@ Nermin Zakhas asked me a question via PM, and as this forum is not a consultancy service, I will post so all experts can contribute.
Dear Dr. Alan,
I am trying to prepare magnetite nanoparticles using coprecipitation method and NaoH. Malvern analyzer showed good size. The zetapotential showed positive 17 mv. Can you please help me with might went wrong ? Is not magnetite nanoparticles prepared with alkaline medium should be negatively charged ? How can i overcome this ?
Thanks a lot for your time and help.
What is your reason for thinking you have 'gone wrong'? Many metal oxides will be positively charged at pH's around neutral or even alkaline (you don't tell me the actual pH). At acidic pH's (e.g. < 2) then magnetite will dissolve. At higher pH's, Fe(OH)3 will precipitate. There is a limited pH range over which magnetite will not chemically interact with its surroundings. The major factor affecting zeta potential is pH.
If you have genuine nanoparticles (< 100 nm) then your colloidal suspension should be transparent (probably colored though) with no precipitation.
I have the same view concerning the transparency (of the diluted dispersion) and of the pH dependency of the zeta potential. The concentrated dispersion will be black.
Unfortunately Nermin miss to mention some information (preparation, Reimers und Khalafalla (1972) with surfactant(?),stoichiometry (excess of NaOH(?), stabilisation,....). The question indicates charge stabilization (without surfactant), e.g. by "digesting" of the freshly prepared particles with HNO3 which result in the positive zeta potential separation and cleaning similar to the procedure presented in the articles.
Article Tuning the relaxation behaviour by changing the content of c...
Article Dependence of Brownian Relaxation on the Volume Fraction and...
Zeta potential is a property of the complete system, so any specific counter ions, salts, surfactants, pH all influence the measured zeta potential. The original poster might also find http://www.materials-talks.com/blog/2017/07/27/isoelectric-points-of-nanomaterials-qa/ of interest.
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