Key Concepts for section IV (Electrokinetics and Forces)
1: Debye layer, Zeta potential, Electrokinetics
2: Electrophoresis, Electroosmosis
3: Dielectrophoresis
4: Inter-Debye layer force, Van-Der Waals force
5: Coupled systems, Scaling, Dimensionless Numbers
Goals of Part IV:
(1) Understand electrokinetic phenomena and apply them
in (natural or artificial) biosystems
(2) Understand various driving forces and be able to
identify dominating forces in coupled systems
Motion
of particles
inand
E and
B field.
Motion of (bio)
Particles
in Electric
Magnetic
field
Electrophoresis
Motion of charged particles in an electric field
Dielectrophoresis
Motion of (neutral) particles in an electric field gradient
Magnetophoresis
Motion of magnetic particles (with magnetic dipole) in a magnetic
field
Electric dipole
Q +
d
ẑ
Simple electric
dipole
Q
r
p = Qdzˆ
r r
r⋅p
r
= 0, Φ (r ) =
4πε r 3
net
-Q -
(r >> d )
Dipole in uniform electric field
+
-
-
+
-
+
Dipole in non-uniform electric field
+
-
-
+
-
+
Induced dipole by external field
⎛ 3σ 0 ⎞
Φ in = − E0 ⎜
⎟z
⎝ σ i + 2σ 0 ⎠
Φ out
σo , ε
Induced dipole
E
0
θ
σi, ε
Eo R 3 ⎛ σ i − σ 0 ⎞
= − E0 z + 2 ⎜
⎟ cos θ
{
r ⎝ σ i + 2σ 0 ⎠
external field
1444
424444
3
induced dipole field
σo > σ i
E0
++
+
+
+
++
-- -
σo < σ i
-
E0
--
--
++
+
+
+
++
z
Positive DEP
(σ > σ 0 )
Negative DEP
(σ < σ 0 )
Positive / negative DEP, etc.
+
+ pr - +
+
+
- r +
- p +
+
+
- +
r
E
r
E
Particle moves toward
the high field region.
Particle moves away from
the high field region.
DEP force is independent of the direction of the field.
Motion
of (induced) dipoles
in
an
+r - r +
+ p - p +
+
+
+ electric field +
+
+
r
E
r
E
Nanoparticles : Emerging tools for Bioengineering
Introduction : Use of nanoparticles and vesicles in
modern biotechnology
Image removed due to copyright restrictions.
Photo of EviDots (TM) vials - 490nm to 680nm.
From www.evidenttech.com (Evident Technology)
Nanoparticles : Emerging tools for Bioengineering
Image removed due to copyright restrictions.
Electron microscope photo of Qdot core-shell nanocrystals.
From www.qdots.com (Quantum Dots Corporation)
Introduction 2 : Cancer targeting using nanoparticles
Gao, Cui, Levenson, Chung and Nie, Nature Biotechnology 22, 969 (2004)
Courtesy of Leland W. K. Chung. Used with permission.
The problem of colloid (nanoparticle) stability
The problem of colloid /
nanoparticle stability
Image removed due to copyright
restrictions.
Figure 4 in A. Yethiraj and A. van
Blaaderen. Nature 421, 513
(2003)
M. M. Baksh, M. Jaros, J. T.
Groves, Nature 427, 139
(2004)
Coagulation / Flocculation
Courtesy of J. T. Groves. Used with permission.
Source: Figure 2b in Baksh, M. M., M. Jaros, and J. T. Groves. "Detection of Molecular Interactions
at Membrane Surfaces through Colloid Phase Transitions." Nature 427 (January 8, 2004): 139-141.
Measurement of W (inter-bilayer distance)
in different salt concentration
140
100
Hydrophobic tail
CaCl2
KCl
W (A)
Polar head
(phosphate for the
lecithin layers
used above)
60
W
20
0.1
0.2
conc(N)
0.3
0.4
Interlayer distance in lipid layers, separated by aqueous solution containing varying amounts of electrolyte
as determined by K.J. Palmer and F.O. Schmitt.
Figure by MIT OCW.
Schulze-Hardy Rule
Critical coagulation concentrations for hydrophobic
solutions (millimoles per dm3)
As2S3 (-ve sol)
AgI (-ve sol)
Al2O3 (+ve sol)
LiCl
58
LiNO3
165
NaCl
43.5
NaCl
51
NaNO3
140
KCl
46
KCl
49.5
KNO3
136
KNO3
60
KNO3
50
RbNO3
126
K acetate
110
AgNO3
0.01
CaCl2
0.65
Ca(NO3)2
2.40
K2SO4
0.30
MgCl2
0.72
Mg(NO3)2
2.60
K2Cr2O7
0.63
MgSO4
0.81
Pb(NO3)2
2.43
K2 oxalate
0.69
AlCl3
0.093
Al(NO3)3
0.067
K3[Fe(CN)6]
0.08
1 2 Al2(SO4)3
0.096
La(NO3)3
0.069
Al(NO3)3
0.095
Ce(NO3)3
0.69
Figure by MIT OCW.
Source: “Introduction to Colloid and Surface Chemistry”
By Duncan J. Shaw (Butterworth Heinemann)
Electrostatic
interaction
withininelectrolyte
solution
Interactions
and forces
micro / nanoscale
+ +
+ - -- +
- +
+ + -- +
+
−1
+ +
κ
h
+ +
+ - -- +
- +
+ + -- +
+
+ +
weak or no interaction
+ +
+ +
+
- -- +
+ - -- +
- + - +
- + + - + -+ -+
+
+
+ - + +
+ +
significant repulsive interaction
(inter-Debye layer repulsion)
Van der Waals Forces (attractive forces)
London Dispersion Forces (F. London, 1930)
Van der Waals Forces
h
weak or no interaction
-
+
+
h
Non-polar molecules
(induced dipole)
Attractive interaction
Midplane
φw
φw
ρm
φw
ρo
φ
Electrolyte
Electrolyte
φm
φ
h/2
h
x
λD
Potential distribution resulting from the overlap of double layers from opposing plates.
Figure by MIT OCW.