Size/Shape – Property Relationships of Non - Carbonaceous Inorganic Nanoparticles and their Environmental Implications - Environmental and Human Health Impacts of Nanotechnology - page 89

all nanoparticulate systems. In some systems, chemical processes not included in
the classical models, such as photocatalyzed oxidation, may affect dissolution
(Stouwdam et al. , 2007 ; Aldana et al. , 2001 ).
Other experimental results indicate that small size does not always result
in higher rates of dissolution. In one study of zinc oxides in aqueous systems,
the same mass of nanoparticles and bulk solids dissolved at the same rate, even
though the increased surface area and smaller size of the nanoparticles would
warrant otherwise (Franklin et al. , 2007). On the other hand, the nanoparticles in
the experiment were highly aggregated, which may have lessened surface or size
related effects.
In some cases, dissolution at the nanoscale may be slower. Indeed, studies
on various calcium phosphate minerals (Tang et al. , 2001, 2003, 2004a, 2004b,
2004c, 2005; Tang and Nancollas, 2002) display a phenomenon of self-inhibited
dissolution occurring primarily at the nanoscale, in which dissolution rates dwindle
over time.
To understand one of the means by which inhibited dissolution is possible, it is
useful to consider the opposite process of nanoparticle growth from a solution. The
energy of particle formation, ∆ G form , can be expressed as:

∆

GGG
form

=+ ∆ ∆

v s

where G v is the negative energy term describing the spontaneous tendency of

solute to precipitate as part of a solid particle, and

∆

∆ G s describes the excess free

energy to form a new solid-liquid interface. G v depends upon the degree of satura-

tion of the solvent. Assuming spherical particle morphology, both energy terms are
functions of r , the radius of the particle. For a given level of saturation, there is a
critical radius, r *, above which the magnitude of

∆

∆ G v will be greater than that of

∆ G s and a particle can form (Tang et al. , 2001 ).

An analogous critical radius is believed to exist for dissolution processes. In dis-
solution, which occurs in an undersaturated solution, there is a favourable energetic
driving force for units of the solid particle to become solute. However, the forma-
tion of etch pits can increase the area of the solid-liquid interface, which is ener-
getically disfavoured. For such a system, there is a critical radius (of etch pit) at
which dissolution is energetically allowed. Below this radius, dissolution is inhibited.
Such inhibition phenomena have been observed in a number of systems and par-
ticularly well studied for various biologically relevant calcium phosphates (Tang
et al. , 2003, 2004a, 2004b, 2004c, 2005; Tang and Nancollas, 2002). It is therefore
reasonable to expect the possibility of such inhibited dissolution occurring for
nanoparticles of sparingly soluble compounds, because the nanoparticle dimen-
sions may be below this critical radius. Currently, there is limited data to confi rm
or disprove these expectations.

3.5.2 Effects of Nanoparticle Morphology

Nanoparticles released into the environment will not only vary in size but also in
their morphology, which may strongly affect dissolution. This particularly applies
to cases in which the nanoparticles are crystalline. When nanoparticles of the same