Dipolar orientation

An alternative method of studying the molecular motions of a polymeric chain is to measure the complex permitivity of the sample, mounted as dielectric of a capacitor and subjected to a sinusoidal voltage, which produces polarization of the sample macromolecules. The storage and loss factor of the complex permitivity are related to the dipolar orientations and the corresponding motional processes. The application of the dielectric thermal analysis (DETA) is obviously limited to macromolecules possessing heteroatomic dipoles but, on the other hand, it allows a range of frequency measurement much wider than DMTA and its theoretical foundations are better established. 

Next, we consider the second-order perturbation approximation. Since (AU)0 averaged over dipolar orientations vanishes... 

Fluorescence anisotropy is generally used to provide information about the dipolar orientational dynamics occurring after excitation of a system. This technique has successfully been used to probe ultrafast dynamics of energy transfer in organic conjugated dendrimers. The detected emission intensities Tar and Ter for parallel and perpendicularly polarized excitation respectively, were used to construct an observable emission anisotropy R(t) in accordance with the equation [121] ... 

These results indicate that, compared to bulk water, interfacial water exhibits unique oscillations in density with distance from the surface and preferential dipolar orientations. Both simulations report density values which are unreasonable. Part of this problem arises from attempting to fix the water density based on the average cell volume and the number of water molecules an approach which... 

Monte Carlo and Molecular Dynamics simulations of water near hydrophobic surfaces have yielded a wealth of information about the structure, thermodynamics and transport properties of interfacial water. In particular, they have demonstrated the presence of molecular layering and density oscillations which extend many Angstroms away from the surfaces. These oscillations have recently been verified experimentally. Ordered dipolar orientations and reduced dipole relaxation times are observed in most of the simulations, indicating that interfacial water is not a uniform dielectric continuum. Reduced dipole relaxation times near the surfaces indicate that interfacial water experiences hindered rotation. The majority of simulation results indicate that water near hydrophobic surfaces exhibits fewer hydrogen bonds than water near the midplane. 

Fig. 6.77. Calculations done using the statistical mechanical theory of electrolyte solutions. Probability density p(6,r) for molecular orientations of water molecules (tetrahedral symmetry) as a function of distance rfrom a neutral surface (distances are given in units of solvent diameter d = 0.28 nm) (a) 60H OH bond orientation and (b) dipolar orientation, (c) Ice-like arrangement found to dominate the liquid structure of water models at uncharged surfaces. The arrows point from oxygen to hydrogen of the same molecule. The peaks at 180 and 70° in p(0OH,r) for the contact layer correspond to the one hydrogen bond directed into the surface and the three directed to the adjacent solvent layer, respectively, in (c). (Reprinted from G. M. Tome and G. N. Patey, ElectrocNm. Acta 36 1677, copyright 1991, Figs. 1 and 2, with permission from Elsevier Science.

In Fig. 4, we show the single particle dipolar orientational relaxation (CM(t)) of bound water molecules (within 4.35A from any surfactant headgroup), comparing it to its relaxation in bulk. 

Consequently, polarisation due to dipolar orientation is directly proportional to the local field strength and inversely proportional to temperature. We can think of the quantity ifikT as an orientational polarisability and simply add... 

As frequency increases, all dispersion curves decrease to virtually zero. This indicates that EB is produced by the dipolar-orientational mechanism whereas the anisotropy of the dielectric polarizability of the macromolecules is virtually imper-ceptile in the Kerr effect. The displacement of curves towards higher frequencies with... 

If no ion is present in the system (q, = 0) then the above expression gives the sum of the Keesom-dipolar orientation and Debye-induction contributions to the total van der Waals forces between two molecules. A third contribution to van der Waals forces is also present,... 

Thus, the Keesom dipolar orientation interaction coefficient, CP, can be written as... 

The values in Table 2.3 indicate that the most important contribution to van der Waals interactions results from the London dispersion interactions. Keesom dipolar orientation interactions are only operative for strongly polar and hydrogen-bonding substances such... 

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