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Investigations on the Behaviour and Properties of Different Types of Unilamellar Vesicles
(2005)
- The present work is a study that concentrates on spontaneous or quasi-spontaneous formation of vesicles in the case of single chained oleate surfactant and mixed vesicles of catanionic aggregation of oleate with some cationic surfactants, with a focus on the phase behaviour. Aqueous solution of 200 mM Na-oleate was isotropic micellar and was low viscous. Addition of octanol and geraniol as co-surfactants continuously lowered the mean spontaneous curvature on micellar interface, since they have a small head group area which increases the packing parameter. The systems undergo a phase transition with increasing the concentration of co-surfactants from micellar phase to vesicular phase via a two-phase region. For the system with geraniol, on the other hand, it was observed that at higher concentration of surfactant, the system behaves as in the case of octanol; but when the concentration was lower than 100 mM, the system was transforming from L1 phase to vesicular phase without forming any biphasic region. DLS measurements and microscopic observations clearly revealed that both the co-surfactants form spherical vesicles with Na-oleate. Cryo-TEM observations for the systems with both co-surfactants showed that the vesicles are unilamellar and spherical. The rheological measurements show that the shear viscosity increases with increasing the concentration of co-surfactants in L1 phase. In Lves phase the viscosity increases very rapidly with co-surfactant concentration. The effects of an organometallic [Ti(OBu)4] have been studied on the vesiculation of surfactant and co-surfactant system. The kinetic measurements by monitoring the turbidity it has been revealed that the time for the reduction of turbidity increases with increasing the metal precursor in the composition. SAXS measurements revealed that size and ordering of the vesicles remain almost the same with increasing the TTB concentration. Rheological measurements illustrate that viscoelastic property decreases with increasing the concentration of TTB and the system transforms from stiff gel state to liquid state with higher TTB concentration. Catanionic interaction between two oppositely charged parent-surfactants oleic acid and decyltrimethyl ammonium hydroxide (C10TAOH) is like an acid-base interaction. Phase behaviour of their interaction revealed that a narrow region with equimolar mixing composition of two components form an isotropic partially bluish vesicular phase in the lower concentration range. DLS measurements and microscopic observation by cryo-TEM indicate that equimolar composition and XC10TAB = 0.4 at 75 mM have the vesicular morphology. Sizes obtained by DLS and cryo-TEM are in good agreement. In order to see an alternate way of unilamellar vesicle formation catanionic systems have been investigated. For this study four different cationic surfactants were selected to interact with an anionic surfactant, Na-oleate. Phase diagram and measurements of electrical conductivity clearly revealed that admixing of C8TAB with Na-oleate does not form any phase separation and Lves phase. Other three cationic surfactants form Lves phase which is separated from L1 phase by a two-phase region which indicates that all the phase transformations are following the first order phase transition rule. In the cases of C10TAB and C12TAB, only one vesicular phase has been observed. For C10TAB the mixing composition of Lves phase was always equimolar and it occurs after exceeding the concentration of 20 mM. But for C12TAB, the vesicular phase was observed in anionic rich region including equimolar composition at low concentration. At higher concentration system was phase separated at equimolar composition. The ternary system of C14TAB shows completely different phase behaviour with aqueous Na-oleate. In this case two Lves phases have been observed. One Lves phase is anionic rich with composition around XC14TAB = 0.3 and other Lves is cationic rich with composition around XC14TAB = 0.6. The rheological measurements showed Newtonian behaviour for the dilute dispersion of vesicles at ≤ 50 mM total concentration. At the total concentration of 100 mM, vesicular compositions with C10TAB and C14TAB systems show a pronounced shear thinning behaviour. The results of DLS measurements further reveal that all catanionic systems with C10TAB, C12TAB and C14TAB form vesicles at the compositions described above. The hydrodynamic radii of vesicles increases with increasing the alkyl chain length of cationic surfactants. Finally the mixed vesicular system composed with anionic aggregation and catanionic aggregation was studied. Here anionic aggregation was Na-oleate with octanol as co-surfactant and catanionic aggregation was 1:1 mixing composition of aqueous Na-oleate with C10TAB. Thermodynamic behaviour of mixed aggregation system revealed that they did not form bigger aggregates at any composition when the concentration was 25 mM. The bigger aggregates were observed in catanionic rich compositions.
