Relationship between Viscosity Coefficients and Volumetric Properties Using a Scaling Concept for Molecular and Ionic Liquids

Abstract

In this work, a scaling concept based on relaxation theories of the liquid state was combined with a relation previously proposed by the authors to provide a general framework describing the dependency of viscosity on pressure and temperature. Namely, the viscosity-pressure coefficient (partial differentialeta/partial differentialp)T was expressed in terms of a state-independent scaling exponent, gamma. This scaling factor was determined empirically from viscosity versus Tvgamma curves. New equations for the pressure- and temperature-viscosity coefficients were derived, which are of considerable technological interest when searching for appropriate lubricants for elastohydrodynamic lubrication. These relations can be applied over a broad range of thermodynamic conditions. The fluids considered in the present study are linear alkanes, pentaerythritol ester lubricants, polar liquids, associated fluids, and several ionic liquids, compounds selected to represent molecules of different sizes and with diverse intermolecular interactions. The values of the gamma exponent determined for the fluids analyzed in this work range from 1.45 for ethanol to 13 for n-hexane. In general, the pressure-viscosity derivative is well-reproduced with the values obtained for the scaling coefficient. Furthermore, the effects of volume and temperature on viscosity can be quantified from the ratio of the isochoric activation energy to the isobaric activation energy, Ev/Ep. The values of gamma and of the ratio Ev/Ep allow a classification of the compounds according to the effects of density and temperature on the behavior of the viscosity.

On the viscosity of two 1-butyl-1-methylpyrrolidinium ionic liquids: Effect of the temperature and pressure

“…The scaling thermodynamic postulates that for a given liquid, properties such as viscosity, diffusion coefficient, electrical conductivity or structural relaxation time can be expressed solely as a function of the variable Tν γ [60,61]:…”

“…The higher the γ value, the stronger dependence on density of the transport property of the fluid [62]. Furthermore, when γ is close to 0 it is assumed that the dynamics are controlled by the temperature [60,[62][63][64]. This parameter reflects the repulsive interactions as well as the contributions from internal molecular modes, and attractive intermolecular forces for complex molecules or ions [16,61].…”

Molecular Dynamics Study of Thermodynamic Scaling of the Glass-Transition Dynamics in Ionic Liquids over Wide Temperature and Pressure Ranges

“…12,13 A class of materials called room-temperature ionic liquids, or simply ionic liquids (ILs), whose melting points are below or near room temperature, [14][15][16][17] has attracted much attention in the research community because they possess many desirable properties for applications such as high thermal stability and negligible vapor pressure. Recently, successful TV γ scaling of viscosity in ionic liquids has been also reported, 9,11 where γ is found to be at around 2-4. The structures of systems are determined by the combinations of cation-cation, cation-anion, and anion-anion interactions, and the scaling parameter γ is different from the r -1 dependence of the Coulombic term.…”

“…[1][2][3][4] Although supercooled liquids and glasses including ionic systems show complicated heterogeneous dynamics, [5][6][7] an existence of a simple scaling behavior of the dynamic properties, x ) T (TV γ ) (where T is temperature and V is the specific volume), has been reported for many systems, [8][9][10][11] where x can be the relaxation time τ or viscosity η, while γ is a material constant.…”

Scaling of the dynamics of flexible Lennard-Jones chains

“…Since both alkanes [19,35,36] and polymers [15,37] have been shown to obey power-law density scaling, we simulated a general viscous model liquid of linear, flexible Lennard-Jones chains (LJC). The model has been used extensively for viscous polymer melts close to the glass transition [38][39][40][41][42].…”