Polarized interface

FIG. 9 Simulated electrical potential and space charge density profiles at the water-1,2-DCE interface polarized at/= 5 in the absence (a) and in the presence (b) of zwitterionic phospholipids. The supporting electrolyte concentrations are c° = 20 mM and c = 1000 mM. The molecular area of the phospholipids is 150 A, and the corresponding surface charge density is a = 10.7 xC/cm. The distance between the planes of charge associated with the headgroups is d = 3 A. 

Keywords Fluorescence probing Hemimicelle Micellar fluidity Micelle Organized assemblies in solution and interfaces Polarity parameter Pyrene... 

Heterophase assemblages of mixed ionic/electronic conductors of the type A/AX/AY/A under an electric load are the simplest inhomogeneous electrochemical systems that can serve to exemplify our problem. Let us assume that the transport of cations and electrons across the various boundaries occurs without interface polarization and that the transference of anions is negligible. For the other transference numbers we then have... 

Fig. 16. Nyquist plot of the impedance response of an electrode. The equivalent electrical circuit is shown above the plot. Ra is the solution resistance, Cp the electrode/solution interface capacitance, and Rp the electrode/solution interface polarization resistance.

The results obtained after having examined several latices are summarized in Figure 14. The values found for A do not differ significantly for the homopolymer ana the VeoVa copolymer. For the vinyl acetate copolymer the area is distinctly increased as a result of increased interface polarity (39). 

The chemical nature of a solid determines its adsorptive and wetting properties. Now, the energy of immersion mainly depends on the surface chemistry but also, to some extent, on the nature of the bulk solid. For example, the interaction between water and silica has contributions from the bulk Si02 together with contributions from the silanol groups of the interface. Polar molecules are very sensitive to the local surface chemistry, whereas nonpolar molecules are more sensitive to the bulk composition. Interactions between a bulk Hquid and a bulk solid through an interface are often described in terms of Hamaker constant [16]. Immersion calorimetry in apolar liquids was proposed to estimate the Hamaker constant [17]. The sensitivity of immersion calorimetry to the surface polarity has justified its use for characterising the surface sites. 

The electrochemical cell used by Akamatsu et al. [36] to measure hydrogen permeability is shown in Fig. 8.10. A gas mixture of hydrogen and argon is introduced at the sample/ gas interface. Polarization measurements are performed in the potential range between —100 and 600 mV, so the palladium cathode is not oxidized. Hydrogen partial pressures range from 0.10 to 20.5 kPa. 

Interface polarization Dipole stretching Ferroelectric hysteresis Electric domain wall resonance Electrostriction Kezoelectricity Nuclear magnetic resonance Ferromagnetic resonance Ferrimagnetic resonance... 

The cell-suspension spectra are known to show a so-called (3-dispersion (183), which is observed in the frequency range 100 kHz-10 MHz and can be interpreted as the interfaee polarization. This dispersion is usually described in the framework of different mixture formulas and shelled models of particles (14, 70, 72, 183, 184). In the example of biological cells, the interface polarization is connected to the dielectrie permittivity and conduetivity of the cell structural parts. 

S. Murakami, H. Naito, Electrode and interface polarizations in nematic liquid crystal cells. 

Wang, H.R, E. Borguet, and K.B. Eisenthal (1998). Generalized interface polarity scale based on second harmonic spectroscopy. J. Phys. Chem. B 102, 4927 932. 

Shen, Y.R. and Ostroverkhov, V. 2006. Sum-frequency vibrational spectroscopy on water interfaces Polar orientation of water molecules at interfaces. Chem. Rev. 106 1140-1154. 

The results for the dissociation of water at the metal/solution interface show the well-known double-layer structure that is at the heart of most electrochemical systems. While the negative charge is delocalized, it still acts to polarize the surface. The proton which forms exists as either a hydronium (H3O+) or a Zundel (H5O2 ) ion both of which are about one solvation shell removed from the surface. This is known as the inner-layer Helmholtz layer. The chemistry that occurs at the interface polarizes the surface, which ultimately leads to a potential across the interface. In an actual system, the electrolyte plays an important role in establishing the potential as well as in potentially altering the structure and chemistry that occur at the interface. 

Unlike the interphase peak, the other main depolarization peaks observed, respectively, at + 35°C and + 0.4°C, do occur within a well defined concentration interval and are no more detectable outside a given liquid-crystalline region we interpreted these peaks as structure-peaks due to the presence of either liquid-crystalline structure. It should be noted that, since liquid-crystalline meso-phases of w/o microemulsions are both lyotropic and thermotropic, the structure peaks depend on the polarizing temperature, whereas the interphase peak does not. The above result indicates that each liquid—crystalline phase has a threshold temperature above which that given structure is destroyed. Considering the behavior of both the activation energy and the relaxation time of the interface--polarization process (Fig. 6), we may conclude that, as far as the interphase peak is concerned, a transition occurs at c = 0.580. 

In this context, the possibility to tune tire piezo- and pyroelectricity of specific composites (Floss et al. 2000) by means of separate poling of the inorganic particles and of the polymer crystallites should also be mentioned. In addition, piezo-, pyro-, and ferroelectric polymers such as PVDF and its relevant copolymers may be optimized by controlling fire poling of the amorphous and of the crystalline phase, as well as of the interface between fiiem (Maxwell-Wagner interface polarization) separately (Rollik et al. 1999). Furthermore, it is possible to follow the examples of the classical electret transducers (witti polymeric space-charge electrets) or of the dielectric-elastomer transducers (sometimes also called electro-electrets) and to... 

In this border layer, dissociation and increased mobility of the alkali ions are possible. This assumption is supported by the reduction in volume resistance by more than two magnitudes of power that occurs parallel to the increase in dielectric loss factor and dielectric constant. Additional losses are caused by interface polarization due to the increased mobility in the charge carriers. 

Eor another example at the liquid/liquid interface. Steel and Walker used two different solvatochromic probe molecules, para-nitrophenol (PNP) and 2,6-dimethyl-para-nitrophenol (dmPNP), to study the polarity of the water-cyclohexane interface. These probes give spectral shifts as a function of bulk solvent polarity that are very similar because both solutes are mainly sensitive to the nonspecific solvent dipolar interactions. However, when these two dye molecules are adsorbed at the water/cyclohexane interface, they experience quite different polarities. The more polar solute (PNP) has a maximum SHG peak that is close to that of bulk water, and thus it reports a high-polarity environment. In contrast, the less polar solute (dmPNP) reports a much lower interface polarity, having a maximum SHG peak close to that of bulk cyclohexane. Clearly, the more polar solute is adsorbed on the water side of the interface, keeping most of its hydration shell, and thus reports a higher polarity than does the nonpolar solute. Other examples of the surface polarity dependence on probe molecules are discussed in Ref. 363. 

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