Download Electrokinetics

By Dan Janiak and Mark Hanna
September 15 2003
Electrokinetics
• Electroosmosis- Mark
• Electrophoresis- Dan
Electric Double Layer
Solid
Liquid
• 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
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
Electroosmotic Flow Profile
• 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
Electroosmosis + and +
• 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
Basics of Electrophoresis
-
+
In the presence of
an electric field,
molecules in a
solution will move.
Medium
Cathode
Anode
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 Basics
D.M. Hawcroft
Electrophoresis
Molecular Structure
• Size
• Shape
• Charges
+
+
+
+
1
+
+
+
+
+
+
+
+
+
+
-
2
-
+
+
-
-
+ + - -
3
+
- + + +
+
-
-
-
-
-
4
-
-
-
-
-
Distribution
Properties of the medium and
buffer solution
Medium
Paper
Cellulose acetate
Gels
Sample molecules must
be solvated, ionized.
Concentration and pH
play a large role in
electrophoresis.
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
Summary
Electrokinetics
Electroosmosis (Surface Interaction)
Electrophoresis (Separation)
Processes occur simultaneously