Dielectric Interaction

Dielectric interactions take place when ions polarize molecules of a solvent with a high dielectric constant. 

It is important to know the influence of the physicochemical parameters of the mobile phase (dipole moment, dielectric constant, and refractive index) on solvent strength and selectivity. The main interactions in planar chromatography between the molecules of the mobile phases and those of solutes are caused by dispersion forces related to the refractive index, dipole-dipole forces related to the dipole moment, induction forces related to a permanent dipole and an induced one, hydrogen bonding, and dielectric interactions related to the dielectric constant. Solvent strength depends mainly on the dipole moment of the mobile phase, whereas the solvent selectivity depends on the dielectric constant of the mobile phase. 

The term polarity refers to the ability of a sample or solvent molecule to interact by combination of dispersion, dipole, hydrogen bonding, and dielectric interactions (see Chapter 2 in reference 5). The combination of these four intermolecular attractive forces constitutes the solvent polarity, which is a measure of the strength of the solvent. Solvent strength increases with polarity in normal phase, and adsorption HPLC decreases with polarity in reversed-phase HPLC. Thus, polar solvents preferentially attract and dissolve polar solute molecules. 

To answer this question, let us first consider a neutral molecule that is usually said to be polar if it possesses a dipole moment (the term dipolar would be more appropriate)1 . In solution, the solute-solvent interactions result not only from the permanent dipole moments of solute or solvent molecules, but also from their polarizabilities. Let us recall that the polarizability a of a spherical molecule is defined by means of the dipole m = E induced by an external electric field E in its own direction. Figure 7.1 shows the four major dielectric interactions (dipole-dipole, solute dipole-solvent polarizability, solute polarizability-solvent dipole, polarizability-polarizability). Analytical expressions of the corresponding energy terms can be derived within the simple model of spherical-centered dipoles in isotropically polarizable spheres (Suppan, 1990). These four non-specific dielectric in-... 

A problem of molecular-shaped SCRF models is the absence of an analytical solution for the reaction field. One line of development was the search for an approximate expression for the dielectric interaction energy of a solute in a molecular-shaped cavity, without the need for explicit calculation of the solvent polarization. These models were summarized as generalized Born (GB) approximations [22,30]. The most popular of these models... 

Comparison of the barrier heights in the gas phase with those found in the liquid phase can elucidate the role of solvent internal pressure on conformational interconversion when dielectrical interactions are minimal. The magnitude of these interactions can be estimated by considering an activation volume, AV, for the process, defined as,... 

Some empirical polarity / polarizability descriptors which were proposed to measure the ability of the compound to influence a neighbouring charge or dipole by virtue of dielectric interactions are reported below. 

This term is a measure of the exoergic balance (i.e. release of energy) of solute-solvent and solute-solute dipolarity / polarizability interactions. This term, denoted by n, describes the ability of the compound to stabilize a neighbouring charge or dipole by virtue of nonspecific dielectric interactions and is in general given by -> electric polarization descriptors such as -> dipole moment or other empirical - polarity / polarizability descriptors [Abraham et al, 1988]. Other specific polarity parameters empirically derived for linear solvation energy relationships are reported below. 

Dielectric interactions resulting from electrostatic attraction between the solute molecules and a solvent of high dielectric constant. 

When developing theoretical models with which to interpret and predict the solvatochromic behavior of molecules. a distinction must be made between specific and nonspecific effects. The former are caused by specific interactions, such as complexation or hydrogen bonding with the solvent. The latter is a sum of different solute-solvent dielectric interactions. 

Despite all these problems, flexoelectricity has, from time to time, been found wanting when the results of an experiment could not fully be interpreted by the dielectric interaction alone though its relevance or responsibility could not always be proved. 

The orientation process in a LC polymer is a result of its dielectric interaction with an electric field. The duration of this process depends on the polymerization degree, the greater the polymerization degree, the lon-... 

If Ae were negative but not very small, the dielectric interaction would prevent the deformation of the director configuration. We estimate the dielectric anisotropy Ae, which will make the flexoelectric effect disappear, in the following way. The dielectric energy is... 

When a nematic liquid crystal is confined, such as when sandwiched between two parallel substrates with alignment layers, in the absence of external fields, the orientation of the liquid crystal director is determined by the anchoring condition. When an external electric field is applied to the liquid crystal, it will reorient because of the dielectric interaction between the liquid crystal and the applied field. If the dielectric anisotropy is positive (Ae > 0), the hquid crystal... 

In order to model the liquid crystal director configuration, we must first know how Uquid crystals interact with externally applied electric fields. Many liquid crystal devices make use of uniaxial nematic liquid crystals which are dielectrics. We consider the electric energy of nematic liquid crystals in externally applied electric fields through dielectric interaction. A typical liquid crystal device cell is shown in Figure 7.1, where the liquid crystal is sandwiched between two parallel substrates with transparent electrodes. The electric energy of the liquid crystal is given by [11-13]. 

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