Modeling the dynamic viscosity of associating and polar fluids via the use of density scaling

It is well-known that certain dynamical properties such as the dielectric relaxation times and the viscosity of liquids can be graphically superpositioned onto a single master curve as a function of the thermodynamic potential (ρ^γ/T), where T is the temperature, ρ is the density, and γ is a state-independent scaling exponent. We presently applied the aforementioned thermodynamic scaling to the viscosity of typical hydrogen-bonding formers and polar fluids such as water, hydrogen sulfide, ammonia, methanol and an ionic liquid: [bmim][PF₆]. Unlike previous studies on density scaling of transport properties, a more suitable reduction and normalization of the viscosity was introduced here in order to obtain improved correlations of viscosity over much wider temperature and pressure ranges encompassing the zero-density limit, the high-density region, the gas-liquid saturation line and the vicinity of the critical point. A calculation procedure is also described here to optimize the value of the scaling exponent γ that ensures the best super-positioning of all experimental isotherms considered for each substance.