Interactions and Self-Assembly in Ionic Liquid Solutions
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Ionic liquids, low-melting organic salts, have generated interest in diverse fields due to their unique properties. The extensive intermolecular interactions between their ions and other solute or solvent molecules render ionic liquids useful solvents or additives for various applications. However, the knowledge on how different types of interactions within the systems lead to certain structures and properties of ionic liquid-based materials, and how such interactions drive the self-assembly process remains limited. Ionic liquids are utilized as synthesis and dispersion media for nanoparticles as well as for surface functionalization. Ionic liquid and nanoparticle hybrid systems are governed by a combined effect of several intermolecular interactions between their constituents and lead to various self-organized structures. (He, Z.; Alexandridis, P., Nanoparticles in ionic liquids: interactions and organization. Phys. Chem. Chem. Phys. 2015, 17 (28), 18238-18261. doi:10.1039/C5CP01620G) Such hybrid materials exhibit interesting synergisms with novel and enhanced performance, which render them useful as novel materials especially in the catalysis, electrochemistry and separations fields. (He, Z.; Alexandridis, P., Ionic liquid and nanoparticle hybrid systems: Emerging applications. Adv. Colloid Interface Sci. 2017, 244, 54-70. doi:10.1016/j.cis.2016.08.004) Amphiphiles can self-assemble into ordered structure in water and its mixtures with various polar organic solvents. Ionic liquids are expected to affect the amphiphile micellization due to their ionic nature and hydrogen bonding ability. Poly(ethylene oxide)-poly (propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymer (Pluronic) micellization in aqueous solution with the addition of protic ionic liquid ethylammonium nitrate (EAN) was studied in terms of thermodynamics parameters obtained through proper experimental design applying isothermal titration calorimetry. The presence of EAN promoted micellization, and decreased the critical micelle concentration (CMC). The effects of the protic ionic liquid EAN, its corresponding classic salt, ammonium nitrate (NH 4 NO 3 ), and a representative aprotic ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF 4 ) on PEO-PPO-PEO block copolymer (Pluronic P123) micellization in aqueous solution have been compared. The results show that the protic ionic liquid EAN displays similar effects with the classic salt NH 4 NO 3 on promoting PEO-PPO-PEO micellization, but less significant. The aprotic ionic liquid BmimBF4 exhibits opposite effects with protic ionic liquid by hindering PEO-PPO-PEO micellization, as it has a lower tendency to form ion-water complexes compared to EAN, and higher tendency to assist the dissolution of PEO-PPO-PEO. The intermolecular interactions within ionic liquid-containing systems are complex and hard to assess by direct methods. Our work provides a way to assess the interactions during micellization through the change of thermodynamics, and systematically compares for the first time the effects of protic ionic liquid with classic salt and with aprotic ionic liquid on micellization in aqueous solution. The understandings on the interactions and thermodynamics of ionic liquid-modulated self-assembly process provides guidance on the rational design of novel ionic liquid-based materials, facilitating emerging applications in broad areas.