Polarization molecular origin

Now let us examine the molecular origin of Molecular polarity may be the result of either a permanent dipole moment p or an induced dipole moment ind here the latter arises from the distortion of the charge distribution in a molecule due to an electric field. We saw in Chap. 8 that each of these types of polarity are sources of intermolecular attraction. In the present discussion we assume that no permanent dipoles are present and note that the induced dipole moment is proportional to the net field strength at the molecule ... 

This chapter has given an overview of the structure and dynamics of lipid and water molecules in membrane systems, viewed with atomic resolution by molecular dynamics simulations of fully hydrated phospholipid bilayers. The calculations have permitted a detailed picture of the solvation of the lipid polar groups to be developed, and this picture has been used to elucidate the molecular origins of the dipole potential. The solvation structure has been discussed in terms of a somewhat arbitrary, but useful, definition of bound and bulk water molecules. 

Optical nonlinearities can be explained by considering the interaction of strong electric fields with matter. If the fields have optical frequencies, the phenomena resulting from the nonlinear interactions are called nonlinear optical phenomena. Most texts on nonlinear optics (e.g., Refs. 22-25) begin the discussion of this area from considerations of macroscopic relations between the vector quantities P (the polarization vector), D (the displacement vector), and E (the electric field vector). Chemists, however, consider the molecular origin of physical phenomena, so the description of NLO phenomena that follows starts from consideration of the behavior of a single molecule in a strong electric field. 

The Van der Waals attractive forces are forces of molecular origin, though at their basis lie electrical interactions. By their nature, these forces are caused by molecular polarization under the influence of fluctuations of charge distribution in the neighboring molecule and vice versa. These forces are also known as London s dispersive forces. The potential energy of molecular interaction (the London attraction energy) is equal to... 

FIGURE 2.56 Typical structure of the IL inside electrified pores of the CE)C-1200 material. Blue C-C bonds, red BMI+, and green PFs. (a) Local structure near a positive surface (+0.5 V), the anionic density is enhanced, (b) A single anion in a nanotube-like pore positively polarized (+0.5 V). (c) Same as (a) but near a negative surface (-0.5 V). (Reprinted by permission from Macmillan Publishers Ltd. Nature Materials Merlet, C. et al. 2012. On the molecular origin of supercapacitance in nanoporous carbon electrodes. 11 306-310, copyright 2012.)... 

The time dependence of the dielectric properties of a material (expressed by e or CT ) under study can have different molecular origins. Resonance phenomena are due to atomic or molecular vibrations and can be analyzed by optical spectroscopy. The discussion of these processes is out of the scope of this chapter. Relaxation phenomena are related to molecular fluctuations of dipoles due to molecules or parts of them in a potential landscape. Moreover, drift motion of mobile charge carriers (electrons, ions, or charged defects) causes conductive contributions to the dielectric response. Moreover, the blocking of carriers at internal and external interfaces introduces further time-dependent processes which are known as Maxwell/Wagner/Sillars (Wagner 1914 Sillars 1937) or electrode polarization (see, for instance, Serghei et al. 2009). 

The molecular origins of a macroscopic polarization P are dipole moments p . Hence, for molecules and/or particles in a volume V, the polarization can be calculated to... 

The spontaneous polarization is originated in the C2 symmetry of the SmC phase perpendicular to the molecular long axis. The molecular structure and the phase sequence of the world s first FLC, DOBAMBC (p-decyloxybenzylidene-p -amino-2-methylbutylcinnamate) synthesized by Meyer etal. [1], is shown in Fig. 5.1.3. 

A number of studies have shown that various other types of polysaccharide are capable of reducing oil-water interfacial tensions and forming stable emulsions (e.g. galactomannans, pectin, and chitosan) [23, 107-109]. Nevertheless, there is still some debate about the molecular origin of their surface activity (e.g. non-polar regions on the polysaccharide molecule itself, protein contaminants, or protein moieties bound to the polysaccharides), and about whether their ability to form stable emulsions is primarily due to their surface activity or the ability to thicken the aqueous phase [23]. 

We therefore conclude that there is some evidence that certain components of crude oils, including asphaltenes and resins, can adsorb at the air-liquid surface of non-polar hydrocarbon liquids. However, the molecular origin of such surface activity is far from clear, especially in those cases where significant polarity is present. Moreover, there appears to be little direct evidence that, for example, adsorption of asphaltenes and resins occurs at gas-crude oil surfaces. The main challenge therefore remains the difficult task of verifying that such surface activity is actually present in crude oils. 

In addition to the liquid structure of pure ILs, the time scales and molecular processes involved in the solvation of polar solutes in ILs have been examined by Kim and coworkers, who established the existence of multiple time scales in the relaxation of the solute using classic MD methods. Solvation dynamics in ILs and aqueous IL solutions have also been investigated in detail by Margulis and co-workers. In particular, they have been able to identify the molecular origins of the red edge effect as arising from the dynamic heterogeneity of these liquids on time scales relevant to optical spectroscopy. 

The immediately reacting part, Pu, is due to the shift of the electron clouds and the deformation of the molecular skeletons which occur within times corresponding to frequencies in the UV- and IR-range respectively. The retarded part, Por, arises for polar molecular fluids and originates from the orientation of the permanent dipoles. 

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