The pHpzc (point of zero charge) is the pH at which the surface of a material has no net electrical charge. The behavior of pHpzc in cationic and anionic dyes can be different depending on the specific dye and the pH of the system.

In general, cationic dyes have a positive charge and are attracted to negatively charged surfaces. Therefore, in a system with a negatively charged surface, the pHpzc of the surface will be lower than the pH of the dye. At pH values below the pHpzc, the surface will be positively charged and attract the negatively charged cationic dye. Conversely, at pH values above the pHpzc, the surface will be negatively charged and repel the positively charged dye.

Anionic dyes, on the other hand, have a negative charge and are attracted to positively charged surfaces. Therefore, in a system with a positively charged surface, the pHpzc of the surface will be higher than the pH of the dye. At pH values below the pHpzc, the surface will be negatively charged and repel the negatively charged anionic dye. Conversely, at pH values above the pHpzc, the surface will be positively charged and attract the negatively charged dye.

It is important to note that the behavior of pHpzc can also depend on other factors such as the nature and concentration of electrolytes in the system, the specific chemical composition of the dye and the surface, and the presence of other chemical species that may interact with the dye or the surface. Therefore, the behavior of pHpzc in cationic and anionic dyes at lower and higher pH values than pHpzc can be complex and may require experimental investigation or modeling to fully understand.

Dr. Salim Jassam in your explanation of cationic dye and charged surfaces , do you mean :Conversely, at pH values above the pHpzc, the surface will be negatively charged and repel the positively charged dye?

When pH of the solution is lower than pHpzc of the adsorbent, the surface charge of the adsorbent becomes positive charge and attract the anionic dyes molecules. However, when the pH of the solution is higher than pHpzc of the adsorbent, the surface charge of the adsorbent becomes negative and attract the cationic dyes and vice versa.

@Salis A Musa:

The pH zero point charge (pH ZPC) is the pH value at which the surface of a charged solid particle or colloid has zero net charge. It is a critical parameter in many fields, including chemistry, materials science, and environmental science.

When a solid surface is immersed in water, it may become charged due to the dissociation of surface groups, such as hydroxyl (OH-) or carboxylic (COOH) groups. The surface charge can be either positive or negative, depending on the pH of the solution. At the pH ZPC, the number of positive and negative charges on the surface is equal, resulting in a net charge of zero.

The pH ZPC can be determined experimentally by measuring the surface charge of a solid particle or colloid as a function of pH. Typically, this is done using techniques such as electrophoretic mobility, zeta potential, or surface titration.

The pH ZPC is an important parameter in many applications, such as mineral processing, wastewater treatment, and soil science, as it can affect the adsorption, aggregation, and transport of particles in aqueous systems.

The pHpzc can be used to understand and manipulate the net surface charge of a material to be used as an adsorbent in solution, when the pH of the solution containing the adsorbent is less than the pHpzc, the net surface charge of the adsorbent becomes positive because of the adsorption of excess H+ and the net surface charge becomes negative when the pH of solution is higher than the pHpzc, because of the desorption of H+. The situation where the pH is lower than the pHpzc favors the adsorption of anions on the surface of the adsorbent which is predominantly positively charged due to coulombic attraction