Its agglomerate principally because of the high surface energy that this kind of nanomaterial have, so due to this they tend to agglomerate to diminish this energy.

Dear Sergio Kogikoski jr ,

What happens to surface energy when we use reducing or capping agent?

In that case usually there is the counter balance of energy on the surface of the nanomaterial, and around it is created a region with a smaller surface energy, which will prevent the agglomeration.

Does Vander Waals force plays a role anyway?

i am still wandering , when we use ultrasonication, in order to reduce these Van der Waals attraction ! whats exactly going on with this partials!

Stochastic forces drive the process a great deal, even neutral particles tend to aggregrate due to the greater entropy of the system when the particles are aggregated.

High surface area to volume ratio of nanoparticles provides a very high surface energy. To minimize its surface energy the nanoparticles create agglomeration. Uncontrolled agglomeration of nanopartiles many occur due to attractive van der Waals forces between particles.

Hello Bhaumic

The answers shown above are correct. With smaller sizes, the particles have higher relative surface area and higher energy. I wish to add one aspect.

With smaller size, higher relative surface area, and higher relative numbers of surface atoms. Such surface atoms have unsaturated coordinations (not complete coordination) and each atom has vacent coordination sites. A phenomenon so called dangling bonds.

More bonds need to be formed per each surface atom. They try to make bonds, and such bonds tend to form between adjacent particles (bonds between surface atoms of each) This causes agglomeration.

It should also be noted that even if you have smaller size particles by sonication or other techniques, the small particles are still subject to agglomeration afterwards. Normally, we use surfactants to keep particles away (similar to micelles. Surfactants also stabilize the surface atoms.

I hope this adds some thing to the correct answers shown by colleagues above.

Bets wishes

I totally agree with Hikmat hilal's answer. Yes we have experienced these aggregation in nano cluster synthesis. Its more of surface charge ( surface energy) so by aggregation particles try to become more stable. but this can change by using anchoring groups or surfactants (CTAB). Sonicating might help for a short period of time but it will not helpful for long period because its gonna aggregate again. but this is a common problem in nano material synthesis

Thank you researchers for your kind input....

But still I have a confusion with Vander Waals force...whether they take part...

Bhaumik

The issue is mainly related to relative surface area, higher energy, coordination unsaturation, unstable atoms, etc. Simply that.

Vander Waals Forces will exist there in one case:, if bonding in the solid is a vander Waals type, then we say incomplete number of bonds and consequently higher energy.........

So coordination unsaturation means real chemical bonds, metallic, covalent & ionic, (or vanderWaals forces as well if they are involved, such as ice, molecular solids, Solid CO2, .... and other solids involving Week London Forces).

Another idea to know is: Even in case of liquids, the same philospophy still applies, the phenomenon is then called surface tension. Higher relative surface area then gives higher energy and higher unstability, and more coordination unsaturation (unformed bonds). So minimum surface area attained in order to maximize bonds. Such bonds can me chemical/ metallic bonds (such as in mercury) or vander Waals bonds between atoms or molecules (such as in water) or oil or ......

The subject involves different types of bonds and forces, with same logic.

I think one of the main cause is high surface energy due to high S/V ratio, which also mentioned by the researchers. There are several methods by which we can reduced the surface energy and prevent the agglomeration between NP.

Nano particles do have a lot of surface coordinatively unsaturated sites and surface energy. No materials with high energy and un-saturated sites are stable, To get the stability they tend to get agglomerated.

Respected researchers, I am just curious to know, Is this high surface energy of nanoparticles is that dominant to overcome the surface charge of particles.?

From a thermodynamic perspective, the increased surface energy of nano- and microstructurs makes them unstable. They still can be formed by virtue of slow kinetic transformations, but this does not eliminate the need for such materials to transform to more stable structures in every possible way. Changes in unit cell types, phase transformations, agglomeration are examples of these ways. Agglomeration, from one side, leads to a decrease in entropy of the system, but from another side it leads to negative enthalpy changes (due to bond formation). As long as the enthalpy factor outweigh the entropy factor, agglomeration can take place, and in this case the overall result is a negative Gibbs energy change which forms the driving force for agglomeration. Once this driving force is eliminated (ΔG=0) the structure becomes thermodynamically stable.

thankx Mr.Shahwan

regarding van der Waals, of course these attractions are a part of the interaction balance, even if they dominate more for larger particles. For stabilization repulsive forces have to be larger than attractive like van der Waals forces.

Ultrasonication does not changes the interaction balance, it only delivers energy to overcome attraction forces to deagglomerate.

It depends on the nature of your nanoparticles, your aspect ratio and your concentration. For particles who are like long fibers (high aspect ratio; aspect ratio>1), if the concentration is higher than 1/aspect ratio, then you are in a very concentrated regime and you can no longer neglect the interactions between the partciles; thus agglomerates or aggregates can be formed. If the concentration is

I think Rabih you confuse it with hydrodynamic interactions. Nevertheless, of course there is a difference between agglomeration in diluted and in concentrated regime.

Up to what size of nanoparticles the high surface energy effect and form the agglomerates...???

With smaller sizes agglomeration should occur more readily. With smaller size there is higher relative surface and more surface atoms (less unstable system), thus agglomeration may occur. 

In case of very small particles (~5 nm) it may be difficult to see separate particles. In many cases the particles give larger ones. This is why people may need to use surfactants to prevent agglomeration or particle growth. 

Question:What is the main reason that nano particles get agglomerated

It would be best to describe a particle as a nano part of atom or matter or smallest unit of an element. That being said, it is realistic to say that ionized particle (s) or ions) do attract each other on solid surfaces to form ionic bond or cluster or agglomorate.  Single minute or nano atom would first selectively land on a substarte area of high surface energrgy. The singled atom would then  attaract another similar atom forming atomic molecules just as in the case of adatoms of hydrogen ions which ataracts each other to form hydrogen molecular gas (H + + H+ = H2 ) . All things being equal the nano atoms now adatoms  (  clusters) formed on the preferential sites  of the substratre ( i.e, x, y planes, respectively )  will grow  into micostructure with defined grain boundries and spread, expanding toward the low energy surface area of the substrate ( the Z plane). Eventaully the atomic cluters deposited  and expanding from their respective preferential sites on the substrate would simultaneously merged into the low energy section of the substrate ( the Z plane ) forming a complete first atomic ( film) coverage on the substrate surfaces.  After the atomic first layer growth film layer was completed, the second waves of the cluster film deposition continues, atomic layer by layer until a required homogeneous useful atomic thickness layers, was achieved. 

Hi,

I have a question. Can you suggest some surfactants which can be used in DCM as solvent to avoid agglomeration of nano particles..

I have  a question  can any help me ? Ni nano particle completed dissolved in water how i will get it as solid nature 

因垂丝汀

@Palaniappan Nagarajan,

Hi, Try and centrifuge the suspension. Hope this helps

Our experience shows that the sonication of metal nanoparticles when they are suspended in organic solvent does help in breaking up the agglomeration as shown in SEM data 

Above answers is really convincing.

Does pH have any influence on agglomeration effect ?? Could anyone explain thermodynamics aspect of nanop.using gibbs free energy and entropy.

The sedimentation and agglomeration is one of the major problems in preparation nano-lubricants owing to very strong van der Waals interactions, which disrupt the large-scale commercialization. If the attractive force is higher than the repulsive force, the particles will colloid and the suspension is unstable. If the particles have a high repulsive force, the suspension will be stable.

Van Der Waals forces and hydrogen bonds increases particle-particle interactions which then induces aggregation.

van der Waals forces increase with mass (size) of particles, not all particles are able to form hydrogen bonds. The smaller the size (higher interfacial area) the larger the potential for aggregation, which will happen during synthesis and upon contact of dry powders.

From the thermodynamic point of view, increase in surface-to-bulk ratio causes increase in excess energy in overall considered system. Therefore, a tendency to decrease an amount of free surfaces becomes stronger with dropping particles’ size down.

Kinetically, a variety of local forces act to bring the nano-particles together and then to adhere. These may be Van-der-Vaals, electrical, chemical, etc. forces. Being brought together, particles usually complete their consolidation by interdiffusion which may eliminate an interface completely.

Nanoparticle aggregation and agglomeration need to be differentiated although it is usually used interchangeably. Agglomerations are weakly bonded particles while aggregates indicate strongly bonded or fused particles. Nanoparticles agglomerate in aqueous biological matrices like phosphate buffered saline and other cell medias. This might be due toneutral pH or perhaps due to high ionic strength. Albumin can also influence agglomeration.

Nanoparticles are generally described as primary particles are generally aggregated (aggregates are particles combined by covalent bonds). Into relatively stable aggregates and these aggregates are frequently clustered into less stable arrangements called agglomerates (Agglomerates are particles held together by van der Waals attraction and polar bonds) Depending on the synthesis conditions and the surface chemistry, the nanoparticles tend to form soft or hard agglomerates. Hard agglomerates consist of smaller particles which are connected to each other by sinter necks. They can be destroyed only by high energy milling. Soft agglomerates are accumulations of isolated particles which are connected to each other by attractive physical interactions like van-der-Waals or hydrogen bridge forces. Soft agglomerates can be disrupted into smaller particles by shear forces generating mechanical stress gradients

Why Albumin can also influence agglomeration ?

Please explain@ Omer Iqbal

This is a team work on Tribology. We have been succesful to hold in suspension an impoprtant nanoparticles in Engine Oil. To have colladial is not enough, it needs also to reduce Friction of Coefficient according to the reference oil. We got it and we try to improve in IRP (international research project). Now it is confidential and we will publish soon.

Because the inherent interparticle repulsive forces are insufficient to overcome attractive forces which allows approaching particles to aggregate. The particle motion may be due to diffusion or shear forces in the liquid.

Aggregation does not imply covalent bonding (as incorrectly stated in previous answers).

There are other reasons but this in the main one.

Note, "aggregation" means different things to different people. Even in the most highly regarded text books on colloid science written by eminent colloid scientists present contrasting definitions of aggregation/agglomeration/flocculation etc. There is no generally accepted definition.

NTCs tend to tangle with each other due to the presence of a van der Walls force between them. Carbon nanomaterials of large surfaces tend to agglomerate in clusters to reduce their surface energy during composite treatment.

The main reason of the agglomerated of the nanoparticles is the extremely high surface area to volume ratio of nanoparticles in addition of taking into account the environmental factors that can effect on this ratio.

1- The environmental factor. One example is that adding bad solvent into the nanoparticle solution will cause agglomeration of the nanoparticles due to the minimization of the interface. Another example is the temperature impact. When the temperature is cooled down, the enthalpy change is negative, to compensate this change, the system will try to maximize the entropy by separating the media (gas/liquid molecules) from the "huge" nanoparticles. All these make convenience for the occurring of above-mentioned agglomerations.

2- Its agglomerate principally because of the high surface energy that this kind of nanomaterial have, so due to this they tend to agglomerate to diminish this energy.