Change in surface potential

A quantitative treatment for the depletive adsorption of iogenic species on semiconductors is that known as the boundary layer theory [84,184], in which it is assumed that, as a result of adsorption, a charged layer is formed. Doublelayer theory is applied, and it turns out that the change in surface potential due to adsorption of such a species is proportional to the square of the amount adsorbed. The important point is that very little adsorption, e.g., a 0 of about 0.003, can produce a volt or more potential change. See Ref. 185 for a review. 

FIG. 28 Changes in contact potential of mica relative to a hydrophobic tip as a function of relative humidity. The tip-sample distance during measurements was maintained at 400 A. At room temperature the potential first decreases by about 400 mV. At -30% RH it reaches a plateau and stays approximately constant until about 80% RH. At higher humidity the potential increases again, eventually becoming more positive than the initial dry mica surface. The changes in surface potential can be explained by the orientation of the water dipoles described in the previous two figures. 

The measurement of the concentration dependence of the changes in surface potential was also used to find critical concentration for forming a micellar solution. Near this critical concentration, one can observe a very strong fall in A%. ... 

The form of the kinetic equation depends on the way in which the surface potential X varies with electrode potential E. When the surface potential is practically constant, the first factor in Eq. (14.24) will also be constant, and the potential dependence of the reaction rate is governed by the second factor alone. The slope b of the polarization curve will be RT/ F (i.e., has the same value as that found when the same reaction occurs at a metal electrode). When in another case a change in electrode potential E produces an equally large change in surface potential (i.e., E = x + const), while there is practically no change in interfacial potential. Then Eq. (14.24) changes into... 

The change in surface potentials of the LB films of types a and b under step illumination with He-Cd laser light of 441.6 nm was measured by SSPM with a gold... 

The surface potential arises because the lipid molecule orients with polar part toward the aqueous phase. This effects a change in dipole at the surface. There would thus be a change in surface potential when a monolayer is present, as compared to a clean surface. The surface potential, AV, is... 

If in Problem 5 the change in surface potential arising upon contact between the two phases is taken into consideration, i.e., 8xS, what is the value of %s The value of 8%s is +0.27 V. (Gamboa-Aldeco)... 

Surface Potential. Shah and Schulman have proposed that interaction between dipoles of uncharged lipids in mixed monolayers should result in a change in surface potential, AV. Linearity of the relation of AV to composition of the lecithin-cholesterol monolayer was taken to indicate absence of interaction (17). We do not agree with Shah and Schulman, since surface potential does not appear to be a valid criterion for assaying interaction between dipoles of uncharged lipids. Except for the speculations of Shah and Schulman (17, 18), there is neither theoretical nor experimental evidence that dipole-dipole interactions have... 

By measuring a physical process related to the electron distribution in the surface, it may be possible to link the photosorption process at the surface with a certain electron transition in the surface layer of the catalyst lattice. Photoelectric emission, thermal afterglow, photoconductivity and Hall effect, changes in surface potential or electron spin resonance are such processes. 

Thus the maximum change in the surface potential, os the two surfaces are brought together, is proportional to the proximity to the isoelectric point If the surfaces are veiy near to isoelectric conditions when widely separated, then the change in surface potential... 

The change in surface potential shown by amines, on passing from the alkalino to the acid side, is much smaller than that shown by the acids when salts are formed.1 At about 20 5 sq. A. the potential on alkaline solutions is usually about 700 mv. there are, however, considerable differences in potential between films on solutions containing different salts of similar pH, and even on different concentrations of the same salt. The potential is slightly less, as a rule, for the films on acid solutions and here also different salts give different results, the variations amounting to 100 to 150 mv. 

The rate of change of Tc is greater for propylammonium than for butylammonium 50 K per log unit compared with 14 K per log unit. This still corresponds to a change in surface potential of only a few millivolts, so the qualitative behavior is the same as in the butylammonium system, namely one of approximately constant surface potential with respect to electrolyte concentration. The quantitative difference between the two slopes was important for our purposes in that it enabled us to make d = 43.6 A gels under easily controllable conditions at c = 0.5 M, T = 4°C. 

The model explains the continuing reaction as due to diffusive penetration of the surface. The simple equation used implies that the depth of penetration is small relative to the size of the particle. Remember, we are dealing with very slow processes. For any segment, the rate is proportional to the square root of time. The observed kinetics arise partly because there is a braking effect due to the change in surface potential as a result of reaction and partly because of the smearing effect of the heterogeneity. 

The change in surface potential at the air interface has been studied for a variety of electrolyte solutions in water [6, 10]. The results are reported as A% where... 

Table 8.4 Typical Values for the Change in Surface Potential at the Aqueous Solution Air Interface After Addition of an Organic Solute...

Fig. 8.15 Plot of the change in surface potential Ax against surface excess F for 1-propanol in water at the solution air interface [11].

Tihe interaction of a soluble constituent A with a spread monolayer of an insoluble constituent B was first studied by Schulman and Rideal (1). A was injected into the liquid substrate, and its penetration into B was studied by measuring the change in surface potential, AV, and surface pressure, An, of the spread monolayer at constant area. Assuming that the resulting mixed (A + B) monolayer was ideal, the surface density of A was inferred to be proportional to An. A positive An was taken as evidence for interaction between A and B. 

Determined from the change in surface potential with temperature and the 2pK model. The listed enthalpy is the average of... 

Change in surface potential and hydration of head groups... 

Ideally, if the layer formed by particles is stable, the signal coming from the solid can be subtracted from the final spectra, and the presence of these bands does not hamper the detection and interpretation of bands from adsorbed sp>edes. However, in practice subtraction is often difficult due to the evolution of the signal of the solid with time or solution composition. Phenomena such as re-entrainment of particles by flowing solution, or swelling/ shrinkage due to the change in surface potential can explain this problem. 

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