Tim Dinh I suspect that your particles are hydrophilic and do not want to disperse in hexane. They'll not wet and will clump and float. Take a look at this webinar (registration required):

Dispersion and nanotechnology

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

A surfactant will help but they're difficult to find for hexane and you can't 'modify the pH' of hexane as pH is an aqueous concept. I think you need to functionalize the SiO2 surface with say a silane to make it hydrophobic.

Tim Dinh

In the reverse osmosis process, your dispersion is filtered under pressure through a semi-permeable membrane. The driving force of the concentration of nanoparticles on the membrane surface is pressure.

The aggregation you are asking about is a spontaneous process of decreasing the Gibbs energy of the surface of nanoparticles in a disperse system.

What you have is not aggregation on the membrane, but the concentration of nanoparticles.

Dear all, please have a look at the following documents, hope they are helpfull. My Regards

https://doi.org/10.1021/acs.langmuir.0c00729

10.1039/C9RA09672H

Tim Dinh

You ask: 'Do you have any insight for why the silica nanoparticles aggregated in the aqueous solution if they are hydrophilic?'. My guess would be that they're small and inadequately stabilized. Try either a change of pH (and measure the zeta potential, if you can) or try the addition of a polymer such as 50 kDa PEG or PEI in order to sterically stabilize the system.

It shows the selected solvent is not a good medium for the silica nanoparticle (the one that you have) dispersion. Problem with the reproducibility may be because the nano particle in the original work may be synthesized in different way (reactants, medium, capping agents etc) which provided a different surface characteristics to the nanoparticle. Instead of functionalization of the nanoparticle you may add some dispersing agent to the solvent nanoparticle mixture that can effectively reduce the solvent-particle repulsion. You may look into some papers regarding tape casting in which you can find several dispersing agents for silica nanoparticles.

Dear Alan F Rawle silica nanoparticles have exceptional dispersion stability in water also with a zeta potential of 0.

Titus Sobisch Yes, that may be the case, but is not universally true. It depends on the size of the silica, any surface modification of the silica, the form (e.g. fumed versus crystalline), the pH of the external solution and so on. I only suggested that the poster look at ZP and not that it's a universal solution (e.g. certainly not true with steric stabilization). A simple vial experiment may be just as useful as a single ZP number obtained in a pseudo-scientific manner.