Viscosity scaling of the self-diffusion and velocity cross-correlation coefficients of two functionalised ionic liquids and of their non-functionalized analogues

Abstract

Ion self-diffusion coefficients have been measured for ionic liquids based on the cations N-acetoxyethyl-N,N-dimethyl-N-ethylammonium ([N_112,2OCO1]⁺) and its non-functionalised analogue, N,N-dimethyl-N-ethyl-N-pentylammonium ([N₁₁₂₅]⁺), and N,N-dimethyl-N-ethyl-N-methoxyethoxyethylammonium ([N_112,2O2O1]⁺), and its analogue, N,N-dimethyl-N-ethyl-N-heptylammonium ([N₁₁₂₇]⁺) and the bis(trifluoromethanesulfonyl)amide anion. The functionalised chain on an ammonium cation has the same length, in terms of the number of atoms, as the non-functionalised chain of the corresponding analogue. For [N_112,2OCO1][Tf₂N] and [N₁₁₂₇][Tf₂N], the cation and anion self-diffusion coefficients are equal, within experimental error, whereas for [N₁₁₂₅][Tf₂N], the cation diffuses more quickly, and for [N_112,2O2O1][Tf₂N], it is the anion that diffuses more quickly than the ether-functionalised cation. But these differences are relatively small, just beyond experimental error. The data are used to calculate velocity cross-correlation coefficients (VCC or f_ij) and distinct diffusion coefficients (D^d_ij). Both the self-diffusion and distinct diffusion coefficients are analysed in terms of (fractional) Stokes–Einstein–Sutherland equations. Though the self-diffusion coefficients, as with the conductivity and viscosity, show marked differences in absolute terms between the functionalised and non-functionalised forms, being higher for the ethoxy-substituted IL and lower for the acetoxy-substituted IL, these are largely removed by scaling with the viscosity. Thus the transport properties are better understood as functions of the viscosity rather than the temperature and density, per se. The presence of the alkoxy-substituted side chains is known to change the local mesoscopic liquid structure, but it appears once this is done, the transport properties scale correspondingly. In the case of the acetoxy-substituted IL, this is also largely the case, but the Nernst–Einstein deviation parameter, Δ, which depends on the difference between the anion–cation VCC and the mean of the cation–cation and anion–anion VCCs, is smaller than that of its analogue salt, and also temperature dependent.