Temperature Of Maximum Density Line Research Articles

Liquid state anomalies and the relationship to the crystalline phase diagram.

A relationship between the observation of a density anomaly and the underlying crystalline phase diagram is demonstrated. The crystal phase diagram and temperature of maximum density (TMD) lines are calculated over a range of parameter space using a Stillinger-Weber potential. Relationships between the loci of density maxima in the PT plane for the liquid state and the underlying crystalline phase diagram are investigated. Two key potential parameters are systematically varied in order to control the balance between the model two- and three-body interaction terms, and the relative effects of varying the potential parameters analyzed. The respective TMD lines diverge at extreme values with one set of lines showing a reentrant behavior. For each parameter set the TMD lines are extrapolated to T=0K. The corresponding pressures are related to the crystalline phase diagram and are found to lie on or near specific crystal-crystal coexistence lines for a wide range of potential parameters. The density anomaly is observed to vanish corresponding to regions in the crystal phase diagram which lack crystal-crystal coexistence lines potentially offering a new interpretation for the emergence of anomalous behavior.

Phase behaviour of a continuous shouldered well model fluid. A grand canonical Monte Carlo study.

The phase behavior of the continuous shouldered well model fluid proposed by Franzese [J. Mol. Liq. 136 (2007) 267] was examined using the Monte Carlo computer simulations in the grand canonical ensemble. The essential parts of the vapour-liquid and liquid-liquid coexistence envelopes were obtained. The Widom lines departing from coexistence envelopes were calculated using maxima of the fluctuations of the number of particles as a function of chemical potential along various isotherms. The region embracing anomalies in the properties of the model was located using the approximate criterion that involves the excess pair entropy.. The temperature of maximum density line was built by performing canonical Monte Carlo simulations. Our results are consistent with previous results from molecular dynamics constant pressure-constant temperature simulations and provide wider insight into the phase behavior of the model by using the chemical potential as the external parameter.

Density anomaly of charged hard spheres of different diameters in a mixture with core-softened model solvent. Monte Carlo simulation results

Very recently the effect of equisized charged hard sphere solutes in a mixture with core-softened fluid model on the structural and thermodynamic anomalies of the system has been explored in detail by using Monte Carlo simulations and integral equations theory [J. Chem. Phys., 2012, 137, 244502]. Our objective of the present short work is to complement this study by considering univalent ions of unequal diameters in a mixture with the same soft-core fluid model. Specifically, we are interested in the analysis of changes of the temperature of maximum density (TMD) lines with ion concentration for three model salt solutes, namely sodium chloride, potassium chloride and rubidium chloride models. We resort to Monte Carlo simulations for this purpose. Our discussion also involves the dependences of the pair contribution to excess entropy and of constant volume heat capacity on the temperature of maximum density line. Some examples of the microscopic structure of mixtures in question in terms of pair distribution functions are given in addition.

Liquid–Liquid Coexistence in NaCl Aqueous Solutions: A Simulation Study of Concentration Effects

In this paper we investigate by means of molecular dynamics computer simulations how the hypothesized liquid-liquid critical point of water shifts in supercooled aqueous solutions of salt as a function of concentration. We study sodium chloride solutions in TIP4P water, NaCl(aq), for concentrations c = 1.36 mol/kg and c = 2.10 mol/kg. The liquid-liquid critical point is found up to the highest concentration investigated, and its position in the P-T plane shifts to higher temperatures and lower pressures upon increasing concentration. For c = 2.10 mol/kg it is also located very close to the temperature of maximum density line of the system. The results are discussed and compared with previous results for bulk TIP4P water and for c = 0.67 mol/kg NaCl(aq) and with experimental findings. We observe a progressive shrinkage of the low-density liquid region when the concentration of salt increases; this suggests an eventual disappearance of the liquid-liquid coexistence upon further increase of NaCl concentration.

Molecular dynamics studies on the thermodynamics of supercooled sodium chlorideaqueous solution at different concentrations

In this paper we compare recent results obtained by means of molecular dynamicscomputer simulations on the thermodynamics of TIP4P bulk water and onsolutions of sodium chloride in TIP4P water. The concentrations studied arec = 0.67, 1.36 and2.10 mol kg − 1. The results are checked against change of water–salt potential and size effects. Thesystems are studied in a wide range of temperatures, going from ambient temperature tothe supercooled region. Analysis of simulated state points, performed on the isochores andon the isotherm plane, allowed the determination of the limit of mechanical stability and ofthe temperature of maximum density lines. While the presence of ions in thesystem does not affect the limit of mechanical stability with respect to the bulk, itcauses the temperature of the maximum density line to shift to lower pressureand temperature upon increasing concentration. The occurrence of minima inthe trend of potential energy as a function of density and the inflections in thelow temperature isotherms suggest the presence of liquid–liquid coexistence forbulk water and for the sodium chloride solutions at all concentrations studied.

Liquid crystal phase and waterlike anomalies in a core-softened shoulder-dumbbells system

Using molecular dynamics we investigate the thermodynamics, dynamics, and structure of 250 diatomic molecules interacting by a core-softened potential. This system exhibits thermodynamic, dynamic, and structural anomalies: a maximum in density-temperature plane at constant pressure and maximum and minimum points in the diffusivity and translational order parameter against density at constant temperature. Starting with very dense systems and decreasing density the mobility at low temperatures first increases, reaches a maximum, then decreases, reaches a minimum and finally increases. In the pressure-temperature phase diagram the line of maximum translational order parameter is located outside the line of diffusivity extrema that is enclosing the temperature of maximum density line. We compare our results with the monomeric system showing that the anisotropy due to the dumbbell leads to a much larger solid phase and to the appearance of a liquid crystal phase.

Effect of concentration on the thermodynamics of sodium chloride aqueous solutions in the supercooled regime

Molecular dynamics simulations are performed on two sodium chloride solutions in TIP4P water with concentrations c=1.36 mol/kg and c=2.10 mol/kg upon supercooling. The isotherms and isochores planes are calculated. The temperature of maximum density line and the limit of mechanical stability line are obtained from the analysis of the thermodynamic planes. The comparison of the results shows that for densities well above the limit of mechanical stability, the isotherms and isochores of the sodium chloride aqueous solution shift to lower pressures upon increasing concentration while the limit of mechanical stability is very similar to that of bulk water for both concentrations. We also find that the temperature of maximum density line shifts to lower pressures and temperatures upon increasing concentration. Indications of the presence of a liquid-liquid coexistence are found for both concentrations.

Temperature of maximum density line of a polarizable water model.

Monte Carlo simulations of liquid water were performed with a polarizable model in a narrow range of temperatures around the temperature of maximum density (TMD) and a broad pressure range, extended also to negative pressures. In order to infer features of the phase diagram relevant for the explanation of the anomalous behavior of water the pressure dependence of the TMD line was analyzed. The TMD is found to increase with decreasing pressure up to 274.8 K at -32 bar, and decrease upon further stretching. This behavior excludes the possibility of a reentrant liquid-gas spinodal line.

Application of Statistical Physics to Understand Static and Dynamic Anomalies in Liquid Water

We present an overview of recent research applying ideas of statistical mechanics to try to better understand the statics and especially the dynamic puzzles regarding liquid water. We discuss recent molecular dynamics simulations using the Mahoney–Jorgensen transferable intermolecular potential with five points (TIP5P), which is closer to real water than previously-proposed classical pairwise additive potentials. Simulations of the TIP5P model for a wide range of deeply supercooled states, including both positive and negative pressures, reveal (i) the existence of a non-monotonic temperature of maximum density line and a non-reentrant spinodal, (ii) the presence of a low-temperature phase transition. The take-home message for the static aspects is that what seems to “matter” more than previously appreciated is local tetrahedral order, so that liquid water has features in common with SiO2 and P, as well as perhaps Si and C. To better understand dynamic aspects of water, we focus on the role of the number of diffusive directions in the potential energy landscape. What seems to “matter” most is not values of thermodynamic parameters such as temperature T and pressure P, but only the value of a parameter characterizing the potential energy landscape—just as near a critical point what matters is not the values of T and P but rather the values of the correlation length.

Interplay between time-temperature transformation and the liquid-liquid phase transition in water.

We study the new water model proposed by Mahoney and Jorgensen [J. Chem. Phys. 112, 8910 (2000)], which is closer to real water than previously proposed classical pairwise additive potentials. We simulate the model in a wide range of deeply supercooled states and find (i) the existence of a nonmonotonic "nose-shaped" temperature of maximum density line and a nonreentrant spinodal, (ii) the presence of a low-temperature phase transition, (iii) the free evolution of bulk water to ice, and (iv) the time-temperature-transformation curves at different densities.

Equation of state of supercooled water simulated using the extended simple point charge intermolecular potential

We carry out extensive molecular dynamics simulations in order to evaluate the thermodynamic equation of state of the extended simple point charge model of water (customarily described by the acronym SPC/E) over a wide range of temperature and density, with emphasis on the supercooled region. We thereby determine the location of the temperature of maximum density (TMD) line and the liquid spinodal line. In particular, we find that the experimental TMD line lies between the TMD lines of the SPC/E and ST2 models of water, so perhaps the behavior of these two models of simulated water “bracket” the behavior of real water. As temperature decreases, we find (i) that maxima appear in isotherms of the isothermal compressibility as a function of density, (ii) that isotherms of the internal energy as a function of volume display negative curvature and (iii) that the pressure of the liquid–vapor spinodal decreases. We compare the results to corresponding behavior found from simulations of the ST2 model of water and find that the behavior of SPC/E, when shifted to higher values of temperature and pressure (ΔP≈50 MPa and ΔT≈80 K), approximates that of ST2. We discuss the implications of our results for the hypothesis that a critical point occurs in the phase diagram of supercooled water. Finally, we argue that the results of our simulations are not inconsistent with the possibility that C′ exists for SPC/E water.