A calorimetry and light scattering study of the formation and shape transition of mixed micelles of EO20PO68EO20 triblock copolymer (P123) and nonionic surfactant (C12EO6)

The interaction between the nonionic surfactant C12EO6 and the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer EO20PO68EO20 (P123) has been investigated by means of isothermal titration and differential scanning calorimetry (DSC) as well as static and dynamic light scattering (SLS and DLS). P123 self-assembles in water into spherical micelles at ambient temperatures. At raised temperatures, the DSC data revealed a sphere-to-rod transition of the P123 micelles around 60 degrees C. C12EO6 interacts strongly with P123 micelles in aqueous solution to give mixed micelles with a critical micelle concentration (cmc) well below the cmc for pure C12EO6. The presence of C12EO6 also lowers the critical micelle temperature of P123 so aggregation starts at significantly lower temperatures. A new phenomenon was observed in the P123-C12EO6 system, namely, a well-defined sphere-to-rod transition of the mixed micelles. A visual phase study of mixtures containing 1.00 wt % P123 showed that in a narrow concentration range of C12EO6 both the sphere-to-rod transition and the liquid-liquid phase separation temperature are strongly depressed compared to the pure P123-water system. The hydrodynamic radius of spherical mixed micelles at a C12EO6/P123 molar ratio of 2.2 was estimated from DLS to be 9.1 nm, whereas it is 24.1 nm for the rodlike micelles. Furthermore, the hydrodynamic length of the rods at a molar ratio of 2.2 is in the range of 100 nm. The retarded kinetics of the shape transition was detected in titration calorimetric experiments at 40 degrees C and further studied by using time-resolved DLS and SLS. The rate of growth, which was slow (> 2000 s), was found to increase with the total concentration.111215911592

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.10661239124

Interaction Between Peo-ppo-peo Triblock Copolymers And Ionic Surfactants In Aqueous Solution Studied Using Light Scattering And Calorimetry

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. 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