By Dan Janiak and Mark Hanna

September 15 2003

Electrokinetics

• Electroosmosis- Mark• Electrophoresis- Dan

Electric Double Layer

• These layers form automatically when an electric field is applied to the system

• The attraction between + and – can be used to move the fluid along the channel

• These layers form automatically when an electric field is applied to the system

• The attraction between + and – can be used to move the fluid along the channel

Solid

Liquid

Electroosmosis

• Negative charges evenly distributed on liquid/solid interface driven by E field

• Positive ions attracted to moving negative ions, are pulled along dragging the liquid with them

• Negative charges evenly distributed on liquid/solid interface driven by E field

• Positive ions attracted to moving negative ions, are pulled along dragging the liquid with them

Electroosmotic Flow Profile

• Relatively uniform flow profile compared to pressure driven flow

• Relatively uniform flow profile compared to pressure driven flow

Flow Profiles Compared

• Pressure driven flow is not uniform due to indirect driving force

• Electroosmotic driven flow is more uniform due to a direct driving force on each atom

• Pressure driven flow is not uniform due to indirect driving force

• Electroosmotic driven flow is more uniform due to a direct driving force on each atom

Electroosmosis + and -

+• Generated naturally

when a potential field is applied

• Flow can be controlled with voltage adjustments

• Flat, symmetrical flow profile

+• Generated naturally

when a potential field is applied

• Flow can be controlled with voltage adjustments

• Flat, symmetrical flow profile

-• Sensitive to solution

and surface chemistry• Requires homogenous

fluid and constant surface conditions

• Flow needs to be monitored so it is possible to account for these changes

-• Sensitive to solution

and surface chemistry• Requires homogenous

fluid and constant surface conditions

• Flow needs to be monitored so it is possible to account for these changes

Basics of Electrophoresis+-

Cathode Anode

Medium

In the presence of an electric field, molecules in a solution will move.

Factors influencing electrophoresis

Promoting Factors Retarding Factors Potentially either

Voltage Physical resistance

pH

Current Viscosity Buffer ions

High surface charge

Interactions Buffer additives

Low mass Low surface charge

Buffer concentration

Molecular dissociation

High mass

Molecular association

Non-spherical shape

Electrophoresis, the BasicsD.M. Hawcroft

Electrophoresis

Molecular Structure

• Size

• Shape

• Charges

Distribution1 2 3 4+

++

+

++

+

+

+

+

+

+

+++ + +

+ + ++ +

-

+

-

-

-

-

-

-

-

-

-

-

-

- -

-

-

-

-

-

--

-

-

- -

Properties of the medium and buffer solution

MediumSample molecules must be solvated, ionized.

Concentration and pH play a large role in electrophoresis.

Sample molecules must be solvated, ionized.

Concentration and pH play a large role in electrophoresis.

Paper Cellulose acetateGels

Paper Cellulose acetateGels

Movement of molecules

E * Q E = Strength of applied electric field

Q = Molecular charge

Molecules with greater charge densities move faster and farther than ones with smaller charge densities

Molecules move toward electrodes of opposite polarity

= V / E

Movement of molecules (cont’d)

Restriction of Movement

Size

Shape ( Rod, Elliptical, Cone)

Entanglements

Viscosity

Nature of buffer solution and sample ions

Support medium

Temperature

Nature of buffer solution and sample ions

Support medium

Temperature

Summary

Electrokinetics

Electroosmosis (Surface Interaction)

Electrophoresis (Separation)

Processes occur simultaneously

Electrokinetics

Electroosmosis (Surface Interaction)

Electrophoresis (Separation)

Processes occur simultaneously