PH-sensitive vesicles and rheological properties of PFLA/NaOH/H <inf>2</inf>O and PFLA/LiOH/H<inf>2</inf>O systems

Abstract

The formation of pH-sensitive vesicles and the rheological properties of the mixtures of perfluorolauric acid (PFLA) and its salts (PFL-Na and PFL-Li) neutralized via NaOH or LiOH were investigated in aqueous solution. When the right mixing ratios of the ionized to nonionzed PFLA molecules with a very high Krafft point are established, vesicles can spontaneously form at room temperature. The vesicles spontaneously formed in the PFLA/PFL-Na/H2O system with the rigid fluorocarbon chains were determined by atomic force microscopy images. Compared to those of hydrocarbon amphiphiles, these vesicle samples, which can be kept for 2 years at room temperature, are more stable. The phase transition from the vesicle phase to the lamellar lyotropic liquid crystal phase with the increase of pH was determined by freeze-fracture transmission electron microscopy images in the PFLA/PFL-Li/H2O system. The system of perfluoro fatty acid vesicles exhibits much more interesting rheological properties, compared to hydrocarbon fatty acid vesicles. The perfluoro fatty acid vesicle solutions display a much higher yield stress and viscoelastic properties, which depend on two factors: (i) the fluorinated alkyl chains of PFL-, which are in the crystalline state at room temperature because of their rigid chains compared to analogous hydrocarbon chains, and (ii) the packing of the vesicles, which is very dense. This is the first time that pH-sensitive vesicles exhibiting birefringence were constructed through ionizing perfluoro fatty acid, which may direct primarily toward acquiring an understanding of the mechanism of vesicles depending on the right mixing ratios of the ionized to the nonionzed perfluoro fatty acid molecules with a very high Krafft point and secondarily to expand the development of fluorinated surfactants in both fundamental research and practical applications. © 2011 American Chemical Society.