Comparison of ionic liquid and salt effects on the thermodynamics of amphiphile micellization in water

Abstract

Ionic liquids are low-melting salts that have novel properties and are very useful as solvents or additives on the basis of the intermolecular interactions operating between the ionic liquid ions and various solute or solvent molecules. In this work, we compare the effects of ionic liquids and salt on the micellization of a representative amphiphilic block copolymer poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO), Pluronic P123 (EO20PO70EO20). Ethylammonium nitrate (EAN) is used as a representative protic ionic liquid, to compare with its corresponding classic salt, ammonium nitrate (NH4NO3), and with 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4) as a representative aprotic ionic liquid. The protic ionic liquid EAN is similar with the classic salt NH4NO3 in promoting PEO-PPO-PEO micellization; they both increase the micellization entropy and lower the micellization enthalpy at higher concentration. The hydrogen bonding between EAN and PEO, and the ethyl group on the EAN cation, both assist the solvation of PEO-PPO-PEO molecules, which partially offsets their dehydration caused by the ionic liquid ions. These lead to a less significant change of the micellization thermodynamics parameters by EAN compared to NH4NO3. The aprotic ionic liquid BmimBF4 exhibits effects that are opposite to those of the protic ionic liquid EAN. The micellization enthalpy of PEO-PPO-PEO significantly increases in the presence of BmimBF4. The hydrogen bonds formed between BmimBF4 and PEO-PPO-PEO, along with nonpolar butyl clusters formed by Bmim cations assist the solvation of Pluronic P123, which in turn hinders micellization. This work compares for the first time the effects of a protic ionic liquid with a classic salt and with an aprotic ionic liquid on amphiphile micellization in aqueous solution in terms of thermodynamics.