As-Produced Intrinsic Physico-Chemical Properties and Appropriate Characterization Tools - Safety of Nanomaterials along Their Lifecycle - page 14

Zeta potential values can be related to the stability of NOAAs dispersion. High
zeta potential values (positive or negative) usually confirm dispersion stability and
small values (positive or negative) indicate potential agglomeration/aggregation of
the system.
1.9 SOLUBILITY/DISPERSIBILITY
According to ISO 7579, solubility is the maximum mass of a nanomaterial that is
soluble in a given volume of a particular solvent under specified conditions. A nano-
object is expected to exhibit greater solubility and faster dissolution than a bigger
material of the same composition. Solubility mainly depends on temperature, pres-
sure, and pH of a solvent. Dissolution of a NOAA can have a great effect on its
behavior, which definitely affects the biological and environmental distribution of
the material (Oyabu et al. 2008). Although changes in particle size can provide some
indications of dissolution, the use of mass spectroscopies, such as ICP-MS, that
monitor the concentration of dissolved NOAAs is preferred.
For a nanomaterial, it can be difficult to distinguish between when it is dispersed
and when it is dissolved because of the small particle size. It is crucial to recognize
that solubility and dispersibility are two different phenomena, and it is important to
differentiate between them.
Dispersibility is described as the degree to which a solid material is uniformly
distributed in another material (a dispersing medium), and the resulting dispersion
remains stable. In general, the main difference between solubility and dispersibility
is that solubility requires the molecules of a solid phase to be strongly disassociated
by the process, while there is no significant disassociation involved in a phase that
has been dispersed into another. The influence of dispersibility on toxicity results is
not yet fully understood (Powers et al. 2009).
1.10 CONTROL OF PROPERTIES BY SURFACE
FUNCTIONALIZATION
Two materials, nanoGEM.SiO 2 .FITC and nanoGEM.Ag_50.EO, were used to
exemplify characterization methods in the previous sections. Another 16 nanoma-
terials, all of roughly spherical shape, were characterized by the same methods
of the nanoGEM project. Three compositions (SiO 2 , ZrO 2 , and Ag) with primary
particle sizes from 7 nm to 134 nm (determined by TEM) were functionalized by
organic molecules to control the surface chemistry. Relatively short molecules
mainly aimed at surface charge reversal: trioxadecanoic acid, amino, phosphate, or
citrate end groups. Relatively uncharged polymers aimed to impart steric function-
alization: polyethylene glycols (PEG), polyvinylpyrrolidones (PVP), and ethylene
oxides (EO). A combination of both mechanisms was tested using strongly charged
acrylic polymer on ZrO 2 . The characterization substantiated the successful control
of size, composition, surface charge, and surface reactivity (Table 1.2). Full charac-
terization results including the generation of particle-free reference samples, assays
of reactivity, and approaches to the spectroscopy of impurities are accessible online
(Hellack et al. 2013).