Vesicle formation in oleyldimethylamine oxide/sodium oleate mixtures

Vesicle formation in a mixture of oleyldimethylamine oxide (OleylDMAO) and sodium oleate (NaOl) was investigated by viscoelastic measurements and cryoscopic transmission electron micrograph (cryo-TEM) observations. The viscoelastic properties changed with increasing mole fraction of NaOl (X-NaOl) from the Maxwell behavior of OleylDMAO solutions (X-NaOl=0) suggesting a transient network of long flexible chains. For X-NaOl=0.2 and 0.4 mixtures, both the shear storage modulus G' and the shear loss modulus G' showed weak dependences on angular frequency omega with a relation G'>G'. From cryo-TEM observations, vesicles coexisted with threadlike micelles in mixtures of X-NaOl=0.2 and 0.3. As X-NaOl increased further (X-NaOl=0.5 and 0.6), threadlike micelles disappeared and the coexistence of vesicles and globular micelles was observed. At X-NaOl=0.5, the viscosity decreased remarkably, which was consistent with the disappearance of threadlike micelles. The results indicated that vesicles were formed by the addition of NaOl to OleylDMAO solutions, contrary to the expectation of a decrease of the packing parameter with the introduction of electric charges.