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Interaction between salt-free catanionic TTAOH-LA mixtures and nonionic polymers: surface tension and rheological study.

By mixing tetradecyltrimethylammonium hydroxide (TTAOH) and lauric acid (LA) in water, the salt-free cationic/anionic (catanionic) surfactant system TTAOH/LA/H2O can be constructed. Based on our recent presentation of the TTAOH/LA/H2O ternary phase diagram, we investigate the interaction between TTAOH-LA mixtures and two kinds of nonionic polymers, Pluronic F127 (PEO-PPO-PEO triblock copolymer, EO(97)PO(69)EO(97)) and polyvinylpyrrolidone (PVP), by means of surface tension and rheological measurements. It is found that the presence of F127 has an obvious influence on the surface activity of the TTAOH/LA/H2O system in the TTAOH-rich side. Upon addition of F127, a smaller critical micellar concentration (cmc) and a higher surface tension after cmc (gamma(cmc)) were observed, which should be due to the complex formation between F127 and TTAOH-LA mixtures at the air/water interface. F127 is also found to associate strongly with TTAOH-LA mixtures in bulk aqueous solutions, as revealed by rheological measurements. The association leads to a significant decrease in micelle length and makes the vesicular phase more fluid. The influence of PVP on the surface activity and rheological properties of the TTAOH/LA/H2O system, however, is much less obvious compared with that of F127. In the TTAOH-rich side, only a very limited effect has been observed, indicating a rather weak association between PVP and surfactants. As the LA content increases, the influence of PVP on rheological properties of the TTAOH/LA/H2O system becomes detectable in the wormlike micellar region and vesicular region. However, the influence of PVP remains weaker compared with that of F127.