  In recent years there has been significant progress in the development of
artificial ion pumping membranes. Ion pumps based on asymmetric nano-pores have
been shown to operate as ionic current rectifiers, thus pumping a net ion flux
against a concentration gradient even when driven with unbiased ac signals.
However, since ion transport relies on charged nano-pores, it is selective to
either cations or anions, and thus cannot pump both cations and anions
simultaneously. In this paper, we present a model for an electronically active
membrane which is based on a flashing ratchet mechanism. The model includes
adjacent electrolyte reservoirs and ion-ion interactions, which were not
accounted for in prior similar models, and thus provides a better understanding
of the driving mechanism and potential capabilities and limitations. It is
shown that unlike most other proposed ion pumps, the ratchet-based ion pump
(RBIP) drives both cations and anions in the same direction and up a
concentration gradient. This process, referred to as ambipolar ion pumping, is
shown to be highly robust for many electrolyte compositions and input signals.
The membrane is composed of alternating conductive thin layers (electrodes),
separated by insulating layers in an asymmetric design. With insulating layers
thickness of 70 and 30 nm and an input signal amplitude of 0.5 V, the device
drives a salt flux of 0.03 mol/m^2 s in a mildly saline solution (10 mM). Thus,
RBIPs may pave the way for many exciting future applications involving
ambipolar ion pumping, most notably for desalination.
