Natural Colloids and Nanoparticles in Aquatic and Terrestrial Environments - Environmental and Human Health Impacts of Nanotechnology - page 132

The stability of colloidal particles in an aquatic environment is often higher than
expected on the basis of zeta potential and ionic strength, which is likely related to
a steric stabilization effect induced by NOM surface coating (Jekel, 1986) and pos-
sibly to the hydration effect explained in Section 4.5.4.1. Direct measurements of
forces between colloidal particles are scant, though some have been performed by
AFM (Assemi et al. , 2004 ; Mosley et al. , 2003 ; Sander et al. , 2004 ). Assemi et al.
(2004) investigated the interaction forces between a goethite coated mica surface
(positively charged) and silica colloidal probe (negatively charged). The adsorption
of humic substances onto a goethite coated mica surface (imparting negative
charge) induces repulsion between the goethite surface and silica colloidal probe.
In addition, a high repulsion force was observed at short separation distances (typi-
cally 2 -3 nm) and was attributed to steric forces induced by the sorption of humic

substances. Mosley et al. (2003) investigated the effect of adsorbed NOM, solution
pH and ionic composition on the force-distance curve between natural colloids
represented by surface fi lm of iron oxides precipitated onto spherical SiO 2 particles.
At low ionic strength, the interparticle forces were dominated by electrostatic
repulsion from the dissociation of functional groups on the NOM. At small separa-
tion distances (

<

10 nm) another repulsive force originating from steric interferences
of NOM molecules, and at shorter distances from the hydration effect, were also
present. At high ionic strength or low pH conditions, steric repulsion forces domi-
nate due to the absence of electrostatic forces (Mosley et al. , 2003 ). Sander et al.
(2004) investigated the effect of adsorbed layer of humic acid on interparticle forces
in natural colloids (iron oxide and alumina) (Sander et al. , 2004 ). Figure 4.10 a shows
the interaction forces, measured by atomic force microscopy at different pH condi-
tions, between a silica particle (4- 5

<

µ m in diameter) and a fl at quartz plate, both

6
5
4
3
2
1
0

Iron oxide

Iron oxide

pH 4.0
pH 6.1
pH 9.2

pH 4.0
pH 6.1
pH 9.0

Humic Acid layer
encountered

1

0

5 0 5 0 5
Separation (nm)
(a)

10 20 30 40 50

Separation (nm)

(b)

Figure 4.10 Interaction forces as a function of separation distance at 0.001 M NaCl and at
different pH conditions for (a) iron oxide coated silica and (b) iron oxide coated silica in the
presence of adsorbed layer of SRHA molecules. (Reprinted with permission from S. Sander,
L.M. Mosley, K.A. Hunter, Investigation of interparticle forces in natural waters: effects of
adsorbed humic acids on iron oxide and alumina surface properties, Environmental Science
& Technology , 38 , 4791-6. Copyright 2004, American Chemical Society.)