Biomedical Engineering Reference
In-Depth Information
Suspension
Flocculated
Dispersed
Weakly flocculated
Strongly flocculated
(ii)
(ii)
(iii)
FIGURE 3.7
Schematic of three general states of particle dispersion in liquid media. (Adapted from Lewis, J.A.
J. Am. Ceram.
Soc.,
83 [10] 2341-2359 (2000).)
difference between stable (dispersed) suspensions, weakly flocculated suspensions, and
strongly flocculated suspensions.
When additives for suspension stability such as deflocculants and/or other dissolved
species are present in the medium, Equation 3.2 must be altered to accommodate their
effects. This is shown in Equation 3.9 [34]:
V
total
=
V
vdw
+
V
elect
+
V
steric
+
V
struct
(3.9)
where:
V
total
= total potential energy (
V
T
)
V
vdw
= attractive potential energy (van der Waals;
V
A
)
V
elect
= repulsive potential energy (electrostatic interaction between double layers;
V
R
)
V
steric
= repulsive potential energy (steric interactions between particle surfaces)
V
struct
= attractive or repulsive potential energy (nonadsorbed species in solution)
When additives are used, the energy associated with each of these can play a critical role
in the stabilization of the suspension. Figure 3.8 schematically illustrates these mecha-
nisms [29,34]:
• Electrostatic stabilization.
Repulsive electrostatic double-layer forces at the solid/
liquid interface are responsible for the stabilization of the suspension. This
approach, which is what has been considered up to this point and shown in Figure
3.3, is more effective with aqueous media owing to the ready generation of M-OH
2
+
and M-O
−
electrolytes, as shown in Reactions
1
and
2
[19].
• Steric stabilization.
The addition of what usually are long-chain organic species
effectively coat the particles with an adlayer sufficiently thick to (1) overcome van
der Waals attraction between particles and (2) avoid adsorbing on multiple par-
ticles (
bridging flocculation
[36]). Alternatively, short-chain, functionalized, polar
polymers also can achieve the same outcome. The polymer adlayer provides effec-
tive interparticle steric repulsion. This approach can be used with both aqueous
and organic media.
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