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International audience; Nearly forty years ago, Ashurst and Hoover [1] have shown that the shear reduced residual viscosity of dense fluids composed of soft spheres can be described by a function of rg/T only, where T is the temperature and r the density. g is a parameter which is a measure of the repulsion steepness of the interaction potential between particles. More recently, it has been found that this so-called "thermodynamic scaling" holds for a large class of real liquids [2] and is applicable even for moderately dense fluids [3]. To further extent this approach to larger class of fluid systems including mixtures, we have first performed numerous molecular dynamics simulations on model fluids in which the interaction were described by a Mie n-6 potential where n is the repulsion exponent (varying between 8 and 36). To determine unambiguously the optimal g, we have defined a general strategy based on a modified Avramov's relation between the reduced residual viscosity and rg/T. Using that scheme, it has been found that the thermodynamic scaling holds very well for such model fluids and that the optimal g is simply related to the repulsion exponent n of the Mie potential, as expected. Then, we have performed simulations on mixtures of these model fluids. First, it has been found that the thermodynamic scaling of the viscosity is well respected in binary mixture of Mie fluids. Second, it has been observed that a simple logarithmic mixing rule yields an excellent estimate of the g value in mixtures. Finally, it has been noticed that these findings hold very well even when dealing with real (alcohols, alkanes ...) fluid mixtures. [1] Ashurst, W. T.; Hoover, W. G., Physical Review A 1975, 11, 658-678. [2] Roland, M.; Hensel-Bielkowka, S.; Paluch, M.; Casalini, R., Rep. Prog. Phys. 2005, 68, 1405-1478. [3] Galliero, G.; Boned, C.; Fernandez, J., Journal of Chemical Physics 2011, 134, 064505.

International audience; Sulfur hexafluoride is a very important compound in many fields of technology. It is a relatively inert compound that is very stable over a wide range of temperatures and pressures. From the modeling point of view, the sulfur hexafluoride molecule is quite large and relatively spherical. For these reasons, from a fundamental point of view sulfur hexafluoride can be considered as a model fluid which could provide a stringent test on the applicability of models aiming at representing its thermophysical properties. Viscosity at very low density states (i.e. in the "diluted-gas" conditions) having been carefully studied and modeled by Quiñones-Cisneros et al1, we focus in this work to the viscosity behavior for the denser states. Thus, the purpose of this work is to check how recent approaches are able to represent the sulfur hexafluoride viscosity over a large range of thermodynamic conditions with a limited number of adjustable parameters. With this intention, three recent approaches with important physical background are considered: a free-volume model2, a thermodynamic scaling model3,4 and a correlation5 representing the dynamic viscosity using the equivalent Lennard-Jones 12-6 (LJ) fluid. The experimental database employed for adjustment (1562 data points) considers a large temperature (225.18 to 473.15 K) and pressure intervals (0.0264 to 51.21 MPa). The absolute average deviation is 3.8% for the Lennard-Jones model (1 parameter; see Fig.1 close to the critical temperature), 1.7% for the free volume model (3 parameters) and 1.5 % for the thermodynamic scaling model (6 parameters). Thus, it is shown that when physically based approaches are employed, a limited number of parameters are sufficient to represent accurately the shear viscosity of sulfur hexafluoride. Furthermore it has been confirmed, using the thermodynamic scaling approach, that the repulsive steepness of the sulfur hexafluoride interaction potential is higher than usually found for non polar spherical molecule.

International audience; In their seminal paper, Ashurst and Hoover1 have shown that the shear viscosity of simple dense fluids can be expressed as a (unknown) function of TV only, so-called "thermodynamic scaling", where T is the temperature, V the volume and  a parameter that is characteristic of the repulsion steepness of the fluid particles. More recently it has been shown that this thermodynamic scaling holds for a large class of liquids2 and can be extended to deal with moderate density3. The simultaneous description of thermodynamics and transport properties with the same underlying effective "molecular" model is usually difficult to achieve, even for very simple molecules. Recently4,5 it has been shown that using Mie potentials to describe the interaction between the atoms/molecules may be an efficient choice to tackle that problem without using too many molecular parameters. Thus, a viscosity database of the Mie n-6 fluid (n being the repulsion exponent) has been constructed for a wide range of thermodynamic conditions using extensive non-equilibrium molecular dynamics (NEMD) simulations (for n varying between 7 and 48). Then, it has been verified that the thermodynamic scaling approach can be applicable for such a model simple fluid. As expected, it has been found that the optimal  parameter is closely related to the n value as well known for purely repulsive potential1. It is worth emphasizing that such a result provides an interesting way to estimate, from viscosity measurements, the steepness of the effective repulsion occurring between the molecules of a given pure real fluid. In addition the applicability of the thermodynamic scaling on model (Mie) and real (alcohols, alkanes ...) mixtures has been verified. Rather interestingly, it has been found that the value of  in mixtures can be simply deduced from the values of the pure components of the mixture.

International audience

International audience; This work reports new density data (178 points) of 1-butanol at twelve temperatures between 293.15 and 403.15K (every 10 K), and fifteen pressures from 0.1 up to 140MPa (every 10 MPa). An Anton Paar vibrating tube densimeter, calibrated with an uncertainty of ±0.5 kgm−3 was used to perform these measurements. The experimental density data were fitted with the Tait-like equation with low standard deviations. In addition, the isobaric thermal expansivity and the isothermal compressibility have been derived from the Tait-like equation.

International audience; Introduction Modern lubricants are complex formulations, which are continually developed to meet increasing requirements for performance and durability [1]. In the scope of the investigation of more benign lubricants, as potential replacements of oils currently used, we have focused our attention on the diesters (adipates), which have excellent properties for industrial applications. Diesters are obtained by reacting a linear or branched diacid, with a monofunctional alcohol. In the case of adipates, 1,6-hexanoic diacid, also named adipic acid, is used. In the present work dimethyl adipate (DMA), diethyl adipate (DEA) and diisobutyl adipate (DiBA) have been studied over wide pressure and temperature intervals. The analysis of the effect of the molecular size is performed in terms of volumetric properties (density, isothermal compressibility and thermal isobaric expansivity), together with viscosity and viscosity index. Experimental The density  as a function of pressure and temperature was measured with a high pressure vibrating tube densimeter Anton Paar DMA HPM which is adequate to perform measurements up to 140 MPa. The calibration of this equipment has been performed with vacuum, water and decane [2]. Viscosity at atmospheric pressure was measured with a rotational automated viscometer Anton Paar Stabinger SVM3000. We have verified the viscosity uncertainty of this viscometer with two Canon reference fluids S60 and N100, finding that the deviations are lower than 1%. Results and Discussion Density measurements have been performed from 293.15 to 403.15 K and up to 140 MPa. The experimental density values have been correlated successfully using a Tait-type equation. In Figure 1 it can be seen the molecular structure of dialkyl adipates and the density behavior. At fixed temperature and pressure, density follows the trend DMA > DEA > DiBA. So, it decreases with the molecular size (molar weight or number of carbon atoms). The behavior of the isothermal compressibility for two temperatures can be also seen in Figure 1. This property decreases with pressure and increases with temperature for the three adipates. Furthermore, an increase on T occurs when the molecular size of the molecule increases. Similar behavior has been previously found for other fluids such as pentaerythritol ester lubricants [3]. The effect of changing the temperature or the pressure has more impact on the values of the isothermal compressibility than on the isobaric thermal expansivity.