Ecotoxicology of Manufactured Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 266

Stabilization of
nanoparticles
by surfactants

Binding to suspended
particles

Binding to NOM Aggregation Binding to NOM

and other colloids and other colloids

Dissolution

Biological degradation,
hydrolysis, photolysis

M n+

M n+

M n+

Sedimentation

Figure 7.3 Processes affecting the fate and transport of nanoparticles in aquatic systems.

7.2.1 Particle Dispersion and Aggregation

The dispersion of nanoparticles to form stable suspensions is a desirable step prior
to toxicity testing. Unfortunately, some areas of the ecotoxicological literature have
confused this process with that of dissolution . Dissolution is the process by which
solids (including nanoparticles) dissolve in a solvent to form a solution containing
ions or solvated molecules. This process results in a reduction in size of the particle
and ultimately its disappearance. Conversely, dispersion results in the formation of
a liquid phase suspension in which the individual nanoparticles are present as sepa-
rate entities surrounded by solvent molecules. Solubilising agents assist the dissolu-
tion of particles whereas dispersing agents assist the formation of a nanoparticle
suspension. Nanoparticles are a sub-set of colloids and are mainly assemblages of
molecules or atoms. For such nanoparticles it is therefore appropriate to use the
term disperse rather than dissolve. Some potential exceptions include fullerenes
and dendrimers, which are molecules in their own right. Dissolution may be cor-
rectly applied to these nanomaterials when referring to the solvation of individual
molecules by a solvent to form a solution. In practice, however, most fullerenes
are poorly soluble (especially in aqueous media) and are more likely to form
semi-stable suspensions of nanoparticles by the aggregation of many fullerene
molecules.
The stability of nanoparticles in aqueous media has been covered in detail in
chapters 2 and 4 . To briefl y summarise, nanoparticles may form stable suspensions
in solution if their surface charge is suffi cient to repel other particles or if there are
steric factors (such as the presence of long chain polymers attached to the particle
surface) that inhibit particle aggregation. The Derjaguin, Landau, Verwey and
Overbeek (DLVO) theory of colloid stability in aqueous electrolyte solution is