The Role of Experimental and Co-ion Specific Factors in Membrane Permselectivity

The apparent permselectivity of an ion exchange membrane is a critical ion transport property that influences the efficiency of electric field-driven membrane technologies and often is measured using a pseudo-steady state measurement technique. This study first examined three major experimental factors that might affect the accuracy of permselectivity measurement. Among them, temperature had a small influence on apparent permselectivity properties for two commercially available membranes (i.e., Selemion CMV and CMI-7000s), as the value of apparent permselectivity decreased by approximately 2% as temperature increased from 14oC to 31oC. Membrane potential measurement fluctuations contributed approximately 0.2% to 0.5% uncertainty to apparent membrane permselectivity. Deviations from target sodium chloride solution concentrations of 10ppm/L introduced approximately 0.015% to 0.1 error, respectively, in apparent permselectivity. The magnitudes of these uncertainties typically are comparable to the magnitude of the measurement variability associated with disassembling and reassembling the measurement cell between replicate measurements made on the same sample, so the overall influence of the experimental factors considered in this study on apparent permselectivity is expected to be generally small.
At the same time, the permselectivity, co-ion sorption coefficient and co-ion diffusion coefficient of a lab-prepared CEM, XLAMPS was also characterized using four different salts: sodium chloride, sodium bromide, sodium nitrate and sodium perchlorate. Co-ion species were also found to intrinsically affect membrane permselectivity via both co-ion sorption and co-ion diffusion. The membrane permselectivity of XLAMPS, a lab-prepared CEM, follows the trend: α_NaCl>α_(NaNO_3 )>α_NaBr>α_(NaClO_4 ). The co-ion sorption coefficient of XLAMPS follows the trend: K_(NaClO_4 )>K_(NaNO_3 )>K_NaBr>K_NaCl, whereas the co-ion diffusion coefficient of XLAMPS follows the opposite trend: D_NaCl^m>D_NaBr^m>D_(NaNO_3)^m>D_(NaClO_4)^m. These three trends, combined together, reveal a competing mechanism between co-ion sorption and co-ion diffusion in determining the relative magnitude of membrane permselectivity. The salt sorption coefficient is connected with bare co-ion radius and co-ion excess polarizability through a continuum dielectric model. According to the model, large bare co-ion radius and large co-ion excess polarizability are related with lower co-ion sorption energy barrier and higher co-ion sorption coefficient. However, for sodium perchlorate, the formation of NaClO4-EO complexation contributes to its high sorption coefficient as well. The relative magnitude of salt diffusion coefficients of sodium bromide and sodium nitrate can be inversely connected with their hydrated radii. Whereas for sodium chloride and sodium perchlorate, hydrated radii might not be the only influencing factor. Repulsive interactions might exist between sodium chloride and membrane polymer matrix, and attractive interactions might exist between sodium perchlorate and membrane polymer matrix. The former will increase salt diffusion coefficient in membrane and the later will reduce salt diffusion coefficient in membrane.