Natural Colloids and Nanoparticles in Aquatic and Terrestrial Environments - Environmental and Human Health Impacts of Nanotechnology - page 131

0.5

0.0

80

60

-0.5

40

20

0

0 0 0 0 0 0

Separation (nm)

-1.0

0 20 40

Separation (nm)

60 80 100

Figure 4.9 Force vs separation curve (•) between a standard silicon nitride (Si 3 N 4 ) tip and
a bare mica surface ( ) between a neutral tip and hydrophobic mica surface prepared depos-
iting a monolayer of CTAB on the surface of freshly cleaved mica surfaces. (Reprinted from
Chemical Physics Letters , 375 , 540-6, O. Teschke and E.F. de Souza, Measurements of long-
range attractive forces between hydrophobic surfaces and atomic force microscopy tips.
Copyright 2003, with permission from Elsevier.)

2001 ; Eriksson et al. , 1989; Ruckenstein and Churaev, 1991). It is possible for the
surface of particles dispersed in environmental waters to have some degree of
hydrophobicity and the hydrophobic force has been postulated to contribute to the
aqueous aggregation of clay particles (Zbik and Horn, 2003).

4.5.4.3 Steric Interactions

Adsorption of natural organic matter to colloidal particles is a well known process
in aquatic systems as discussed previously in Section 4.4.3 (Gibson et al. , 2007 ;
Tipping and Higgins, 1982; Wilkinson et al. , 1997a). Adsorbed NOM molecules can
play a very important role in the aggregation and deposition phenomena. In some
cases, adsorbed NOM molecules (e.g. polysaccharides) can induce aggregation by
bridging mechanisms (Section 4.5.4.4), while in other cases NOM molecules such
as fulvic and humic acids can enhance colloidal stability (Wilkinson et al. , 1997a )
by a mechanism known as steric stabilization. The adsorbed molecule chains extend
some distance into the water, giving increased stability to colloidal particles. As
particles approach each other, the adsorbed layer of NOM comes into contact,
resulting in the interaction between these molecules. As these molecules are
hydrated, any interaction will induce hydration repulsive forces as described in the
previous section. The steric stabilization effect increases with the load with NOM
or the thickness of the adsorbed layer.