Surface dielectric constant

The physical properties of saturated fluorocarbons and their analogous hydrocarbons differ in many respects [4 5] Sahirated fluorocarbons have the lowest dielectric constants, surface tensions, and refractive indexes of any liquids at room... 

In Eq. (6) Ecav represents the energy necessary to create a cavity in the solvent continuum. Eel and Eydw depict the electrostatic and van-der-Waals interactions between solute and the solvent after the solute is brought into the cavity, respectively. The van-der-Waals interactions divide themselves into dispersion and repulsion interactions (Ed sp, Erep). Specific interactions between solute and solvent such as H-bridges and association can only be considered by additional assumptions because the solvent is characterized as a structureless and polarizable medium by macroscopic constants such as dielectric constant, surface tension and volume extension coefficient. The use of macroscopic physical constants in microscopic processes in progress is an approximation. Additional approximations are inherent to the continuum models since the choice of shape and size of the cavity is arbitrary. Entropic effects are considered neither in the continuum models nor in the supermolecule approximation. Despite these numerous approximations, continuum models were developed which produce suitabel estimations of solvation energies and effects (see Refs. 10-30 in 68)). 

The density of the perfluorocarbons is usually more than twice that of their hydrocarbon counterparts. The low dielectric constants, surface/interface tensions and refractive indices of perfluorocarbons reflect their nonpolar character as liquids and solvents.6... 

FIGURE 10.10 (a) Plot of the attraction and repulsion forces acting on two spherical particles as a function of particle separation for constant dielectric constant, surface potential, and double layer thickness resulting from 1 1 salt (b) Plot of the sum... 

Very recently, the separation of polar analytes has also been performed by using pure water under subcritical conditions. Subcritical water has several unique characteristics. For example, the dielectric constant, surface tension, and viscosity of water are dramatically decreased by raising the water temperature while a moderate pressure is applied to keep water in the liquid state. At 200 -250°C, the values of these physical properties are similar to those of pure methanol or acetonitrile at ambient conditions. Therefore, subcritical water may be a potential mobile phase for polar analytes. SFC mobile phases other than CO2 are reviewed separately in this encyclopedia. 

Solvent Boiling point (°C) Viscosity (cP) Refractive index UV cutoff (nm) Dielectric constant Surface tension (dyne/cm)... 

Liquid water has many other properties that exhibit anomalous values, e.g. dielectric constant, surface tension, diffusion coefficient, viscosity, etc. Aqueous solutions also exhibit unusual properties, some of which will be discussed in Chapter 3. In this introduction, we have mentioned only a few properties, the interpretation of which can be given by some simple theoretical arguments. We shall discuss these properties again in Chapter 2. 

Dielectric Behavior of Adsorbed Water. Determination of the dielectric absorption of adsorbed water can yield conclusions similar to those from proton NMR studies and there is a considerable, although older literature on the subject. Figure XVI-7 illustrates how the dielectric constant for adsorbed water varies with the frequency used as well as with the degree of surface coverage. A characteristic relaxation time r can be estimated... 

Similar, very detailed studies were made by Ebert [112] on water adsorbed on alumina with similar conclusions. Water adsorbed on zeolites showed a dielectric constant of only 14-21, indicating greatly reduced mobility of the water dipoles [113]. Similar results were found for ammonia adsorbed in Vycor glass [114]. Klier and Zettlemoyer [114a] have reviewed a number of aspects of the molecular structure and dynamics of water at the surface of an inorganic material. 

By using an effective, distance-dependent dielectric constant, the ability of bulk water to reduce electrostatic interactions can be mimicked without the presence of explicit solvent molecules. One disadvantage of aU vacuum simulations, corrected for shielding effects or not, is the fact that they cannot account for the ability of water molecules to form hydrogen bonds with charged and polar surface residues of a protein. As a result, adjacent polar side chains interact with each other and not with the solvent, thus introducing additional errors. 

VISCOSITY, SURFACE TENSION, DIELECTRIC CONSTANT, DIPOLE MOMENT,... 

Table 5.18 Viscosity, Dielectric Constant, Dipole Moment, and Surface Tension...

Substance Viscosity, mN Dielectric constant, e Dipole moment, D Surface tension, mN m ... 

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