Dielectric constant of liquid water

It is now well understood that the static dielectric constant of liquid water is highly correlated with the mean dipole moment in the liquid, and that a dipole moment near 2.6 D is necessary to reproduce water s dielectric constant of s = 78 T5,i85,i96 holds for both polarizable and nonpolarizable models. Polarizable models, however, do a better job of modeling the frequency-dependent dielectric constant than do nonpolarizable models. Certain features of the dielectric spectrum are inaccessible to nonpolarizable models, including a peak that depends on translation-induced polarization response, and an optical dielectric constant that differs from unity. The dipole moment of 2.6 D should be considered as an optimal value for typical (i.e.. 

The dielectric constant of liquid water at room temperature is 80. This means that two opposite electrical charges in water attract each other with a force only 1/80 as strong as in air (or a vacuum). It is clear that the ions of a crystal of sodium chloride placed in water could dissociate away from the crystal far more easily than if the crystal were in air, since the electrostatic force bringing an ion back to the surface of the crystal from the aqueous solution is only 1/80 as strong as from air. It is accordingly not surprising that the thermal agitation of the... 

The very high dielectric constant of liquid water results from its peculiar structure. The structure shown in Figure 3.9 is not a completely rigid one, since individual water molecules can rotate fairly freely on their axes. The structure thus has a high polarizability, in that it easily accommodates itself to an electric field. This high polarizability results in the high dielectric constant. 

All the calculations have been performed at SCF level with a basis set equal to the Dunning/Huzinaga valence double-zeta. All the calculations in solution have been performed using the lEF version of the PCM method and they refer to a medium having dielectric constant e = 78.5 corresponding to the static dielectric constants of liquid water at 298 K. 

Hamelin J, Mehl JB, Moldover MR. 1998. The static dielectric constant of liquid water between 274 and 418 K near the saturated vapor pressure. Int. J. Thermophys. 19 1359-1380. 

Mata et al have determined the dynamic polarizability and Cauchy moments of liquid water by using a sequential molecular dynamics(MD)/ quantum mechanical (QM) approach. The MD simulations are based on a polarizable model of liquid water while the QM calculations on the TDDFT and EOM-CCSD methods. For the water molecule alone, the SOS/TDDFT method using the BHandHLYP functional closely reproduces the experimental value of a(ffl), provided a vibrational correction is assumed. Then, when considering one water molecule embedded in 100 water molecules represented by point charges, a(co) decreases by about 4%. This decrease is slightly reduced when the QM part contains 2 water molecules but no further effects are observed when enlarging the QM part to 3 or 4 water molecules. These molecular properties have then been employed to simulate the real and imaginary parts of the dielectric constant of liquid water. 

Figure 22.15 Experimental values for the dielectric constant of liquid water as a function of temperature.

Table 12.2 gives the properties of sub- and supercritical water. Subcritical water is the water that is in a state under a pressurized condition at temperatures above its boiling point under ambient pressure and below the critical point Tc = 374°C Pc = 22.1 MPa, pc = 320 kg/cm ). The dielectric constant of liquid water decreases with increasing temperature (Nanda et al., 2014b). At temperatures from 277 to 377°C, the dielectric constant becomes as low as those of polar organic solvents. The ionic product of water is maximized at temperatures between 227 and 372°C depending upon the pressure (Kruse and Dinjus, 2007). Thus, subcritical water acts as acid and/or base catalysts for reactions, such as hydrolysis of ether/ester bonds, and also as a solvent for the extraction of low molecular mass products (Brunner, 2009). 

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