Enhancing Ion Transfer in Overlimiting Electrodialysis of Dilute Solutions by Modifying the Surface of Heterogeneous Ion-Exchange Membranes

Table 1

Some characteristics of commercial MK-40, MA-40, and surface-modified MK-40/Nf and MA-40M membranes.

Membranes	Cation exchange		Anion exchange
	MK-40	MK-40/Nf	MA-40	MA-40M
Ion exchange groups, bulk	–SO3^−	–SO3^−	=NH, ≡N	=NH, ≡N
Idem, surface	–SO3^−	–SO3^−	=NH, ≡N	–N^+(R)3
Thickness, μm	480±10	500±10	470±20	470±20
Ion exchange capacity^1, mM cm^−3 wet	1.7±0.1	1.7±0.1	3.2±0.08	3.2±0.08
Surface fraction of ion exchange material^2, %	22±3	100	19±4	19±4
Contact angle^3, degrees	55±3	64±3	50±2	47±2
Specific electrical conductivity in 0.5 M NaCl solution^4, mS cm^−1	7.7±0.3	8.3±0.3	7.5±0.3	5.5±0.3
Diffusion permeability^5, 10^−8 cm^2 s^−1	6.7±0.4	6.6±0.4	7.0±0.4	6.7±0.4

¹Ion exchange capacity was determined for wet membranes in the sodium form (cation exchange membranes) or in the chloride form (anion exchange membranes).
²Surface fraction of ion exchange material was determined for a wet membrane using SEM images by the method described in [21].
³Contact angle on the surface of wet membrane was found 20 s after placing a test drop, according to the method described in [18].
⁴Specific electrical conductivity of membrane in 0.5 M NaCl solution was determined by a difference method using a clip cell [28].
⁵Diffusion permeability of membranes was determined in H₂O/IEM/0.5 M NaCl system using the cell described in [29]; the modified side of MK-40/Nf membrane was facing the NaCl solution; the left- and right-hand sides of MK-40, MA-40, and MA-40M membranes are symmetrical.