The ionic liquid 1-ethyl-3-methylimidazolium acetate [C(2)C(1)Im][OAc] shows a great potential to dissolve strongly hydrogen bonded materials, related with the presence of a strong hydrogen bond network in the pure liquid. A first step towards understanding the solvation process is characterising the hydrogen bonding ability of the ionic liquid. The description of hydrogen bonds in ionic liquids is a question under debate, given the complex nature of this media. The purpose of the present article is to rationalise not only the existence of hydrogen bonds in ionic liquids, but also to analyse their influence on the structure of the pure liquid and how the presence of water, an impurity inherent to ionic liquids, affects this type of interaction. We perform an extensive study using ab initio molecular dynamics on the structure of mixtures of the ionic liquid 1-ethyl-3-methylimidazolium acetate with water, at different water contents. Hydrogen bonds are present in the pure liquid, and the presence of water modifies and largely disturbs the hydrogen bond network of the ionic liquid, and also affects the formation of other impurities (carbenes) and the dipole moment of the ions. The use of ab initio molecular dynamics is the recommended tool to explore hydrogen bonding in ionic liquids, as an explicit electronic structure calculation is combined with the study of the condensed phase.

Some room-temperature ionic liquids can hold stable suspensions of nanoparticles without additional surface-active agents [1] through mechanisms of solvation and stabilization that are not understood at present, particularly for metallic nanoparticles. These systems are relevant for applications in catalysis, lubrication, electrochemical devices, and chemical processes. We address this issue by studying the interactions and ordering of ionic liquids around metallic nanoparticles using molecular dynamics simulations, which is a suitable tool because the arrangement of the ions around a 2 nm particle is difficult to observe experimentally. The fundamental obstacle to modeling resides in the description of the interactions between metals and ionic fluids, a problem not only for nanometer-scale objects but for extended surfaces as well. In this work we devised an original strategy to represent accurately the molecular interactions and gain insight into the solvation and stabilization mechanisms of nanoparticles in ionic liquids.Experimental studies of metallic nanoparticles in ionic liquids provide different clues about the stabilization of the colloid. Some postulate an electric double layer (the Deryagin-Landau-Verwey-Overbeek model) in which a first solvation shell of anions surrounds the metal cluster, followed by a less ordered layer of cations, and so on.[2] Other studies present evidence of close interactions of the nanoparticles with the cations, through deuterium exchange on positively charged imidazolium rings [3] and through surface-enhanced Raman spectroscopy on gold nanoparticles in imidazolium liquids.[4] Correlations have been established between the size of metallic nanoparticles synthesized in situ with the anion volume.[5] Still other studies suggest that nanoparticles are solvated in nonpolar regions formed by aggregation of the hydrophobic alkyl side chains of the ions, as there is a relationship between the length scale of the structural heterogeneities of the ionic liquid [6] and the size of nanoparticles synthesized therein. [7] Measurements of the thickness of the electrostatic double layer of ionic liquids at metal surfaces have been performed [9] yielded an interfacial layer with one-ion thickness of 3.3 to 5 . This is consistent with the Debye length of the order of 1 estimated for an electrolyte with a concentration around 4 or 5 m, such as a pure ionic liquid, and constitutes an argument against DLVO-type stabilization. However, measurements on macroscopic flat surfaces may not be immediately transposed to nanoparticle suspensions.Suspensions of metallic nanoparticles in an ionic liquid are governed by three kinds of molecular interaction: ion-ion, metal-metal, and metal-ion, which are all nontrivial and each offers its own difficulties to a description. We adopted an atomistic description for both the nanoparticle and for the ionic liquid, providing a high level of detail regarding the interactions and conformations. We considered a ruthenium nanoparticle in [C 4 C 1 im][Ntf 2 ], 1-buty...

New experimental densities are reported for squalane and for branched pentaerythritol tetra(2ethylhexanoate), PEB8, in the compressed liquid state over the temperature range from (278.15 to 353.15) K and for pressures up to 45 MPa. The reliability of the technique and the procedure up to 25 MPa has been verified in our previous works, and in the present work they were checked up to 45 MPa by comparing our experimental densities for heptane with literature data. A total of 297 density values have been measured with a high-pressure vibrating tube densimeter. The correction factor for density due to the sample viscosity has been considered. This factor ranges from 1 × 10 -4 to 1 × 10 -3 g‚cm -3 for PEB8 and from 4 × 10 -5 to 6 × 10 -4 g‚cm -3 for squalane over the entire Tp interval. The pressure and temperature dependencies of squalane and PEB8 densities could be accurately represented by the Tammann-Tait equation with standard deviations of 3 × 10 -5 g‚cm -3 for squalane and 9 × 10 -5 g‚cm -3 for PEB8. These density data were used to analyze the isothermal compressibility, the isobaric thermal expansivity, and the internal pressure of squalane and PEB8.

We present a comprehensive molecular dynamics simulation study on 1-butyl-3-methylimidazolium ionic liquids and their fluorinated analogs. The work focused on the effect of fluorination at varying anions. The main findings are that the fluorination of the cations side chain increases overall structuring, especially the aggregation of cation side chain. Furthermore, large and weakly coordinating anions tend to occupy on-top positions of the cation and decrease the aggregation of cation side chains, most likely due to enhanced alkyl-anion interaction.

Properties of the surface of ionic liquids, such as surface tension, ordering, and charge and density profiles, were studied using molecular simulation. Two types of modification in the molecular structure of imidazolium cations were studied: the length of the alkyl side chain and the presence of a polar hydroxyl group at the end of the side chain. Four ionic liquids were considered: 1-ethyl-3-methylimidazolium tetrafluoroborate, [C(2)C(1)im][BF(4)]; 1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate, [C(2)OHC(1)im][BF(4)]; 1-octyl-3-methylimidazolium tetrafluoroborate, [C(8)C(1)im][BF(4)] and 1-(8-hydroxyoctyl)-3-methylimidazolium tetrafluoroborate, [C(8)OHC(1)im][BF(4)]. The surface tension was calculated using both mechanical and thermodynamic definitions, with consistent treatment of the long-range corrections. The simulations reproduce the available experimental values of surface tension with a maximum deviation of ±10%. This energetic characterization of the interface is completed by microscopic structural analysis of orientational ordering at the interface and density profiles along the direction normal to the interface. The presence of the hydroxyl group modifies the local structure at the interface, leading to a less organized liquid phase. The results allow us to relate the surface tension to the structural ordering at the liquid-vacuum interface.

The experimental measurements of dynamic viscosity for squalane, pentaerythritol tetra-2-ethylhexanoate (PEB8), and pentaerythritol tetranonanoate (PEC9) have been performed using a Ruska rolling-ball viscometer over the temperature range of 303.15−353.15 K and a pressure range of 0.1−60 MPa. A total of 2016 experimental measurements of the rolling time have been obtained for the determination of 252 dynamic viscosity data. The available literature viscosity data for squalane at high pressures have been compared with the new measurements, and an average deviation of 1.5% has been obtained, which is within the experimental uncertainty (±3%). The higher viscosity values are reached for PEB8, followed by PEC9. For the pentaerythritol esters, it has been observed that the dynamic viscosity increases with the branching degree of the molecule. The relative change in viscosity with temperature is biggest for PEB8 and, consequently, the poorer viscosity index (VI) values and higher temperature coefficients have been obtained for this fluid. The lowest pressure−viscosity coefficient and the highest VI have been obtained for PEC9.

The choice of cation and anion in an ionic liquid (IL) as well as the design of ion side chains determine the fundamental properties of ILs, which permits creating tailor-made lubricants and lubricant additives. So, the study of the influence of molecular structure on thermophysical properties of ionic liquids is essential for their use in lubrication. Recent results from the literature, essentially based on ammonium, phosphonium, or imidazolium cations, are promising from the tribological point of view, but still new investigations should be performed, for example, in elastohydrodynamic lubrication (EHL), for which calculations of the universal pressure-viscosity coefficient, a film , and central thickness are needed. In this work viscosity and density data from the literature on broad pressure and temperature ranges for the ILs [C 4 C 1 im]PF 6 , [C 4 C 1 im]Tf 2 N, [C 4 C 1 im]BF 4 , [C 8 C 1 im]PF 6 , [C 8 C 1 im] BF 4 , [C 6 C 1 im]PF 6 , and [C 6 C 1 im]Tf 2 N are used to determine their a film values over a wide temperature range. The American Gear Manufacturers Association relation of the central thickness with the pressure viscosity coefficient is used to estimate the film-generating capability of these lubricants. Furthermore, an overview of the literature data on tribological and physical properties of the ionic liquids is presented.

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.