Double layer zeta potential

ScheUman, JA Stigter, D, Electrical Double Layer, Zeta Potential, and Electrophoretic Charge of Double-Stranded DNA, Biopolymers 16, 1415, 1977. 

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)... 

Inner Potential (Stern) In the diffuse electric double layer extending outward from a charged interface, the electrical potential at the boundary between the Stern and the diffuse layer is termed the inner electrical potential. Synonyms include the Stern layer potential or Stem potential. See also Electric Double Layer, Zeta Potential. 

Photon correlation spectroscopy can also be used to measure electrophoretic mobility and zeta potentials of suspended particles. The sample is subjected to an electric field that causes charged particles to migrate to one of the electrodes. Two coherent laser beams intersect within the sample, giving rise to a series of interference fringes. The fluctuations in the signal intensity are analyzed in the manner described above to calculate the mobility distribution of the particles and then the potential difference across the diffuse part of the double layer (zeta potential). 

Zeta potentials of the oxides and virus are presented in Table IV. Using these values, double-layer interaction potentials were evaluated and are presented in Table V. They are repulsive in all cases. 

For example, dissolved carbonate species present in our 0.02 I buflFer system, predominantly CO2, H2CO3 , HCOa, COa, and various car-bonato complexes, had a marked elfect concerning adsorption to transition metal oxides. The zeta potential of CuO in 0.02 I buffer was —17.6 6.1 mV, while in 0.02M NaCl that contained only traces of total dissolved carbonate (approximately lO M), it was -f32.0 it 5.8 mV. This shows marked alteration of the electrical structure of double layers by some carbonate species. The same effects were seen to lesser extents on Fc203 and Mn02 (8). Double-layer interaction potentials calculated with zeta potentials measured in 0.02 I reaction buffers matched adsorption free energy differences well, and these potentials included the effects of carbonate species. Where effects of dissolved constituents have not been accounted for intrinsically, predictions made on the basis of the DLVO-Lifshitz considerations alone must be made with care. 

The nature of the potential depends upon the pressure applied as well as nature of the liquid. It has been suggested that the streamffig potential results due to forcible flow of liquid wWch tends to separate the oppositely charged layers of the electric double layer. Streaming potential is related to zeta potential by the equation. 

Zeta potential is defined as the electric double layer (EDL) potential located at the shear plane between the Stem layer and the diffuse layer of the EDL that is formed in the neighborhood of a charged solid-liquid interface. Zeta potential is an experimentally measurable electrical potential that characterizes the strength and polarity of the EDL of the charged solid-liquid interface. Depending on the solid surface and the solution, zeta potentials values are within a range of —100 mV to - -100 mV for most solid-liquid interfaces in aqueous solutions. 

Several effects, due to the existence of the double layer on the surface of most particles suspended in Hquids, can be used to measure the so-called zeta potential. Table 1 gives a simplified summary of the effects. 

Fig. 8. Electrical double layer of a sohd particle and placement of the plane of shear and 2eta potential. = Wall potential, = Stern potential (potential at the plane formed by joining the centers of ions of closest approach to the sohd wall), ] = zeta potential (potential at the shearing surface or plane when the particle and surrounding Hquid move against one another). The particle and surrounding ionic medium satisfy the principle of electroneutrafity.

The well-known DLVO theory of coUoid stabiUty (10) attributes the state of flocculation to the balance between the van der Waals attractive forces and the repulsive electric double-layer forces at the Hquid—soHd interface. The potential at the double layer, called the zeta potential, is measured indirectly by electrophoretic mobiUty or streaming potential. The bridging flocculation by which polymer molecules are adsorbed on more than one particle results from charge effects, van der Waals forces, or hydrogen bonding (see Colloids). 

This equation is a reasonable model of electrokinetic behavior, although for theoretical studies many possible corrections must be considered. Correction must always be made for electrokinetic effects at the wall of the cell, since this wall also carries a double layer. There are corrections for the motion of solvated ions through the medium, surface and bulk conductivity of the particles, nonspherical shape of the particles, etc. The parameter zeta, determined by measuring the particle velocity and substituting in the above equation, is a measure of the potential at the so-called surface of shear, ie, the surface dividing the moving particle and its adherent layer of solution from the stationary bulk of the solution. This surface of shear ties at an indeterrninate distance from the tme particle surface. Thus, the measured zeta potential can be related only semiquantitatively to the curves of Figure 3. 

Eleetrostatie eharaeterization of partieles is eommonly determined via their eleetrokinetie or zeta potential i.e. the potential of a slipping plane, notionally loeated slightly away from the partiele surfaee approximately at the beginning of the diffuse part of the double layer using, for example, eleetrophoresis. In some eases, zeta potential ean be used as a eriterion for aggregation. 

Electroviscous effect occurs when a small addition of electrolyte a colloid produces a notable decrease in viscosity. Experiments with different salts have shown that the effective ion is opposite to that of the colloid particles and the influence is much greater with increasing oxidation state of the ion. That is, the decrease in viscosity is associated with decreased potential electrokinetic double layer. The small amoimt of added electrolyte can not appreciably affect on the solvation of the particles, and thus it is possible that one of the determinants of viscosity than the actual volume of the dispersed phase is the zeta potential. 

In order to describe the effects of the double layer on the particle motion, the Poisson equation is used. The Poisson equation relates the electrostatic potential field to the charge density in the double layer, and this gives rise to the concepts of zeta-potential and surface of shear. Using extensions of the double-layer theory, Debye and Huckel, Smoluchowski,... 

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. 

The charges present on the insulator surface in contact with the solution give rise to an accumulation of ions of opposite sign in the solution layer next to the surface, and thus formation of an electric double layer. Since straightforward electrochemical measurements are not possible at insulator surfaces, the only way in which this EDL can be characterized quantitatively is by measuring the values of the zeta potential in electrokinetic experiments (see Section 31.2). 

The potential governing these electrokinetic effects is clearly at the boundary (the face of shear) between the stationary phase (the fixed double layer) and the moving phase (the solution). This potential is called the electrokinetic potential or the zeta potential. An electrokinetic phenomenon in soil involves coupling between electrical, chemical, and hydraulic gradients. 

The electroosmotic pumping is executed when an electric field is applied across the channel. The moving force comes from the ion moves in the double layer at the wall towards the electrode of opposite polarity, which creates motion of the fluid near the walls and transfer of the bulk fluid in convection motion via viscous forces. The potential at the shear plane between the fixed Stem layer and Gouy-Champmon layer is called zeta potential, which is strongly dependent on the chemistry of the two phase system, i.e. the chemical composition of both solution and wall surface. The electroosmotic mobility, xeo, can be defined as follow,... 

The charge or zeta ( ) potential of the filler particle (i.e. the charge at the plane of shear between the particle s diffuse double layer and the bulk liquid phase) can be obtained by measuring its mobility in an applied electric field of known magnitude. The mobility is a function of the field gradient and is therefore expressed as a speed per unit potential gradient (/im/s/V/cm). Mobility and therefore zeta potential are both a function of pH (Figure 6.4). 

The microelectrophoretic mobility (jUe) is related to zeta potential ( ) via one of two equations. When the diameter of the particle is small relative to the thickness of the electrical double layer, the Huckel equation applies ... 

The electrokinetic potential (zeta potential, Q is the potential drop across the mobile part of the double layer (Fig. 3.2c) that is responsible for electrokinetic phenomena, for example, elecrophoresis (= motion of colloidal particles in an electric field). It is assumed that the liquid adhering to the solid (particle) surface and the mobile liquid are separated by a shear plane (slipping plane). The electrokinetic charge is the charge on the shear plane. 

The surface potential is not accessible by direct experimental measurement it can be calculated from the experimentally determined surface charge (Eqs. 3.1 - 3.3) by Eqs. (3.3a) and (3.3b). The zeta potential, calculated from electrophoretic measurements is typically lower than the surface potential, y, calculated from diffuse double layer theory. The zeta potential reflects the potential difference between the plane of shear and the bulk phase. The distance between the surface and the shear plane cannot be defined rigorously. 

There is no experimental way to measure y. (As we mentioned before, the zeta potential - as obtained, for example, from electrophoretic measurments - is in a not readily definable way - smaller than y.) But as discussed in section 3.3 we can obtain the surface charge (Eq. 3.2) and then compute the surface potential y on the basis of the diffuse double layer model with Eq. (3.8a) Eq. (3.8a) in simplified form for 25° C is... 

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