This study demonstrates the effective separation of alkali metal cations using a Supported Liquid Membrane (SLM) containing lipophilic, negatively charged borate moieties, operating under electro dialysis conditions. The selectivity of the membrane is essentially based on differences in dehydration energy and mobility between ion species. The system favors the ion species with the largest crystal radius, despite its lower mobility. In mixtures of K⁺ and Na⁺, the SLM separates K⁺ from Na⁺ with a separation efficiency ranging from ~20% to 90%, depending on the feed solution composition. With solutions containing either K⁺ or Na⁺ and Li⁺, the K⁺/Na⁺ over Li⁺ separation efficiency is nearly 100%. Addition of 15-crown-5 derivative does not improve SLM behavior, but slows down the K⁺ current by approximately 30% whereas the Na⁺ current remains unaffected. As supported by simulations, the free K⁺ and Na⁺ ratio in the membrane (and with that the current ratio) is entirely defined by partitioning and the feed concentration ratio, regardless the presence of 15-crown-5. As a result, the current ratio of two ion species can be described exclusively in terms of their feed concentrations and crystal radii because the latter parameter defines both partitioning and mobility.