Water density anomaly

Mahoney, M. W. Jorgensen, W. L., Quantum, intramolecular flexibility, and polarizability effects on the reproduction of the density anomaly of liquid water by simple potential functions, 7. Chem. Phys. 2001,115, 10758-10768... 

The O H stretching spectra of ethanol trimers and larger clusters cannot be conformationally resolved in a slit jet expansion [65, 77, 157], VUV-IR spectra [184] are even broader, sometimes by an order of magnitude, and band maxima deviate systematically by up to +50 cm 1 from the direct absorption spectra. We note that ethanol dimers and clusters have also been postulated in dilute aqueous solution and discussed in the context of the density anomaly of water ethanol mixtures [227], Recently, we have succeeded in assigning Raman OH stretching band transitions in ethanol-water, ethanol water, and ethanol water2 near 3550, 3410, and 3430cm, respectively [228],... 

Mahoney MW, Jorgensen WL (2000) A five-site model for liquid water and the reproduction of the density anomaly by rigid, nonpolarizable potential functions, J Chem Phys, 112 8910-8922... 

Model for Liquid Water and the Reproduction of the Density Anomaly by Rigid, Non-polarizable Potential Functions. 

Mahoney MW, Jorgensen WL. A five-site model for liquid water and the reproduction of the density anomaly by rigid, nonpolar-izable potential functions. J. Chem. Phys. 2000 112 8910-8922. Rick SW. A reoptimization of the five-site water potential TIP5P for use with Ewald sums. J. Chem. Phys. 2004 120 6085-6093. Horn HW, Swope WC, Pitera JW, Madura JD, Dick TJ, Hura GL, Head-Gordon T. Development of an improved four-site water model for bio-molecular simulations UP4P-Ew. J. Chem. Phys. 2004 120 9665-9678. 

Table II. Density Anomaly (ct,) Values of Characteristic Features in the Water-Column Profiles...

The density anomaly of the Baltic Sea is chiefly caused by a significant excess of calcium bicarbonate discharged from the rivers. The resulting density deviation of brackish water in the Baltic Sea can be estimated by the empirical relation between absolute salinity 5a, determined from density measurements, and practical salinity 5, determined from conductivity (Millero and Kremhng, 1976),... 

Table 2 predicts that, of the biphase processes, the aqueous version will attain particular importance because of the many advantages of water as the support. As a solvent, water has numerous anomalies (e.g., density anomaly, the only non-toxic and liquid hydride of the non-metals, pressure-dependence of the melting point, dielectric constant), and its two- or even three-dimensional structure is still not well understood (cf. Sections 2.1 -2.3). Some of the known properties are listed below ... 

Water is an unusual and poorly understood liquid. Its first extraordinary property is a density anomaly. If solid water or ice is heated above its melting point, the density first increases, reaches a maximum density at about 4°C, and then decreases. Ice thus swims on fiquid water, and living organisms in lakes are not killed by a sinking ice cover in winter. Simple model calculations in two dimensions using the Mercedes Benz logo as the water model and small computers as tools reproduce... 

The unique features of individual water molecules (discussed in the preceding chapter) give rise to many anomalous properties of liquid water. Commonly attributed to the presence of an extensive hydrogen-bond network, these anomalies teach us a lot more about water itself Anomalies are observed in many properties, ranging from a density maximum at 4°C, the temperature dependence of isobaric specific heat and isothermal compressibility to a host of dynamic properties. Here we discuss some of them, with the emphasis on collective properties that are relevant to our study of complex systems discussed later. Understanding these anomalies is still the subject of considerable research activity. 

The density anomaly is one of the oldest known and one of the most quoted puzzles in the behavior of water [1 ]. Unlike other simple liquids, which expand upon heating... 

While in the liquid phase such a regular arrangement is disrupted by the entropic disorder that fevors the presence of molecules with many different coordination numbers, the influence of ice-like 4-coordinated stmctures remains significant at temperatures between 242 and 320 K. As discussed earher, the density anomaly can be understood by the decrease of 3-coordinated water molecules in favor of 4-coordinated ones till 4°C and the decrease of 5-coordinated ones in favor of 4-coordinated below 4 °C. As discussed in this chapter, many such anomalies can be understood by using the simple phenomenological approach, although it is not quantatively rigorous. 

Lynch GC, Pettitt BM (1997) Grand canonical ensemble molecular dynamics simulations Reformulation of extended system dynamics approaches. J Chem Phys 107 8594-8610 Madura JD, Pettitt BM, Calef DF (1988) Water under high pressure. Mol Phys 64 325 Mahoney MW, Jorgensen WL (2000) A five-site model for liquid water and the reproduction of the density anomaly by rigid, nonpolarizable potential functions. J ChemPhys 112 8910-8922 March RP, Eyring H (1964) Application of significant stmcture theory to water. J Phys Chem 68 221-228 Martin MG, Chen B, Siepman JI (1998) A novel Monte Carlo algorithm for polarizable force fields. 

The presence of the LLCP can affect the liquid s behavior at pressures and temperatures far away from the LLCP s location. This is evident in Fig. 5a which shows that the LLCP is located at Tc 245 K and Pc 190 MPa but, for example, the line extends up to T >= 300 K and P -20 MPa. Similarly, Fig. 5b shows that the LLCP is located at Tc 0.38 and Pc 0.25 but the icf line extends at least up to T >= 0.6 and P 0.4. The extent from the LLCP to which the anomalous properties of the liquid can be observed are particularly relevant to the case of water. It has been hypothesized that a LLCP exists in water at low temperature [24,48], recently estimated to be located at Tc 223K and Pc 50 MPa [49], and that it is such a LLCP that causes the anomalous properties observed in water at normal pressures. Among these anomalies are the density anomaly, increase of diffusivity upon isothermal compression, and increase of kt T P) and Cp T, P) upon isobaric cooling [42,50]. 

Water, both in the liquid and solid (ice) phase, is very peculiar, with properties that differ from most substances. A growing list of currently 69 anomalous properties has been compiled by Chaplin [1]. For example, water is the only substance that can be found in nature in the solid, liquid, and gas phases [2]. In the solid phase, it can exist in a wide variety of crystalline phases. Water is also well known for its density anomalies (such as the liquid s density maximum at 277.13K and the solid s density minimum at 70K [3]), diffusion anomalies... 

For both a = 2.1 and a = 3.3, in the reentrant-fluid region coexisting with the bcc solid, a density anomaly occurs, that is, the number density decreases upon cooling at constant pressure. This region is bounded from above by the temperature of the maximum density line (see Figs. 4 and 5). Similarly to water, the region of density anomaly is encompassed by the region of anomalous diffusion that in turn is enclosed by that of structural anomaly. A compendium of anomalous behaviors of the YK system with a = 3.3 is shown in Fig. 6. 

In order to better understand liquid polyamorphism [73,74], a systematic study was carried out on the effects of A, the ratio of characteristic energies on the existence of a LL transition, the positive or negative slope of the line of first-order LL transition in the P, T) plane, and the relationship, if any [58], between the LL transition and density anomalies. Calculations were performed in parallel for both confined and bulk water, and a spherically symmetric potential with two different length scales called the Jagla potential with both attractive and repulsive parts was used [58,64,65]. The potential is defined as... 

Figure 29.13(b) shows a more subtle volumetric property of w ater called the density anomaly. Heating simple liquids expands them because the greater thermal motions push the molecules apart. In this regard, water is normal above 3.984 °C. But below that temperature, heating increases water s density. 3.984 °C is called the temperature of maximum density. The maximum density is subtle (see the inset in Figure 29.13(b)) the density increase from 0 °C to 4 °C is less than 1% of the density increase upon melting. 

---