I am dealing with ZnO nanoparticles and their surface modification and would like to know why nanoparticles should be dispersed in a solvent prior to surface modification.
What could be the chemistry between nanoparticles and solvent?
I would like to add: it is not just the agglomeration of the particles (in fact, probably this is the least important problem for dry nanoparticles). The much more serious disadvantage is that dry nanoparticles (in agglomerates) are not "naked", their surface is not readily available for being functionalized!
Why not?
Because on their surface, you have adsorbed gas (air), water, and air pollution (mainly oily stuff).
While in dispersion, the solvent has replaced the nanoparticle surface / gas, oil, stuff interface (if well dispersed), and created a particle surface / solvent interface. Now, your solvent also contains the chemicals with which you want to modify the nanoparticle surface - fine, great! go ahead!
But if your nanoparticles are dry, the reactive agents FIRST have to remove the particle / air (contamination) interface which is a process called "dispersion", which requires a lot of energy!
For better understanding, maybe you want to study my non-equilibrium thermodynamical theory of dispersion. (in my RG pages, if you don't find, ask me, I send you the links)
As mentioned by Maarten, the challenge in the nanoparticles modification is to break down agglomerates to individual particles, if you look at the cohesive energy of nanopaticles, you can see that they have strong inter-particles interactions. Ultrasonic dispersion and deagglomeration in solvents is found to be effective to breakdown the most of the agglomerates of nanoparticles. yet, some challenges remain, for example, if you want to modify a particles with hydrophilic surface nature with a hydrophobic component, then you need to find a solvent that can dissolve the hydrophobic component and yet have strong interactions with hydrophilic solid surface.
i agree with all explanation and really the agglomerates of nanoparticles is big problem. so for the Question my opinion the solvent has different properties for each type which helping more and easy to control of nano-particle disperse. for example for ZnO is the best disperse in ethanol because related the the polarity which easily to helping of dispersing. at same time Ethanol has a polar -OH group with it. This enables it to be a polar solvent,The O-H bond is quite polar, due to high electronegativity of oxygen atom compared to that of hydrogen. after that you can modify the surface of a particles as you want to be as a final surface for application. good luck
In order to functionalize the surface you need to separate nano particles as much as possible that's why you need to disperse them in a solvent (system). If you try to funct. the agglomerates, probably it will not be a homogeneous treatment and you may not repeat it.
At that point mixing is of significance, ultrasonication is a very common method but high shear mixing is more effective and takes couple of minutes (even at high nano-filler concentrations)
I would like to add: it is not just the agglomeration of the particles (in fact, probably this is the least important problem for dry nanoparticles). The much more serious disadvantage is that dry nanoparticles (in agglomerates) are not "naked", their surface is not readily available for being functionalized!
Why not?
Because on their surface, you have adsorbed gas (air), water, and air pollution (mainly oily stuff).
While in dispersion, the solvent has replaced the nanoparticle surface / gas, oil, stuff interface (if well dispersed), and created a particle surface / solvent interface. Now, your solvent also contains the chemicals with which you want to modify the nanoparticle surface - fine, great! go ahead!
But if your nanoparticles are dry, the reactive agents FIRST have to remove the particle / air (contamination) interface which is a process called "dispersion", which requires a lot of energy!
For better understanding, maybe you want to study my non-equilibrium thermodynamical theory of dispersion. (in my RG pages, if you don't find, ask me, I send you the links)
Everything stated so far may help to deal with your modification problem. But ZnO is a special candidate concerning its reactivity towards surface modifications. Most likely your sample will be modificated even freshly received from the manufacture (not on purpose) or it will be impossible to de-agglomerate the particle. HATR-IR measurements may help to analyse the premodification and the desired modification. I guess you will find a lot of acetate. Recently I have published a study dealing with the preparation and modification of very small ZnO nanoparticle (M. Schmitt, Nanoscale, 2015, 7, 9532-9544.). Depending on your application you may find opportunities to use non-modified bare ZnO.
Kind Regards
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