CHEE2940 Lecture 18 - Colloid Stability
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Transcript of CHEE2940 Lecture 18 - Colloid Stability
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CHEE2940: Particle Processing
Lecture 18: Colloid stability This Lecture Covers DLVO and extended DLVO theories Force measurements Effect of interparticle forces on suspension
behaviour Chee 2940: Colloid stability and dispersion
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18.1 INTRODUCTION
Important property of colloidal dispersions is •
•
•
Tendency of the particles to aggregate
Principal cause of aggregation is
The van der Waals attractive forces (and hydrophobic attraction & polymer bridging)
Stability against aggregation is
The EDL repulsive force (and other forces: steric repulsion, hydration)
Chee 3920: Colloid stability and dispersion 1
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18.2 DLVO THEORY OF COLLOID STABILITY
Was independently developed by Deryagin and Landau (1939) in Russia, and Verwey and Overbeek (1948) in the Netherlands.
•
•
•
Explains the effect of salts on stability of colloidal systems (known from the Faraday time).
Involves estimation of the total interparticle interaction energy, VT, from the van der Waals and EDL interactions as
Chee 3920: Colloid stability and dispersion 2
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T vdW dlV V Ve= +
Total Interaction Repulsion Attraction= +
VvdW … van der Waals interaction energy between two particles (Lecture 16)
( )12vdWARV DD
= −
Vedl … electrical double-layer interaction energy between two particles (low potential - Lecture 17)
( ) ( )20 02 expedlV D R Dπεε ψ κ= −
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where D … distance between particle surfaces R … particle radius
A … Hamaker constant 0ψ … particle surface potential
ε0… permittivity of vacuum (8.854×10-12 C J-1 m-1) ε … dielectric constant of solution (=80 for water)
κ … Debye constant For high surface potential 0ψ , the EDL energy is
( ) ( )202 expedlV D R Dπεε γ κ= −
where the reduced potential, γ, is given as Chee 3920: Colloid stability and dispersion 4
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04 tanh4
B
B
k T ezez k T
ψγ
=
z … valency of ions
e… electronic charge (1.602×10-19 C) … Boltzmann constant (= 1.381×10Bk
-23 J/K) T … absolute temperature tanh … hyperbolic tangent function
( ) ( ) ( )( ) ( )
exp exptanh
exp expx x
xx x
− −=
+ −
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Effect of salt concentration and pH on surface forces between a particle and a substrate measured with AFM
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-6
-4
-2
0
2
4
6
0 5 10 15Separation distance, D (nm)
Inte
ract
ion
ener
gy, V
T (x
10-1
9 J)
1.0E+08 2.0E+08 5.0E+08
7.0E+08 1.0E+09 5.0E+09
Debye constant, κ (1/m)
Effect of the Debye constant (salt conc.) on the total interaction energy. A = 6.1x10-20J, ψ0 = -50 mV, T = 25oC, R = 0.1 micron. Chee 3920: Colloid stability and dispersion 7
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Salt concentration determined from the Debye constant
κ (1/m) 1×108 2×108 5×108 7×108 1×109 5×109
CNaCl (M) 0.00189 0.00755 0.0472 0.0925 0.189 4.719
Important properties of the interaction energies
van der Waals energy is almost independent of salt concentration.
•
•
EDL energy strongly depends on salt conc.
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Total interaction energy can be regulated by changing the added salt concentration.
•
•
• There exists an energy (maximum) barrier of
aggregation at low salt concentration.
There exist two local minima in the total energy at high salt concentration.
o Primary minimum (deep) at short distances.
o Secondary minimum (shallow) at long distances.
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Chee 3920: Colloid stability and dispersion 10
-6
-4
-2
0
2
4
6
0 5 10 15Separation distance, D (nm)
Inte
ract
ion
ener
gy, V
T (x
10-1
9 J)
1.0E+08 7.0E+08 5.0E+09
Debye constant, κ (1/m)
Barrier of aggregation
Secondary minimum
Primary minimum
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Critical Coagulation Concentration (CCC)
Is the minimum salt concentration required to produce coagulation of a colloid suspension
•
•
•
Salt concentration lower than the CCC produces stable suspensions
Mathematical description for the CCC:
o Critical coagulation occurs if the barrier of coagulation is reduced to zero, giving
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( ) 0T CCCV D = (The condition of zero barrier)
0CCC
T
D D
dVdD =
=
(The condition of maximum)
( ) ( )202 exp
12TARV D R DD
πεε γ κ= − + −
Solving the above equations gives •
( )202 exp 1
12ARR 0κπεε γ − − = and 1CCCDκ =
Chee 3920: Colloid stability and dispersion 12
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• From Lecture 17: 1/ 2 1/ 222 2 2
0 0
1000 2000A i i A
B B
N e z c N e z ck T k T
κεε εε
∞ = =
∑
( ) [ ] 1/ 22 22
00
20002 exp 1
12A
B
N e z CCCARRk T
πεε γεε
− =
[ ] ( )( )
32 40
2 2 2
2881000exp 2
B
A
k TCCC
A N e zπ εε γ
=
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A = 1×10-19 J
Chee 3920:
Sur
face
pot
entia
l (m
V)
Dependence of critical coagulation on CCC and surface potential and salt valency. The colloids are to be stable above and to left of each curve and
coagulated below and to the right.
CCC (mol/L)
Colloid stability and dispersion 14
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Critical coagulation concentration in mmol/L for hydrophobic colloids (sols)
As2S colloid (-) AgI colloids (-) Al2O3 colloids (+) LiCl 58 LiNO3 165 NaCl 43.5NaCl
51 NaNO3 140 KCl 46 KCl 49.5 KNO3 136 KNO3 60
KNO3 50 RbNO3 126 K acetate
110 AgNO3 0.001
CaCl2 0.65 Ca(NO3)2 2.40 K2SO4 0.30MgCl2
0.72 Mg(NO3)2 2.60 K2Cr2O7 0.63MgSO4 0.81 Pb(NO3)2 2.43 K oxalate 0.69AlCl3 0.093 Al(NO3)3 0.067 K3[Fe(CN)6] 0.08 Al2(SO4)2
0.096 La(NO3)3 0.069
Al(NO3)3 0.095 Ce(NO3)3 0.069
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18.3 EXTENDED DLVO THEORY
DLVO theory considered only two forces: •
•
•
o (vdW & EDL forces: DLVO forces)
Deviation from the DLVO theory has been observed, due to additional (non-DLVO) forces
Non-DLVO forces include: o Hydrophobic forces between hydrophobic
surfaces (long range, up to 100 nm) o Hydration repulsion between hydrophilic
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surfaces (short range, up to 10 nm) o Polymeric bridging attraction (flocculation) o Steric repulsion (due to polymers/surf’tants)
Total interaction energy (force) •
T vdW edl non DLVOV V V V −= + +
• Non-DLVO forces have been determined by
subtracting the DLVO force from the (total) measured force.
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In absence of EDL repulsion: VT = Vvdw + Vsteric VT
Inte
ract
ion
ener
gy Vsteric
Vsteric
VT
In presence of EDL repulsion: VT = Vvdw + Vedl + Vsteric
Vvdw + V
VvdW
Schematic interaction energy for sterically stabilised particles.
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Stabilisation of colloidal systems: Create a total repulsion between particles by • Electrostatic stabilisation - EDL (charge)
repulsion by changing pH or increasing surface potential (via surface cleaning). Steric stabilisation - repulsion by polymer or surfactant adsorption.
•
•
Destabilisation of stable colloidal systems: Create a total attraction between particles by • Surface hydrophobisation by the surfactant
adsorption or deposition. Increasing salt concentration (not practical).
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18.4 FORCE MEASUREMENTS
There are two major types of equipment for measuring surface forces, including
•
•
o Surface Force Apparatus (SFA) o Atomic Force Microscope (AFM)
Force measurements use Hook’s law: xF k= Measurements of separation distance between surfaces are different.
•
o SFA uses the optical (interferometric) principle. It can give the absolute zero separation.
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o AFM uses the piezoelectric calibration: zero separation cannot be precisely determined.
SURFACE FORCE APPARATUS
Measures the surface force between two large (radii ~ 1 cm, nearly flat) mica surfaces.
Major parts: variable stiffness spring for force measurement, spectrometer for distance measurement.
Sensitivity: 1 nN for force & 0.1 nm for distance Chee 3920: Colloid stability and dispersion 23
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SFA designed by Drs. Israelachvili and Tabor at the Cambridge University (UK) in 1978.
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Picture of SFA Mark II (Israelachvili)
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ATOMIC FORCE MICROSCOPE
Measures the surface force between a small surface (AFM sharp tip with R ~ 10 nm or colloidal probe with R ~ 10 µm) and flat surface.
The surfaces are approached and retracted periodically by the piezoelectric tube
Cantilever deflection is measured by the laser reflection on the position-sensitive photodiode system.
Applied voltage vs photodiode voltage is obtained and converted to force vs distance.
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Operating principle of AFM with a sharp tip
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AFM tip can be replaced by a colloid particle
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AFM PicoForce system at ChemEng Chee 3920: Colloid stability and dispersion 31
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Schematic of our AFM PicoForce system
Chee 3920: Colloid stability and dispersion 32
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A colloid probe: a 14 mm particle glued to an
AFM cantilever used in the force measurement Chee 3920: Colloid stability and dispersion 33
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0 40 80 120 160 200
Separation [nm]
-70
-60
-50
-40
-30
-20
-10
0
Forc
e/R
adiu
s [ m
N/m
]
-25
-15
-5
5
0 50 100 150 200Separation distance (nm)
F/R
(mN
/m)
Pure ethanol17% ethanolPure water
0 40 80 120 160 200Separation [nm]
-70
-60
-50
-40
-30
-20
-10
0
Forc
e/R
adiu
s [ m
N/m
]
-25
-15
-5
5
0 50 100 150 200Separation distance (nm)
F/R
(mN
/m)
Pure ethanol17% ethanolPure water
Steps in the force curves are due to nanobubbles of dissolved gases in water
Effect of soluble gases on attraction between hydrophobic surfaces
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1micron1micron
NanobubblesNanobubbles
Chee 3920: Colloid stability and dispersion 35
AFM image of nanobubbles formed at hydrophobic (graphite) surface in water.
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18.5 EFFECTS OF INTERPARTICLE FORCES ON SUSPENSION BEHAVIOUR
Settling rate and final bed structure depend on interparticle forces
Dense sediment bed & high solid volume fraction
Loose sediment bed & low solid volume fraction
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Summary of effect of interparticle forces on suspension
Chee 3920: Colloid stability and dispersion 37