Particle surface potential determining ions

The possible usefulness of this relationship — which is known as the Hiickel equation — should not be overlooked. Throughout Chapter 11 we were concerned with the potential surrounding a charged particle. Equation (11.1) provides a way of evaluating the potential at the surface, i/ o, in terms of the concentration of potential-determining ions. Owing to ion adsorption in the Stern layer, this may not be the appropriate value to use for the potential at the inner limit of the diffuse double layer. Although f is not necessarily identical to i/ o, it is nevertheless a quantity of considerable interest. 

For a reversible interface, such as Agl/aqueous solution, the electrostatic potential in the solution just outside the surface referred to zero at regions of solution infinitely remote from colloidal particles, the Volta potential, is calculated from the Nernst equation, the concentration of potential determining ions, and the zero-point-of-charge which is not usually the stoichiometric equivalence point. 

Silver iodide particles in aqueous suspension are in equilibrium with a saturated solution of which the solubility product, aAg+ai, is about 10 16 at room temperature. With excess 1 ions, the silver iodide particles are negatively charged and with sufficient excess Ag+ ions, they are positively charged. The zero point of charge is not at pAg 8 but is displaced to pAg 5.5 (pi 10.5), because the smaller and more mobile Ag+ ions are held less strongly than-the 1 ions in the silver iodide crystal lattice. The silver and iodide ions are referred to as potential-determining ions, since their concentrations determine the electric potential at the particle surface. Silver iodide sols have been used extensively for testing electric double layer and colloid stability theories. 

In many colloidal systems, the double layer is created by the adsorption of potential-determining ions for example, the potential 0o the surface of a /Silver iodide particle depends on the concentration of silver (and iodide) ions in solution. Addition of inert electrolyte increases k and results in a corresponding increase of surface charge density caused by the adsorption of sufficient potential-determining silver (or iodide) ions to keep 0O approximately constant. In contrast, however, the charge density at an ionogenic surface remains constant on addition of inert electrolyte (provided that the extent of ionisation is unaffected) and 0O decreases. 

Example. The surfaces of dispersed Agl particles can be considered similarly to an Ag-Agl-aqueous solution reversible electrode (i.e., each phase contains a common ion that can cross the interface). Here both Ag+ and I- will be potential determining ions because either may adsorb at the interface and change the surface potential. In this case, NaN03 is an example of an indifferent electrolyte as far as the electrode potential goes. 

Potential-determining ions are those whose equilibrium between two phases, frequently between an aqueous solution and an interface, determines the difference in electrical potential between the phases. Consider a Agl dispersion in water. There will exist some concentrations of Ag+ and I" such that the surface charge of the Agl particles is zero. This is called the point of zero charge (pzc). It is usually determined by a titration method (called a colloid titration). 

The potential in the diffuse layer decreases exponentially with the distance to zero (from the Stem plane). The potential changes are affected by the characteristics of the diffuse layer and particularly by the type and number of ions in the bulk solution. In many systems, the electrical double layer originates from the adsorption of potential-determining ions such as surface-active ions. The addition of an inert electrolyte decreases the thickness of the electrical double layer (i.e., compressing the double layer) and thus the potential decays to zero in a short distance. As the surface potential remains constant upon addition of an inert electrolyte, the zeta potential decreases. When two similarly charged particles approach each other, the two particles are repelled due to their electrostatic interactions. The increase in the electrolyte concentration in a bulk solution helps to lower this repulsive interaction. This principle is widely used to destabilize many colloidal systems. 

In the second case, the surface potentials of the interacting particles remain constant during interaction. Consider two interacting particles 1 and 2 whose surface charges are due to adsorption of Nt ions (potential-determining ions) of valence Z adsorb onto the surface of particle i (/= 1, 2). If the configurational entropy Sc of the adsorbed ions does not depend on Ni, then the surface potential i/ oi of particle i is given by Eq. (5.10), namely. 

If the dissociation of the ionizable groups on the particle surface is not complete, or the configurational entropy Sc of adsorbed potential-determining ions depends on N, then neither of ij/o nor of cr remain constant during interaction. This type of double--layer interaction is called charge regulation model. In this model, we should use Eqs. (8.35) and (5.44) for the double-layer free energy [ 11-13]. 

The measurement of this surface potential (T g or Pq) is impossible due to the hydrodynamic behavior of the system that generates a thin layer of attached liquid around the particles. However, there is a plane where the shear starts (shear plane), and at this plane the surface potential can be measured and the value is known as the zeta potential ( P ). Besides the indifferent counter- and co-ions in solution, there are also so-called potential determining ions (chemists caU them adsorbing ions). For most systems these are and OH ions that can adsorb directly on the particle surface and alter the -potential. There is a pH value for which the potential becomes zero and is called the isoelectric point (lEP), as shown in Figure 11.6. 

Aringhieri, R., and G. Pardini. 1989. Kinetics of the adsorption of potential-determining ions by positively charged soil particle surfaces. Soil Sci. 147 85-90. 

More detailed information on ion exchange involved in protein adsorption can be derived from titration experiments in systems where the charge of the protein and the sorbent can be varied independently. Currently, we are studying such systems, using bovine plasma albumin (BPA) and cytochrome c as the proteins and silver iodide (Agl) particles as the sorbent. In these systems there are two potential determining ion couples, the H" /oh" couple for the protein and the Ag" "/ " couple for the sorbent. They enable independent control of protein and surface charge. Below, we will briefly discuss some results obtained with the BPA - Agl system. 

Sihca particles in aqueous solution possess a surface charge due to preferential dissolution of surface species and interfacial ion-exchange. The magnitude and sign of the surface charge is dependent on the concentration of the potential determining ions and is a function of pH and ionic strength of the solution. 

In most colloid systems and suspensions the electro-chemical double layer has its origin in a distribution equilibrium of potential determining ions between the particle surface and the sol medium. This has been proved extensively by the work done on the Agl soU, and it has been made plausible for inany other systems (e.g. for suspensions of extremely insoluble oxides in water and other media). ... 

The experimental fact that several colloids lose their stability on extremely prolonged dialysis should evidently not be ascribed to the extreme expansion of the double layer by loss of electrolyte, but to a decrease in the surface potential as a result of the removal of potential determining ions. The lack of stability of suspensions in weakly dissociating liquids may also be explained by a lack of sufficient potential determining ions to give the particles the necessary value of... 

Adsorption-desorption of lattice ions. Silver iodide particles in Ag" " or solutions are the typical example the crystal lattice ions can easily find their way into crystal sites and become part of the surface. They are called potential-determining ions (p.d.i.). 

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