Universality of Viscosity Dependence of Translational Diffusion Coefficients of Carbon Monoxide, Diphenylacetylene, and Diphenylcyclopropenone in Ionic Liquids under Various Conditions

Abstract

Translational diffusion coefficients of diphenylcyclopropenone (DPCP), diphenylacetylene (DPA), and carbon monoxide (CO) in 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIm][NTf2]) and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([EMIm][NTf2]) were determined by the transient grating (TG) spectroscopy under pressure from 0.1 to 200 MPa at 298 K and from 298 to 373 K under 0.1 MPa. Diffusion coefficients of these molecules at high temperatures in tributylmethylphosphonium bis(trifluoromethanesulfonyl)imide ([P4441][NTf2]), and tetraoctylphosphonium bis(trifluoromethanesulfonyl)imide ([P8888][NTf2]), and also in the mixtures of [BMIm][NTf2], N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide ([Pp13][NTf2]), and trihexyltetradecylphosphonium bis(trifluoromethanesulfonyl)imide ([P66614][NTf2]) with ethanol or chloroform have been determined. Diffusion coefficients except in ILs of phosphonium cations were well scaled by the power law of T/η, i.e., (T/η)(P), where T and η are the absolute temperature and the viscosity, irrespective of the solvent species, pressure and temperature, and the compositions of mixtures. The values of the exponent P were smaller for the smaller size of the molecules. On the other hand, the diffusion coefficients in ILs of phosphonium cations with longer alkyl chains were larger than the values expected from the correlation obtained by other ILs and conventional liquids. The deviation becomes larger with increasing the number of carbon atoms of alkyl-chain of cation, and with decreasing the molecular size of diffusing molecules. The molecular size dependence of the diffusion coefficient was correlated by the ratio of the volume of the solute to that of the solvent as demonstrated by the preceding work (Kaintz et al., J. Phys. Chem. B 2013 , 117 , 11697 ). Diffusion coefficients have been well correlated with the power laws of both T/η and the relative volume of the solute to the solvent.

Fast solute diffusivity in ionic liquids with silyl or siloxane groups studied by the transient grating method

“…This is because the bulky structure of the silyl or siloxane group slows down the movement of the cation even though the interaction between the cation and anion and the flexibility of the side chain can have an opposite effect on the viscosity. [15][16]29 This idea is supported by the SiIL diffusion coefficients measured by Chung et al 39 They found that the diffusivity of the trimethylsilylmethyl-substituted imidazolium cation is nearly identical to that of the TFSI anion, whereas cations of ILs generally move faster in AlILs. [40][41][42][43] ILs with the Si-O-Si group showed somewhat lower viscosities than those with the Si group, as was previously reported.…”

“…6,[8][9][14][15] The change in intermolecular interaction is caused by change in ionic nature of the SiIL ions. [15][16]29 Hereafter, this discussion is framed in terms of these three factors. As previously suggested by Shirota et al, the flexibility of the silyl and siloxane groups affect the physical properties of ILs.…”

“…confirmed the high flexibility of the SiOSiIm cation compared to the BMIm cation through DFT calculations. 29 They obtained potential energy surfaces of the rotations along each bond angle in the side chain and found lower activation energies than the butyl group rotation for the same cation ring structure.…”

“…investigated the flexibility of siloxane groups attached to imidazolium cations through density functional theory (DFT) calculations. 29 Their results showed that the rotational barriers of the side chain were lower for the siloxane group than that for the butyl group. These findings for both silicone oil and Si or Si-O-Si group containing ILs imply that SiILs are promising in terms of high solute diffusivity.…”

Transient binding accounts for apparent violation of the generalized Stokes–Einstein relation in crowded protein solutions

“…[70][71][72][73] Despite noting that charge-charge interactions are much more relevant for ionic liquids as for the diffusivity of water in presence of proteins, the deviations from SE behavior are similar in origin. [70][71][72][73] Despite noting that charge-charge interactions are much more relevant for ionic liquids as for the diffusivity of water in presence of proteins, the deviations from SE behavior are similar in origin.…”