Influence of ionic surfactants on the aggregation of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers studied by differential scanning and isothermal titration calorimetry

The interaction between three triblock copolymers of poly(ethylene oxide) and poly(propylene oxide), EOnPOmEOn, and the ionic surfactants sodium dodecyl sulfate, SDS, and hexadecyltrimethylammonium chloride, CTAC, has been studied in dilute aqueous solution using differential scanning calorimetry, DSC, and isothermal titration calorimetry. The length of the PPO block was the same in all three copolymers (68-69 PO units), and the lengths of the PEO groups varied from 5, 20 and 97 EO units. The copolymers are denoted L121, P123, and F127 in order of increasing PEO block size. In dilute aqueous solution P123 and F127 aggregate to form micelles, while the most hydrophobic polymer, L121, forms aggregates which, eventually, separate to give a liquid crystalline phase. Differential scanning calorimetry was used to follow the effect on the copolymer aggregates upon addition of ionic surfactants. Addition of SDS to P123 and L121 increased the temperature for aggregation, but polymer aggregates still formed in 6.2 mmol/L SDS. The effect is different on F 127 where after an initial decrease in the aggregation temperature, the peak in the DSC curve flattens out and disappears at low SDS concentration, as has been observed previously. The addition of CTAC to solutions of the three polymers does not change significantly the aggregation temperature, but the transition peak decreases and eventually disappears in 2-3 mmol/L CTAC. The prominent feature of calorimetric titration curves at 40 degreesC from consecutive additions of surfactant to polymer solution is a well-defined exothermic peak that stems from the disruption of the polymer micelles/aggregates and accompanying hydration of the PPO block. The beginning of the peak indicates the start of binding of the surfactant to the polymer aggregates, and after the end of the peak, the titration curves indicate binding to polymer unimers. At 40 degreesC, about 20 SDS molecules per polymer chain are required to disarrange the P 123 micelles and L121 aggregates, while about 10 suffice to disrupt the F127 micelles. The same amount of CTAC, about 10 molecules per polymer chain, destroys the aggregates of all three polymers.