we have aqueous suspensions of metal oxide nanoparticles showing variation in pH with particle size. What is the reason behind it?
pH play important role in the size of nanoparticles. At high pH reduction rate is high. At low pH (below 5) instead of reduction oxidation will occur. At very high pH (above 9, 10) reduction rate will be too much high resulting an aggregation of your nanoparticles. So that is why pH 8 is desirable in order to synthesized smaller size nanoparticles
Are your NPs naked or functionalized ? in case of functionalization, e.g. CuO@GSH or other bimetallic NPs they show slight variation with pH as change in size changes metal to ligand ratio, and solvent-ligand interaction, in case of bare metal oxides i think there should be no change in pH due to size if they are well purified.
Surface area is changing and due to its change double layer is changing, you can read more about the double layer effect here: https://en.wikipedia.org/wiki/Double_layer_(surface_science)
to identify more accurately you should check zeta potential
https://en.wikipedia.org/wiki/Double_layer_(surface_science)
Metal oxides dissolve in aqueous solution, forming hydroxides - basic (as for CaO) or acidic (as for WO3). So, the pH of sol depends on hydroxides' concentration or nanoparticles’ solubility. All substances partially soluble in water – even "insoluble" metal oxides such as CeO2. The solubility of nanoparticle is a function of its interfacial tension and, according to Ostwald–Freundlich equation, increases with decreasing particle size. Possible, it's an answer?
Dear Shcherbakov,
If NPs are being dissolved and reacting, eventually they must "die" ( so called death, i.e. they must not remain NPs and go to metal ions etc ).
Muhammad Abbas,
Not at all! It means just equilibrium between solid phase and dissolved ions in the solution. The dissolved parts of NPs are small in all cases, but some less in the sol of big NPs. However, the difference affects pH units.
Hope the attached article helps you.
https://www.sciencedirect.com/science/article/pii/S0921883117304260
pH play important role in the size of nanoparticles. At high pH reduction rate is high. At low pH (below 5) instead of reduction oxidation will occur. At very high pH (above 9, 10) reduction rate will be too much high resulting an aggregation of your nanoparticles. So that is why pH 8 is desirable in order to synthesized smaller size nanoparticles
Muhammad Ismail can u provide any link which u have mentioned in comment sir. As we are increasing the pH how particle size reduces?
Particle size may vary with pH as a result of a change in zeta potential (or effective net charge) of the particles in aqueous suspension under the specific solution conditions. Near the isoelectric point (or IEP), many nanoparticles may aggregate, and this IEP is dependent on the material, as well as ionic strength, additives, etc.
https://www.materials-talks.com/blog/2017/07/27/isoelectric-points-of-nanomaterials-qa/
Sir, Why the particle size changing ,is any fundamental behind that process after changing pH.
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