chee2940 lecture 18 - colloid stability

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

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

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


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= −


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

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

− −=

+ −


Effect of salt concentration and pH on surface forces between a particle and a substrate measured with AFM


-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

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.


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.



-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

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


( ) 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 =


• 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

=


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

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


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


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.


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.

Chee 3920: Colloid stability and dispersion
19

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).


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.


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

SFA designed by Drs. Israelachvili and Tabor at the Cambridge University (UK) in 1978.


Picture of SFA Mark II (Israelachvili)



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.

Operating principle of AFM with a sharp tip


AFM tip can be replaced by a colloid particle


AFM PicoForce system at ChemEng Chee 3920: Colloid stability and dispersion 31

Schematic of our AFM PicoForce system


A colloid probe: a 14 mm particle glued to an

AFM cantilever used in the force measurement Chee 3920: Colloid stability and dispersion 33

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


1micron1micron

NanobubblesNanobubbles


AFM image of nanobubbles formed at hydrophobic (graphite) surface in water.

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


Summary of effect of interparticle forces on suspension


chee2940 lecture 18 - colloid stability

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