Water, permittivity

N is the number of point charges within the molecule and Sq is the dielectric permittivity of the vacuum. This form is used especially in force fields like AMBER and CHARMM for proteins. As already mentioned, Coulombic 1,4-non-bonded interactions interfere with 1,4-torsional potentials and are therefore scaled (e.g., by 1 1.2 in AMBER). Please be aware that Coulombic interactions, unlike the bonded contributions to the PEF presented above, are not limited to a single molecule. If the system under consideration contains more than one molecule (like a peptide in a box of water), non-bonded interactions have to be calculated between the molecules, too. This principle also holds for the non-bonded van der Waals interactions, which are discussed in Section 7.2.3.6. 

The first modification is to simply scale the dielectric permittivity of free space (T( ) by a scale factorD to rn ediate or dam pen thelong range electrostatic interactions. Its value was often set to be between 1.0 and 7H.0, the macroscopic value for water. A value of D=2..5, so that u=2..5Ug, wasoften used in early CIIARMM calculation s. 

Grade 1—T1 Flex strength, MPa Water absorption, % Permittivity, 1 MHz Dissipation factor, 1 MHz Impact strength, J/m Dielectric breakdown parallel to laminations, kV... 

R = 8.314 J/(mol-K) T = temperature, K and F = 9.65 x lO" C/mol. The relative permittivity is a measure of the conductance of the pure bulk material relative to a vacuum. In the salt water in the beaker example, the pure bulk material would be water, which has a relative permittivity of about 80. 

Fig. 20.12 Double layer consisting of (a) a layer of water dipoles and a layer of ions that may be regarded as (b and c) two capacitors in series (after Bockris and Reddy ). Note that A rH are the capacitances of the regions of high and low permittivities, respectively...

Meanwhile, the R-R coupling (see Sect. 2.2) has evidently found general acceptance as the main reaction path for the electropolymerization of conducting polymers The ionic character of the coupling species explains why polar additives such as anions or solvents with high permittivity accelerate the rate of polymerization and function as catalysts. Thus, electropolymerization of pyrrole is catalyzed in CHjCN by bromide ions or in aqueous solution by 4,5-dihydro-1,3-benzenedisulfonic acid The electrocatalytic influence of water has been known since the work... 

In Eqs. (27) and (28), p is the contribution of the substrate water molecules, p that of the adsorbate polar head, and p that of the hydrophobic moiety of the adsorbed molecules. Consistently, 8i, 82, and 83 are the effective local permittivities of the free surface of water and of the regions in the vicinity of the polar head and of the hydrophobic group, respectively. The models have been used in a number of papers on adsorbed monolayers and on short-chain substances soluble in water. " Vogel and Mobius have presented a similar but more simplified approach in which p is split into two components only. " Recently some improvements to the analysis using Eq. (27) have been proposed. " An alternative approach suggesting the possibility of finding the values of the orientation angle of the adsorbate molecules instead of local permittivities has been also proposed.""... 

The low permittivity of these liquids compared with water inhibits dissociation of the acids so that cement formation demands much more reactive basic oxides. Oxides and hydroxides that are capable of cement formation are ZnO, CuO, MgO, CaO, Ca(OH)2, BaO, CdO, HgO, PbO and BiaOj (Brauer, White Moshonas, 1958 Nielsen, 1963). In practice these are confined to two calcium hydroxide and special reactive forms of zinc oxide. 

This hypothesis received support from the electrical studies of Braden Clarke (1974) and Crisp, Ambersley Wilson (1980), who attributed maxima in curves of permittivity and conductivity against time to the liberation of water and its subsequent reabsorption into the matrix (Figure 93a,b). Crisp, Ambersley Wilson (1980) also considered that these maxima were due to generation of both water and ionic zinc species. Subsequently, as the reaction proceeds the zinc ions are fixed as insoluble zinc eugenolate. 

Figure 9.3a Permittivity/time curves (e) of cements, showing maxima. Prepared from ZnO ignited at 600°C with A-.-A dry eugenol eugenol+1% water 0-- 0...

Ion pairs can form only when the distance of closest approach, a, of the two ions is less than r . For 1 1 electrolytes for which = 0.357 nm, this condition is not always fulfilled, but for others it is. The fractions of paired ions increase with increasing concentration of solutions. In nonaqueous solutions which have lower values of permittivity e than water, the values of and the fractions of paired ions are higher. In some cases the values of coincide with the statistical mean distance between the ions (i.e., the association of the ions is complete). 

An interface between two immiscible electrolyte solutions (ITIES) is formed between two liqnid solvents of a low mutual miscibility (typically, <1% by weight), each containing an electrolyte. One of these solvents is usually water and the other one is a polar organic solvent of a moderate or high relative dielectric constant (permittivity). The latter requirement is a condition for at least partial dissociation of dissolved electrolyte(s) into ions, which thus can ensure the electric conductivity of the liquid phase. A list of the solvents commonly used in electrochemical measurements at ITIES is given in Table 32.1. 

Dielectric constant Also called permittivity. The dielectric constant of a substance is the ratio of the attractive force between two opposite charges measured in a vacuum to that force measured in the substance. The high dielectric constant of water makes it a good solvent for ionic compounds. 

On the assumption that = 2, the theoretical values of the ion solvation energy were shown to agree well with the experimental values for univalent cations and anions in various solvents (e.g., 1,1- and 1,2-dichloroethane, tetrahydrofuran, 1,2-dimethoxyethane, ammonia, acetone, acetonitrile, nitromethane, 1-propanol, ethanol, methanol, and water). Abraham et al. [16,17] proposed an extended model in which the local solvent layer was further divided into two layers of different dielectric constants. The nonlocal electrostatic theory [9,11,12] was also presented, in which the permittivity of a medium was assumed to change continuously with the electric field around an ion. Combined with the above-mentioned Uhlig formula, it was successfully employed to elucidate the ion transfer energy at the nitrobenzene-water and 1,2-dichloroethane-water interfaces. 

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