The origin of anomalous positive heat capacity change upon micellization of Pluronic triblock copolymer F108 in aqueous solutions: Effect of PEO-PPO diblock impurities

Abstract

It is well understood that the heat of micellization is monotonically decreased along with increasing temperature. However, it was found (Langmuir 2008, 24, 13858.) that the heat of micellization for Pluronic triblock copolymer in aqueous solutions abnormally increases along with the temperature at high Pluronic concentration region. The origin of this anomalous positive heat capacity change upon micellization of Pluronic F108 in aqueous solutions was explored by using differential scanning calorimetry, chromatography and dynamic light scattering. The impurities of commercial Pluronic F108, used as received, in aqueous solution form large aggregates at the temperatures well below its critical micelle temperature (CMT). At high F108 concentrations (>2.4wt.%), the aggregates almost completely dissociate and then integrate into micelles, resulting in anomalous positive relationship of heat of micellization (ΔH) with critical micelle temperature (CMT). By contrast, at low F108 concentrations (<2.4wt.%), the aggregates do not, or only partly, participate the micellization. When the impurities of F108 are separated and removed by using high performance liquid chromatography, the ΔH of purified F108 aqueous solutions monotonically decreases along with an increase in CMT. That is, a typical linear relationship of ΔH vs. CMT is resumed. It is interesting to find out that the ΔH of 0.05wt.% purified F108 aqueous solution (234.1kJ/mol) is higher than that of unpurified F108 (174.2kJ/mol) by 25.6%.