Posted by There are a large number of hydroxyl groups and unsaturated residual bonds in different states on the surface of the silica nanoparticles, making the silica nanoparticles hydrophilic and oleophobic and easy to agglomerate. They must be functionally modified to improve performance and range of applications.
The purpose of silica nanoparticle surface modification mainly includes the following three aspects:
- One is to improve or increase the dispersion between silica nanoparticles and compatibility with other substances because the surface-modified nanoparticles can weaken the charging effect of surface-active hydroxyl groups and the hydrophilicity of surface groups. Thus, it prevents particles from agglomeration or to achieve compatibility with organic substances;
- The second is to cover the surface of the silica with active groups to improve or control its surface activity through surface modification, thereby providing the possibility of further grafting or functionalization of nanoparticles;
- The third is to broaden the scope of the application of silica nanoparticles. Surface-modified nanoparticles can produce new functions, such as drug delivery and release, and stimulus responsiveness.
The surface modification of silica nanoparticles can be divided into two categories based on whether there is a chemical reaction between the surface hydroxyl group and the modifier: physical modification and chemical modification. The physical modification changes the ratio of hydroxyl groups on the surface of silica, while chemical modification needs to change the chemical properties of nanoparticles.
1. Physical modification of silica nanoparticles
Adsorption, encapsulation, and coating of silica nanoparticles by polymers or inorganic substances are the main ways of physical modification of silica nanoparticles. The combination of the hydroxyl functional group on the surface of the silica nanoparticles and the modifier is carried out through the interaction of hydrogen bonds, van der Waals forces, or coordination processes, which means physical modification is a kind of physical adsorption modification.
The surface deposition method is the main method for the physical modification of the silica nanoparticles. The coating layer is deposited on the surface of the silica nanoparticles by substances, and there is no chemical binding in this process.
2. Chemical modification of silica nanoparticles
The surface chemical modification of silica nanoparticles is based on the chemical reaction between a large number of hydroxyl groups or unsaturated bonds on the surface of the particles and the modified molecules.
Chemical modification has become a major method for surface modification because of its simplicity and strong firmness. Modification molecules can be divided into organic molecules and inorganic molecules. The commonly used chemical modification methods are as follows:
(1) Coupling agent method (such as silane coupling agent). One of the most common and basic methods.
(2) Esterification method.
(3) Surface graft polymerization method. This method can prepare hybrid materials with complementary properties and mutual promotion effects, so it is an important research topic with application prospects in the field of materials science.
3. Physicochemical modification of silica nanoparticles
It is the ultimate goal to prepare nanomaterials with excellent properties and wide applications, so physical and chemical simultaneous modification in the modification process is also possible. In order to obtain silica nanoparticles with small particle size and extremely high surface energy, and make the nanoparticles tightly combine with the organic chain in the organic system, the synthesized silica sol can be directly added to the organic system that needs to be modified. Organic materials that are used as modified molecules can be added during the preparation of silica nanoparticles as well.
The advantage of this method is to avoid the agglomeration of silica nanoparticles before modification, to ensure the crystal structure and bulk composition of the nanoparticles, to improve the dispersion of the silica nanoparticles, to make the particle size smaller and more uniformly dispersed.