Molecular packing and lateral interactions of distearoylphosphatidylcholine with dihexadecyldimethylammonium bromide in Langmuir monolayers and vesicles

The behavior of distearoylphosphatidylcholine (DSPC) mixed with dihexadecyldimethylammonium bromide (DHDAB) in the monolayer was investigated by means of Langmuir trough, interfacial thermodynamic analysis, and Brewster angle microscopy. It was found that the cationic surfactant, DHDAB, was miscible with DSPC and a condensing effect, indicating stronger contraction of area per molecule and stronger ordering molecular packing, appeared in the mixed DSPC/DHDAB monolayers. Condensed structures of the mixed monolayers were visible as the molar fraction of DHDAB (X_DHDAB) ≥ 0.7. The negative deviations of excess area and mixing Gibbs free energy were obtained, and their minimum values occurred at X_DHDAB = 0.3, suggesting that a DSPC/DHDAB monolayer with this composition exhibited the most pronounced intermolecular interactions with a compact molecular arrangement than the monolayers with separation between individual components. Furthermore, characteristics of mixed DSPC/DHDAB vesicles dispersed in water were studied by dynamic light scattering, transmission electron microscopy, and fluorescence polarization. The DSPC vesicles added with DHDAB showed zeta potentials of about +50 mV and narrower size distributions than those of pure DSPC vesicles. DSPC formed more rigid membranes than DHDAB, and the minimum disordering effect on membrane packing of vesicles was found at X_DHDAB = 0.5, which was the most stable vesicle composition against aggregation. By contrast, the condensing effect and the increase of intermolecular attraction in mixed DSPC/DHDAB monolayers may be related to the stability enhancement of mixed vesicles as compared with the stability of pure component vesicles.