The thermodynamics of the activation for viscous flow of aqueous [C6mim][Ala] (1-hexyl-3-methylimidazolium alanine salt)

•Viscosity B-coefficient and apparent molar volume for [C6mim][Ala] (aq) are obtained.•Thermodynamic parameters of activation for viscous of [C6mim][Ala](aq) are obtained.•ΔH2≠0 implies the activation process of solute is an isocoulombic reaction.•A semi-empirical method to estimate viscosity of [C6mim][Ala](aq) was put forward.

The values of density and viscosity for aqueous solution of ionic liquid (IL) based on alanine [C6mim][Ala] with various molalities were measured at T = (288.15–328.15) K with interval 5 K. The activation energy, Eη, for viscous flow of the solution was calculated. In terms of Jones-Dole equation, the viscosity B-coefficients with large positive value and dB/dT < 0 were obtained and these facts implied that the ionic liquid is water-structure-making. According to Feakins, the contribution of the solute per mole to the free energy of activation for viscous flow of the solution, Δμ†0, or was called as the standard molar activation free energy, was obtained at T = (288.15–328.15) K. The relationship between Δμ2†0 and the temperature is linear, so that the standard molar activation entropy ΔS2†0 and enthalpy, ΔH2†0, were obtained. Under the constant molality of solution, ΔH2†0 is a temperature-independent constant. It implies the activation process of the solute for viscous flow of aqueous [C6mim][Ala] is an isocoulombic reaction. On the basis of Eyring’s theory, a semi-empirical method to estimate the viscosity of aqueous [C6mim][Ala] was put forward and the estimated viscosity values of aqueous [C6mim][Ala] are in good agreement with the corresponding experimental ones.

Graphical abstractThe estimated values, ηest/Pa·s, and the corresponding experimental ones are highly correlated and extremely similar. It shows that the semi-empirical method which used to estimate the viscosity of solution is better.Figure optionsDownload full-size imageDownload as PowerPoint slide