Dielectric screening effects

At a constant total salt concentration, this steep dissociation increase is a clear manifestation of the dielectric screening effect of the solvent, i.e., the solvent s dielectric constant shows a threefold increase as the density doubles. Furthermore, the isochoric temperature effect on the degree of dissociation is also pronounced, even though the range of temperature is much smaller, i.e., 1.05 < T, < 1.4 (solubility of NaCl in water is larger than 10 M in the range of state conditions analyzed here). 

Also use constant dielectric Tor MM+aiul OPLS ciilciilatimis. Use the (lislance-flepeiident dielecinc for AMBER and BlO+to mimic the screening effects of solvation when no explicit solvent molecules are present. The scale factor for the dielectric permittivity, n. can vary from 1 to H(l. IlyperChem sets tt to 1. .5 for MM-r. Use 1.0 for AMBER and OPLS. and 1.0-2..5 for BlO-r. 

Before running a molecular dynamics simulation with solvent and a molecular mechanics method, choose the appropriate dielectric constant. You specify the type and value of the dielectric constant in the Force Field Options dialog box. The dielectric constant defines the screening effect of solvent molecules on nonbonded (electrostatic) interactions. 

As is obvious from the above relation, the screening effect of the substrate on interactions causes, in the limiting case z —> 0, the change in the surface-normal components by a factor of 2e/( +e) and in the surface-parallel components by a factor of 2/(1 + ), i.e., the renormalization ratio is equal tO . As the substrate dielectric constant increases, the interaction of surface-parallel components monotonically decays to zero, whereas the interaction of surface-normal components is enhanced reaching the maximum, viz. the interaction with a double dipole moment, for a metal (at e -> oo).130 Analogous renormalization factors are also of significance in the treatment of island-like particles on a dielectric substrate.131 With such effects included, formula (3.3.9) can be rewritten as ... 

Sections III. 1-III. 3 have described some basic discrete molecular and continuum treatments of solute-solvent interactions. There are many variants and refinements of these that have not been discussed, such as the use of effective dielectric constants66 or the implementation of dielectric screening.155 156 For ionic solutions, it is sometimes preferred to find the reaction field potential via the Poisson-Boltzmann rather than the Poisson equation,132 157 since the effects of the other ions can readily be incorporated into the former.158... 

With this background, we have proposed and developed a new purely electrical method for imaging the state of the polarizations in ferroelectric and piezoelectric material and their crystal anisotropy. It involves the measurement of point-to-point variations of the nonlinear dielectric constant of a specimen and is termed scanning nonlinear dielectric microscopy (sndm) [1-7]. This is the first successful purely electrical method for observing the ferroelectric polarization distribution without the influence of the screening effect from free charges. To date, the resolution of this microscope has been improved down to the subnanometer order. 

This longitudinal relaxation time differs from the usual Debye relaxation time by a factor which depends on the static and optical dielectric constants of the solvent this is based on the fact that the first solvent shell is subjected to the unscreened electric field of the ionic or dipolar solute molecule, whereas in a macroscopic measurement the external field is reduced by the screening effect of the dielectric [73]. 

The dynamics of the EET process is expressed in terms of a rate constant, k, which depends on several factors spectral properties of the D/A molecules, electron coupling between them, and the account of the screening effect of the solvent as a dielectric medium. In the so called weak coupling regime, the rate constant is predicted by the following Forster equation ... 

In order to treat the combined effects of added salt and dielectric boundaries on a manageable level, we use screened Debye-Hiickel (DH) interactions between all charges. In the presence of a dielectric interface, the Green s function can in general not be calculated in closed form [114] except for (i) a metallic substrate (with a substrate dielectric constant e =oo) and (ii) for e =0 (which is a fairly accurate approximation for a substrate with a low dielectric constant). For two unit charges at positions r and r one obtains for the total electrostatic interaction including screening and dielectric boundary effects... 

This approximate form of Gss(z R1 R2) shows a general property of van der Waals interactions when formulated in the approximation (small differences in dielectric response, neglect of retardation) used here. The interaction is independent of length scale. If we were to change all the sizes and separations by any common factor, both the numerator RfR and the denominator z6 would change by the same factor to the sixth power. In reality, because retardation screening effectively cuts off interactions at distances of the order of nanometers, it makes sense to think of this inverse-sixth-power interaction only for particles that are the angstrom size of atoms or small molecules. 

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