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

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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

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VT = VvdW + Vedl

Total Interaction = Repulsion + Attraction VvdW … van der Waals interaction energy between two particles (Lecture 16) AR VvdW ( D ) = − 12 D

Vedl … electrical double-layer interaction energy between two particles (low potential - Lecture 17)

Vedl ( D ) = 2πεε 0 Rψ 0 exp ( −κ D ) 2

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where D … distance between particle surfaces R … particle radius A … Hamaker constant ψ 0 … particle surface potential -12 -1 -1 ε0… permittivity of vacuum (8.854×10 C J m ) ε … dielectric constant of solution (=80 for water) κ … Debye constant

For high surface potential ψ 0 , the EDL energy is Vedl ( D ) = 2πεε 0 Rγ exp ( −κ D ) 2

where the reduced potential, γ, is given as Chee 3920: Colloid stability and dispersion

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 ezψ 0  4k BT γ= tanh   ez  4k BT  z … valency of ions -19 e… electronic charge (1.602×10 C) -23 k B … Boltzmann constant (= 1.381×10 J/K) T … absolute temperature tanh … hyperbolic tangent function exp ( x ) − exp ( − x ) tanh ( x ) = exp ( x ) + exp ( − x ) Chee 3920: Colloid stability and dispersion

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Effect of salt concentration and pH on surface forces between a particle and a substrate measured with AFM Chee 3920: Colloid stability and dispersion

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6

Interaction energy, VT (x10

-19

J)

4

2

0

-2

Debye constant, κ

(1/m)

1.0E+08

2.0E+08

5.0E+08

7.0E+08

1.0E+09

5.0E+09

-4

-6 0

5 10 Separation distance, D (nm)

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Effect of the Debye constant (salt conc.) on the total interaction -20 o energy. A = 6.1x10 J, ψ0 = -50 mV, T = 25 C, R = 0.1 micron. Chee 3920: Colloid stability and dispersion

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Salt concentration determined from the Debye constant κ (1/m) CNaCl (M)

1×108

2×108

5×108

7×108

1×109

5×109

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. Chee 3920: Colloid stability and dispersion

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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. Chee 3920: Colloid stability and dispersion

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6

4 Interaction energy, VT (x10

-19

J)

Barrier of aggregation

2

Secondary minimum

0

-2

Primary minimum Debye constant, κ

-4

1.0E+08

7.0E+08

(1/m) 5.0E+09

-6 0

Chee 3920: Colloid stability and dispersion

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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 Chee 3920: Colloid stability and dispersion

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VT ( DCCC ) = 0 (The condition of zero barrier)

 dVT  = 0 (The condition of maximum)    dD  D = DCCC AR 2 VT ( D ) = − + 2πεε 0 Rγ exp ( −κ D ) 12 D

• Solving the above equations gives

ARκ 2πεε 0 Rγ exp ( −1) − = 0 and κ DCCC = 1 12 2

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• From Lecture 17: 1/ 2 1/ 2 2 2 2 2 1000 N Ae ∑ zi ci∞   2000 N Ae z c  κ =  =  εε 0 k BT  εε 0 k BT   

AR  2000 N Ae z [CCC ]  2πεε 0 Rγ exp ( −1) =   12  εε 0 k BT 

1/ 2

2 2

2

288π ( εε 0 ) k BT γ [CCC ] = 2 2 2 A N Ae z 1000exp ( 2 ) 2

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4

13

Surface potential (mV)

A = 1×10

-19

J

CCC (mol/L)

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. Chee 3920: Colloid stability and dispersion

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Critical coagulation concentration in mmol/L for hydrophobic colloids (sols)

As2S colloid (-) LiCl 58 NaCl 51 KCl 49.5 50 KNO3 K acetate 110 CaCl2 0.65 MgCl2 0.72 0.81 MgSO4 AlCl3 0.093 Al2(SO4)2 0.096 Al(NO3)3 0.095 Chee 3920: Colloid stability and dispersion

AgI colloids (-) LiNO3 165 NaNO3 140 136 KNO3 RbNO3 126 0.001 AgNO3 Ca(NO3)2 2.40 Mg(NO3)2 2.60 Pb(NO3)2 2.43 Al(NO3)3 0.067 La(NO3)3 0.069 Ce(NO3)3 0.069

Al2O3 colloids (+) NaCl 43.5 KCl 46 KNO3 60 K2SO4 K2Cr2O7 K oxalate

0.30 0.63 0.69 K3[Fe(CN)6] 0.08

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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 Chee 3920: Colloid stability and dispersion

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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) VT = VvdW + Vedl + Vnon− DLVO

• Non-DLVO forces have been determined by subtracting the DLVO force from the (total) measured force. Chee 3920: Colloid stability and dispersion

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Chee 3920: Colloid stability and dispersion

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Interaction energy

In absence of EDL repulsion: VT = Vvdw + Vsteric Vsteric

VT Vsteric

VT

VvdW

Vvdw + V In presence of EDL repulsion: VT = Vvdw + Vedl + Vsteric

Schematic interaction energy for sterically stabilised particles. Chee 3920: Colloid stability and dispersion

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Chee 3920: Colloid stability and dispersion

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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). Chee 3920: Colloid stability and dispersion

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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: F = kx • Measurements of separation distance between surfaces are different. o SFA uses the optical (interferometric) principle. It can give the absolute zero separation. Chee 3920: Colloid stability and dispersion

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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

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SFA designed by Drs. Israelachvili and Tabor at the Cambridge University (UK) in 1978. Chee 3920: Colloid stability and dispersion

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Picture of SFA Mark II (Israelachvili) Chee 3920: Colloid stability and dispersion

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Chee 3920: Colloid stability and dispersion

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Chee 3920: Colloid stability and dispersion

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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. Chee 3920: Colloid stability and dispersion

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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

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Schematic of our AFM PicoForce system Chee 3920: Colloid stability and dispersion

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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

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0

-20

5

-30

F/R (mN/m)

Steps in the force curves are due to nanobubbles of dissolved gases in water

Force/Radius [ mN/m]

-10

-40

-5

Pure ethanol -15

17% ethanol

Pure water

-50

-25 0

-60

50 100 150 200 Separation distance (nm)

-70 0

40

80

120

Separation [nm]

160

200

Effect of soluble gases on attraction between hydrophobic surfaces Chee 3920: Colloid stability and dispersion

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Nanobubbles 1micron AFM image of nanobubbles formed at hydrophobic (graphite) surface in water. Chee 3920: Colloid stability and dispersion

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18.5 EFFECTS OF INTERPARTICLE FORCES ON SUSPENSION BEHAVIOUR

Dense sediment bed & high solid volume fraction

Loose sediment bed & low solid volume fraction

Settling rate and final bed structure depend on interparticle forces Chee 3920: Colloid stability and dispersion

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Summary of effect of interparticle forces on suspension

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