from small to big: ion transport at interfaces
Anne Benneker is a PhD Student in the Soft Matter, Fluidics and Interfaces Group. Her supervisor is professor Rob Lammertink from the Faculty of Science and Technology.
The production of clean drinking water and energy are two main technological challenges ahead of us. Charge selective interfaces are important in a multitude of these technologies, such as electrochemical cells and electrodialysis. In electrodialysis, selective ion transport through charge selective membranes is driven by the application of an external electric potential. At low to moderate electric field strengths the system shows Ohmic behaviour, where an increase in driving potential yields a linear increase of the ion flux through the membranes. As a result of severe concentration polarization, at elevated applied potentials the ionic current through the membrane is limited by the diffusive transport of ions through a boundary layer at the membrane. At these elevated potentials, the ion flux (current) is not linearly dependent on the driving force (potential), and the efficiency of the electrodialysis process drops. However, electrodialysis processes also show an overlimiting behaviour, in which the ion flux is increased again as a function of increasing potential. The origin of the overlimiting current can be explained by different mechanisms, amongst which are water splitting and the occurrence of current induced convection. In current induced convection, the external electric field acts on the space charge that is developed at the membrane interface. The resulting vortical fluid movement enhances charge transport to the interface by mixing the depleted boundary layer with the adjacent bulk solution.
In this work, the effect of external properties such as temperature and membrane geometry on charge transport in electrodialysis systems in different regimes is investigated. Numerical modelling of a non-isothermal formulation of the Poisson-Nernst-Planck and Navier-Stokes equations was employed to study the effect of temperature (gradients) on ion transport through charge selective nanochannels. Experiments on electrodialylsis systems are conducted in the Ohmic, limiting and overlimiting regimes and at different length scales. Visualization of the formation of ion depletion zones and hydrodynamic vortices is done in microfluidic experiments, while the effects of temperature on charge transport are investigated in macro-scale electrodialysis stacks. Different charge selective materials are investigated; (i) nanochannels (charge selective as a result of their overlapping Debye layers), (ii) commercially available polymeric charged membranes and (iii) patterned, charged hydrogels. These experiments yield knowledge on the development of ion depletion zones and the influence of geometry and temperatures on the onset of hydrodynamic instabilities near membrane interfaces.
The effect of temperature and temperature gradients on the selectivity and total transport through cation selective nanochannels is investigated numerically using a non-isothermal formulation of the Poisson-Nernst-Planck and Navier-Stokes equations. An enhanced selectivity of nanochannels was found when a temperature gradient is aligned with the applied electric field, as a result of temperature dependence of ion diffusivity and solvent viscosity. Additionally, for asymmetric nanochannels the current rectification is enhanced when a temperature gradient is applied.
Temperature gradients can be of potential use in enhancing the efficiency of desalination by electrodialysis. Stacks of commercially available ion exchange membranes are used for the investigation of the influence of temperature and temperature gradients on macro-scale systems. Efficiency and selectivity of (reverse) electrodialysis in the Ohmic and limiting regime are measured as a function of applied temperature gradients. It is found that increasing the temperature of one feed stream can enhance the ion transport in the system and thus enhances the efficiency of the separation. The direction of the temperature gradient is of limited influence in the Ohmic regime, but can enhance the selective transport in the limiting current regime.
For the investigation of the development of ion depletion zones in electrodialysis at the microscale, two different microfluidic platforms are employed. Glass chips containing patches of charge selective nanochannels are used for the investigation of the effect of geometry on the development of hydrodynamic vortices at the membrane interface. Fluorescence imaging is combined with particle tracking techniques to quantify the development of ion depletion zones and mixing of the boundary layer. Geometry is of large influence on the efficiency of the electrodialysis process, as it can result in electroosmotic transport of ions towards the charge selective interface. The geometry of the membrane is of influence on the distribution of the electric field lines, and can enhance electrohydrodynamic effects at the interface.
This work shows possible strategies in enhancing the efficiency of water desalination by electrodialysis, but has implications for all fields in which selective charge transport is of importance.
