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

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 (Delta G degrees), standard enthalpy change (Delta H degrees), and standard entropy change (Delta S degrees) of the micelle shuttle are extracted from the temperature dependence of the micelle distribution. Both Delta H degrees and Delta S degrees 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][TPSI]/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.