Molecular self assembly in flurocarbon surfactant / water systems

Abstract

The work presented is a continuation of the study of a homologous series of tetrabutylammonium perfluoroalkylcarboxylate surfactants in water. These systems showed phase behaviour uncharacteristic of ionic perfluorocarbon surfactant systems in that they exhibit a clouding phenomenon with increasing temperature. This behaviour was ascribed to the tight association of the large, hydrophobic counterions with the poiar head group region. In this study a series of perfluorocarbon surfactants have been synthesised in which the hycirophobicity of the counterion is varied. The counterion is W (CH2CH2CH2CH3)(CH3)
41 whilst the surfactant ion remains unchanged throughout the series as periluorodecanoate. The number of butyl chains. 'n' controls the hydrophobicity of the counterion and, in these experiments, n = 4, 2, 1 and 0. The phase diagrams and the
detailed phase structures have been investigated using optical polarising microscopy, 2H NMR spectroscopy and small angle x-ray scattering.
As n decreases, the phase diagrams change, recovering the "generic" phase behaviour more usual for a perfluorocarbon surfactant - water system. X-ray measurements show that in the n = 4 (tetrabutylammonium perfluorodecanoate) system, all the phases (both liquid crystalline and non-liquid crystalline) have uniform mean interfacial curvature. The phases
observed are L 1 (vesicles), L. and L 2. With decreasing counterion hydrophobicity, the population of counterions associated with the interface decreases, introducing greater curvature into the system. The mean curvature of the phase structures also becomes nonuniform.
For n = 0 (tetramethylammonium perfluorodecarioate) no classical mesophases are observed. Much of the liquid crystalline region is taken up with a random mesh intermediate phase, Mh 1 (0) and an extensive rhombohedral mesh intermediate phase, Mh1 (R3 m). Phase behaviour intermediate between the two extremes is observed at n = 1 (butyltrimethylammonium perfluorodecanoate). In this system, the clouding phenomenon is not observed but there is a two phase region of L 1 + L. at low concentration and high temperatures. The phase structures also possess uniform mean curvature. In keeping with
the less hydrophobic systems, the L. phase is less temperature sensitive at high concentrations.
This work has shown that the but'l groups of the counterion are, in part, responsible for the unusual phase behaviour observed in the TBA surfactants. The hydrophobic nature of the counterion has a major impact on the structures formed even at high dilution. This nature probably drives the counterion to the interface which affects the type of mesophase formed
at higher concentrations and also determines its stability with respect to temperature and concentration.