Surface Modification of Nanoparticles

Particles in the nanometer-size range have a strong tendency to agglomerate due to van der Waals interactions. It is therefore important to develop synthetic methods by which the particles can be stabilized, that is, where repulsive forces between the particles can be provided to balance this attraction (Fig. 1(A)) [48]. Generally two types of stabilization are used to prevent agglomeration of nanoparticles, namely, electrostatic stabilization and steric stabilization by adsorbed

Figure 1. Particles in a colloid (A) uncharged particles are free to collide and agglomerate and (B) charged particles repel each other.

molecules or steric hindrance [49-50]. Electrostatic stabilization involves the creation of an electrical double layer arising from ions adsorbed on the surface and associated counterions that surround the particle. Thus, if the electric potential associated with the double layer is sufficiently high, the Coulombic repulsion between the particles will prevent their agglomeration (Fig. 1(B) and Fig. 2) [51-52].

Steric stabilization can be achieved by the adsorption of large molecules such as polymers at the surface of the particles (Fig. 3). Indeed, the coil dimensions of polymers are usually larger than the range over which the attraction forces between colloidal particles are active. Two distinct effects describe this type of stabilization (the volume restriction contribution and the osmotic diffusion), and they both contribute to the interaction free energy [46, 53-54]. First, the fact that the adsorbed molecules are restricted in motion causes a decrease in the configurational entropic contribution to the free energy (Fig. 3). Second, the local increase in concentration of polymer chains between approaching particles results in an osmotic repulsion, since the solvent reestablishes equilibrium by diluting the polymer molecules and separating the particles. This can be described by the energy of free mixing of polymer segments and solvent molecules, calculated by the Flory-Krigbaum theory [55].

The conformation of adsorbed polymers tends to be controlled by the strength of segment/surface interactions [56], which may be described by the classical loop-train-tail model [57-58]. For an effective particle stabilization, it is important that the polymer form a complete, dense layer around the particle [59]. Then, the polymeric stabilizer must have sufficient tail length and adsorb uniformly enough to screen the attractive interaction between the particles.

Another requirement for good particle stabilization is the use of appropriate solvents. The stabilizing polymer has to possess high affinity with the solvent in order to solvate the particles and form an extended layer for screening the van der Waals attraction between the particles. It is well known, however, that adsorption is generally stronger when the affinity of the polymer to the solvent is low [60-61]. Therefore, block or graft copolymer micelles, in the presence of a selective solvent that solubilizes one of the blocks,

Positive counter ion

Positive counter ion

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