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8.8 Properties of colloids
1) Optical properties
3.1 Tyndall effect
1) Tyndall effect
1857, Faraday first observed the optical properties of
Au sol
1971, Tyndall found that when an intense beam of light
is passed through the sol, the scattered light is
observed at right angles to the beam.
sol
solution
Dyndall Effect:
particles of the colloidal
size can scatter light.
Rayleigh scattering equation:
The greater the size and
the particle number per
unit volume, the stronger
the scattering intensity.
light with shorter wave
length scatters more
intensively.
I 
24 A V
I K
2
2
4
CV
4
2
 n n 

 n  2n 



2
1
2
1
 K 'Cr 3
2
2
2
2
2
Ag sol with different particle size shows different color!
I K
CV
4
 K 'Cr 3
mythical lighting of Beautiful water
and golden sands
2) application
Richard A. Zsigmondy
1925 Noble Prize
Germany, Austria,
1865-04-01 - 1929-09-29
Colloid chemistry
(ultramicroscope)
principle of ultramicroscope
For particles less than 0.1
m in diameter which are
too small to be truly
resolved
by
microscope,
the
under
light
the
ultramicroscope, they look
like stars in the sky. Their
differences in size are
indicated by differences in
brightness.
2) Distinguishing solutions from sols
3) The number of particles can be counted and it is also
possible to decide the shapes of the particles roughly.
Slit-ultramicroscope
Filament, rod, lath, disk, ellipsoid
4) Detect concentration and size of the particles
3
1
3
2
For two colloids with the same
concentration:
I1
r

I2
r
For two colloids with the same
diameter:
I1
c1

I2
c2
From: Noble Lecture, December, 11, 1926
3.2 Dynamic properties of colloids
1) Brownian Motion:
1827, Robert Brown observed that pollen grains
executed a ceaseless random motion and traveled a zigzag path.
In 1903, Zsigmondy studied
Brownian
motion
using
ultramicroscopy and found that
the motion of the colloidal
particles is in direct proportion to
T, in reverse proportion to
viscosity of the medium, but
independent of the chemical
nature of the particles.
Vitality?
For particle with diameter > 5
m, no Brownian motion can be
observed.
Wiener suggested that the Brownian motion arose
from molecular motion.
Although motion of molecules can not be observed
directly, the Brownian motion gave indirect evidence
for it.
Unbalanced collision from
medium molecules
2) Diffusion and osmotic pressure
Fickian first law for diffusion
x
dm
dC
  DA
dt
dx
Concentration gradient
Diffusion coefficient
1905 Einstein proposed that:
kT RT
D

f
Lf
f = frictional coefficient
For spheric colloidal particles,
f  6r
RT 1
D
L 6r
Stokes’ law
Einstein first law for diffusion
C
C1
A
D
½x
B
1
1
 1

m   xC1  xC2    x(C1  C2 ) 
2
2
 2

(C  C2 )
 dC 
D
D 1
x
 dx 
x  2 Dt
C2
E
½x F
 dC  (C1  C2 )


x
 dx 
(C1  C2 )
1
D
t   x(C1  C2 )
2
x
x
RT t
L 3r
Einstein-Brownian motion equation
x
RT t
L 3r
suggests that if x was determined using ultramicroscope,
the diameter of the colloidal particle can be calculated. The
mean molar weight of colloidal particle can also be
determined according to:
4 3
M  r L
3
Perrin calculated Avgadro’s constant from the
above equation using gamboge sol with diameter of
0.212 m,  = 0.0011 PaS. After 30 s of diffusion,
the mean diffusion distance is 7.09 cm s-1
L = 6.5  1023
Which confirm the validity of Einstein-Brownian motion
equation
Because of the Brownian motion, osmotic pressure also
originates
n
  RT
V
3) Sedimentation and sedimentation equilibrium
1) sedimentation equilibrium
diffusion
Buoyant
force
Mean concentration:
(C - ½ dC)
Gravitational
force
b’
b
a
dh
C
a’
The number of
colloidal particles:
dC
(C 
) AdhL
2
Diffusion force:
  CRT
d  RTdC
The diffusion force exerting on each colloidal particle
Ad
RTdC
fd 

dC
CdhL
(C 
) AdhL
2
The gravitational force exerting on each particle:
fg 
4 3
r (    0 ) g
3
f g  fd
C1 LV
ln

(    0 )( h2  h1 ) g
C2 RT
Heights needed for half-change of concentration
systems
Particle diameter / nm
h
O2
0.27
5 km
Highly dispersed Au sol
1.86
2.15 m
Micro-dispersed Au sol
8.53
2.5 cm
Coarsely dispersed Au sol
186
0.2 m
This suggests that Brownian motion is one of the
important reasons for the stability of colloidal system.
2) Velocity of sedimentation
Gravitational force exerting on a particle:
4 3
f g  r (    0 ) g
3
When the particle sediments at velocity v, the resistance
force is:
f F  fv  6rv
When the particle sediments at a
constant velocity
2 r 2 (   0 )g
v
9

fF  fg
Times needed for particle to settle 1 cm
radius
time
10 m
5.9 s
1 m
9.8 s
100 nm
16 h
10 nm
68 d
1 nm
19 y
For particles with radius less than 100 nm, sedimentation is
impossible due to convection and vibration of the medium.
3) ultracentrifuge:
Sedimentation for colloids is usually a very slow process.
The use of a centrifuge can greatly speed up the process by
increasing the force on the particle far above that due to
gravitation alone.
1924, Svedberg invented ultracentrifuge, the r.p.m of which
can attain 100 ~ 160 thousand and produce accelerations of
the order of 106 g.
Centrifuge acceleration:
a  x
2
Fc   xM
2
Fb   xM 0  MV 0 x
2
2
dx
Fd  Lf
dt
For sedimentation with constant velocity
dC M 2 x

(1  V 0 )dx
C
RT
C2
C1
M 
(1  V 0 ) 2 ( x22  x12 )
2 RT ln
Therefore, ultracentrifuge can be used for determination of
the molar weight of colloidal particle and macromolecules
with aid of ultramicroscope.
rotor
light
Quartz
window
balance
cell
Sample
cell
bearing To optical
system
The first ultracentrifuge,
completed in 1924, was capable
of generating a centrifugal force
up to 5,000 times the force of
gravity.
Theodor Svedberg
1926 Noble Prize
Sweden
1884-08-30 - 1971-02-26
Disperse systems
(ultracentrifuge)
Svedberg found that the size and
weight of the particles determined their
rate of sedimentation, and he used this
fact to measure their size. With an
ultracentrifuge,
he
determined
precisely the molecular weights of
highly complex proteins such as
hemoglobin.