Electrostatic potential electrolyte concentration, effect

When the concentration of the inert electrolyte is low, the electrostatic potential at the reaction site differs from that in the bulk and changes with the applied potential. This results in two effects [4] ... 

Schematic forms of the curves of interaction energies (electrostatic repulsion Vr, van der Waals attraction Va, and total (net) interaction Vj) as a function of the distance of surface separation. Summing up repulsive (conventionally considered positive) and attractive energies (considered negative) gives the total energy of interaction. Electrolyte concentration cs is smaller than cj. At very small distances a repulsion between the electronic clouds (Born repulsion) becomes effective. Thus, at the distance of closest approach, a deep potential energy minimum reflecting particle aggregation occurs. A shallow so-called secondary minimum may cause a kind of aggregation that is easily counteracted by stirring.

If the difference in energy level between a free ion and one bound to the surface of the metal is Y, and the difference in level between a free ion and a hydrated one is W, then the difference in energy level between the hydrated ion, and the ion at the surface of the metal is W—Y. The energy level of the ions in solution depends, however, on the concentration of these ions this produces the well-known effect of concentration on electromotive force. Gurney gives9 the strength of the double layer, i.e. the difference in electrostatic potential set up between metal a and electrolyte s, as... 

The details of the influence that electrostatic surface forces on the stability of foam films is discussed in Section 3.3. As already mentioned, the electrostatic disjoining pressure is determined (at constant electrolyte concentration) by the potential of the diffuse electric layer at the solution/air interface. This potential can be evaluated by the method of the equilibrium foam film (Section 3.3.2) which allows to study the nature of the charge, respectively, the potential. Most reliable results are derived from the dependence foam film thickness on pH of the surfactant solution at constant ionic strength. The effect of the solution pH is clearly pronounced the potential of the diffuse electric layer drops to zero at certain critical pH value. We have named it pH isoelectric (pH ). As already mentioned pH is an intrinsic parameter for each surfactant and is related to its electrochemical behaviour at the solution/air interface. Furthermore, it is possible to find conditions under which the electrostatic interactions in foam films could be eliminated when the ionic strength is not very high. 

The effect, on dispersion and de-agglomeration in water, of electrostatic repulsion force arising from the surface potential and the double layer 1/x around particles has been investigate. Several suspensions of polystyrene latex in an agglomerated state were prepared where j/ and 1 x were controlled by the pH and electrolyte concentration respectively. These were accelerated in a convergent nozzle to give an external force and the resulting dispersions were examined by optical microscopy. It was found that the dispersion was enhanced with an increase in y/and 1/x. 

It follows from eqs. (VII.23) and (VII.24) that values of n and A depend on electrolyte concentration. Concequently, the electrolyte concentration defines the height as well as position of potential barrier (see Fig. VII-10), which characterize film stability. The addition of electrolyte to colloidal system results in compression of electrical double layer, and, hence, in compression of the region of the effective action of electrostatic repulsion... 

The importance of electrostatic retardation increases with the surface potential, i.e. with the adsorption surfactant molecules. Especially in some practical systems of high background electrolyte, only at densely packed adsorption layers the electrostatic retardation will set in. This state of adsorption has not been taken into consideration so far. With increasing background electrolyte concentration counterions build the Stem layer. The charge of the adsorption layer is compensated partially by the diffuse layer and the Stem layer (Eq. 2.5) which decrease with the increased amount of counterions in the Stem layer. Simultaneously, the Stem potential is lowered and the electrostatic retardation becomes less effective. This aspect was discussed already by Kretzschmar et al. (1980). Consequently, the electrostatic retardation can exist in NaCl solution while it can disappear under certain conditions in CaCl2 solutions. 

Realisation of microflotation is ensured by opposite signs of the charges of the particle and the bubble and at low electrolyte concentration the effect of electrostatic attraction forces is extended over large distances and the depth of the potential well increases. Since particles and bubbles usually carry negative charges, it is expedient to use cation-active surfactants, which are predominantly adsorbed at bubbles in order to ensure contactless flotation. 

The bulk transport of ions in electrochemical systems without the contribution of advection is described by Poisson-Nernst-Planck (PNP) equations (Rubinstein, 1990).The well-known Nernst-Planck equation describes the processes of the process that drives the ions from regions of higher concentration to regions of lower concentration, and electromigration (also referred to as migration), the process that launches the ions in the direction of the electric field (Bard and Faulkner, 1980). Since the ions themselves contribute to the local electric potential, Poisson s equation that relates the electrostatic potential to local ion concentrations is solved simultaneously to describe this effect. The electroneutrality assumption simplifies the mathematical treatise of bulk transport in most electrochemical systems. Nevertheless, this no charge density accumulation assumption does not hold true at the interphase regions of the electric double layer between the solid and the Uquid, hence the cause of most electrokinetic phenomena in clay-electrolyte systems. 

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