Surface dipole orientation, measurement

Equations (25) to (29) concern the case of neutral adsorbates, where there is no ionic double layer to contribute to the surface potential. In the case of charged (i.e., ionic) adsorbates, the measured potential consists of two terms. The first term is due to dipoles oriented at the interface, which may be described by the above formulas, and the second term presents the potential of the ionic double layer at the interface from the aqueous... 

Every liquid interface is usually electrified by ion separation, dipole orientation, or both (Section II). It is convenient to distinguish two groups of immiscible liquid-liquid interfaces water-polar solvent, such as nitrobenzene and 1,2-dichloroethane, and water-nonpolar solvent, e.g., octane or decane interfaces. For the second group it is impossible to investigate the interphase electrochemical equilibria and the Galvani potentials, whereas it is normal practice for the first group (Section III). On the other hand, these systems are very important as parts of the voltaic cells. They make it possible to measure the surface potential differences and the adsorption potentials (Section IV). 

Another means of measuring the properties of insoluble films at the air-water interface is through the use of surface potentials. Surface potential (AF) measures the charge separation created by the vector component of the surfactant s molecular dipole that is perpendicular to the air-water interface. Thus, the surface potential yields information about the orientation of the surfactant molecules. Surface potential values are often expressed alternatively as surface dipole moments /i according to (2), where n is the... 

Figure 4. Detection of the change in photo-induced surface dipole moments in highly oriented A-S-D triads in artificial photosynthetic reaction centers as the local surface potential change in nano-domains measured by SSPM.

Two additional feature can be incorporated into Eqs. (7.32)—(7.35) the dipole orientation distribution and the concentration distribution in systems consisting of many dipoles. The orientation of the dipole with respect to the surface, described by angles Q = (8, ), affects E and all the other measurables derived from it.(33) Consider a concentration distribution of dipoles in both orientation and distance from the surface specified by C(0, , z). Since the dipoles all oscillate incoherently with respect to one another, the integrated intensity J due to this distribution is simply ... 

According to Bockris and Habib, the potential difference at the metal/solution interface at pzc is a result of the contribution of two components the surface potential (electron overlap) of the metal go and solvent dipoles oriented at the electrode surface, go- The value of go cannot be experimentally measured because the absolute value of the electrode potential is not known. However, the value of go can be estimated from the relation... 

In much of the above analysis, the relative magnitude of the surface and bulk contribution to the nonlinear response has not been addressed in any detail. As noted in Section 3.1, in addition to the surface dipole terms of Eq. (3.9), there are also nonlocal electric-quadrupole-type nonlinearities arising from the bulk medium. The effective polarization is made of a combination of surface nonlinear polarization, PNS (2co) (Eq. (3.9)), and bulk nonlinear polarization (Eq. (3.8)) which contains bulk terms y and . The bulk term y is isotropic with respect to crystal rotation. Since it appears in linear combination with surface terms (e.g. Eq. (3.5)), its separate determination is not possible under most circumstances [83, 129, 130, 131]. It mimics a surface contribution but its magnitude depends only upon the dielectric properties of the bulk phases. For a nonlinear medium with a high index of refraction, this contribution is expected to be small since the ratio of the surface contribution to that from y is always larger than se2(2co)/y. The magnitude of the contribution from depends upon the orientation of the crystal and can be measured separately under conditions where the anisotropic contribution of vanishes. 

To be consistent with the experimental data for hydration forces measured between silica interfeces, Xm has to be of the order of 4 A, a value which is compatible with Eiq. (24) for a random orientation of the neighboring water molecules. Therefore, in what follows, we will employ only the value A, =4 A. For the van der Waals interactions, we will assume in all calculations A//-X.3 10 21 J and 2t= 15 A, as obtained previously form the fit with Eq. (43b). The magnitudes of the surface dipoles will be considered 4 Debyes (about twice that of a water molecule) and it will be assumed that each dipole occupies on the surface an area of 50 A" and that they are located at a distance A =1 A below the first water monolayer. 

In the interpretation of the electrochemical dipole moment, it must be borne in mind that the adsorbate affects the surrounding solution. Thus, in the vicinity of an adsorbate with a dipole moment, the solvent dipoles will be oriented preferentially in the opposite direction, partially canceling the effect of the adsorbate dipole. The measurements give the total change in the surface dipole moment,... 

Figure 11.2 Left Schematic illustrating use of double stranded DNA to mediate distance between fluorophore and nanoparticle surface. Middle Symbols show the average values of experimentally measured fluorescence lifetime. The last measurement was done in the absence of gold nanoparticles as a calibration. The dashed and dashed-dotted curves display the calculated fluorescence lifetime for the molecular dipole oriented radially or tangentially with respect to the gold nanoparticle. Right Fluorescence signal corresponding to the measurements presented the middle panel. Reprinted with permission from reference [22]. (2007) American Chemical Society.

As with metals, the Helmholtz layer is developed by adsorption of ions or molecules on the semiconductor surface, by oriented dipoles or, especially in the case of oxides, by the formation of surface bonds between the solid surface and species in solution. Recourse to band-edge placement can be sought through differential capacitance measurements on the semiconductor-redox electrolyte interface [29j. 

The presence of an electrical potential drop, i.e., interfacial potential, across the boundary between two dissimilar phases, as well as at their surfaces exposed to a neutral gas phase, is the most characteristic feature of every interface and surface electrified due to the ion separation and dipole orientation. This charge separation is usually described as the formation of the ionic and dipolar double layers. The main interfacial potential is the Galvani potential (termed also by Trasatti the operative potential), AJinner potentials chemical nature of the contacting phases in the equilibrium, but it is not a measurable quantity. 

In the case of a polymer-coated metal substrate in a humid air atmosphere (for simplificity, only the situation of a polymer that is not highly oriented and has rather a small dipole potential is considered), a situation for the correlation of the corrosion potential with the Volta potential difference A R° f measured here (outer polymer surface and the probe as reference) could be derived analogously to the situation of an electrolyte-covered metal substrate (Eq. (10) with xpoI the surface dipole potential of the polymeric phase, which should be constant for a given polymer and a given gas phase as long as the polymer surface is stable). 

Many of the dynamic processes occurring on the electrode surface take place without the accompanying electron transfer process. Such processes can be represented by adsorption-desorption or change of electric dipole orientation. We need to gain sensitive access to the non-faradaic processes to track the non-fara-daic dynamics of molecular assembhes on electrode surfaces. The following non-faradaic processes can be the targets of the ER measurements. 

Similar to an electrolyte-covered metal surface, an electrode potential of the inner interface would be measured. However, the physical meaning of this electrode potential is not as obvious, as it cannot be interpreted by conventional electrochemical kinetics. The electrode potential could in the absence of any faradaic currents be determined by dipole orientation of segments of the polymer chain. If, however. 

---