Dear Rozita Foulady,

You may want to review the following info:

Zeta potential is the electrical potential at the slipping plane. This plane is the interface which separates mobile fluid from fluid that remains attached to the surface. Zeta potential is a scientific term for electrokinetic potential in colloidal dispersions. In the colloidal chemistry literature, it is usually denoted using the Greek letter zeta (ζ), hence ζ-potential. The usual units are volts (V) or, more commonly, millivolts (mV).

https://en.wikipedia.org: Zeta potential - Wikipedia

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The paper presents the influence of fucoidan (FD) on stability of alumina suspensions in the presence of cationic surfactant hexadecyltrimethylammonium bromide (CTAB). The research results show that fucoidan adsorbs on the alumina surface and that the adsorption decreases in the CTAB presence. This is due to formation of the polymer-surfactant complexes characterized by lower affinity for the alumina surface than pure fucoidan. The complex formation was confirmed by the densiometric studies where the increase of the CTAB/FD surface tension in comparison to pure CTAB was observed. It was established that fucoidan possesses great stabilizing efficiency regardless of pH. Furthermore, stability of the fucoidan/alumina system increased after CTAB addition due to the presence of non-adsorbed complexes between the alumina particles. The results indicate that fucoidan could be successfully used as a stabilizer of colloidal suspensions where the presence of surfactant is required, that is in cosmetic and pharmaceutical industries.

https://www.researchgate.net/publication/341887215_Stabilizing_properties_of_fucoidan_for_the_alumina_suspension_containing_the_cationic_surfactant

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Alteration of Zeta potential and membrane permeability in bacteria: a study with cationic agents

In the present study, we have tried to establish the correlation between changes in Zeta potential with that of cell surface permeability using bacteria (Escherichia coli and Staphylococcus aureus). An effort has been made to establish Zeta potential as a possible marker for the assessment of membrane damage, with a scope for predicting alteration of cell viability. Cationic agents like, cetyl trimethyl ammonium bromide and polymyxin B were used for inducing alteration of Zeta potential, and the changes occurring in the membrane permeability were studied. In addition, assessment of poly-dispersity index (PDI), cell viability along with confocal microscopic analysis were performed. Based on our results, it can be suggested that alteration of Zeta potential may be correlated to the enhancement of membrane permeability and PDI, and it was observed that beyond a critical point, it leads to cell death (both Gram-positive and Gram-negative bacteria). The present findings can not only be used for studying membrane active molecules but also for understanding the surface potential versus permeability relationship.

https://springerplus.springeropen.com/articles/10.1186/s40064-015-1476-7

I will talk about the PZC, the reasoning is the same as the zeta potential.

Firstly : The uncalcined sample which means the presence of CTAB, is more basic and less acidic than the calcined.

So, the PZC will be higher than the value of pH=7.

Secondly : The presence of aluminum makes the sample more acidic.

So, more there is aluminum, more the sample is acidic,and by consequence : low is the PZC (much less than pH=7 for the calcined sample ).

Mohammed Nadir Miloud It has been known for many years that alumina surfaces have a higher PZC in aqueous media compared to silica surfaces.

e.g., Article The Zero Point of Charge of Silica-Alumina Oxide Suspensions