You state 'There is no additive in the dispersion' and this is the root cause of your problem...

See (registration required):

Dispersion and nanotechnology

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W190528DispNanoMat

Priyabrata Sahoo Your question headline mentions zeta potential but your question itself doesn't. How do you know the zeta potential is near zero?

You asked "Is it because of the less mass of the nanosheet compared to bulk?" - please explain. Certainly, the aggregation kinetics will be slower if you have less material. i.e., if you prepare dispersions of unstabilized particles at different concentrations, the more concentrated ones will aggregate faster.

Please provide more information about your dispersions. Also, please define "stable" and "long duration of time". There's no such thing as a truly stable dispersion of hydrophobic particles that relies only on electrostatics.

John Francis Miller I measured the zeta potential and it was around -2mV.

Size of the Nano sheet is less compared to bulk material. Means, bulk material is heavy and has higher chance of aggregation. That why I asked, I am not sure about this.

Dispersions are stable more than a month.

Alan F Rawle Thank you for suggesting the webinar. But unfortunately, I didn't get my answer.

"This is the root cause of your problem"- I didn't get. Why this is a problem?

If you don't mind, can you please explain it in detail?

Priyabrata Sahoo

I do not believe you measured zeta potential (ZP) but some other property (e.g. electrophoretic or acoustic mobility) and you are using some/many assumptions in a conversion to ZP. This distinction is important because at higher concentrations, you will have particle-particle interactions and hence reduced mobility (which may be 'transformed' to a 'ZP' closer to 0 mV). The mobility of sheets is totally different to that of spheres or more compact objects.

Why do you believe that ZP is a measure of 'stability'? [Whatever you mean by that word 'stability'] -echoing the comments of John Francis Miller .

Stability is conferred through charge (erroneously called electrostatic) or steric means. ZP is irrelevant to steric stability as the slipping plane is in the protective polymer 'coat'. Both stabilization mechanisms involve additives or change in pH. Hence DI water is about the worst material to dilute any system as it washes out/dilutes any stabilizing agents. The 'mother liquor' is the correct dilution medium.

Your comment 'bulk material is heavy and has higher chance of aggregation' is completely illogical. A density difference corresponds to settling propensity. Aggregation involves van der Waals forces and diffusion (in a powder, this is solid-solid diffusion).

Pedantically I also cannot understand 'Dispersions of the bulk materials are not stable though. There are no other additives/surfactants in the dispersion. Measured Zeta potential value is -2mV'. You talk of 'dispersions' and 'bulk materials' (both plural) yet only have one single 'zeta potential' measurement (- 2 mV) - without indicating pH or other important parameters such as concentration. I would expect many 'ZP' measurements.

Have you verified the instrument performance with the appropriate transfer standard?

Priyabrata Sahoo

The surface property of WO3 and Ni(OH)2 can change in the aqueous environment considering the hydroxyl group it possesses. It is obvious that your nanosheets were not stable (showed near zero value) in Zeta potential analysis.

Abdul Azeez Nazeer

Why do you say 'It is obvious that your nanosheets were not stable (showed near zero value) in Zeta potential analysis'. (And why an upper-case 'Z'?). Systems can be perfectly stable at an inferred ZP of 0 mV. And, anyway, what is 'Zeta potential analysis'?

Alan F Rawle Thank you for your time and valuable suggestion.

First of all, let me clarify all the things once again. I have the WO3 and Ni(OH)2 dispersions similar to graphene dispersion. after that I measured zeta potential using “Malvern Zetasizer nano“ obviously With “well diluted dispersion”. It was showing -2mV Zeta potential( i dont know whether it is electrophoretic or acoustic mobility).

And the dispersion is stable for more than a month. What I mean by ”stabilty” is that materials and solvents are not phase seperated from each other. In my case, the two phases are completely seperated after one month where top portion Of the dispersion is only the solvent And the bottom Portion contains materials.

secondly, there is no other polymers/surfactants/ stabilisers in the dispersion. so, how steric stabilization is happening here, can you please explain. I am new to the field.

Dispersion of bulk materials means, mixing the materials with solvents and sonication for 5mins, Like the way we prepare the dispersion for afm/tem sample. This dispersion rather I should say mixture is stable for 5-10 mins. I didn’t not measure any zeta potential for this mixture.

I really want know “what makes my dispersions of exfoliated materials so stable.” It could be zeta, could be some other things which I don’t know. I would love to hear from you if you can address my question.

Alan F Rawle Zeta potential is the potential difference between the surface of the particle and the medium surrounding it. In a very unstable particle, the molecules transfer between the surface and the medium and thus the potential difference would be zero. If the particle is very stable, molecules of the particle would remain intact with the surface. Hence the potential difference between the aqueous medium and the particle would be high (away from zero).

Here in this sample, the nanosheets have hydroxyl groups which are prone to react with aqueous environment consequently losing its chemical form. Hence the Zeta potential is near zero.

if this is what you were asking about, Professor.