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EVEN 3321
Fall 2011EVEN 3321
Objectives
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1. To understand what colloids are & why they are important in environmental engineering.
2. To understand the electric double layer theory ofcolloidal surface charge.
3. To understand thedifference between electrostatic repulsive forces & van der Waals’ attractive forces between colloidal particles.
� Primarycharge must be balanced by counter ions nearthe surface & in solution.
� (see next slide)
� Result is an electric double layer:
� Fixed or Stern layer of counter ions
� Diffuse layer of a mixture of charged ions.
� (see next slide)
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Electrical double layer of negatively charged colloid
Surface charge
(or primary charge)
BACK
11
Electric double layer theory (cont.)
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� Fixed & diffuse layers are separated by a shear surface.
� The fixed layerwill movewith colloid if it is subjected to anelectric field.
� (see next slide)
� Counter-ions in fixed layerare attracted electrostatically.
� However, counter ions can diffuse away from fixed layerdue to Brownian motion.
� (see next slide)
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Electrical double layer of negatively charged colloid
Surface charge
(or primary charge)
BACK
13
Electric double layer theory (cont.)
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� Competing forces of electrical attraction & diffusion(due to concentration difference) spread charge overthe electrical double layer.� Conc. of counter ions is greatest at surface & decreases
with distance from surface.� (see next slide)
� The primarycharge produces an electric potentialbetween the surface & the solution.� Theelectric potential is greatest at the surface &
decreases with distance from the surface.� (see next slide)
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Electrical double layer of negatively charged colloid
Surface charge
(or primary charge)
BACK
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Electric double layer theory (cont.)
� As two negatively-charged colloids come closer (r smaller), the electrostatic repulsive force between thetwo primary charges (same sign) increases (Frepel
� (see next slide)
1/r2).
� The electrostatic repulsive forces are counteracted by anintermolecularattractive force.
� Theattractive “van der Waals’ force” decreases rapidlywith distance from surface (-Fattract
� (See next slide)
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1/r6).
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Effect separating distance between colloidson forces of interaction between them
BACK
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Electric double layer theory (cont.)� Weakvan derWaals’ intermolecularattractive forces arisewhen
moleculesare in veryclose proximity (a few angstroms – 10-10m).� “Synchronized” induced dipoles result in weak electrical attraction
between molecules:
• If two colloids can be brought sufficiently close so van der Waals’ forces are greater than electrostatic repulsive forces, the two colloids will coagulate together.
o (see next slide)
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Effect separating distance between colloidson forces of interaction between them
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Electric double layer theory (cont.)
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� Todestabilize & coagulate colloidal particles:
� Need to provide kinetic energy (by stirring) to overcome theenergy barrier, or
� Reduce the energy barrier by some means.� (see next slide)
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Effect separating distance between colloidson forces of interaction between them
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Electric double layer theory (cont.)
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� One way to decrease energy barrier is to increase ionconcentration in solution (high ionic strength).
� This decreases thickness of the electric double layer.� (see next slide)
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Effect of ionic strength on energy barrier thatprevents coagulation of colloids
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Colloid electrokinetic properties� To predict conditions that will destabilize colloids, it is
useful to estimate their surface charge.
� The surface charge of colloids can be estimated by experimentally measuring their electrophoretic mobility (essentially theirvelocity in an applied electric field).
+ E -
V
v
colloid with negative surface charge
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Instrument for measuring electrophoretic mobility &
zeta potential
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Colloid electrokinetic properties
(cont.)
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� A colloid’selectrophoretic mobility is directly related to itszeta potential.
� (see next slide)
� A colloid’s surface charge (coulombs/m2) can be estimatedfrom it’s zeta potential.
� Zeta potential measurementsare used to characterize effectiveness of lowering energy barrier between colloids:
� by adding electrolyte
� by adjusting pH
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Electrical double layer of negatively charged colloid
Surface charge
(or primary charge)
BACK
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Effect of ionic strength on energy barrier thatprevents coagulation of colloids
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“Point of zero charge” & pHpzc
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� The pzc “point of zero charge” or “isoelectric point”occurs when the colloid surface charge is zero.
� Surface charge changes with pH:
� The pH at point of zero charge is called pHpzc.
� Colloidsare generally least stable (i.e., tend tocoagulate readily) at pHpzc.
� (see next slide)
Effect of pH on surface charge of clay, iron, &
aluminum colloids
pHpzc30
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Colloid destabilization &
coagulation
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� Destabilizing colloids allows them to coagulate into largerparticles that can be removed by settling.
� Four basic mechanisms for coagulating colloids:
� Electrical double layer compression.
� Charge neutralization.
� Entrapment in precipitate.
� Interparticle bridging.
(1) Electrical double layer
compression� High electrolyteconcentration:
increases concentration of ions in double layer
decreases double layer thickness
decreases energy barrier
increases colloidal coagulation
� Ions with higher charge (e.g., Al3+) are moreeffectivethan ions with lowercharge (e.g., Na+).
� (see next slide)
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Effect of ionic strength on energy barrier thatprevents coagulation of colloids
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“alum”
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(2) Charge neutralization
5+
� Addition of hydrophobic moleculesof oppositechargethatcanadsorbonto the colloids: