Zeta potential concentration

One can write acid-base equilibrium constants for the species in the inner compact layer and ion pair association constants for the outer compact layer. In these constants, the concentration or activity of an ion is related to that in the bulk by a term e p(-erp/kT), where yp is the potential appropriate to the layer [25]. The charge density in both layers is given by the algebraic sum of the ions present per unit area, which is related to the number of ions removed from solution by, for example, a pH titration. If the capacity of the layers can be estimated, one has a relationship between the charge density and potential and thence to the experimentally measurable zeta potential [26]. 

Response to Electric and Acoustic Fields. If the stabilization of a suspension is primarily due to electrostatic repulsion, measurement of the zeta potential, can detect whether there is adequate electrostatic repulsion to overcome polarizabiUty attraction. A common guideline is that the dispersion should be stable if > 30 mV. In electrophoresis the appHed electric field is held constant and particle velocity is monitored using a microscope and video camera. In the electrosonic ampHtude technique the electric field is pulsed, and the sudden motion of the charged particles relative to their counterion atmospheres generates an acoustic pulse which can be related to the charge on the particles and the concentration of ions in solution (18). 

Overbeek and Booth [284] have extended the Henry model to include the effects of double-layer distortion by the relaxation effect. Since the double-layer charge is opposite to the particle charge, the fluid in the layer tends to move in the direction opposite to the particle. This distorts the symmetry of the flow and concentration profiles around the particle. Diffusion and electrical conductance tend to restore this symmetry however, it takes time for this to occur. This is known as the relaxation effect. The relaxation effect is not significant for zeta-potentials of less than 25 mV i.e., the Overbeek and Booth equations reduce to the Henry equation for zeta-potentials less than 25 mV [284]. For an electrophoretic mobility of approximately 10 X 10 " cm A -sec, the corresponding zeta potential is 20 mV at 25°C. Mobilities of up to 20 X 10 " cmW-s, i.e., zeta-potentials of 40 mV, are not uncommon for proteins at temperatures of 20-30°C, and thus relaxation may be important for some proteins. 

It also follows from what was said that a zeta potential will be displayed only in dilute electrolyte solutions. This potential is very small in concentrated solutions where the diffuse edl part has collapsed against the metal surface. This is the explanation why electrokinetic processes develop only in dilute electrolyte solutions. 

FIGURE 31.3 Zeta potentials of a glass surface as functions of the concentrations of salt solutions with the cations (from bottom to top) K, Ca +, Al, Th ". ... 

As at other interfaces, the effective snrface charge of colloidal particles depends on the total concentration and composition of the solution, particnlarly on polyvalent or snrface-active ions that may be present. When the zeta potential is reduced below a certain critical (absolute) value, which is approximately 25 to 30 mV, the colloidal solution becomes nnstable. 

Flocculating agents can be simple electrolytes that are capable of reducing the zeta potential of suspended charged particles. Examples include small concentrations (0.01-1%) of monovalent ions (e.g., sodium chloride, potassium chloride) and di- or trivalent ions (e.g., calcium salts, alums, sulfates, citrates or phosphates) [80-83], These salts are often used jointly in the formulations as pH buffers and flocculating agents. Controlled flocculation of suspensions can also be achieved by the addition of polymeric colloids or alteration of the pH of the preparation. 

The zeta-potentials of Table III are greater than those in Figure 9 because the concentration of OLOA-1200 in solution was appreciably higher. The results are quite consistent at the same concentration in solution. These findings show that the zeta-potential in organic media is not a function of how much dispersant is adsorbed, but how much dispersant is left in solution. This is in contrast to the steric barriers, which depend on how much is adsorbed and not on how much is left in solution. 

However in Table IV we see no increase in W at 1%, and only a small increase at 2% of dispersant. The value of W increases rapidly at about the same concentration that the conductivity increases, the counterion concentration increases and the zeta-potential increases. At OLOA-1200 levels of 3.5% and higher the stability ratio exceeds 5x10, with half-times in excess of seven months these stability ratios developed when zeta-potentials were -120 mV or more. 

The electrostatic barrier developed only after enough dispersant adsorbed that a concentration of dissolved dispersant of about 0.1% or more remained in the oil phase, where counterions developed as evidenced by increased conductivity, the development of large negative zeta potentials, steeply rising stability ratios, and complete deflocculation. 

Further, since both the zeta potential and the electric field depend on the ionic concentration and pH of the fluid (the tissue... 

Zeta potential of Al203 (corundum) in solutions of various electrolytes. The concentration unit is equivalents per liter. 

In addition to its pH, the concentration of a buffer also affects the mobility of proteins. At high concentrations the zeta potential of the protein is reduced resulting in a shorter distance of migration. However, because higher concentrations of buffer give improved resolution, a compromise concentration has to be found and buffers with ionic strength (pi) varying from 0.025 to 0.075 are frequently used. 

Liquid-solid contact Liquid-liquid contact Statistical distribution due to ion concentration fluctuations Double layer (zeta potential) disruption Volta potential (for electron conducting materials) Electrolytic (galvanic) potential (for ionic systems)... 

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