The increment of the temperature of maximum density of water by addition of small amounts of tert-butanol: Experimental data and microscopic description revisited.

The temperature of maximum density, TMD, of aqueous solutions of tert-butanol has been experimentally determined in the pressure range of 0-300 bars and up to 0.025 tert-butanol mole fraction. At atmospheric pressure, this quantity increases for low alcohol mole fractions, reaches a maximum at intermediate concentrations, and then quickly falls. The new experimental results are basically in agreement with previous data in the literature by Wada and Umeda [G. Wada and S. Umeda, Bull. Chem. Soc. Jpn. 35, 646 (1962)], except at very low mole fractions, where these authors reported a stronger density anomaly. Our measurements also confirm the known effect of pressure, p, on the variation in the temperature of maximum density with respect to that of pure water, ΔTMD: this quantity increases with p over the whole composition range. In addition, molecular dynamics simulations were performed between 0 and 2000 bars and from 238 to 328 K using a recently proposed model for the tert-butanol/water system. It has been found that our model reproduces qualitatively the experimental behavior of the ΔTMD, but for pressures above 1000 bars. A detailed structural analysis showed that the addition of tert-butanol promotes the low density water structure, and this promotion is somewhat hampered as the temperature increases at high pressure (ΔTMD > 0) and mostly independent of temperature at low pressures (ΔTMD < 0). Our analysis shows that the ultimate factor determining changes in the TMD is the temperature dependence of the low density water structure enhancement. We have also carried out a local structure analysis in which in addition to solid-like structures, low density liquid water ones have also been considered.

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.

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.

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.

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, ß, 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 ß 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.

A simple methodology for analyzing association effects on response functions via Monte Carlo simulations.

A simple methodology was developed to analyze association effects on the thermodynamic response functions for a pure self-associated fluid via Monte Carlo simulations. The procedure essentially involves expressing the residual energy and volume of the fluid in terms of these properties for two hypothetical fluids consisting of monomers and associated molecules, respectively. This allows the thermodynamic response functions to be expressed in a perturbative form as a combination of the values for the property in the monomeric fluid and the contribution of association (the perturbative term). The proposed methodology was used to determine both contributions to the isobaric heat capacity and to the temperature and pressure derivatives of the volume for OPLS methanol along the 50 MPa isobar from 220 to 1500 K. Based on the results, both terms exert a substantial influence on the isobaric heat capacity; by contrast, the association term for the volumetric properties is negligible. These results are consistent with those of a previous work involving simulations with the same model under identical thermodynamic conditions but a different approach. They are also compared with others previously reported in context. Moreover, a comprehensive study of the different types of clusters present in the fluid was performed and the results were related to thermodynamic properties. A strong correlation between the heat capacity of the monomeric fluid and this structural analysis was found.

Melting point and phase diagram of methanol as obtained from computer simulations of the OPLS model.

In this work, the melting point and the phase diagram of methanol is determined via computer simulations using the OPLS model. The three different solid structures that are found experimentally were considered. By computing the free energies of both the fluid phase and the three different solid structures (alpha, beta, gamma), the initial solid-solid and fluid-solid coexistence points were determined. By performing Gibbs-Duhem integration, the complete coexistence lines were evaluated. In this way, it was possible to compute, for the first time, the complete phase diagram for a potential model of methanol. It is found that the optimized potential model for liquid simulations (OPLS) provides reasonable predictions for the densities of the three solid polymorphs, although they tend to be somewhat low when compared with the experiment. Overall the model provides a qualitatively correct description of the phase diagram of methanol. The beta solid, which is thermodynamically stable in the experimental phase diagram of methanol, is found to be metastable in the phase diagram of the model. The alpha phase is stable at low pressures and the gamma phase is stable at high pressures, in agreement with experiment. Thus, the model is able to predict the existence of the gamma solid at high pressure. From free energy calculations we found that the melting point of the model at room pressure is 215 K. That was further confirmed by direct coexistence simulations. Thus, the model presents a melting point about 40 K above the experimental value of 175 K. Thus the OPLS model provides a reasonable description of the phase diagram of methanol, but it could probably be modified to improve the phase diagram predictions.

Highly precise experimental device for determining the heat capacity of liquids under pressure.

An experimental device for making isobaric heat capacity measurements of liquids under pressure is presented. The device is an adaptation of the Setaram micro-DSC II atmospheric-pressure microcalorimeter, including modifications of vessels and a pressure line allowing the pressure in the measurement system to be set, controlled, and stabilized. The high sensitivity of the apparatus combined with a suitable calibration procedure allows very accurate heat capacity measurements under pressure to be made. The relative uncertainty in the isobaric molar heat capacity measurements provided by the new device is estimated to be 0.08% at atmospheric pressure and 0.2% at higher levels. The device was validated from isobaric molar heat capacity measurements for hexane, nonane, decane, undecane, dodecane, and tridecane, all of which were highly consistent with reported data. It also possesses a high sensitivity as reflected in its response to changes in excess isobaric molar heat capacity with pressure, which were examined in this work for the first time by making heat capacity measurements throughout the composition range of the 1-hexanol+n-hexane system. Finally, preliminary measurements at several pressures near the critical conditions for the nitromethane+2-butanol binary system were made that testify to the usefulness of the proposed device for studying critical phenomena in liquids under pressure.