Counterion identity effects on the self-assembly processes in a series of perfluorinated surfactant - water mixtures

Zhou, Rongrong (2003) Counterion identity effects on the self-assembly processes in a series of perfluorinated surfactant - water mixtures. Doctoral thesis, University of Central Lancashire.

[thumbnail of Thesis document] PDF (Thesis document) - Submitted Version
Restricted to Repository staff only
Available under License Creative Commons Attribution Non-commercial Share Alike.



The effects of counterion on the lyotropic liquid crystalline phase behaviour of some quaternary ammonium salts of perfluorodecanoic acid in water have been studied using a combination of optical polarising microscopy (OPM), deuterium nuclear magnetic resonance (211 NMR) and cryo-transmission electron microscopy (cryo-TEM). The results from the phase diagram studies flill into two groups. Firstly the ammonium (A) and tetramethylanmionium (TMA) counterions show a phase behaviour with nematic (N) and random mesh (MJi 1 (0)) phase which possess non-uniform interfacial curvature.
The second group of surfactants with counterions, butyltrimethylammonium (BTMA), dibutyldimetylammonium (DBDMA), and tetrabutylammonium (TBA), form only a classical lamellar phase (La). For both DBDMA and TBA lower consolute behaviour
has been observed. At fixed concentration in all five systems cryo-TEM visualises isotropic liquid phase structures that vary from sphere / rod micelles for A and TMA to vesicles / bilayer pieces for the other surfactants. These results are consistent with a reduction of the interfacial curvature of the aggregates, a phenomenon explained by a closer association of the counterion with the micellar surface. This is postulated to be driven by an increase in the size and hydrophobicity of the counter-ion from A to TBA.
The correlated mesh phase (Mh i (R3 m)) is unique to the TMA surfactant of this series. It is extremely stable, both thermally and in surfactant concentration, when compared to such phases formed in other surfactant systems. To further elucidate the effect of countenon and investigate the mechanisms that stabilise mesh intermediate phases the effects of additives on the Mh 1 (R3 m) have been studied. The evolution of the phase behaviour and lyotropic phase structures formed in the TMA system have been studied upon the addition of salt (tetramethylammonium chloride, TMAC1), cosurfactant (111,1H-perfluoro-I-decanol, CiooI) and oil (perfluorooctane, C goil) using OPM, 211 NMR and small angle X-ray scattering (SAXS). Upon addition of a third component the Mh1(R3 m) is lost and a Mii j (0) stabilised. Further addition of the third component
drives the formation of L. phase in the TMACI and C 10o1 addition systems. The structure of Mli 1 (R 3 m) phase is essentially unaltered over that of the binary phase irrespective of the amount or type of third component added. In the case of TMAC1 addition the formation of the Mh 1 (0) is driven by a reduction in the electrostatic interlayer interaction via a screening effect of the added counter-ions. In the C10o1 and C8oil addition systems the phase transition is driven by an alteration in the packing of the hydrophobic interior of the aggregates, which is termed a 'hydrophobic packing constraint'. SAXS experiments show that upon the transition from the Mh 1 (R3 m) to Mh1 (0) phase the surface area per molecule is conserved (within experimental error) and occurs more rapidly in the presence of C 10o1 and Cgoil indicating that the stability of the former over the latter requires a well defined intra-layer topology. The Mh 1(0) to L. transition is driven by a closure of defects as the additive concentration decreases the surface area per surfactant molecule via enhanced counter-ion binding in the TMACI experiments, or surface charge dilution in the C 10o1 addition systems.

Repository Staff Only: item control page