CAN THE FORMATION OF VESICLES FROM SINGLE-CHAIN PERFLUOROALKYLATED AMPHIPHILES BE PREDICTED

The formation of vesicles from single-chain perfluoroalkylated amphiphiles was recently reported. We wanted to know whether this aggregation behaviour, which is unusual for single-chain surfactants, was predictable using the amphiphile self-organization model described by J. Israelachvili, D.J. Mitchell and B.W. Ninham, J. Chem. Soc., Faraday Trans. 2, 72 (1976) 1525-1568. The pertinent surface-activity data and geometrical characteristics needed to calculate the packing parameter P were determined for two single-chain perfluoroalkylated amphiphiles (a neutral phosphoramidate and a zwitterionic phosphocholine derivative) and for their hydrogenated analogues. It was found that the polar-head surface area remained identical for fluorinated and hydrogenated amphiphiles while, as expected, the hydrophobic chain volume of the former was significantly larger. The structures predicted from these results are vesicles in the case of the perfluoroalkylated amphiphiles (P=0.7) and micelles in the case of their hydrogenated analogues (P=0.5), in agreement with our previously reported experimental results. The formation of vesicles instead of micelles from F-alkylated single-chain amphiphiles can thus be explained by their truncated-cone geometry. It is also suggested that the bulkiness of the fluorinated chains brings them closer together, thus increasing lateral hydrophobic interactions.