Colloidal particles generally carry a negative charge associated with their nature. When these are dispersed in any medium, they attract positively charged particles, which end up forming a layer the thickness of ions: the Stern layer (lower image).
On this fixed layer ( Stern Layer ), depending on the dimensions of the colloidal particle, be it solid, liquid or gaseous, other neighboring ions will be added. Most of them will remain positive, because they experience the great negative charge of the colloidal particle; however, negative particles will also appear, giving rise to an electrical double layer.
The potential z can acquire certain values, positive or negative, according to the sign of the charge carried by the colloidal particles. Many of them show ζ values in a range of -60 mV to 60 mV.
If ζ is below or above this range of values, the stability of the colloidal particles will be excellent, which means that they will remain dispersed without aggregating.
Meanwhile, those particles that have a value of ζ between -10 mV and 10 mV, will be prone to agglomerate; as long as they are not covered by films of molecules covalently anchored to their surfaces. It is then said that the colloidal state is “broken”.
These values must be reported with the pH indications and the solvent in which they were determined. For example, ζ will vary enormously with the addition of an acid, since it contributes H + ions that sneak between the double layer that surrounds the particles. This results in a positive increase in the values of ζ.
When ζ has a value of 0, we are talking about the isoelectric point of the solution. Therefore, it is the region where the particles will tend to agglomerate much more. The addition of salts shows the same effect: the added ions will decrease or compress the double layer, causing agglomeration to occur.
The potential z is responsible for the colloidal particles of equal charges repel each other. It is then said that the degree of coagulation is zero, since there is no opportunity for them to interact.
As such charges are neutralized, the particles will begin to interact by Van der Walls forces, until coagulation takes place.
Electrophoresis is a method used to estimate the value of the Z potential of a particle in suspension.
When an electric field is applied, the electrically charged particles will move towards the electrode that has a charge opposite to their own. Electrophoretic mobility is directly related to the velocity of the particle in electrophoresis, and inversely to the voltage gradient.
Thus, the zeta potential is subject to electrophoretic mobility, which in turn depends on the viscosity of the solution, the difference in voltages applied between the electrodes, and the dielectric constant of the solution.
This displacement is analyzed by the incidence of a laser beam, whose radiation is scattered by the moving particles and causes variations in their frequency. These changes in laser frequency are related to electrophoretic mobility, and finally, to the z potential.
The greater the electrophoretic mobility, the greater the z potential of the particles in question.
Determination of surface changes
Measurements of ζ make it possible to establish whether there have been surface changes in the colloidal particles. It is understood by these changes to the interactions between two or more aggregates.
For example, if particles A and B mix, and ζ changes for both, it means that they are interacting; and therefore, that their surfaces undergo changes with respect to their loads.
Most colloids, particles, bacteria and pyrogens are negatively charged. A filter medium can be modified to give a positive z potential.
Filter elements with positive z potential have the advantage of removing small, negatively charged organisms with radii less than microns.
Demineralized water has a pH scale between 5 and 8. Therefore, most of the dissolved particles in them acquire a negative charge. This allows it to be removed from the water by interacting with the positively charged filter medium.
Rivers show fluctuations in a short period of time in the quality of the water they transport. This determines that it is necessary to determine the optimal dose of coagulant necessary for water purification, the determination of the zeta potential being useful in this regard.
It was determined that with a zeta potential value between -2.28 and + 1.2 mV in the coagulated water, low values of turbidity and color are obtained in it.
Then, it is possible to achieve an optimal behavior of the water coagulation and / or flocculation processes, by making determinations in the coagulated water of the zeta potential as an indicator of the destabilization of colloids and other particles.
The value of the zeta potential has a positive correlation with the dose of coagulant applied in the purification of the water.
Elaboration of paintings
The dispersion of paint pigments is a necessary requirement to obtain a good quality product. The agglomeration of the pigments produces the formation of grains, which reduce the quality of the paint, since they make it difficult to apply.
In addition, the gloss and texture of the paint depend on the way in which the particles that make it up are dispersed. The measurement of the zeta potential serves to control the composition of the paint, allowing the optimal addition of additives necessary for a correct dispersion of the pigments.
Fluidization of a suspension
Carrageenan is a negatively charged polyelectrolyte used as a fluidizing agent. The polyelectrolyte adsorbs to the surface of the insoluble particles and reverses the flocculation, as soon as the potential value z reaches a critical value.
This system has been used in the suspension of aluminum hydroxide antacid. The decrease in z potential correlates with the viscosity of the suspension.