Biomedical Engineering Reference
In-Depth Information
agglomerate together, which would lead to decrease or even loss of any associated
properties. Therefore, an understanding of the colloidal stabilization mechanism
is crucial to the design and synthesis of these nanoparticles in a well-defi ned
way. The preservation of long-term stability, without agglomeration or precipita-
tion, would be the primary prerequisite for any application related to magnetic
nanoparticles.
Derjaguin, Landau, Verwey, and Overbeek developed the DLVO theory in 1940s
to explain the stability mechanism of colloids in suspension. Their theory sug-
gested that the balance between two opposing forces - that is, electrostatic repulsion
and van der Waals attraction - would determine whether the particles agglomerated
together or were separated from each other. This theory is also extendable for the
interpretation and formulation of nanosized nanoparticles. In general, three major
approaches have been discussed for the stabilization of nanoparticles in terms of
the stabilizers used: (i) electrostatic repulsion; (ii) steric stabilization; and (iii) elec-
trosteric stabilization. The scheme is shown in Figure 16.9 .
Nanoparticles dispersed in an aqueous solution are able to generate an elec-
trostatic repulsion due to the positively or negatively charged surfaces, while
uncharged particles are free to form colloids and to aggregate. In solution, the
presence of a net charge on a particle will affect the distribution of ions surround-
ing that particle, and result in an electrical stern layer and diffuse double- layer
around the particles. When a particle moves due to either gravity or to an applied
electric fi eld, ions within the boundary will move with it accordingly. However, a
boundary remains where ions do not travel with the particle - this is termed the
“surface of hydrodynamic shear” or the “slipping plane”. The potential that exists
at this boundary is known as the “zeta potential”; this is a critical parameter to
determine the stability of a colloidal suspension. When all of the particles have a
large negative or positive potential, they will repel each other to keep suspension
stable, whereas fl occulation happens with a low potential value. The ionic strength,
temperature, pH value, or the addition of an electrolyte would each affect the zeta
potential and, in turn, the stability of the particles.
In the case of steric stabilization, bulky functional organic molecules or poly-
mers are adsorbed onto the particle surface so as to form a protective layer among
nanoparticles, and thus prevent the particle surfaces from coming into close
(a) (b)
Figure 16.9
Schematic representation of (a) electrostatic
repulsion; (b) steric stabilization; and (c) electrosteric
stabilization of colloidal nanoparticles.
(c)
