Thermodynamics and Mechanism of the Block Copolymer Micelle Shuttle between Water and an Ionic Liquid

Abstract

The micelle shuttle utilizing block copolymer micelles as nanocarriers for transportation between water and a hydrophobic ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]), is examined in detail. Rhodamine B, a dye with a high molar absorptivity and fluorescence quantum yield, is conjugated to a short poly(1,2-butadiene) homopolymer and then loaded in amphiphilic poly((1,2-butadiene)-block-ethylene oxide) (PB-PEO) block copolymer micelles. The round-trip transportation of the micelles between water and the ionic liquid is simply triggered by temperature; it is fully reversible, quantitative, and without leakage. Quantitative fluorescence analysis reveals that the micelle distribution in the biphasic system has a very strong temperature dependence, which is favorable for control of the transportation. The standard Gibbs free energy change (DeltaG(o)), standard enthalpy change (DeltaH(o)), and standard entropy change (DeltaS(o)) of the micelle shuttle are extracted from the temperature dependence of the micelle distribution. Both DeltaH(o) and DeltaS(o) are positive, indicating an entropy-driven process. The slow yet spontaneous micelle shuttle is explored under quiescent conditions to understand the transfer kinetics. Both of the two-way transfers involve three steps, formation of micelle-concentrated [EMIM][TFSI]/water droplets in the initial phase, sedimentation/creaming of the droplets to the interface, and diffusion of the micelles to the destination phase. A detailed mechanism for the transfer is therefore proposed.