Positive surface potential

The phase-11 islands have several interesting properties. First, they have a positive surface potential relative to the surrounding unperturbed water film. The potential is highest immediately after formation and decays with time to zero. Another interesting property is the shape of the islands. Their boundaries are often polygonal, bending in angles of 120°,... 

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. 

In this case (Fig. 14.10), we observe a more complicated distribution of species. At low pH, the H+ activity and positive surface potential drive SOJ to sorb according to the reaction,... 

In an attempt to clarify matters further by studying a specific example, let us evaluate the total force, acting per unit length of two parallel cylinders of radius R, symmetrically charged to a positive surface potential and... 

This postulate has several implications regarding the mechanism of alkyne hydrogenation these will be discussed in Sect. 4.3. It should be noted, however, that there is as yet little or no direct evidence for structure L, although analogous structures are known to exist with organometallic complexes [161], Such a structure is also consistent with the positive surface potentials observed for acetylene adsorption on evaporated nickel films [88]. 

The AV-A results for NH4OH substrates exhibit a unique behavior not seen with the other basic subsolutions. At 60 sq. A. per molecule, the AV values are negative and remain so with compression up to 40 sq. A. per molecule. Beyond this point, the curves rise sharply toward positive surface potentials at maximum compression, potentials of +75 mv. are recorded for stearic acid. Montagne reports +40 mv. for stearic acid films at maximum compression over NH4OH solutions at pH 11 (41). 

Kenrick found that many organic substances cause a considerable positive surface potential—the outer layer being more positive than the inner half of the double layer, just as most insoluble organic substances have a positive surface potential most of the following results are due to Frumkin. 

Based on the above, a RI-SS relationship representing a particular colloidal system would be constant, assuming that pH is constant, because the RI-SS relationship is expected to be pH dependent. Furthermore, at a certain pH value, a colloidal system that exhibits both negative and positive surface potential would be expected to coflocculate when the net electrical potential is zero or the system is at its PZC (Schofield and Samson, 1953 Quirk and Schofield, 1955 Evangelou and Garyotis, 1985). Under these conditions, the settling characteristics of the suspended solids would be independent of RI (Fig. 9.14a and b). 

Reciprocity failure in aggregate photoreceptors has been described by Mey et al. (1979), as shown in Fig. 3. The exposure wavelength was at the absorption maximum, 680 nm. A loss in sensitivity of 0.14 log z was observed for positive surface potentials and 0.12 log z for negative surface potentials. The reciprocity failure was explained by Langevin (1903) recombination in conjunction with a field-dependent photogeneration process. 

Figure 3 Reciprocity failure in an aggregate photoreceptor with low-intensity emission-limited exposures (solid circles) and 6 ns flash exposures (open circles). The measurements were made with positive surface potentials (upper figure) and negative potentials (lower figure).

Nowadays, measures against silica contamination are often undertaken, and this has become a standard procedure, too obvious to be specified in scientific papers. Plastic labware was used in the synthesis of iron (hydr)oxides in [462]. Contact with sources of silica was avoided in a cleaning procedure of alumina flat plates in [463]. Attempts to avoid silica are not always successful. An abnormally low lEP was found in a study of the force between two single-crystal sapphire platelets [464]. The authors explained the positive surface potential at pHs as low as 6.7 by the sorption of silica or other species from the solution. 

When a metal comes into contact with a polar liquid, a redistribution of charge occurs at the newly formed interface. At the metal air interface, the electrons spill out of the metal atom lattice, giving rise to a positive surface potential, x - At the metal solution interface, this spillover is not the same because the same electrons encounter the electron density associated with the solvent molecules. Similar considerations apply to the liquid phase where the molecules usually adopt a special orientation at the liquid air interface giving rise to the surface potential of the solvent or the solution, x - This orientation may change upon contact with the metal phase. Finally, there is usually a redistribution of charge between the two phases. The exact nature of this process depends on the composition of the two phases, as will be seen from the following discussion. 

As the PAA concentration increases from zero to 2 x 10 g/l, the binding of carboxylate anionic groups to the hematite surface effectively reduces the positive surface potential, and a decreased stability ratio is observed. As the PAA concentration is increased beyond 2 x 10 g/l, the continued adsorption of negative carboxylate groups is able to reverse the interfacial potential, resulting in an increase in the stability ratio. 

Small organic molecules, for example phthalate, show specific interaction with hematite surface hydroxyl groups. Phthalate forms strong complexes on the hematite surface, thereby reducing the initially positive surface potential. Wlien sufficient phthalate is present in the suspension, hematite surface potential can be reversed as a result of phthalate adsorption. 

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