How to View the Phenomenon of Nanoparticle Agglomeration

Nanotechnology, as a high-tech that influences the direction of human development in the 21st century, has wonderful and bright application scenarios. Nanocomposites have already become an important part of nanomaterials engineering due to their excellent comprehensive characteristic. “Nanocomposites” refer to the composites with dispersed phase scale that at least has one dimension less than 100 nm, i.e., nanoparticles dispersed into conventional three-dimensional solids. Nanocomposites obtained by this method, especially those with organic-inorganic molecular interactions, have become one of the hot spots in nanomaterials research today due to their superior performance and wide application prospects. However, nanoparticles themselves are very easy to agglomerate, so the primary problem to obtain ideal organic-inorganic nanocomposites is how to disperse nanoparticles into organic polymers. Studies have shown that effective dispersion and surface modification of nanoparticles with appropriate physical and chemical methods can solve this problem.

1. Agglomeration principle of nanoparticles

1.1 Surface effect of nanoparticles: The change in nature of the nanoparticle is caused by the ratio between the quantity of surface atoms to the total amount of atoms in nanoparticles which increases sharply as the particle size becomes smaller. Nanoparticles have a high surface area. When the particle size of nanoparticles is below 10 nm, the ratio of surface atoms increases rapidly, and when the particle size drops to 1 nm, the ratio of surface atoms is up to 90% or more. With almost all of the atoms are concentrated on the surface of the particles in a highly activated state, this results in insufficient coordination number of surface atoms and high surface energy, which makes these atoms extremely easy to combine with other atoms and stabilize them. It shows that the nanoparticles are highly chemically active and exhibit strong surface effects.

1.2 Brownian motion

The collision of particles and solvents gives the particles the same kinetic energy as the surrounding particles, so the small particles become faster. The small nanoparticles will often collide with each other during Brownian motion, they will join together due to the attraction and form secondary particles. Secondary particles move slower than single particles, but still can collide with other particles, which in turn can form larger agglomerates until they are too large to move on, eventually settle down.

1.3 Influence of van der Waals forces and hydrogen bonds

Particles suspended in solution are generally subjected to van der Waals forces and are prone to agglomeration. Van der Waals force is inversely proportional to particle diameter, therefore due to their small size, nanoparticles have a stronger van der Waals force. Common nanoparticles such as SiO2 particles are composed of rigid, solid, and extremely fine spherical particles. When they are formed, many particles are fused together, while the shape being very irregular. There are many -OH on the surface of nano-SiO2 molecules. Water molecules can easily generate hydrogen bonds with the -OH on the surface, forming a hydrophilic strong polar surfaces. They can easily generate agglomerative groups due to the attraction of hydrogen bonds and van der Waals forces, at the same time easy to separate with impact and re-agglomerate again.

2. Dispersion method

The dispersion of nanoparticles in solvents belongs to sol, and if the colloidal particles can still maintain the dispersed state after a long time, the system is stable. In order to achieve a stable state in the sense of colloid chemistry, there are two ways: (1) To make the particles carry the same sign of electrical charge and repel each other. (2) By adsorption of a substance such as a polymer on the surface of the particles to prevent the particles from approaching each other, the combination of these two mechanisms is called “electrostatic steric hindrance stabilization effect”. Physical and chemical methods of dispersion are commonly used today.

2.1 Physical dispersion method

Commonly used Physical dispersion methods are mechanical dispersion and ultrasonic dispersion. The mechanical method mainly adopts mechanical stress to purposefully activate particle surface to change its surface crystal structure and physicochemical properties, while ultrasonic dispersion method is a popular research field in recent years. The use of ultrasound can effectively break the soft agglomeration of nanoparticles, and the powder is dispersed in medium as more uniform small agglomerates from the strong impact, shearing and grinding.

2.2 Chemical dispersion method

Chemical dispersion method is to select one or more suitable dispersants to improve the dispersibility of suspended solids and add dispersants in suspended solids, so that it attaches on the surface of the particles. It can change the nature of particle surface, thus changing the interaction between the particles and liquid phase medium,as well as particles between particles, creating a stronger repulsive force between the particles. Commonly used dispersants mainly include the following categories:

2.2.1 Surfactants Surfactants are composed of two parts: lipophilic and hydrophilic groups, which are amphiphilic molecules, including long-chain fatty acids, cetyl trimethyl ammonium bromide (CTAB), etc. The role of this type of dispersant is mainly a steric hindrance effect, with the hydrophilic group attached on the surface of powders and hydrophobic chain into the solvent.

2.2.2 Small molecules of inorganic electrolytes or inorganic polymers such as sodium silicate, sodium aluminate, ammonium citrate, etc. Such dispersants can be dissociated and charged, and the attachment of the powders on the surface can increase the surface potential of the particles, then increasing the electrostatic repulsion.

2.2.3 Polymers These dispersants have a large molecular weight and are adsorbed on the surface of solid particles. Their long polymer chains are fully extended in the medium to form an adsorption layer of several nanometers to tens of nanometers, resulting in a steric hindrance effect that can successfully prevent particles from aggregating with each other.

3. Surface modification of nanoparticles

“Surface modification of nanoparticles” is to change the structure and state of nanoparticle surface with physical and chemical methods, giving the particles new functions and improving their physical properties, then achieving the control for nanoparticle surface. Many scholars have researched and explored in this field and proposed various surface modification methods, which can be divided into two major categories according to their principles: surface physical modification and surface chemical modification; and seven small categories according to their processes: surface coverage modification, local chemical modification, mechanochemical modification, outer film modification, high energy surface modification, and precipitation reaction modification.

3.1 Surface physical modification

Surface physical modifications mainly include polymer deposition in solution or melt, adsorption modification, monomer inclusion polymerization modification, surfactant coverage modification, outer-layer film modification and high energy modification, etc. In recent years, the application of surfactants has been reported more frequently. By adding polymeric surfactants to make them adsorb around the sol particles to make the steric hindrance potential energy between the particles,thus making the inter-particle potential barrier larger and achieving the purpose of preventing agglomerates.

3.2 Surface chemical modification

Surface chemical modification in general is to change the surface structure and state of nanoparticles by chemical reaction between nanoparticle surface and modifier to achieve the purpose of surface modification. The method is a more reliable but complicated, which is covalently fixed by chemical bonding, mainly including esterification reaction method surface modification, coupling agent surface coverage modification and surface grafting polymer modification.

3.2.1 Esterification reaction The most important thing to modify the surface of nanoparticles by esterification reaction is to *change the original hydrophilic and oleophobic surface into a lipophilic and hydrophobic surface.

3.2.2 Coupling agent surface coverage method Generally, the surface of inorganic nanoparticles can be treated with coupling agent to produce good compatibility with organic materials. The effective coupling agent molecular structure should be a bifunctional group compound that can chemically react with inorganic surfaces at one end and react or have compatibility with organics or high polymers at the other end.

3.2.3 Surface grafting is a method of linking polymer to the surface of inorganic nanoparticles through chemical reaction, it can be divided into coupling grafting method, particle surface polymerization growth grafting method, and simultaneous polymerization and surface grafting method. Coupling grafting method is realized by the direct reaction between the high-energy groups on nanoparticle surface and macromolecules. Particle surface polymerization and growth grafting method is the direct polymerization from the surface of inorganic particles as the initiator induce growth and complete polymer inclusion on the surface of particles. Simultaneous polymerization and surface grafting method requires inorganic nanoparticle surface to be strong in capturing free radical. While a monomer completes polymerization under the action of initiator, it is immediately captured by strong free radicals on the surface of inorganic nanoparticles, so that the polymer chains are chemically linked to the surface of inorganic nanoparticles.

4. Conclusion

In summary, the agglomeration problem of nanoparticles is the key technical problem in the synthesis process of nanocomposites*. And now, with the rapid development of science and technology and the tenacious struggle of many scientists, nanocomposites have been widely used for clinical practice in the medical field. Nanocomposites can be used as drug nanocarriers in medicine,and they are more used in aerospace, national defense, transportation, sports and other fields. With nanocomposites being the * attraction,the strategies of developing new material in developed countries all over the world are putting the development of nanocomposites in an important position.

References:

[1] Review article by Zhu Yanping. Revised by Xu Lianlai, Li Changfu. Research Progress on Nanoparticle Agglomeration (M) Tianjin: Stomatological Hospital of Tianjin Medical University. 2005.338-340