Effect of counterion hydration numbers on the development of Electroconvection at the surface of heterogeneous cation-exchange membrane modified with an MF-4SK film

Abstract

The transport of sodium, calcium, and magnesium ions through the heterogeneous cationexchange membrane MK-40, surface modified with a thin (about 15 μm) homogeneous film MF-4SK. By using chronopotentiometry and voltammetry techniques, it has been shown that the combination of relatively high hydrophobicity of the film surface with its electrical and geometrical (surface waviness) heterogeneity creates conditions for the development of electroconvection, which considerably enhances mass transfer in overlimiting current regimes. The electroconvection intensity substantially depends on the degree of counterion hydration. Highly hydrated calcium and magnesium ions involve in motion a much larger volume of water as compared with sodium ions. When constant overlimiting direct current is applied to the membrane, electroconvective vortices in 0.02 M CaCl₂ and MgCl₂ solutions are generated already within 5–8 s, a duration that is the transition time characterizing the change of the transfer mechanism in chronopotentiometry. The generation of vortices is manifested by potential oscillations in the initial portion of chronopotentiograms; no oscillation has been observed in the case of 0.02 M NaCl solution. More intense electroconvection in the case of doubly charged counterions also causes a reduction in the potential drop (Δφ) at both short times corresponding to the initial portion of chronopotentiograms and long times when the quasi-steady state is achieved. At a fixed ratio of current to its limiting value, Δφ decreases in the order Na⁺ > Ca²⁺ > Mg²⁺.