The Interaction between PEO-PPO-PEO Triblock Copolymers and Ionic Surfactants in Aqueous Solution Studied Using Light Scattering and Calorimetry

Abstract

Properties of nonionic triblock copolymers of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) (EOnPOmEOn) in aqueous solution and their interaction with the ionic surfactants sodium dodecyl sulfate and hexadecyltrimethylammonium chloride have been investigated by static and dynamic light scattering, high sensitivity differential scanning, and isothermal titration calorimetry. The studied copolymers (denoted P123 and F127) have the same hydrophobic PPO central block (m = 68), but different length of the endblocks, n = 20 and 97. At 40 °C, the copolymers are associated into micelles with hydrodynamic radius of 9.8 nm (P123) and 12.5 nm (F127) composed of a hydrophobic PPO core and a water-swollen PEO corona. The different copolymer/surfactant systems have been investigated at a constant copolymer concentration of 1 wt % and with varying surfactant concentration up to about 120 mM. When ionic surfactants are added to the PEO−PPO−PEO block copolymer micellar systems, three concentration regimes are observed in the results from the complementary experimental techniques. At low surfactant concentrations (<1−2 mM), single surfactant molecules associate with the copolymer micelle forming a large copolymer-rich complex that becomes increasingly charged. The relaxation time distributions from dynamic light scattering are monomodal and the electrostatic interaction is evidenced in both the static and the dynamic light scattering results. In the intermediate surfactant concentration regime, two types of copolymer−surfactant complexes coexist, one large copolymer-rich complex and one small complex consisting of one or a few copolymer chains and rich in surfactant. This indicates a peel-off mechanism behind the disintegration of the copolymer micelles. The titration calorimetric data present an exothermic signal as the PPO blocks become rehydrated when the copolymer micelles break-up. At high surfactant concentrations, only the small surfactant-rich complexes are present in the systems.