@ Amid, with the increase in the salt concentration of the solution , the electrical double layer is compressed and the zeta potential decreases. After a certain point, the electrical double layer will collapse and it becomes the same as the surrounding media, leaving the particles prone to agglomeration effects. In general, with the addition of only 0.1 M salt, the zeta potential decreased to values ranging from −10 to −30 mV. With further increase in salt concentration up to 1 M, only a slight decrease in zeta potential was observed, and the minimum zeta potential was reported to be about −10 mV.

@J. C. Tarafdar Sir appreciated your response.

But I have not used any salt for dispersing MgO in water.

It was only MgO in water.

Any insight about considering such situation.

Thanks

Amid Laxman Sadgar MgO reacts with water - slowly, but it reacts - forming Mg(OH)2. This is pH dependent with lower pH's favoring more dissolution (to Mg2+ ions) and higher pH's more precipitation. What was the particle size of the MgO? Are you observing settling of this sparingly soluble material? Zeta potential without an associated measured pH is virtually valueless. Higher concentrations of MgO will tend to be more alkaline. I assume you measured the pH for all your suspensions. What was the trend of pH with increasing 'MgO' concentration? BTW, DI water is about the worst diluent for nanomaterials as any stabilizers and surfactants may be diluted out causing aggregation and agglomeration (and then, probably, settling).

@Alan F Rawle Sir. Thank you for your detailed reply and thought on it.

The particles size which was determined by TEM is ~100 nm, and with DLS hydrodynamic diameter is ~300-350 nm.

The particles were not settle while measurements but found settling after keeping as such for overnight.

I haven't measured the pH of solutions. But I think it may be around 8-9.(Not sure).

My further experiments were in water only so I decided to go with water as a diluent.

Sir, any suggestions based on the above information.

Amid Laxman Sadgar Remember that you do not measure zeta potential. It's inferred from the measurement of (electrophoretic) mobility. At higher concentrations then you may have hindered mobility via particle-particle interactions. This hindered or lowered mobility may be interpreted as decreased zeta potential. You need to measure the pH's at the same time you change your MgO concentration.

Amid Laxman Sadgar I also expect the pH to increase somewhat with increasing MgO concentration.

@Alan F Rawle Sir.

Thank you for the explanation.

In addition to the comments above, the Zetasizer actually records the conductivity (in mS/cm) for each measurement, and you could use that value to observe if there is a change in ionic strength between your different preparations.

https://www.materials-talks.com/debye-screening-how-it-affects-zeta/

https://www.materials-talks.com/zeta-potential-in-salt-solution-or-any-other-ions/