Water dipolar properties

The dissolving of electrolytes in water is one of the most extreme and most important solvent effects that can be attributed to electric dipoles. Crystalline sodium chloride is quite stable, as shown by its high melting point, yet it dissolves readily in water. To break up the stable crystal arrangement, there must be a strong interaction between water molecules and the ions that are formed in the solution. This interaction can be explained in terms of the dipolar properties of water. 

The relationship between the herbicidal activity of 1,2,5-oxadiazole iV-oxides and some physicochemical properties potentially related to this bioactivity, such as polarity, molecular volume, proton acceptor ability, lipophilicity, and reduction potential, were studied. The semi-empirical MO method AMI was used to calculate theoretical descriptors such as dipolar moment, molecular volume, Mulliken s charge, and the octanol/water partition coefficients (log Po/w) <2005MOL1197>. 

Water absorption can also cause significant changes in the permittivity and must be considered when describing dielectric behavior. Water, with a dielectric constant of 78 at 25°C, can easily impact the dielectric properties at relatively low absorptions owing to the dipolar polarizability contribution. However, the electronic polarizability is actually lower than solid state polymers. The index of refraction at 25°C for pure water is 1.33, which, applying Maxwell s relationship, yields a dielectric constant of 1.76. Therefore, water absorption may actually act to decrease the dielectric constant at optical frequencies. This is an area that will be explored with future experiments involving water absorption and index measurements. 

Monte Carlo and Molecular Dynamics simulations of water near hydrophobic surfaces have yielded a wealth of information about the structure, thermodynamics and transport properties of interfacial water. In particular, they have demonstrated the presence of molecular layering and density oscillations which extend many Angstroms away from the surfaces. These oscillations have recently been verified experimentally. Ordered dipolar orientations and reduced dipole relaxation times are observed in most of the simulations, indicating that interfacial water is not a uniform dielectric continuum. Reduced dipole relaxation times near the surfaces indicate that interfacial water experiences hindered rotation. The majority of simulation results indicate that water near hydrophobic surfaces exhibits fewer hydrogen bonds than water near the midplane. 

According to the electrostatic model the solvation is due to electrostatic interaction between the charged ions and the dipolar solvent molecules. Thus the solvating and ionizing properties of a solvent are considered as being due primarily to the dipole moment of the solvent molecules. Thus, ionic compounds such as sodium chloride are insoluble in non-polar solvents such as carbon tetrachloride. Actually, rather than the dipole moment the field action of the dipoles should be considered. This approach might explain why acetonitrile (p = 3.2) is poor in its ionizing properties compared to water (p = 1.84). However, no numerical values are available for this quantity. 

Lee, Rasaiah, and Hubbard presented one of the first molecular dynamic studies of the effect of an external field on the properties of a dipolar fluid between charged walls. They simulated a film of 206 Stock-mayer fluid molecules (a Lennard-Jones core in which a point dipole is imbedded) between two flat walls under the influence of external electric fields of intensities ranging from 0 to 4 V/nm. We summarize their results here because they can be used as a reference point for the more complicated case of water. 

Figure 9,1 Molecular interaction potentials in Stockmayer s (1941) model for H2O vapor, (a) antiparallel dipolar moments (b) parallel dipolar moments. Reprinted from D. Eisemberg and W. Kauzmann, The Structures and Properties of Water, 1969, by permission of Oxford University Press.

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