Surface potential, effective

Electrostatic Interaction. Similarly charged particles repel one another. The charges on a particle surface may be due to hydrolysis of surface groups or adsorption of ions from solution. The surface charge density can be converted to an effective surface potential, /, when the potential is <30 mV, using the foUowing equation, where -Np represents the Faraday constant and Ai the gas law constant. 

Fig. 5. Variation of ion concentrations, C (-), and effective surface potential, / (-), with distance from a positively charged surface. The layer of...

The interactions between bare mica surfaces in 10 and 10 M KNO solutions were determined at pH = 3.5. In both cases an exponential type relation F(D) = 0-lcD was indicated, with decay lengths 1/k = 1.4 nm and 8 nm for the two salt concentrations, respectively, but with an effective surface potential tp = 40 mV, considerably lower than its value at the higher pH used in the PEO experiments (figure 6a, curve (a)). The lower value of p is probably the result of a lower net degre of ionization of the mica surface in the presence of the large H1" concentration (the low pH was used to ensure full ionization and polyelectrolyte). 

The rate constants for the reaction of a pyridinium Ion with cyanide have been measured in both a cationic and nonlonic oil in water microemulsion as a function of water content. There is no effect of added salt on the reaction rate in the cationic system, but a substantial effect of ionic strength on the rate as observed in the nonionic system. Estimates of the ionic strength in the "Stern layer" of the cationic microemulsion have been employed to correct the rate constants in the nonlonic system and calculate effective surface potentials. The ion-exchange (IE) model, which assumes that reaction occurs in the Stern layer and that the nucleophile concentration is determined by an ion-exchange equilibrium with the surfactant counterion, has been applied to the data. The results, although not definitive because of the ionic strength dependence, indicate that the IE model may not provide the best description of this reaction system. 

Reaction models. The effective surface potentials ( j x) as defined... 

The rate constants for the reaction of N-dodecyl-3-carbamoyl-pyridinlum ion with cyanide in both cationic and nonionic o/w microemulsions have been measured as a function of phase volume. Added salt has no effect in the cationic system, but the rate constants in the nonionic system depend upon ionic strength as would be expected for a reaction between two ions. In order to compare the two microemulsions, the ionic strength in the reaction region has been estimated using thicknesses of 2-4A. The former produces values of the effective surface potential which yield... 

Table II shows that morpholinium laurate is markedly less effective in enhancing mechanical stability than are the other laurates which have been investigated. This is attributed to specific counterion adsorption, with a consequent reduction of the effective surface potential at the rubber-water interface.

ASYMPTOTIC BEHAVIOR OF POTENTIAL AND EFFECTIVE SURFACE POTENTIAL 37... 

Comparing Eqs. (1.25) and (1.177), we find that the effective surface potential... 

We obtain the scaled effective surface potential Y for a plate having a surface potential i/ o (or scaled surface potential jo) immersed in a solution of general electrolytes [9]. Integration of the Poisson-Boltzmann equation for the electric potential i//(x) is given by Eq. (1.65), namely,... 

Equation (1.200) is the required expression for the scaled effective surface potential (or the asymptotic constant) V and reproduces Eq. (1.197) for a sphere of radius a having a surface potential i/ o in a symmetrical electrolyte solution of valence z. The relative error of Eq. (1.200) is less than 1% for ko > 1. 

The effective surface potential i/ eff or scaled effective surface potential Y = ze jieffl kT of a cylinder in a symmetrical electrolyte solution of valence z can be obtained from the asymptotic form of the potential around the cyhnder, which in turn is derived from Eq. (1.157) as [7]... 

The potential distribution outside the surface charge layer of a soft particle with surface potential j/g is the same as the potential distribution around a hard particle with a surface potential xj/g. The asymptotic behavior of the potential distribution around a soft particle and that for a hard particle are the same provided they have the same surface potential xj/o- The effective surface potential is an important quantity that determines the asymptotic behaviors of the electrostatic interaction between soft particles (see Chapter 15). 

We introduce the effective surface potential i/ eff and the effective surface charge 2eff defined by... 

If plates 1 and 2 are immersed in a mixed solution of 1-1 electrolyte of bulk concentration and 2-1 electrolyte of bulk concentration then the effective surface potential i/ o and the scaled effective surface potential 7, of plate i (i = 1 and 2) are given by Eqs. (1.185) and (1.186),... 

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