A van der Waals-EoS-based model for the dynamic viscosity of ionic liquids

A density-independent thermodynamic model was presently developed to adequately represent the dynamic viscosity of pure ionic liquids (ILs). The present model adopts the simple form of the van der Waals equation of state under the basis of a previously established phenomenological similarity between the P-V-T and T-η-P surfaces for non-ionic fluids; however, we found that, in the particular case of pure ionic liquids, the surface P−f−T (where f is the fluidity, the reciprocal of viscosity) rather than the T-η-P surface conforms much better to the P-V-T surface. The resulting model was successfully validated during the representation of experimental dynamic viscosities of three families of imidazolium-based ILs ([C_Xmim][BF₄], [C_Xmim][PF₆] and [C_Xmim][Tf₂N]), four pyridinium-based ILs ([bmpy][BF₄], [empy][EtSO₄], [Et₂Nic][EtSO₄] and [hemmpy][Tf₂N]) and two ammonium-based ILs ([cpmam][MeSO₄] and [4bam][doc]) within a temperature range varying from 0 to 80 °C and at pressures from 1 up to 3,000 bar thus covering a wide viscosity range of 10–19,610 mPa-s.