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