When the -SCF3 group in BU 7 was altered to its oxidised sulfoxide and sulfone variants, a decrease in the transport activity was found for 9, while for 10 no significant transport was observed (green, violet and pink curves in Figure ). On one hand, an increase in the binding strength can slow down the transport process if binding is too strong, but on the other hand, the Ka of BU 2 for Cl -is higher than that of either 9 or 10, yet 2 is still among the most active anionophores. An alternative explanation for the lower activity of BUs 9 and 10 can be proposed by considering that -SOCF3 and -SO2CF3 groups could be strongly hydrated, thereby impeding the detachment of the corresponding BUs from the membrane interface as required for the diffusion through the membrane.