Orientational dielectric compensation

If a substance with a relative dielectric constant e, is placed between the plates, the field will be less by this factor. This is because of the partially compensating field that is induced within the dielectric by dipole orientation, as suggested by Figure 11.3b. Therefore, in the presence of the dielectric, the field is given by... 

Whenever two phases come in contact with one another, an interfacial region forms within which physical and chemical characteristics of each phase are disturbed relative to interior (bulk) regions of each phase. At the air-water interface, for example, the directional orientation of water molecules is more pionounced than in bulk solution, in order to compensate for the lack of hydiogen-bonding partners on the gas-phase side of the interface. As a consequence, the dielectric constant and other solvent characteristics that influence liemical reactions are perturbed to some degree. Solute molecules added to mi water or solvent-water systems may reside predominately in one phase or the other, or may concentrate in the interfacial region. Whether or not solute molecules are surface-active depends on the relative energies of possible dilute solute, solute-solvent, and solvent-solvent interactions (Tanford, 1980). 

Helices that form pores will be amphiphilic because it is more favorable to have situated in the inner side of the pore hydrophilic amino acid side chains, while the outer side of the pore represents a more favorable environment for hydrophobic amino acid side chains since these are in contact with lipids. Some authors point to the possibility that such a structure contains hydrogen bonds between amino acid residues and the main chain in order to compensate opposite charges and oppositely oriented dipoles. A comparison between the strength of different interactions in the structure of soluble and membrane proteins leads to the conclusion that because of the decreased strength of hydrophobic interactions and increased strength of electrostatic interactions (because of the reduced dielectric constant), the electrostatic interactions play the main role in stabilizing the structure of membrane proteins. ... 

Molecules consisting of atoms with different values of the electronegativity are polar. The dipole moment of a chemical bonding is a vector and therefore, a compensation or increase of the bond moments in a molecule can be observed [1, 2], Furthermore, the overall dipole moment of a molecule depends on the life time of different conformations. By the dielectric method, only a very small orientation of the molecular dipoles and the time for its reorientation can be measured as static dielectric constant and relaxation time T, respectively. Thereby the static dielectric constant can be produced by switching off an external electrical field in different steps as demonstrated in Fig. 1. 

Figure 13 shows the relaxation map of oriented polystyrene. Hie elementary processes isolated between 9VC and 102°C are characterized by relaxation times following a compensation law, eq. (8) they have the same relaxation time (t. = 0.11 s) at the compensation temperature = 145°C. This mode is the dielectric manifestation of the glass transition. The elementary peak isolated at 13S°C is well-described by a Fulcher-Vogel equation, eq. (7), with a critical temperature Too = 50"C, a thermal expansion coefficient of the free volume of 1.7 x lO" and a pre-exponential factor of 0.74 s. 

---