Colloids and Surfaces A: Physicochem. Eng. Aspects 509 (2016) 109–115 Contents lists available at ScienceDirect Colloids and Surfaces A: Physicochemical and Engineering Aspects journa l h om epage: www.elsevier.com/locate/colsurfa The origin of anomalous positive heat capacity change upon micellization of Pluronic triblock copolymer F108 in aqueous solutions: Effect of PEO-PPO diblock impurities Ya-Hui Hsu a , Hung-Wei Tsui a,b , Shi-Yow Lin c , Li-Jen Chen a,∗ a Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan b Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 10608, Taiwan c Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan h i g h l i g h t s • Heat of micellization of commercial Pluronics abnormally increases with temperature. • Impurities of commercial Pluronic F108 would form aggregates. • Impurity aggregates would dissoci- ate and integrate into micelles upon micellization. • Heat of micellization of F108 increases with temperature after removing impurities. • Heat of micellization of F108 may enhance by 25.6% after removing impurities. g r a p h i c a l a b s t r a c t a r t i c l e i n f o Article history: Received 10 June 2016 Accepted 27 August 2016 Available online 2 September 2016 Keywords: Pluronic Dynamic light scattering Differential scanning calorimetry Critical micelle temperature Heat of micellization Impurity a b s t r a c t It is well understood that the heat of micellization is monotonically decreased along with increas- ing 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 tempera- ture 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 scan- ning 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 (>2.4 wt.%), the aggregates almost com- pletely dissociate and then integrate into micelles, resulting in anomalous positive relationship of heat of micellization (H) with critical micelle temperature (CMT). By contrast, at low F108 concentrations (<2.4 wt.%), 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 H of purified ∗ Corresponding author. E-mail address: ljchen@ntu.edu.tw (L.-J. Chen). http://dx.doi.org/10.1016/j.colsurfa.2016.08.070 0927-7757/© 2016 Elsevier B.V. All rights reserved.