by Professor Shlomo Magdassi
Utilization of nanomaterials very often requires their dispersion in various liquids, in order to enable embedding them homogenously in a device or in a final liquid product. For example, application of metallic nanoparticles or carbon nanotubes in printed electronics is usually based on placing a dispersion of the nanomaterials on various substrates, while the material is kept in its non-aggregated form.
Since most dispersions of nanomaterials are not thermodynamically stable and represent a metastable state as compared to the bulk material, agglomeration and coagulation of these materials tend to occur spontaneously. The driving force of the aggregation is the interaction between the particles or nanotubes. For example, while dispersing carbon nanotubes (CNT) in water, the van der Waals attraction is so strong that it prevents the dispersion of individual CNTs and, therefore, only bundles are present in the liquid. As shown in figure 1a, these bundles eventually sediment, obviously rendering the dispersion useless in various applications, such as those based on coatings.
In general, obtaining dispersions of powder of nanomaterials requires the use of colloids chemistry tools, and can be divided into three stages:
1. wetting of the powder with liquid,
2. breaking the agglomerates of the nanomaterials by applying high shear forces, and
3. stabilizing by proper dispersing agents.
If the synthesis of the nanomaterials results in a liquid dispersion of the material, without going through the drying stage, only the latter stage is of importance.
Wetting of powders can be achieved by a proper selection of the dispersion liquid, or by the addition of a wetting agent. High shear forces can be obtained by proper instrumentation, such as sonicators, high pressure homogenizers, and bead mills. Stabilization of nanomaterials in dispersions is achieved by adding dispersing agents, which increase the energy barrier for aggregation, thus providing their kinetic stability1.
Since the stability of nanomaterials is governed by the balance of various interactions, such as van der Waals attraction and electrical and steric repulsion, the optimal approach to obtain stable dispersions is by using stabilizers which have groups with affinity to the surface of the particles, and groups that provides electro-steric stabilization. The use of proper dispersion agent can lead to the formation of stable dispersions, such as that of CNT presented in Fig 1b.
Figure 1. Unstable (a) and stable (b) dispersion of multi wall CNTs
Evaluation of dispersion quality also presents challenges, especially for nanomaterials which are not simple spherical nanoparticles. We recently reported2 on a rapid and simple process for producing dispersions MWCNTs by using a high pressure homogenization process (HPH)4, and on a simple valuation method for CNT dispersions by centrifugal sedimentation analysis.
Many nanomaterials are produced by the "wet chemistry" processes. In this case the stabilizing agent can be present during the nanoparticles synthesis, or even be one of the reactants, as in formation of gold nanoparticles, while the reducing agent citric acid, also provides electrostatic stabilization. However, as we found in many research projects, such stabilization is not sufficient for stabilizing dispersions containing metallic nanoparticles at high concentration, and in order to achieve this, a steric or electrosteric stabilizer is required3. Such a stabilizer is polyacrilic acid sodium salt, which we used in obtaining dispersions of silver, copper and Cu@Ag core-shell nanoparticles3-7.
Having stable dispersions of these metallic nanoparticles, enabled us to use them in inkjet printing of conductive patterns composed (Fig 2a), in RFID tags (Fig2b) and in several electroluminescent devices.
Figure 2. A printed layer composed of closely packed silver nanoparticles and inkjet printed RFID antenna.
Another field in which dispersion of nanomaterials is of high importance is drug delivery systems. Proper use of dispersion agents in dispersions of organic nanoparticles can lead to improved dissolution and, thus, to improved bioavailability. It can even prevent crystallization of nanomaterials, as we have recently demonstrated for several active materials8,9.
In conclusion, understanding stabilization mechanisms of colloidal systems is of utmost importance in utilizing nanomaterials in material science, as well as in many applications.
1. Kamyshny, A.; Magdassi, S. In Structure and Functional Properties of Colloidal Systems (Surf. Sci. Ser., v. 147); Starov, V., Ed.; CRC Press: Boca Raton-London-New York, 2010 (in press).
2. Azoubel, S.; Magdassi, S. Carbon 48, in press (2010).
3. Kamyshny, A.; Ben-Moshe, M.; Aviezer, S.; Magdassi, S. Macromol. Rapid. Communn, 26, 281. ( 2005).
4. Grouchko, M.; Kamyshny, A.; Magdassi, S. J. Mater. Chem. , 19, 3057 (2009).
5. Magdassi, S.; Grouchko,M.; Berezin, O.; and Kamyshny ,A.; ACS Nano, 4 , 1943-1948 (2010).
6. Layani, M.. ,Grouchko M.., Millo O., Azulay D.;Balberg I..; Magdassi S., ACS NANO, 11,3537-3542 (2009).
7. Grouchko, M..; Kamyshny, A..; Ben-Ami,K.; Magdassi, S., J. Nanopart. Res. 11, 713-716 (2009).
8. Margulis-Goshen, K.; Magdassi, S.; Nanomedicine,5,274-281 (2009).
9. Margulis-Goshen,K.; Donio (Netivi) H.; Major, D. T.; Gradzielski,M.; Raviv,U.; Magdassi,S.; J. Colloid Interface Sci., 342,283-292 (2010).
Copyright AZoNano.com, Professor Shlomo Magdassi (The Hebrew University of Jerusalem)
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