Diego González-Salgado

Isobaric thermal expansivity and thermophysical characterization of liquids and liquid mixturesElectronic Supplementary Information available. See http://www.rsc.org/suppdata/cp/b1/b104891k/

A methodology for the thermophysical characterization of liquids and liquid mixtures based on measurements of density, sound speed and isobaric heat capacity per unit volume at atmospheric pressure as a function of temperature is proposed. Density and sound speed data are used to determine the isentropic compressibility from the Laplace equation. The precision in density measurements allows one to obtain the isobaric thermal expansivity at different temperatures using an incremental procedure with quite acceptable accuracy and precision. The isothermal compressibility and isochoric molar heat capacity are both obtained from the previous properties, using well-known thermodynamic relations. The accuracy of the proposed methodology was checked by determining the above-mentioned properties for liquid n-hexane, n-heptane, n-octane, n-dodecane, n-hexadecane, cyclohexane, and toluene over the temperature range (288.15–333.15) K and comparing the results with selected reported data. The average absolute deviations from the latter showed data obtained with the proposed methodology to be reasonably accurate. The excess quantities for nine binary mixtures of the cyclohexane + n-dodecane system were also determined with a view to assessing precision, which was found to be quite good as regards the dependence on both composition and temperature.

Towards an understanding of the heat capacity of liquids. A simple two-state model for molecular association

A model for the temperature dependence of the isobaric heat capacity of associated pure liquids C(p,m)(o)(T) is proposed. Taking the ideal gas as a reference state, the residual heat capacity is divided into nonspecific C(p) (res,ns) and associational C(p) (res,ass) contributions. Statistical mechanics is used to obtain C(p)(res,ass) by means of a two-state model. All the experimentally observed C(p,m)(o)(T) types of curves in the literature are qualitatively described from the combination of the ideal gas heat capacity C(p)(id)(T) and C(p)(res,ass)(T). The existence of C(p,m)(o)(T) curves with a maximum is predicted and experimentally observed, for the first time, through the measurement of C(p,m)(o)(T) for highly sterically hindered alcohols. A detailed quantitative analysis of C(p,m)(o)(T) for several series of substances (n-alkanes, linear and branched alcohols, and thiols) is made. All the basic features of C(p,m)(o)(T) at atmospheric and high pressures are successfully described, the model parameters being physically meaningful. In particular, the molecular association energies and the C(p)(res,ns) values from the proposed model are found to be in agreement with those obtained through quantum mechanical ab initio calculations and the Flory model, respectively. It is concluded that C(p,m)(o)(T) is governed by the association energy between molecules, their self-association capability and molecular size.

Second-order excess derivatives for the 1,3-dichloropropane + n-dodecane system

An experimental study of the behaviour of the second-order excess derivatives against composition and temperature for the 1,3-dichloropropane + n-dodecane system is presented. To this end, densities and speeds of sound were obtained in the interval 278.15-328.15 K with a step of 1 K. In addition, isobaric molar heat capacities were obtained in the temperature range 283.15-323.15 K. Measurements were performed over the whole composition range and at atmospheric pressure. Isentropic compressibilities were determined using the Laplace equation, and isobaric thermal expansivities were obtained in the interval 283.15-323.15 K from density data. Isothermal compressibilities and isochoric molar heat capacities within 283.15-323.15 K were derived from the above-mentioned properties via appropriate thermodynamic relations. The excess quantities for these properties were calculated using the ideality criterion first proposed by Benson and Kiyohara. Destruction of the 1,3-dichloropropane dipolar order during mixing appears to be the main effect to which the excess properties are sensitive.

Calculation of interfacial properties using molecular simulation with the reaction field method: Results for different water models

Coulombic interactions in molecular simulation are usually computed using the Ewald summation technique. This method is reliable for homogeneous and inhomogeneous systems but remarkably time consuming. This means a serious shortcoming in cases where unusually long simulation runs are necessary, for instance, during the calculation of interfacial properties, a subject of increasing interest. In homogeneous systems, the reaction field (RF) method can be alternatively used, reducing not only the computation time but also the difficulty of its implementation. However, it cannot be applied for inhomogeneous systems, at least from a strict formal point of view. In this paper, an analysis of the discrepancies in the computation of interfacial properties of water using the RF method is performed using constant volume biphasic Monte Carlo simulations, considering several of the most popular models available. The results show good quantitative agreement, within the simulation uncertainty, with the values obtained f...

Fluid-solid equilibrium of carbon dioxide as obtained from computer simulations of several popular potential models: the role of the quadrupole.

In this work the solid-fluid equilibrium for carbon dioxide (CO2) has been evaluated using Monte Carlo simulations. In particular the melting curve of the solid phase denoted as I, or dry ice, was computed for pressures up to 1000 MPa. Four different models, widely used in computer simulations of CO2 were considered in the calculations. All of them are rigid non-polarizable models consisting of three Lennard-Jones interaction sites located on the positions of the atoms of the molecule, plus three partial charges. It will be shown that although these models predict similar vapor-liquid equilibria their predictions for the fluid-solid equilibria are quite different. Thus the prediction of the entire phase diagram is a severe test for any potential model. It has been found that the Transferable Potentials for Phase Equilibria (TraPPE) model yields the best description of the triple point properties and melting curve of carbon dioxide. It is shown that the ability of a certain model to predict the melting curve of carbon dioxide is related to the value of the quadrupole moment of the model. Models with low quadrupole moment tend to yield melting temperatures too low, whereas the model with the highest quadrupole moment yields the best predictions. That reinforces the idea that not only is the quadrupole needed to provide a reasonable description of the properties in the fluid phase, but also it is absolutely necessary to describe the properties of the solid phase.

Association effects in pure methanol via Monte Carlo simulations. I. Structure

A methodology for the determination of the oligomers residing in a pure associated fluid was developed in the framework of the molecular simulation technique. First, the number of hydrogen bonds between each pair of molecules of the fluid is computed by using a specific criterion to define the hydrogen bonding formation. Secondly, sets of molecules linked by hydrogen bonds are identified and classified as linear chains, cyclic aggregates, branched linear chains, branched cyclic aggregates, and the rest of clustering. The procedure is applied over all the configurations produced in usual Monte Carlo simulations and allows the computation of the following properties characterizing the structure of the fluid: the fraction of molecules in the monomer or associated state, the fraction of each type of aggregate with a given size (and of molecules belonging to them), and the most probable and the average cluster size for each type. In addition, the degree of branching in branched linear chains and the type of ring in branched cyclic clusters can be obtained. In this work, all these quantities were computed for OPLS methanol using NpT Monte Carlo simulations at atmospheric pressure for 298.15 K (room conditions) and from 800 K to 350 K (gas phase), and along several supercritical isobars: 25, 50, 100, 200, and 500 MPa from 250 K to 1000 K. An analysis of the results has provided a comprehensive structural picture of methanol over the whole thermodynamic state space.

Solid-Solid and Solid-Fluid Equilibria of the Most Popular Models of Methanol Obtained by Computer Simulation

The ability of the most popular models of methanol (H1, OPLS, L2, and L1) for the prediction of the solid-solid and the solid-fluid equilibria was analyzed in detail in this work by using molecular simulation. The three solid phases (α, β, and γ) detected experimentally as being thermodynamically stable, as well as the fluid phase, were considered for the calculations. It turns out that all the models provide similar results. The α, γ, and fluid phases were found to be thermodynamically stable for a certain range of temperatures and pressures, whereas the β phase was always metastable. The coexistence curves (α-fluid, α-γ, γ-fluid) corresponding to all the models took the same shape except for some slight differences about their locations. From a qualitative point of view, it can be considered that the four models give a reasonable prediction of the phase diagram of methanol. However, there are important quantitative discrepancies. The melting points fell in the interval 214-223 K, whereas the γ phase was predicted to be stable at pressures above 12 × 10(4) bar. These results are quite different in relation to the experiments since the melting point of methanol is 175.6 K and the γ phase is stable at 3.5 × 10(4) bar at room temperature. In addition, the values of the melting enthalpy obtained by the different models are very similar but about 50% higher than the experimental value. Therefore, it is clear that there is room for improvement. Reducing the stability of the α phase with respect to the other phases seems to be a necessary condition to construct an improved potential.

Temperature of maximum density and excess thermodynamics of aqueous mixtures of methanol

In this work, we present a study of representative excess thermodynamic properties of aqueous mixtures of methanol over the complete concentration range, based on extensive computer simulation calculations. In addition to test various existing united atom model potentials, we have developed a new force-field which accurately reproduces the excess thermodynamics of this system. Moreover, we have paid particular attention to the behavior of the temperature of maximum density (TMD) in dilute methanol mixtures. The presence of a temperature of maximum density is one of the essential anomalies exhibited by water. This anomalous behavior is modified in a non-monotonous fashion by the presence of fully miscible solutes that partly disrupt the hydrogen bond network of water, such as methanol (and other short chain alcohols). In order to obtain a better insight into the phenomenology of the changes in the TMD of water induced by small amounts of methanol, we have performed a new series of experimental measurements and computer simulations using various force fields. We observe that none of the force-fields tested capture the non-monotonous concentration dependence of the TMD for highly diluted methanol solutions.

A new intermolecular potential for simulations of methanol: The OPLS/2016 model

In this work, a new rigid-nonpolarizable model of methanol is proposed. The model has three sites, located at the same positions as those used in the OPLS model previously proposed by Jorgensen [J. Phys. Chem. 90, 1276 (1986)]. However, partial charges and the values of the Lennard-Jones parameters were modified by fitting to an adequately selected set of target properties including solid-fluid experimental data. The new model was denoted as OPLS/2016. The overall performance of this model was evaluated and compared to that obtained with other popular models of methanol using a similar test to that recently proposed for water models. In the test, a certain numerical score is given to each model. It was found that the OPLS/2016 obtained the highest score (7.4 of a maximum of 10) followed by L1 (6.6), L2 (6.4), OPLS (5.8), and H1 (3.5) models. The improvement of OPLS/2016 with respect to L1 and L2 is mainly due to an improvement in the description of fluid-solid equilibria (the melting point is only 14 K higher than the experimental value). In addition, it was found that no methanol model was able to reproduce the static dielectric constant and the isobaric heat capacity, whereas the better global performance was found for models that reproduce the vaporization enthalpy once the so-called polarization term is included. Similar conclusions were suggested previously in the analysis of water models and are confirmed here for methanol.

Association effects in the {methanol + inert solvent} system via Monte Carlo simulations. I. Structure

In this work, the clusters residing in the {methanol + inert solvent} binary system have been characterized using a specific methodology in the framework of Monte Carlo molecular simulations. The cluster classification scheme considered distinguishes into five types: linear chains, cyclic clusters or isolated rings, branched linear chains, branched cyclic clusters, and composite rings. The procedure allows one to compute the next rich structural information: the fraction of molecules in the monomer or associated state, the fraction of each type of aggregate with a given size (and of molecules belonging to them), and the most probable and average cluster size for each type; likewise, the degree of branching in branched linear chains and the size distribution of the inner ring in branched cyclic clusters can be quantified. Specifically, all these properties were obtained for the {Optimized Potential for Liquid Simulation methanol + Lennard-Jones spheres} system at 298.15 K and 1 bar throughout the composition range. The results have provided a complete structural picture of this mixture describing comprehensively the effect of dilution into the hydrogen-bonded network of the pure associated fluid.