- Phase Behavior and Structural Transitions in The Mixtures of Cationic Surfactants and Hydrophobic Counterions (2003)
- Anionic hydrophobic counterions with certain geometry adsorb onto the surface of cationic surfactant micelles and they minimize the repulsion between the headgroups, so the charge density on the surface is reduced. As a result of this, the micelle spontaneously changes its morphology due to a new packing for the head groups. The adsorption of 2-hydroxy-1-naphthoic acid 2,1 HNC and 6-hydroxy-2-naphthoic acid 6,2 HNC onto the surface of the cationic surfactant cetyltrimethylammonium hydroxide was studied. The results were compared to the published system 3-hydroxy-2-naphthoic acid 3,2 HNC/CTAOH. When an increasing amount of 2,1 HNC is introduced into a micellar solution of 100 mM CTAOH, one finds low viscous micellar solution, viscoelastic gel (consisting of rod like micelles), turbid region (two phase region), and viscoelastic liquid crystalline gel (consisting of multilamellar vesicles MLV with yield value). The complex viscosity (0.01 Hz) of 100 mM CTAOH rises by six orders of magnitude as the rodlike micelles form.It decreases then to the turbid region, and then rises again approximately six orders of magnitude. The second rising of the complex viscosity is accompained by the formation of a liquid crystalline phase which consists of multilamellar vesicles. This has been proven by DICM, FF-TEM and Cryo-TEM. The vesicles were polydisperse and ranged from 100 to 1000 nm in diameter. SANS detected the transition in the microstructure which was caused by changing the concentration of 2,1 HNC in the system. SANS calculations show results similar to that obtained by microscopic methods. Surprising rheological behavior was measured in the rodlike micelle region, at which storage modulus was about one order of magnitude higher than loss modulus and both were parallel in the frequency range 0.001-10 Hz. Such behavior usually indicates the presence of vesicles in the liquid crystal phases. It was proved that other rheological measurements can be used to distinguish the tow types, namely, amplitude sweep measurements, first normal stress difference N1 (Weissenbeg effect), the effect of adding electrolyte, and stress relaxation curves. When 6,2 HNC (new substitution of HNC) is added with an increasing amount to 100 mM CTAOH, a new phase behavior is observed. Here the structure changes from small micelle aggregates into rodlike micelles, and then a two phase region consisting of L1-phase and un-reacted 6,2 HNC is formed. No transition into MLV has been detected. In the case of 3,2 HNC and 2,1 HNC, the hydroxyl and the carboxyl group are neighboring, so they can effectively share in reducing the repulsion between the headgroups while the rings are in interaction with hydrocarbon tails. For 6,2 HNC the hydroxyl group is in position number 6 on the aromatic rings, which means that hydroxyl group is distant from the carboxyl group, thereby, less screening for the cationic charge in the micelle surface is obtained. Substitution of HNC plays a main role in controlling the microstructure and other physical properties such viscosity, Krafft point, ..etc. In the second part of this work, the hydrophobic counterion is fixed (2,1 HNC), and the length of the cationic surfactant‘s chain is changed from C16 into C14, C12, C10 and C8. For the system 2,1 HNC/ tetradecyltrimethylammonium hydroxide TTAOH similar phase behavior as 2,1 HNC/CTAOH is observed. At 2,1 HNC/TTAOH ratio r aproximately 1, formation of MLV is observed. After the neutralization addition of excess amount of 2,1 HNC is possible since the insoluble molecular form of 2,1 HNC becomes solublized in the formed MLV. Conductivity measurements prove that 2,1 HNC stays in the molecular form after the neutralization. A difference in the rheological behavior of the system 2,1 HNC/TTAOH compared to 2,1 HNC/CTAOH is seen. In the rodlike micelles region of 2,1 HNC/TTAOH, the solutions exhibit a short relaxation time compared to 2,1 HNC/CTAOH system. FF-TEM and SANS proved the formation of polydisperse MLV in this system with a maximum diameter of about 2000 nm and wall thickens of about 28 nm. As a result of this work, it is concluded that the role of the hydrophobic counterions with certain geometry could be looked upon as a co-surfactant with a shorter chain length which changes the bending rigidity, of the bilayer. They are surface active species that bind strongly on the micelle surface and change the packing parameter of the headgroups. It is suggested that the hydrophobicity of the counterion plays an important role in deciding the structure of the supramolecular assemblies such as vesicles, or micelles. As a consequence one can change the morphology of micelle species by changing the ratio of counterion /surfactant ion. These studies also suggest that by mixing cationic surfactant and hydrophobic counterion with varying cationic surfactants chain lengths, one can have a control over the supramolecular structures formed.