Interestingly, the para-CF3 compounds 2 and 7 were more active in the assay than the respective 3,5-bis-CF3 and para-SF5 derivatives. Aqueous solubility is less of an issue in these studies due to the lower concentrations required to facilitate transport, therefore it seems likely that lipophilicity plays a crucial role. The presence of two additional trifluoromethyl groups in receptors 4 and 9 greatly enhances the overall lipophilicity of the respective systems, and could result in unfavourable deliverability. A similar degree of lipophilicity is expected for para-SF5 derivatives 5 and 10, and the results suggest that these compounds also exceed the optimal degree of lipophilicity required for efficient chloride transport.