Surface-Potential Data

Surface Potentials for Gases Physically Adsorbed on. Metal Surfaces  

An inspection of the S.P. values reveals that in many cases, e.g., the sys- 

It may be deduced from the data in Tables I-VI that (1) the S.P. for the chemisorption of alkali metals on W is positive and about 2 to 3 v., 

In chemisorption processes the observed surface potentials and the metal properties are not readily correlated as shown by the data for various metal + H2 systems  

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Gouy-Chapman, Stern, and triple layer). Methods which have been used for determining thermodynamic constants from experimental data for surface hydrolysis reactions are examined critically. One method of linear extrapolation of the logarithm of the activity quotient to zero surface charge is shown to bias the values which are obtained for the intrinsic acidity constants of the diprotic surface groups. The advantages of a simple model based on monoprotic surface groups and a Stern model of the electric double layer are discussed. The model is physically plausible, and mathematically consistent with adsorption and surface potential data. 

Co/pH and V o/pH results are sensitive to different aspects of the surface chemistry of oxides. Surface charge data allow the determination of the parameters which describe counterion complexation. Surface potential data allow the determination of the ratio /3 —< slaDL- Given assumptions about the magnitude of the site density Ns and the Stern capacitance C t, this quantity can be combined with the pHp2C to yield values of Ka and Ka2. Surface charge/pH data contain direct information about the counterion adsorption capacitances in their slope. To find the equilibrium constants for adsorption, a plot such as those in Figures 7 and 8 can be used, provided that Ka and Kai are independently known from V o/pH curves. 

Davis et al. [54] designed a graphical extrapolation procedure to calculate parameters of TLM from surface charging data. Similar graphical procedure was used to estimate the ApKa from electrokinetic data [55]. Bousse and Meind [56] postulate combination of potentiometric titration data with surface potential data... 

Preocanin, T., Janusz, W., and Kallay, N., Evaluation of equilibrium parameters of the anatase/aqueous electrolyte solution interface by introducing surface potential data, Colloids Surf. A, 297, 30, 2007. 

Properties of the inverse reconstruction of epicardial potentials from body surface data are discussed. It is shown that epicardial potentials accurately reflect details of the underlying myocardial sources, and are only minimally affected by body shape and by torso inhomogeneities. Initial results demonstrate the ability of the inverse procedure to accurately reconstruct multiple local cardiac events from the low resolution, smooth surface potential data. 

Figure 13 shows similar results but for a different source configuration. There are two activation fronts, one extending from 20° to 30°, and the other from -20° to 30°. Figure 13 (top) shows the surface data and the inverse-reconstructed epicardial potentials. Figure 13 (bottom) shows the actual forward-computed epicardial and body surface potentials for comparison. The surface distribution exhibits only a single potential maximum and does not reflect the two discrete activation fronts. The inverse-reconstructed epicardial potentials, on the other hand, contain two separate maxima which clearly reflect the true nature of the source. The location of the activation fronts and their extent are accurately reflected in the location and extent of the reconstructed maxima. This result demonstrates the capability of the inverse procedure to accurately reconstruct multiple local cardiac events from the low resolution, smooth surface potential data. As already mentioned, this property of the inverse procedure permits detailed examination of regional electrical events within the heart in a fashion that is not possible directly from the body surface potential distribution. 

Relating surface potential data to any molecular groups is rather daring since these groups are at an interface where the dielectric constant e jumps from 80 to 1. Thus much to our surprise but in accordance with previously published data /34/ the surface potential is positive. Looking at a sketched... 

In conclusion optical. X-ray and surface potential data demonstrate that the transition from LE to S involves an intermediate phase. This phase is more compressible than the one established at high pressure, exhibits low... 

Although the molecular interpretation of surface potential data is speculative they can be directly used to calculate dipolar forces. Considering two circular domains with radius R- lOpm and a distance D-3R apart the repulsion energy... 

The polynomials Wa q) q = qik) and Wy q) (q = qi) needed to specify the pair and surface potentials are constructed from the set of such q realized in a data base of 266 proteins with a total of 46100 residues by means of density estimation techniques. 

For the surface potentials, sufficiently many data were available, and no further problems appeared. The resulting potentials are shown in Figure 4. The hydrophobic amino acids are easily recognized as those for which the potential well is at small values of q. 

Pc- (c) Dipole density p. (d) Water contribution to the surface potential x calculated from the charge density Pc by means of Eq. (1). All data are taken from a 150 ps simulation of 252 water molecules between two mercury phases with (111) surface structure using Ewald summation in two dimensions for the long-range interactions. 

Nakagaki1U) has given a theoretical treatment of the electrostatic interactions by using the Gouy-Chapman equation for the relation between the surface charge density oe and surface potential /. The experimental data for (Lys)n agrees very well with the theoretical curve obtained. 

Figure 6-12. Model for Ihe Calculation of the van der Waals potential experienced by a single T6 molecule on a Tfi ordered surface. Each molecule is modeled as a chain of 6 polarizable spherical units, and the surface as 8-laycr slab, each layer containing 266 molecules (only pan of the cluster is shown). Tire model is based on X-ray diffraction and dielectric constant experimental data. The two configurations used for evaluating the corrugation of the surface potential are shown. Adapted with permission front Ref. [48].

The Volta potential is defined as the difference between the electrostatic outer potentials of two condensed phases in equilibrium. The measurement of this and related quantities is performed using a system of voltaic cells. This technique, which in some applications is called the surface potential method, is one of the oldest but still frequently used experimental methods for studying phenomena at electrified solid and hquid surfaces and interfaces. The difficulty with the method, which in fact is common to most electrochemical methods, is lack of molecular specificity. However, combined with modem surface-sensitive methods such as spectroscopy, it can provide important physicochemical information. Even without such complementary molecular information, the voltaic cell method is still the source of much basic electrochemical data. 

The published experimental estimates of the surface potentials of various organic solvents have been derived mainly from the data on the real, asi, and chemical, gSi, energies of solvation of ions ... 

This approach allowed us also to determine the difference in the surface potentials between mutually saturated water and an organic solvent namely, nitrobenzene, nitroethane and 1,2-dichloroethane, and isobutyl methyl ketone (IBMK). The qualitative data show a very strong influence of the added organic solvent on the surface potential of water, while the presence of water in the nonaqueous phase has practically no... 

Calculations for Rp as a function of the relevant experimental parameters (eluant ionic species concentration-including surfactant, packing diameter, eluant flow rate) and particle physical and electrochemical properties (Hamaker constant and surface potential) show good agreement with published data (l8,19) Of particiilar interest is the calculation which shows that at very low ionic concentration the separation factor becomes independent of the particle Hamaker constant. This result indicates the feasibility of xmiversal calibration based on well characterized latices such as the monodisperse polystyrenes. In the following section we present some recent results obtained with our HDC system using several, monodisperse standards and various surfactant conditions. 

In a number of cases, Eqs. (9.12) and (9.13) are in good agreement with experimental data. However, sometimes the quantitative agreement is not as good, and this can be attributed to the approximations made in connection with Eq. (9.12). Therefore, when comparing the calculations with experiment, one can reach certain conclusions as to the way in which the surface potentials vary (i.e., as to the way a metal interacts with the electrolyte). 

Recently, Samec et al. [38] have investigated the same system by the video-image pendant drop method. Surface tension data from the two studies are compared in Fig. 2, where the potential scale from the study [36] was shifted so that the positions of the electrocapillary maxima coincide. The systematic difference in the surface tension data of ca. 3%, cf. the dotted line in Fig. 2, was ascribed to the inaccurate determination of the drop volume, which was calculated from the shape of the drop image and used further in the evaluation of the surface tension [38]. A point of interest is the inner-layer potential difference A (pj, which can be evaluated relative to the zero-charge potential difference A cpp c by using Eq. 

Girault and Schiffrin [6] and Samec et al. [39] used the pendant drop video-image method to measure the surface tension of the ideally polarized water-1,2-dichloroethane interface in the presence of KCl [6] or LiCl [39] in water and tetrabutylammonium tetraphenylborate in 1,2-dichloroethane. Electrocapillary curves of a shape resembling that for the water-nitrobenzene interface were obtained, but a detailed analysis of the surface tension data was not undertaken. An independent measurement of the zero-charge potential difference by the streaming-jet electrode technique [40] in the same system provided the value identical with the potential of the electrocapillary maximum. On the basis of the standard potential difference of —0.225 V for the tetrabutylammonium ion transfer, the zero-charge potential difference was estimated as equal to 8 10 mV [41]. 

What was evident in 1950 was that very few surface-sensitive experimental methods had been brought to bear on the question of chemisorption and catalysis at metal surfaces. However, at this meeting, Mignolet reported data for changes in work function, also referred to as surface potential, during gas adsorption with a distinction made between Van der Waals (physical) adsorption and chemisorption. In the former the work function decreased (a positive surface potential) whereas in the latter it increased (a negative surface potential), thus providing direct evidence for the electric double layer associated with the adsorbate. 

Reactions of hydrophilic anions in microemulsions can be treated in terms of the ion-exchange formalism. The extent of ion binding to microemulsion droplets has been estimated conductimetrically, and the rates of reactions of OH- and F with p-nitrophenyl diphenyl phosphate are consistent with the extent of ion binding and competition between reactive and inert anions. Alternatively the data could be accommodated to variations in the estimated surface potential of the droplet (Mackay and Hermansky, 1981 Mackay, 1982). 

Figure 3 also contains an example of an ISER-flrel plot for a simple specifically adsorbed species, bromide on silver (solid curve). This plot was extracted from bromide coverage-potential data, obtained from differential capacitance measurements, along with the corresponding potential-dependent intensity of the SERS bromide-surface stretching mode at ca. 160 cm"1 (19.). In this case, the maximum (i.e. unity) value of 0r>1 corresponds to a close-packed bromide monolayer, ca. 1.4 x 10"9 mol cm 2. Again, the ISER-0t 1... 

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