Experimental studies of microstructural evolution in mixed cationic/anionic surfactant systems
Various aqueous mixtures of the oppositely charged surfactants combine to form vesicles at a certain mixing ratio. The evolution of aggregate microstructures as well as the dynamics of the formation of vesicles starting with different surfactant pairs of aqueous solutions based on compositions, different molecular structures, tail lengths and addition of monovalent salt has been investigated. ^ The growth of the aggregates during the first two hours after mixing, measured by dynamic light scattering, was found to be sensitive to compositions, tail structures and tail lengths of surfactant pairs, but not to addition of monovalent salt. ^ The morphology of intermediate state aggregates are directly identified using cryogenic transmission electron microscopy. For aqueous solutions of CTAB/SOS and DTAB/SDS, the small vesicles and disks are observed. However, for aqueous solution of CTAB/HDBS, no disks are observed, indicating a fast vesiculation. The molecular architectures of surfactants influence the interplay between the bending and edge energies, resetting evolving microstructures. ^ The bending modulus has been estimated from the size distribution of vesicles imaged by cryo-TEM. All estimated bending rigidities for the present mixed surfactants are around the order of magnitudes of 10−21 or 10−22 J and in range of 0.04∼1.0 kT (±28%); The bending modulus depends on the structures and asymmetry of tail lengths of surfactant pairs. ^ With a decrease in total surfactant concentration in CTAB/SOS/water solutions, disk growth slows down and the transformation to vesicles gets delayed. For lower total surfactant concentrations, only flexible disks are found in the first two hours, while at a higher total surfactant concentration, the coexistence of disks and vesicles is observed. ^ With added salt, the mean diameter of initial vesicles D0 decreases by about 27% and Deq, increases about 25%. This could be attributed to the change of ionic strength, because with addition of 0.5 wt% NaBr, Debye length decreases 20% from 10 Å to 8 Å, leading to a shorter range of the double-layer repulsion. Thus, anionic SOS monomers might penetrate cationic CTAB micelles to assemble into bilayers more easily and rapidly in early stage. ^
"Experimental studies of microstructural evolution in mixed cationic/anionic surfactant systems"
Dissertations and Master's Theses (Campus Access).