High conformational stability of cations in N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium ionic liquid–water mixtures

Abstract

We studied the conformational equilibrium of cations in N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium ([DEME][X], X = BF4 and I) ionic liquid (IL)–water mixtures as a function of water concentration using Raman spectroscopy and molecular dynamics (MD) simulations. We found that the folded form of the [DEME] cation was exceptionally stable, irrespective of the anion species, and its proportion in the mixtures remained almost constant throughout the studied concentration region. This behavior is significantly different from that of imidazolium IL–water mixtures, where the equilibrium shifts depending on the water concentration. Interestingly, the states of the water molecules were unrelated to the stability of the [DEME] cations in the mixtures. We speculated whether the intramolecular attractive interaction between the N and O atoms in the [DEME] cation was the key to maintaining the stability of the folded form.