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Flat-sheet-supported liquid membranes incorporating lasalocid A (a natural ionophore), were previously shown to be permeable to metal cations (Cd 2+ and Zn 2+ ) against a proton gradient (∆pH), which is the driving force of the process [1]. This transport process has been extended to other metal species such as Pb 2+ , Na + , and Ag + and also to the case where two metal species compete for transport. A higher transport flux for Pb 2+ as compared to Cd 2+ and Zn 2+ is observed and partly explained by a higher rate of interfacial complexation owing to the smaller hydration shell of this species. This effect is confirmed by the data obtained with Ag + . However, the size of the metal cation in relation to its hydration shell does not appear as the major parameter to take into account for an estimate of the trans-membrane transport efficiency, as the Na + ions escape to this behavior, which has been considered until now as a general trend for metal cation ionophores.

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