I am synthesizing titania Nanoparticles using Ultrasound (US) and age them in the medium upto a desired time. I have a non-anionic surfactant; a peptizing agent and the medium of suspension is water. The fact is whenever US is given into the medium, there is a major reduction in the particle sizes. But the problem is they readily re-agglomerate the moment sonication is stopped. Nevertheless, particle sizes reduce with time due to aging irrespective of the fact that US is used or not. This re-agglomeration is only seen with US mediums. Most of the papers report acquired kinetic energy of particles increasing their collision frequency and they re-agglomerate. But this observation is 'during' sonication and not post-sonication after say like half an hour. The only explanation I can come up with is that due to high surface-volume ratio of nanoparticles they agglomerate. This can also be true in the case of the non-sonicated dispersion but this is not seen in that case. Is it because there is a controlled reduction of sizes in the medium just using a stirrer and on the other hand US causes rigorous agitation and sudden 'shock' to the medium reducing the sizes abruptly? I also thought about the possible destruction of surfactant layer when US is present. Post sonication, because of the absence of the layer, they agglomerate easily. With time, surfactant helps in reducing the agglomerates caused after sonication.
Any thoughts on this? Any theory/background or paper which can help me form a hypothesis will be really helpful. Thank you so much
Note: 1. Bi-modal distribution is present in the medium throughout
2. All components are added together
3. Method is sol-gel reaction
Thanks
Kailas Naresh
You mention TiO2. There is this example of TiO2 aggregating after cessation of sonication in the following webinar (registration needed) - slides 29 onward:
Dispersion and nanotechnology
https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W190528DispNanoMat
This is due to inadequate zeta potential solved by the optimum concentration of phosphate.
These additional webinars will be useful to study backing up the above:
Adhesion and cohesion
https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W180725AdhesionCohesion
PST & BDAS - An acronym approach to laser diffraction method development
https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W190815PST
You are probably confusing surfactant (wetting agent) with stabilizer (prevents re-agglomeration) in the wetting-separation-stabilization sequence in making a stable dispersion.
I am also wary of a bimodal for a single component system. Is your particle size measurement technique laser diffraction?
I think reduction in the size of nanoparticles increases the surface area which might be the reason for their re-agglomeration.
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