Slightly Viscous Amino Acid Ionic Liquids: Synthesis, Properties, and Calculations

Slightly viscous N-alkyl-substituted glycine ester ionic liquids were prepared via alkylation of glycine ethyl esters with appropriate haloalkanes followed by anion exchange with lithium bis(trifluoromethanesulfonyl)amide. These ionic liquids have been characterized by IR, NMR, elemental analysis, thermal stability, phase behavior, viscosity, and density. Compound 9 crystallizes in the monoclinic system, P2(1)/c. The viscosities of the N,N,N-trialkyl-substituted glycine ester ionic liquids (6, 8, 10) are in the similar to 200 to 400 cP range at 25 degrees C. While exhibiting liquid characteristics analogous to the traditional heterocyclic ionic liquids, these new liquids are much less viscous than known amino acid ionic liquids. The correlation of viscosity and temperature was determined. To understand the influence of the alkyl and ester-substituted groups on viscosity, the electronic distributions and the electrostatic potential surfaces of the glycine-based cations, including glycine (Gly(+)), glycine ethyl ester (GlyET(+)), N,N-dinlethyl glycine (DMGly(+)), N,N-dimethyl glycine ethyl ester (DMGlyET(+)), N,N-dimethyl-N-propyl glycine ethyl ester (DMPGlyET(+)), N,N-dimethyl-N-3-fluoropropyl glycine ethyl ester (DMPFGlyET(+)), and N-butyl-N,N-dimethyl glycine ethyl ester (DMBGlyET(+)) cations have been investigated and analyzed. The possible effects oil the viscosity coming from the intermolecular interactions arising from Coulomb interactions, hydrogen bonding, polarizability effects, and van der Waals interactions are considered.