Soft matter, Fluidics and Interfaces (SFI)

Project Description
Investigation of Ionic Diode Behaviour in Nanoporous Materials

Ionic-diodes are devices which show asymmetric transport of ions depending on the direction of the applied bias. Essentially, this means that the current in one bias (positive or negative) is very different vs. the opposite bias. This opens up the possibility for use as sensors or in desalination applications. Recently, it has been experimentally demonstrated that zeolitic imidazolate frameworks (ZIFs) can demonstrate such behaviour [1].  

The mechanism for this diodic behaviour is, however, still not entirely clear. It has been postulated to be related to cation/anion size-asymmetry and shown to be influenced by pH but the precise interplay of these factors is not yet clear. It is the goal of this masters work to perform detailed investigations into the transport phenomena in these systems, both within the ZIF material and the surrounding fluid reservoirs, under various operating conditions. In this manner, the suitability of these materials for various desalination applications can be assessed. 

The proposed masters work will involve synthesis of ZIF materials in glass (transparent) systems in order to form charge-selective interfaces. After this synthesis, detailed investigation into the ion-transport phenomena will be carried out through direct electrical characterization (conductance), along with characterization of specific ionic species transport through the use of fluorescence-sensitive dyes (fluorescence lifetime imaging microscopy) and other transport mechanisms.  

The specific investigation tasks of this assignment are: 

  • Synthesis of porous plugs/films of ZIF materials in glass capillaries (uniform and asymmetric cross-sections)
  • The role of cation/anion size and charge on diodic behaviour
  • The effect of pH on the resulting ion-transport phenomena
  • Experimental observations will be based on electrical characterization (conductance), fluorescence dye studies (microscopy and lifetime imaging) and fluid velocity (particle tracking)
  • Interpretation of experimental results using continuum level models, such as Teorell-Sievers-Meyers to quantify the effective membrane properties of ZIF.
  • Exploring numerical modeling based on Poisson-Nernst Planck that includes surface reaction chemistry (e.g. surface pK).

 [1] Madrid et al. “Ion flow in a zeolitic imidazolate framework results in ionic diode phenomena.” Chem. Comm. 2016. 52: 2792