Lead salts, double layer

Figure 6.10 Electrostatic double-layer force between a sphere of R = 3 /um radius and a flat surface in water containing 1 mM monovalent salt. The force was calculated using the nonlinear Poisson-Boltzmann equation and the Derjaguin approximation for constant potentials (tpi = 80 mV, ip2 = 50 mV) and for constant surface charge (i/2/Ad so that at large distances both lead to the same potential.

In another vein, double layers play a role In the salt-sieving phenomenon, mentioned In the Introduction to Volume I, and already known to Aristotle. When seawater percolates over a compact sediment of slllcate-like particles, under some conditions the effluent Is potable. Basically the phenomenon is attributable to the negative adsorption of (in this case) anions, leading to the Donnan expulsion of electrolyte, see sec. 3.5b. Over-demand may lead to salt penetration the screening of the double layers around the silica particles (reduction of x ) makes the pores between them effectively wider. For this problem technical solutions had to be found. 

Combination of the attractive van der Waals potential - -1/ and the repulsive double-layer potential ( gives rise to tiie famous DLVO theory of colloid stability. Depending on salt, the net potential can be such as to induce flocculation directly, pose a barrier to flocculation, or lead to a stable suspension. 

Passivation of cathodes in hexamethylphosphotriamide has been studied in greater detail. Measurements of double-layer capacity have revealed that cathodic polarization of electrodes in alkali metal salt solutions or the enrichment of the bulk of these solutions in solvated electrons leads to passivation which decreases the capacity... 

In the dispersion, salt will screen the coulombic repulsion between the particles, and randomize the motion and the packing of the particles. Ionic surfactants can promote ordering at intomediate if they are tightly bound to the latex. Excess surfactant will promote disorder, and salt at very high concentration will suppress the double layer and can lead to flocculation. 

The physics of ILs at surfaces are important for a deeper understanding of the resulting properties and enables the design of appHcations. Each combination of cation and anion can lead to a different behavior on surfaces of sohds, because the molecular structure of each IL has a strong influence of the formation of layers at the interfaces. In aqueous electrolytes the Hehnholtz-model and its further developments are describing the physics in a sufficient way The Gouy-Chapman-model takes the diffusion into account, and the Stem-model combines the formation of a double layer with diffusion. Compared to aqueous solutions of salts, the situation in ILs is different The ions have no solvent environment Their next neighbors are also ions. As a consequence the physics at the interfaces between sohds and ILs cannot be described by the common models. 

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