Manuel M. Piñeiro

Simultaneous estimation of phase behavior and second-derivative properties using the statistical associating fluid theory with variable range approach.

A modified statistical associating fluid theory (SAFT) with variable range version is presented using the family of m-n Mie potentials. The use of this intermolecular potential for modeling repulsion-dispersion interactions between the monomer segments, together with a new method for optimizing the molecular parameters of the equation of state, is found to give a very accurate description of both vapor-liquid equilibria and compressed liquid bulk properties (volumetric and derivative properties) for long-chain n-alkanes. This new equation improves other SAFT-like equations of state which fail to describe derivative properties such as the isothermal compressibility and the speed of sound in the condensed liquid phase. Emphasis is placed on pointing out that the key for modeling the latter properties is the use of a variable repulsive term in the intermolecular potential. In the case of the n-alkanes series, a clear dependence of the characteristic molecular parameters on increasing chain length is obtained, demonstrating their sound physical meaning and the consistency of the new fitting procedure proposed. This systematic method for optimizing the model parameters includes data on the saturation line as well as densities and speed of sound data in the condensed liquid phase, and the results show undoubtedly that the model performance is enhanced and its range of applicability is now widened, keeping in any case a good balance between the accuracy of the different estimated properties.

Thermal conductivity and viscosity measurements of ethylene glycol-based Al2O3 nanofluids

The dispersion and stability of nanofluids obtained by dispersing Al2O3 nanoparticles in ethylene glycol have been analyzed at several concentrations up to 25% in mass fraction. The thermal conductivity and viscosity were experimentally determined at temperatures ranging from 283.15 K to 323.15 K using an apparatus based on the hot-wire method and a rotational viscometer, respectively. It has been found that both thermal conductivity and viscosity increase with the concentration of nanoparticles, whereas when the temperature increases the viscosity diminishes and the thermal conductivity rises. Measured enhancements on thermal conductivity (up to 19%) compare well with literature values when available. New viscosity experimental data yield values more than twice larger than the base fluid. The influence of particle size on viscosity has been also studied, finding large differences that must be taken into account for any practical application. These experimental results were compared with some theoretical models, as those of Maxwell-Hamilton and Crosser for thermal conductivity and Krieger and Dougherty for viscosity.

A study on stability and thermophysical properties (density and viscosity) of Al2O3 in water nanofluid

The dispersion and stability of nanofluids obtained by dispersing Al2O3 nanoparticles (obtained from different sources) in water have been analyzed. The differences arising from different dispersion techniques, the resulting particle size distribution, and time stability among the different samples are evaluated. Then the volumetric behavior up to high pressures (25 MPa) and atmospheric pressure viscosity were experimentally determined. It has been found that the influence of particle size in density is subtle but not negligible, but the differences in viscosity are very large and must be taken into account for any practical application. These viscosity differences can be rationalized by considering a theory describing the aggregation state of the nanofluid.

Simultaneous Application of the Gradient Theory and Monte Carlo Molecular Simulation for the Investigation of Methane/Water Interfacial Properties

This work is dedicated to the simultaneous application of the gradient theory of fluid interfaces and Monte Carlo molecular simulations for the description of the interfacial behavior of the methane/water mixture. Macroscopic (interfacial tension, adsorption) and microscopic (density profiles, interfacial thickness) properties are investigated. The gradient theory is coupled in this work with the SAFT-VR Mie equation of state. The results obtained are compared with Monte Carlo simulations, where the fluid interface is explicitly considered in biphasic simulation boxes at both constant pressure and volume (NPT and NVT ensembles), using reliable united atom molecular models. On one hand, both methods provide very good estimations of the interfacial tension of this mixture over a broad range of thermodynamic conditions. On the other hand, microscopic properties computed with both gradient theory and MC simulations are in very good agreement with each other, which confirms the consistency of both approaches. Interfacial tension minima at high pressure and prewetting transitions in the vicinity of saturation conditions are also investigated.

Enhancement of thermal conductivity and volumetric behavior of FexOy nanofluids

Homogeneous and stable magnetic nanofluids containing iron oxide nanoparticles, α-Fe2O3 (hematite) and Fe3O4 (magnetite) in ethylene glycol, were prepared at concentrations up to 25% in mass fraction. Commercial Hexagonal Scalenohedral-shaped α-Fe2O3 nanoparticles were selected while Fe3O4 nanoparticles were synthesized using a coprecipitation method. The products were characterized by transmission and scanning electron microscopy and x-ray diffraction. The thermal conductivity of both nanofluids was measured as a function of volume fraction and temperature. The results illustrate that the enhanced thermal conductivity of the nanofluids increases with volume fraction but is temperature independent. The experimental results show that both types of nanoparticles in this base fluid present no significant aggregation. These experimental values were also compared with theoretical models. Moreover, the density of these nanofluids was measured as a function of volume fraction, temperature, and pressure. The volu...

Interfacial Properties of Water/CO2: A Comprehensive Description through a Gradient Theory−SAFT-VR Mie Approach

The Gradient Theory of fluid interfaces is for the first time combined with the SAFT-VR Mie EOS to model the interfacial properties of the water/CO(2) mixture. As a preliminary test of the performance of the coupling between both theories, liquid-vapor interfacial properties of pure water have been determined. The complex temperature dependence of the surface tension of water can be accurately reproduced, and the interfacial thickness is in good agreement with experimental data and simulation results. The water/CO(2) mixture presents several types of interfaces as the liquid water may be in contact with gaseous, liquid, or supercritical CO(2). Here, the interfacial tension of the water/CO(2) mixture is modeled accurately by the gradient theory with a unique value of the crossed influence parameter over a broad range of thermodynamic conditions. The interfacial density profiles show a systematic adsorption of CO(2) in the interface. Moreover, when approaching the saturation pressure of CO(2), a prewetting transition is highlighted. The adsorption isotherm of CO(2) is computed as well in the case of a gas/liquid interface and compared with experimental data. The good agreement obtained is an indirect proof of the consistency of interfacial density profiles computed with the gradient theory for this mixture and confirms that the gradient theory is suitable and reliable to describe the microstructure of complex fluid interfaces.

Rheological and volumetric properties of TiO2-ethylene glycol nanofluids

Homogeneous stable suspensions obtained by dispersing dry TiO2 nanoparticles in pure ethylene glycol were prepared and studied. Two types of nanocrystalline structure were analyzed, namely anatase and rutile phases, which have been characterized by scanning electron microscopy. The rheological behavior was determined for both nanofluids at nanoparticle mass concentrations up to 25%, including flow curves and frequency-dependent storage and loss moduli, using a cone-plate rotational rheometer. The effect of temperature over these flow curve tests at the highest concentration was also analyzed from 283.15 to 323.15 K. Furthermore, the influence of temperature, pressure, nanocrystalline structure, and concentration on the volumetric properties, including densities and isobaric thermal expansivities, were also analyzed.

Interfacial properties of the Mie n−6 fluid: Molecular simulations and gradient theory results

In a first part, interfacial properties of a pure monoatomic fluid interacting through the Mie n-6 potential (n=8, 10, 12, and 20) have been studied using extensive molecular simulations. Monte Carlo and molecular dynamics simulations have been employed, using, respectively, the test area approach and the mechanic route. In order to yield reference values, simulations have been performed with a cutoff radius equal to 10sigma, which is shown to be sufficient to avoid long range corrections. It is shown that both approaches provide results consistent with each other. Using the molecular simulations results, it is demonstrated that a unique scaling law is able to provide an accurate estimation of the surface tension whatever the repulsive exponent n, even far from the critical point. Furthermore, it is shown that the surface tension of the Mie n-6 fluid is as well accurately described by a unique Parachors law. Density profiles are shown to be well represented by the tanh mean field profile, with slight deviations for the lowest temperatures and the smallest n. In addition, the interfacial width is shown to increase when n decreases (for a given reduced temperature) and to follow the usual scaling behavior for not too low temperature. In a second part, interfacial properties of the Mie n-6 fluid computed by the gradient theory, coupled with an equation of state based on the Barker-Henderson perturbation theory, have been compared with those obtained by molecular simulations. It is demonstrated that, even far from the critical point, the gradient theory is efficient to compute surface tensions and density profiles of this model fluid, provided the equation of state accurately model the phase behavior of the fluid involved (which is not the case for n=8 in this study).

Description of PVT behaviour of hydrofluoroethers using the PC-SAFT EOS

A description of the volumetric behaviour of segregated hydrofluoroethers has been carried out using PC-SAFT EOS. Characteristic parameters for several molecules were calculated where experimental data were available, and accurate results were obtained for compressed and saturated liquid density estimation, as well as for saturation curves. A functional group contribution scheme is proposed also for the calculation of the mentioned PC-SAFT EOS characteristic parameters, and this approach reproduces adequately the dependence on molecular structure of the parameters calculated in this work, and also gives good estimations when the parameters are extrapolated for other molecules of the same family.

On the Formation of a Third, Nanostructured Domain in Ionic Liquids

The study of solid-fluid transitions in fluorinated ionic liquids using differential scanning calorimetry, rheology, and molecular modeling techniques is an essential step toward the understanding of their dynamics and the thermodynamics and the development of potential applications. Two fluorinated ionic liquids were studied: 1-hexyl-3-methylimidazolium perfluorobutanesulfonate, HMIm(PFBu)SO3, and tetrabutylammonium perfluorobutanesulfonate, NB4(PFBu)SO3. The experimental calorimetric and rheological data were analyzed taking into account the possible mesoscale structure of the two fluorinated ionic liquids. The simulation results indicate the possible formation of three nanosegregated domains-polar, nonpolar, and fluorous-that may have a profound impact on ionic liquid research. In the case of HMIm (PFBu)SO3 the three types of mesoscopic domains can act as interchangeable jigsaw pieces enabling the formation of multiple types of crystals and inducing the observed calorimetry and rheological trends.