Page 247 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

232 Principles and Methods

chemistry are reviewed and interpreted in the content of particles with at
least one characteristic dimension measuring less than 100 nm. We will
begin with a discussion of the relevant interfacial chemical properties and
relationships between particles in aqueous media.

Physicochemical Interactions
In aqueous environments, the nature of particle surfaces is intimately
linked with the solution conditions (pH, ionic strength, temperature, etc.).
Characterization of nanoparticles is therefore relevant largely within the
context of the characteristics of the solution in which the nanoparticle
is suspended. Those properties of particular interest for environmental
analyses include surface charge, the presence of surface functional
groups, Hamaker constant, and interaction energy with water (wetta-
bility). However, the characterization of materials in the nanometer
regime is particularly challenging due to fact that materials in this size
range fall into a gray area where they may, in many cases, be considered
as either small particles or large solutes.

Brownian motion
Particles, molecules, and ions in fluid environments, regardless of size,
possess a Brownian energy (BR) equal to 1.5 kT [3], where k is the
23
Boltzmann constant (1.38 10 J) and T is the absolute temperature in
degrees Kelvin. Brownian potential energy decays exponentially from
this value (1.5 kT) with distance, with a decay length equal to the respec-
tive particle’s radius of gyration, R g . (The radius of gyration is the root
mean square of mass-weighted distances of all subvolumes in a parti-
cle from its center of mass. It has special interest in particle science
because it can be applied to irregularly shaped particles.) Because the
Brownian energy imparted to a given particle originates from collisions
with (primarily) solvent molecules (e.g., water), it becomes more sig-
nificant with decreasing particle size. In contrast, forces such as grav-
ity or those originating from fluid flow (e.g., drag forces), increase with
increasing particle size. Thus, as a transport mechanism, Brownian
motion has particular significance in determining nanoparticle stabil-
ity and mobility in aqueous systems through its influence on the colli-
sion frequency between particles and with stationary surfaces.

DLVO theory
Particle interactions in aqueous environments are generally assessed
within the context of the classical Derjaguin-Landau-Verwey-Overbeek
(DLVO) theory [4]. The DLVO theory expresses the total interaction