Page 293 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

278 Principles and Methods

adsorbed organic layers. As ionic strength decreases and the thickness
of the diffuse double layer increases, the interfacial interactions will be
due primarily to EL interactions, in which the presence of the adsorbed
organic molecules will likely only increase the EL repulsion. Steric inter-
actions are also more likely when the ionic strength is low as the organic
macromolecules may expand under such conditions as a result of
increased electrostatic repulsion between the molecular branches and
with other adsorbed organics.

NOM adsorption and nanoparticle
aggregation
In changing nanoparticle surface chemistry, NOM changes the stabil-
ity and aggregation behavior of the nanoparticle suspension [37, 84, 86,
90]. NOM adsorption affects nanoparticle stability in two principal
ways. First, it modifies the effective interfacial charge and thus the
magnitude and possibly sign of the electrostatic interactions between
surfaces. Second, short-range steric barriers and hydration forces are
produced that prevent interparticle approach and diminish the impact
of attractive van der Waals interactions [87]. The degree to which NOM
adsorption alters nanoparticle stability is dependent on the solution
chemistry and the characteristics of the nanoparticle and the organic
macromolecules. As an example, steric interactions are a strong func-
tion of the molecular weight (an indicator of size) of the NOM; larger
macromolecules result in more substantial steric repulsions. For solu-
+
+
tions composed of monovalent cations like Na and K , NOM adsorp-
tion will generally result in more stable nanoparticle suspensions
(Figure 7.30). For example, Mylon et al. [84] found that the CCC for
NOM-coated hematite nanoscale particles increased from 30 to
107 mM NaCl. This is due to an increase in the nanoparticle surface
charge, as was previously discussed. In this case, adsorption of NOM
to the nanoparticle surface enhances its stability in the presence of
increasing monovalent cation concentrations.
A different interaction behavior sometimes occurs for NOM-coated
2+
particles in the presence of divalent cations such as Ca and Mg . Again
2+
referring to the work of Mylon et al. [84], no significant change in the CCC
(decreased from 2.4 to 1.8) was observed when the NOM-coated nanopar-
ticles were dispersed in a calcium chloride solution. The lack of a change
here may be attributed to conformational changes in the adsorbed organic
layer due to interactions between the Ca and the organic macromole-
2+
cules. This type of interaction however is not universal for all types of
NOM-coated nanoparticles and depends on the characteristics of the
organics. Characteristics and properties that are particularly signifi-
cant in this respect include charge, hydrophobicity, and chemical