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ZETA POTENTIALZETA POTENTIAL

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OutlineOutline

1) Charge Development at Solid Interfaces2) Electrical Double Layers

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Origin of Surface ChargeOrigin of Surface Charge

Immersion of some materials in an electrolyte solution. Twomechanisms can operate.

(1) Dissociation of surface sites.

OH H

OH

OH OH

OO

O OO O

OO O

2

OH

sorpt on o ons rom so ut on.

M

OO O

+ + +

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Origin of Surface ChargeOrigin of Surface Charge

lattice cations or anions on surface, eg. AgI, BaSO4,

(4) Surface charge due to crystal lattice defects, eg. lattice

HO O O O OH

Si

HO O

Al

O

Si

O

Si

OH

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Other Surfaces with ChargeOther Surfaces with Charge

made of a self organizing

lipid bi-layer. Most of the

-

negative charged.

Oil dro lets develo

Bubbles develop negative

negative charge.

charge since cationic species

are more mobile, than anionic

species and accumulate to the

n er ace.

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Electrical Double LayerElectrical Double Layer

- Water moleculesx

Distance from Surface

+

-

-

-

+

+

-

-Surface

Counter-Ions

--

0

otent a

Co-Ions

Helmholtz 100+ ears a o ro osed that surface char e

is balanced by a layer of oppositely charged ions

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GouyGouy--Chapman Model 1910Chapman Model 1910--19131913

x--

Distance from Surface

+

-

-

+ +

--

-

--0

Assumed Poisson-Boltzmann distribution of ions from surface

ions are point charges

ions do not interact with each other

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Stern (1924) / Grahame (1947) ModelStern (1924) / Grahame (1947) Model

Diffusion layer-

-

x

++

-

-

+ +

-

-

-

+

--

-

Shear Plane Gouy Plane Bulk SolutionStern Plane

-

-

0

Gouy/Chapman diffuse double layer and layer of adsorbed

-

charge

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Salt AdditionSalt Addition

Diffusion layer-

-

x

Original Curve

++

-

-

++

-

-

-

+

--

-

+

Shear Plane Bulk SolutionStern Plane

-

-

0+

Gouy Plane

-

Salt will not effect the value of

but will effect the value of

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Salt AdditionSalt Addition

+Diffusion layer-

-

x=0

Original Curve

+

+

+

-

-

-

+

++

--

-

+

+ + --

+

Shear Plane Bulk Solution

-

-0

Gouy Plane

Stern Plane

.

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Debye Length: EDL ThicknessDebye Length: EDL Thickness

Debye-Hckel parameter () describes the decay length

1

2

0

n

i

ii

r

ZC

kT

e

Zi = electrolyte valence

e = elementary charge (C)

Ci = ion concentration (#/m3)n = number of ions

=r

0 = permittivity of a vacuum (F/m)

k = Boltzmann constant (J/K) = temperature

-1

Deb e len th has units o len th

Z P i l Eff f H

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Zeta Potential Effect of pH

OH-H+

++Shear+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

-+

-

+

-+

-

+

-+

-

+

ShearPlane

ShearPlane

-

+

-

+

Low pH: Not-

High pH: Not+

There is a point that-

there is a excess

positive charge.

is a excess negative

charge.

and H+ to completely

balance out the

sur ace c arge

Z t P t ti l Eff t f H

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+

-

+

-

+

-

+

+ Isoelectric Point (IEP):-

+

-

+

-

+V)

+

-

+

-

pH(

+

-

+

-

+

-

+

-

+

-

+-

+

-+

-

+

-

+

-

+-

Z P i l Eff f H

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Question: What will happen to a mixed suspension of Alumina and

3 4 ,

ii

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Zeta PotentialZeta Potential

Increasing50

---- ----

. .ZetaP

20

30

tential( 0

10

V)

-20-10

-50

-40

-

Alumina

pH1 2 3 4 5 6 7 8 9 10 11

Specific AdsorptionSpecific Adsorption

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by electrostatics Have the abilit to shift the isoelectric oint

Can reverse zeta potential

Types of Specific Adsorption Ions Multivalent ions: Ca+2, Mg+2, La+3, hexametaphosphate,

sodium silicate

-

Polyelectrolytes

+

++

+2+

+

+

+ +

+

+

- - - - -

S ifi Ad tiS ifi Ad ti

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PO 3-

Ca2+

pH

Multivalent anions shift IEP to left (apatite supernatant)

Amankonah and Somasundaran, Colloids and Surfaces, 15, 335 (1985).

El t ki ti PhEl t ki ti Ph

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Electrokinetic PhenomenaElectrokinetic Phenomena

E ectrop ores s

Movement of particle in a stationary fluid by an applied

e ec r c e .

Movement of liquid past a surface by an applied electric field

Streaming Potential

charged surface

Sedimentation Potential

Creation of an electric field when a charged particle moves

relative to stationary fluid

Zeta Potential MeasurementsZeta Potential Measurements

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Electrophoresisdetermined b the rate of diffusion electro hroretic

mobility) of a charged particle in a applied DC electric field.

PCS determined by diffusion of particles as measured by photoncorrelation spectroscopy (PCS) in applied field

Acoustophoresis determined by the potential created by a

particle vibrating in its double layer due to an acoustic wave

Streaming Potential determined by measuring the potential created as a fluidmoves past macroscopic surfaces or a porous plug

Zeta Potential Meas rementsZeta Potential Meas rements

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assumed a >> 1 = Debye length

a = particle radiuse ectr ca ou e ayer t c ness muc sma er t an part c e

v = velocity

r= media dielectric constant0 = permittivity of free space

=

0rv

= medium viscosity

E = electric field

0r E = e ectrop oret c mo ty


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Henry Formula (1931)ex anded for arbitrar a

assumed E field does not alter surface charge density (o)

- low 0

r = media dielectric constant

0 = permittivity of free space)a(f2

v 10r

= zeta potential = medium viscosity

E = electric field)a(f

3

21

0r

E = electrophoretic mobility

Hunter, Foundations of Colloid Science, p. 560


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= Debye parametera = particle radius

= zeta potential1

a

pE 0r

= Potential over ca illar VE

r= media dielectric constant

0 = permittivity of free space (F/m) = medium viscosity (Pas)KE= solution conductivity (S/m)

p = pressure rop across cap ary a

Equipment for Measuring Zeta PotentialEquipment for Measuring Zeta Potential

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Electrophoresis - Zeta Mark 21particles ~ .10 microns- 300 m


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Electroacoustic - Colloidal Dynamics Acoustosizer

owders/slurries < 10nm - u to 50 wei ht%


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-

msuspensions

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tream ng otent a aar ys ca granu ar

particles, surfaces

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