Josefa Fernández

High-Pressure Measurements of the Viscosity and Density of Two Polyethers and Two Dialkyl Carbonates

The viscosity and density of four pure liquid compounds (dimethyl carbonate, diethyl carbonate, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether) were measured at several temperatures between 283.15 and 353.15 K. The density measurements were performed up to 60 MPa with an uncertainty of 1×10−4g·cm−3. The viscosity at atmospheric pressure was measured with an Ubbelohde-type glass capillary tube viscometer with an uncertainty of ±1%. At pressures up to 100 MPa the viscosity was determined with a falling ball viscometer with an uncertainty of ±2%. The density (410 experimental values) and viscosity data (184 experimental values) were fitted to several correlation equations.

Relationship between viscosity coefficients and volumetric properties using a scaling concept for molecular and ionic liquids.

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.

Ionic Liquids Based on Phosphonium Cations As Neat Lubricants or Lubricant Additives for a Steel/Steel Contact

After doing several miscibility essays with eight ionic liquids (ILs) and four base oils, the ILs tri(butyl)ethylphosphonium diethylphosphate [P4,4,4,2][C2C2PO4] and trihexyl(tetradecyl)phosphonium tris(pentafluoroethyl)trifluorophosphate [P6,6,6,14][(C2F5)3PF3] were selected to be studied as lubricant additives. The neat IL [P4,4,4,2][C2C2PO4], the base oils, and several blends were characterized in terms of density, viscosity, and thermal stability. The tribological performance of the miscible base oil/IL blends (1 wt %) and the neat [P4,4,4,2][C2C2PO4] were evaluated for the lubrication of an AISI 420 steel-100Cr6 steel contact pair. The friction coefficients and wear volumes obtained are also compared with those corresponding to the pure base oils and their mixtures with conventional additive zinc dialkyldithiophosphate (ZDDP). As neat lubricants, [P4,4,4,2][C2C2PO4] showed the best antifriction ability, whereas in terms of wear, better results were obtained with [P6,6,6,14][(C2F5)3PF3]. However, higher improvements in both friction and wear were found for blends containing [P4,4,4,2][C2C2PO4]. XPS analyses of the worn surfaces lubricated with these mixtures indicated the presence of phosphorus in the tribofilm formed on the wear track. However, this compound was slightly detected on tribosamples lubricated with blends containing [P6,6,6,14][(C2F5)3PF3].

Scaling of the viscosity of the Lennard-Jones chain fluid model, argon, and some normal alkanes

In this work, we have tested the efficiency of two scaling approaches aiming at relating shear viscosity to a single thermodynamic quantity in dense fluids, namely the excess entropy and the thermodynamic scaling methods. Using accurate databases, we have applied these approaches first to a model fluid, the flexible Lennard-Jones chain fluid (from the monomer to the hexadecamer), then to real fluids, such as argon and normal alkanes. To enlarge noticeably the range of thermodynamics conditions for which these scaling methods are applicable, we have shown that the use of the residual viscosity instead of the total viscosity is preferable in the scaling procedures. It has been found that both approaches, using the adequate scaling, are suitable for the Lennard-Jones chain fluid model for a wide range of thermodynamic conditions whatever the chain length when scaling law exponents and prefactors are adjusted for each chain length. Furthermore, these results were found to be well respected by the corresponding real fluids.

pρT measurements of polyethylene glycol dimethylethers between 278.15 and 328.15 K at pressures to 12 MPa

In this paper we present a new experimental apparatus designed to measure pressure–density–temperature (pρT ) properties with a high-pressure vibrating tube densimeter. Data reliability has been verified by comparing our experimental results for methanol, n-heptane, toluene, and HFC-134a with literature data. In this work we also report new experimental densities from 278.15 to 328.15 K, and up to 12 MPa, of triethylene glycol dimethylether (TrEGDME) and tetraethylene glycol dimethylether (TEGDME). The isobaric thermal expansion coefficients, isothermal compressibility, and internal pressure have been calculated. The dependence of these properties on the length of polyethylene glycol dimethylether, CH3O–((CH2)2O)n–CH3, is analyzed.

Analysis of excess enthalpies of ester+ 1-chloroalkanes with two group contribution models: primary parameters

Abstract An analysis of the enthalpies of mixing, obtained experimentally at 298.15 K and at atmospheric pressure, of sixteen binary systems formed by four ethyl esters, from formate to butanoate, and four 1-chloroalkanes, from pentane to octane, is presented in this work. The experimental values of h E have been examined by a version of the UNIFAC model, as modified by Skjold-Jorgensen, as well as by the molecular group contribution model of Nitta. In both cases, the interaction parameters necessary for the prediction of the h E of the binary systems ester/1-chloroalkanes are determined, mainly the parameters involving the groups HCOO, COOC or COO, CH 3 or CH 2 and Cl. The predictions of h E , carried out both with the modified UNIFAC model and with the molecular model of Nitta, are similar. Thus, for the system ethyl formate+ 1-chloroalkanes, errors are lower than 6% with the UNIFAC model and less than 8% with that of Nitta. For the remaining systems, the errors increase as the chain length of the ester increases or as the h E decrease.

Experimental excess volumes of organic carbonate+alkane systems. Estimation of the parameters of the Nitta–Chao model for this kind of binary mixture

Excess volumes at 298.15 K and atmospheric pressure of some organic carbonate+alkane binary systems have been measured using an Anton Paar 602 HP densimeter. For the first time, published data on excess enthalpies, excess volumes, excess Gibbs energies of the above-mentioned binary systems together with the vaporisation enthalpies and the molar volumes of the pure organic carbonates, were used to estimate the interaction parameters between the carbonate group O–CO–O and the methyl and methylene groups, CH3, CH2, respectively. The mean deviations between experimental and theoretical values were smaller than 6% for all the properties. We have also compared our results with those obtained by Garcia etal. with the Original, Tassios etal., Larsen etal. and Gmehling etal. versions of the UNIFAC model and with those obtained by Kehiaian etal. using the DISQUAC model.

Volumetric behaviour of the environmentally compatible lubricants pentaerythritol tetraheptanoate and pentaerythritol tetranonanoate at high pressures

Knowledge of proper lubricant selection and its handling can substantially influence the reliability of a refrigeration system. In this sense the awareness of several thermophysical properties of refrigerants, lubricants, and their mixtures under different conditions of pressure and temperature is highly important for designing refrigeration systems. Polyol ester oils have been proposed as lubricant candidates for refrigeration systems. In this work, we have studied the density of two polyol esters, pentaerythritol tetraheptanoate and pentaerythritol tetranonanoate, in the range 278.15 ≤ T/K ≤ 353.15 and 0.1 ≤ p/MPa ≤ 45. In addition, the behaviour of two other essential volumetric properties, namely the thermal expansion coefficient and the isothermal compressibility coefficient, as well as the internal pressure have been analysed.

PρTx measurements for HFC-134a + triethylene glycol dimethylether system

Experimental densities in the compressed liquid phase are reported for 1,1,1,2-tetrafluoroethane (HFC-134a), triethylene glycol dimethylether (TriEGDME) and six of their mixtures from 293.15 to 373.15 K and at pressures up to 60 MPa. From the experimental results, we have analysed the volumetric behaviour of HFC-134a+TriEGDME. In almost all the measurement range, the density of the refrigerant is greater than that of the polyether. Furthermore, above 333.15 K the densities of the mixtures display a crossover phenomenon with composition. The excess volumes are strongly negative and asymmetrical towards high refrigerant concentrations, becoming more negative when the temperature increases and the pressure decreases.

Interactions and structure of ionic liquids on graphene and carbon nanotubes surfaces

Using quantum methods it was possible to build an atomistic force field for ionic liquids interacting with a graphene surface. Density functional calculations of 1-ethyl-3-methylimidazolium cation and thiocyanate anion interacting with a cluster of carbon atoms representing a graphene surface were performed, at a series of distances and orientations, using the BLYP-D functional. The DFT interaction energies were successfully fitted to a site–site potential function. The developed force field accounts also for the polarization of the graphene surface, therefore the use of induced dipoles to reproduce the interaction energy between charges and a conductor surface is not required. We report molecular dynamics results on the structure of the interfacial layer of the ionic liquid 1-ethyl-3-methylimidazolium thiocyanate at a flat graphene surface and inside single-wall carbon nanotubes of different diameters, including analyses of the positional and orientational ordering of the ions near the surface, and charge density profiles. Both anions and cations are found in the first ordered layer of ions near the surface, with the interfacial layer being essentially one ion thick.