Colloid chemistry

Lecture 8: electrokinetic phenomena

Diffuse Layer:Also called Electrical Double Layer:Ionic concentration not the same as in the bulk solution; there is a gradient in concentration of ions outward from the particle until it matches the bulk

Stern Layer: Rigid layer of ions tightly bound to particle; ions travel with the particle

Plane of hydrodynamic shear: Also called slipping plane: Boundary of the Stern layer: ions beyond the shear plane do not travel with the particle.

Particle surface

Characteristics of Surface Charge: Definitions

Zeta () potential: the electrical potentialthat exists at the slipping plane

Characteristics of Surface Charge: Definitions

diffuse double layer

St

o

Characteristics of Surface Charge: Definitions

Characteristics of Surface Charge

Electrokinetic phenomena result from the differential movement of two phases where the interface is an electrical double layer. The region containing the double layere issheared at some distance from the solid surface creating a thin film associated with the solid. The electrical potential at the shearing plane is the zeta potential.

Positive particle with negative ion atmosphere

St

shear plane

x

Electrokinetic phenomena

surface charge density (amountof surface charge determining

ions adsorbed ) plotted against ion concentration c

Plateau: maximum amount adsorbed (max)at c* implies maximum surface charge

density (max) which, in turn impliesmaximum surface potential (omax)

surface potential oplotted against the

electrolyte concentrationc in the bulk solution

distance x from thesolid surface

towards the bulk solution

zeta potential plotted against the

electrolyte concentrationc in the bulk solution

surface potential oStern potential St

zeta-potential

*

% o

Elektrokinetic phenomena; the electrokinetic (zeta, ) potential

Electrokinetic phenomena

A solid surface in contact with a solution of an electrolyte usually carries an electric charge, o. This gives rise to an electric potential, o, at the surface, and a decreasing potential, , as we move through the bulk solution away from the surface, and in turn this effect the distribution of ions in the liquid.

Two regions are of primary importance: the Stern layerimmediately adjacent to the surface where ion size is important; and outside this region there is a diffuse layer.

Because of difference in charge between the diffuse layer and the solid suface, movement of one relative to the other will cause charge separation and hence generate a potential difference, or alternatively, application of an electrical potential will cause movement of one relative to the other.The relative movement of the solid surface and the liquid occurs at a surface of shear. The potential at the shear planeis known as the zeta () potential and its value can be determined by measurement of electrokinetic phenomena. Zeta potential is almost identical with the Stern potential thus gives a measure of the potential at the beginning of the diffuse layer.

Shear planeShear plane

Electrokinetic Phenomena

Isoelectric point

Zeta Potential Effect of pH

OH-H+

+-+-+-+-+-+-+

+-+-+-+-+-+-+

ShearPlane

ShearPlane

ShearPlane

Low pH: not enough OH- so

there is an excess positive charge.

+-+-+-+-+-+-+

High pH: not enough H+ so there is an excess negative

charge.

There is a point at which there are

enough OH- and H+ions to completely

balance out the surface charge

Zeta Potential Effect of pH

+-+-+-+-+-+-+

+-+-+-+-+-+-+

+-+-+-+-+-+-+

pH

-

(

m

V

)

+ Isoelectric Point (IEP):pH at which = 0

-

(

m

V

)

+

pH

Zeta Potential Effect of pH and ion concentration

Isoelectric point of several common proteins

Electrophoresis a suspended, charged particle moves as a result of an applied electrical field

Sedimentation potential an electrical potential created by the movement of charged particles through a liquid by gravity

Electrosmosis a liquid flows along a charged surface when an electric field is applied parallel to the surface

Streaming potential an electric potential created when a liquid is forced to move along a charged surface

Electrokinetic phenomena

Elektrokinetic phenomena

Electrophoresis - Movement of particle in a stationary fluid by an applied electric field.

Electroosmosis - Movement of liquid past a surface by an applied electric field

Streaming potential - Creation of an electric field as a liquid moves past a stationary charged surface

Sedimentation potential - Creation of an electric field when a charged particle moves relative to stationary fluid

Elektrokinetic phenomena Electrophoresis - Movement of particle in a stationary

fluid by an applied electric field. Electroosmosis - Movement of liquid past a surface by

an applied electric field Streaming potential - Creation of an electric field as a

liquid moves along a stationary charged surface Sedimentation potential - Creation of an electric field

when a charged particle moves relative to stationary fluid

the movement of a charged particle through a liquid under theinfluence of an applied potential difference (electric current)

Electrophoresis

Voltage source

Moving boundary electrophoresis

charged colloidal particles

Paper electrophoresis, continuous separation

sample buffer solution

(Horizontal electrophoresis)

support media: filter paper; cellulose acetate;

gel media: starch gels; polyacrylamide gels; agarose gels; polyacrylamide-agarose gels

Zone electrophoresis (from1950)Zone electrophoresis (from1950)

Schematic of capillary electrophoresis

Detector

Measurement of potential by microelectrophoresis

electrophoreticmobility(V/cm)

m/s)( [ xE

elfvelfu

]==

relfu

=

Smoluchowskiequation

relfu

23=

Hckelequation

(big particlethin double layer)

(small particlethick double layer)

m/s [ tdxd

elfv ]==

/cmV [ UxE ]== lelectric field

strength

electric driftvelocity

The movement of a liquid relative to an immobile charged surfaceof a capillary tube under the influence of an electric field

Electroosmosis

ElectroosmosisMovement of liquid past a surface by an applied electric field

steady state conditions:E = streaming potentiali = flow current

i

glass capillary

Streaming potentialcreation of an electric field as a liquid moves along

a stationary charged surface

glass capillary glass capillary pressure pressure

E = sedimentation potential

EE

Sedimentation potential (Dorn-effect) creation of an electric field when a charged particle

moves relative to stationary fluid

Equipment for Measuring Zeta Potential Equipment for Measuring Zeta Potential

Electrophoresis - Zeta Mark 21;particles ~ 0.10 - 300 m

Electroacoustic - Colloidal Dynamics Acoustosizer; powders/slurries < 10nm - up to 50 weight%

Equipment for Measuring Zeta Potential Equipment for Measuring Zeta Potential

Photocorrelation Spectroscopy - Brookhaven Zeta Plus;dilute suspensions; < 5 m; light scattering technique

Equipment for Measuring Zeta Potential Equipment for Measuring Zeta Potential

Streaming Potential Paar Physica EKA; granular particles, surfaces

Equipment for Measuring Zeta Potential Equipment for Measuring Zeta Potential

Normal protein patterns

Abnormal protein patterns Abnormal protein patterns

Diagnostic applicationsof electrophoresis

Proteins are essential for metabolismidentified by electrophoresis based onsize, shape, and charge of particles.

Applications: clinical diagnosis.