Densities and viscosities of binary mixtures of 2,2-diethyl-1,1,3,3-tetramethylguanidinium bis(trifluoromethylsulfonyl)imide with methanol and ethanol

Abstract

The guanidinium-based ionic liquid (IL), 2,2-diethyl-1,1,3,3-tetramethylguanidinium bis(trifluoromethylsulfonyl)imide ([TMG(C2)2][NTf2]), was synthesized. Densities and viscosities of the IL and its mixtures with methanol or ethanol were reported at temperatures from 293.15 K to 323.15 K. The increment of IL concentration or the decrement of temperature significantly increases both density and viscosity. Excess molar volume (VmE) and viscosity deviation (Δη) of each binary mixture were calculated, and the Redlich–Kister equation was used to fit VmE and Δη with composition. When VmE is negative with the mole fraction of IL (x1) lower than 0.8 for two systems, Δη is negative over the whole concentration range. The positive VmE of the mixture at a high IL concentration indicates that interaction loss occurs. Moreover, structures of the equimolar mixture of IL and alcohol were obtained by DFT calculations, and alcohol–anion, alcohol–cation and cation–anion interactions were revealed. Strong H-bond interaction with the span of 1.61 Å is formed between the H atom of the OH group in alcohol and the O atom in the [NTf2]− anion, while the O atom of the OH group in alcohol interacts with the methyl groups in the [TMG(C2)2]+ cation. Significant cation–anion interactions (<2.40 Å) in the manners of H⋯F and H⋯O with spans less than 2.40 Å are found at the same time. It is the enhanced H-bond interactions that contribute to the negative VmE of the equimolar mixture at non-concentrated regions.