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(l,2-butadiene) homopolymer and then loaded in amphiphilic poly((l,2-butadiene)Woc£- 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 (ΔG°), standard enthalpy change (ΔH°), and standard entropy change (ΔS°) of the micelle shuttle are extracted from the temperature dependence of the micelle distribution. Both ΔH° and ΔS° 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.