Engineered Nanomaterials as Emerging Contaminants in Water - Nanotechnologies for Water Environment Applications - page 571

where ψ is the zeta potential; z is the valence of ions (assuming a symmetric electrolyte);
n is the bulk electrolyte concentration; e is the charge of one electron; and

is the inverse
of the Debye length; r is the particle radius; H is the shortest distance between the
surfaces of two particles; A is the material-specific Hamaker constant. Thus the
tendency for particles to aggregate is a function of zeta potential, which varies with ionic
composition of water, the Hamaker constant and ability for colloids to approach each
other at close distance. NMs of similar zeta potential tend to have greater electrostatic
repulsion than van der Waals attraction; consequently a net repulsive energy barrier
exists and particles remain stable in water (i.e., do not aggregate). The net energy of
interaction ( Total ) varies as a function of the separation distance between two colloids
with a strong attractive vdW at very short separation distances (< 5 nm) and electrostatic
interactions ( EDL ) which can be relevant up to tens of nanomaterials. When the
maximum Total is repulsive, the magnitude of this repulsive interaction ( Total,max ) is
termed the energy barrier. Sufficient external energy through mixing, heating, etc.
would have to be added to overcome the energy barrier before the colloids aggregate.

κ

There are no known mechanistic reasons why engineered NMs would behave
differently from natural nanoparticles (i.e., aquatic colloids) (Lecoanet et al., 2004;
Chen and Elimelech, 2006; Chen and Elimelech, 2007; Zhang et al., 2008-a). Therefore
understanding the surface chemistry (zeta potential and intrinsic properties such as
Hamaker constants) is important in identifying potential interactions between engineered
NMs. It appears that biogeochemical processes may also create polar functional groups
on some NMs (e.g., (Lee et al., 2007)). Some inorganic (i.e., fullerols) or organic (i.e.,
capping ligands on quantum dots) NMs can be “designed” to be stable in water by
functionalizing their surface with polar groups (e.g., carboxyl, hydroxyl). For example,
quantum dots with organic capping ligands or fullerenes with hydroxyl groups are key
determinants affecting their fate in water (Lecoanet et al., 2004; Lecoanet and Wiesner,
2004; Zhang et al., 2008-b). For acidic carboxyl capping ligands on quantum dots,
aggregation occurs rapidly (i.e., small energy barriers exist) at pH levels near or below
the pH IEP of ~ 1.8; deprotonated (pH > pH IEP ) capping ligand-quantum dots are stable
over a wide range of ionic strengths because of the electrostatic repulsion between
different quantum dots. However, certain cations may selectively complex with capping
ligands (e.g., divalent or trivalent metals with thioglycolate capping ligands) and bridge
quantum dots or neutralize the surface charge (Zhang et al., 2008-b). These mechanisms
involving carboxyl functional groups on quantum dots are similar to those of natural
NMs coated with carboxyl-containing humic-acids (e.g., (Tiller and O'Melia, 1993a;
Chen et al., 2006)). Thus a framework exists for understanding NM interactions with
each other based upon classical colloid theory and the decades of its application to
environmental colloids. What remains unknown is if capping ligands from NMs like
quantum dots release in the environment, how rapidly NMs become functionalized via
biogeochemical processes, how engineered NMs interact with humic-acid like material
(i.e., dissolved organic matter present in all environmental waters), how NMs interact