As the lipophilicity and size of BUs increased when going from 2 to 3 and 4, a clear decrease in transport activity was found (green, blue and violet curves in Figure ). A too high lipophilicity might prevent BU from residing at the membrane/aqueous interface, preventing the effective exchange of the anions with the aqueous solution. Additionally, a too large size might slow down the diffusion of the BU through the membrane.