Interaction of Styrene with DODAB Bilayer Vesicles. Influence on Vesicle Morphology and Bilayer Properties

The solubilization of styrene in large unilamellar DODAB vesicles is investigated at a styrene to DODAB molar ratio of 2:1. The combination of various vesicle characterization methods allows a simultaneous look at vesicle morphology (cryo-TEM, DLS) and molecular interactions (micro-DSC, various fluorescence techniques) and gives a complete picture of the DODAB vesicles before and after the addition of styrene. Cryo-TEM and DLS results reveal that the addition of styrene does not break up the DODAB vesicles as an entity, but the peculiar angular DODAB vesicle morphology becomes smoother and the geometries tend to be more curved. The change in morphology is explained by an enhanced bilayer fluidity and the drastic depression of the phase transition temperature as determined from calorimetry and fluorescence experiments. Moreover, micro-DSC scans and fluorescence experiments with two different pyrene probes suggest a nonhomogeneous distribution and partial demixing of solute and bilayer for temperatures below ∼27 °C. Above this temperature, the solute appears uniformly distributed and facilitates molecular motion in the amphiphile aggregate. The diffusion coefficient for the lateral diffusion of an amphiphilic probe is then increased by a factor of 2 compared to the pure DODAB vesicles. The observed solubilization phenomena are rationalized by interactions of the solute with both the hydrocarbon part and the polar headgroup region of the bilayer.