The Interaction between Triblock Copolymers and Surfactants in Dilute Aqueous Solution

Abstract: The solution properties of dilute aqueous solutions of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers and their interaction with surfactants have been investigated using static and dynamic light scattering, combined with small angle X-ray scattering, calorimetry and nuclear magnetic resonance techniques. In aqueous solution, the triblock copolymers P123 and F127 (EO20PO68EO20 and EO97PO68EO97, respectively) form micelles with a PPO core and a PEO corona. The triblock copolymer L121 (EO5PO68EO5) forms vesicles, that are quasi-stationary, by increasing the temperature into a two-phase region. The P123 and the F127 micelles interact with a cationic (hexadecyltrimethylammonium chloride), an anionic (sodium dodecyl sulfate) and a nonionic [hexa(ethylene oxide) monododecyl ether] surfactant. The interaction behavior can be divided into three different surfactant concentration intervals. At low surfactant concentrations the copolymers and surfactants form a spherical mixed complex, which resembles a block copolymer micelle. In the ionic surfactant cases, the low surfactant concentration regime is followed by a narrow concentration interval, where two coexisting complexes (a large block copolymer micelle-surfactant complex and a small surfactant-copolymer complex) are found. At high ionic surfactant concentrations, the copolymer micelles are broken up due to the electrostatic interaction and only small surfactant-copolymer complexes are left. The surfactant-copolymer complex resembles a pure surfactant micelle. In the case of the nonionic surfactant, a single mixed complex is formed over the whole surfactant concentration range investigated. This complex undergoes a structural change and grows in one dimension into a rod-like complex at elevated temperatures (40 °C and above) and at high surfactant concentrations.