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^sub NaOl^) from the Maxwell behavior of OleylDMAO solutions (X^sub NaOl^=0) suggesting a transient network of long flexible chains. For X^sub 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 ω with a relation G'>G''. From cryo-TEM observations, vesicles coexisted with threadlike micelles in mixtures of X^sub NaOl^=0.2 and 0.3. As X^sub NaOl^ increased further (X^sub NaOl^=0.5 and 0.6), threadlike micelles disappeared and the coexistence of vesicles and globular micelles was observed. At X^sub 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.[PUBLICATION ABSTRACT]