be used with confidence for moderately charged nanopores with radius smaller than the Debye screening length of the system. Salt rejection rate has been computed as a function of the applied pressure difference for various one-dimensional (1D) unipolar charge distributions and has been compared with that obtained for a homogeneously charged nanochannel with an identical average volume charge density. The ion rejection capabilities of charged nanopores can be optimized by an appropriate distribution of the fixed charge concentration. When ions are forced to enter the nanopores by the end with the lowest fixed charged concentration, the salt rejection rate exhibits a nonmonotonous variation with the applied pressure. This phenomenon has been attributed to the influence of the inhomogeneous charge distribution on the electric field that arises spontaneously so as to maintain the electroneutrality within the nanopore.