Hi Omar, my guess is that the 0.03% SDS is playing a big part (especially if you are using saline solutions).

The OH-HMWCNTs (OH-CNT) would be more hydrophilic than MWCNT (M-CNTs) because of the OH groups. As such they would not bind SDS as well.

SDS has a very low CMC (critical micellar concentration) and would preferentially bind to the more hydrophobic MCNTs while the OH-CNTs have reached their solubility limits because they don't bind SDS as well.

If this is true then using 1% SDS would make then equally stable in solution.

BTW, do you sonicate your CNTs?

Thank you Dr.steingrimur for your valuable answer. So if this the case, I would like to add one more things. I have another functionalized CNT, namely COOH-CNT.. it is more hydrophilic, and it's still more stable than CNT and OH CNT.

And based on your answer, how can justify the different behaviour at different concentration?

I have sonicated all samples for one hour.

Thank you once again

'If this is true then using 1% SDS would make then equally stable' - SDS has a solubility limit well below 1 % (0.1 % is soluble without problems). It would crystallize out of oversaturated solution.

Hi Omar, I would gather that CNT-COOH, would be more soluble at pH higher than 6.

Although, it might not be as good of a conductor than CNTs.

' would gather that CNT-COOH, would be more soluble at pH higher than 6'

CNT are not soluble - only dispersible

https://www.researchgate.net/project/nanoparticle-solutions-search-for-a-perpetuum-mobile-there-are-no-particle-solutions-only-particle-dispersions

Hi Titus, technically, when you are talking about macromolecules in aqueous media they are soluble only because they are dispersible.

Some proteins are very soluble in aqueous buffer while others are not. These soluble proteins prefer to interact with the aqueous solution than with each other. They are very dispersible.

they are not soluble because dispersible - i.e. dispersible is not cause but also a consequence of preferential interaction with continuous phase. Particles can be dispersible without being soluble - good example is silica / water.

Sodium chloride particles are dispersible in water, but considerable amount is dissolved.

Macromolecules that are dispersible in aqueous media without being soluble eventually fall out of solution. A good example is ammonium sulfate precipitation of proteins.

Hydrated proteins stay in solution in aqueous media because it is more energetically favorable for them to form H-bonds with water than it is to aggregate with other proteins. The H-bonds keep them soluble and dispersed.

NaCl particles are not present in water, only hydrated Na+ and Cl- ions. If you supersaturate water with NaCl then yes you will have NaCl particles that are neither soluble nor dispersible.

and now what?

NaCl particles are dispersible and will not flocculate - do not confuse dispersion with settling

There are no uncharged NaCl particles in water, only their hydrated ions unless the water is supersaturated with NaCl.

I don't understand what the problem is here.