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Membranes 2020, 10, 221 16 of 72 the increment of overlimiting current (Figure 10a, step 4). However, it is not yet known if this excess pressure is directly related to the action of the electric force or indirectly by the forced migration of numerous counter-ions under the high electric field, carrying water in their hydration layer (Figure 10b, step 1), and if this excess pressure within the SCR, applied on counter-ion hydration shells, displaces part of their hydration water out from the SCR (Figure 10b, step 2), collides with inertial non-slipped liquid layers (Figure 10b, step 3) to create the vortices (Figure 10b, step 4). Currently, the SCR is considered in its entirety and the electric field is assumed to be applied to this entire region and not to each ion separately. So, this region behaves like a volume on which the electric force is applied. Consequently, the relatively high space charge values of the extended SCR, developed on the depletion side of ion-exchange membranes during their polarization under high electric field, and the extended SCR thickness are the conditions necessary for the occurrence of electroconvection [51,59,70,71]. Figure 10. Occurrence of electroconvective vortices at membrane interface due to electroosmosis of the second kind (a) as currently accepted (adapted from Nikonenko et al. [21]) and (b) hypothetic, taking into account the ion hydration shell. In addition to mixing the depleted solution at the interface of the membranes with the more concentrated solution in the center of the compartment (bulk solution), these vortices reduce, even eradicate, the diffusion layer. Thus, they reduce mass transfer by diffusion (in the diffusion layer (Figure 4)) which is rather slow and, accelerate the mass transfer by migration (Figure 4). Furthermore, the vortices reduce the phenomenon of dissociation of water molecules by inducing agitation of the solution at the interface of the membrane and thus decreasing the diffusion layer thickness (Figure 11); the total increment of the water splitting in the overlimiting current is really small (around 5%) and this mechanism cannot be considered as a main mechanism [9]. Two types of electroconvective vortexes can be distinguished: relatively small and stable vortexes (low voltage or current intensity, limiting region) and unstable vortexes of larger sizes, appearing at thresholds of 1 V/pair of cells, in the over-limiting region [72] (Figure 11). Contemporary methods of investigations of the electro-osmotic instability such as laser interferometry, flicker noise spectroscopy, particle tracking velocimetry and microfluidic ED platforms coupled with electrochemical methods and modeling allow deeper understanding of this phenomenon [59,73–77]. For more fundamental information and equations about electroconvection please referred to the reviews of Nikonenko et al. [9,51].PDF Image | Electrodialytic Processes
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