8.8 Properties of colloids 8.8.1 Optical property of colloids
Jan 04, 2016
8.8 Properties of colloids
8.8.1 Optical property of colloids
Out-class reading:
Levine pp. 402-405
colloidal systems
lyophilic colloids
lyophobic colloids
sedimentation
Emulsion
Gels
1857, Faraday first observed the optical properties of Au sol
8.8.1 Tyndall effect and its applications
sol solution
Dyndall Effect:
particles of the colloidal size can scatter light.
(1) Tyndall effect
1871, 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.
(2) Rayleigh scattering equation:
The greater the size (V) and the particle number (v) per unit volume, the stronger the scattering intensity.
light with shorter wave length scatters more intensively.
cos1
22
92
21
22
21
22
24
22
0
nn
nn
r
vVII
4
2
cV
KI
Applications
1. Colors of scattering light and transition light: blue sky and
colorful sunset
2. Intensity of scattering light: wavelength, particle size.
Homogeneous solution?
3. Scattering light of macromolecular solution?
4. Determine particle size and concentration?
Distinguishing true solutions from sols
1925 Noble PrizeGermany, Austria, 1865-04-01 - 1929-09-29 Colloid chemistry
(ultramicroscope)
Richard A. Zsigmondy
(3) Ultramicroscope
principle of ultramicroscope
1): Particle size
For particles less than 0.1 m i
n diameter which are too small
to be truly resolved by the ligh
t microscope, under the ultram
icroscope, they look like stars i
n the dark sky. Their differenc
es in size are indicated by diffe
rences in brightness.
The pictures are reproduced from the Nobel Prize report.
Filament, rod, lath, disk, ellipsoid
2) Particle number: can be determined by counting the bright dot in the field of version;
3) Particle shape: is decided by the brightness change when the sol was passing through a slit.
Slit-ultramicroscope
For two colloids with the same concentration:
22
21
2
1
V
V
I
I
For two colloids with the same diameter: 2
1
2
1
c
c
I
I
4) Concentration and size of the particles
From: Nobel Lecture, December, 11, 1926
4
2
cV
KI
8.8.2 Dynamic properties of colloids
(1) Brownian Motion:
1827, Robert Brown observed that pollen grains executed a ceaseless random motion and traveled a zig-zag path.
Vitality?
In 1903, Zsigmondy studied Brownian motion using ultramicroscopy and found that the motion of the colloidal particles is in direct proportion to Temperature, in reverse proportion to viscosity of the medium, but independent of the chemical nature of the particles.
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
x
Fickian first law for diffusion
dx
dcDA
dt
dm
Concentration gradient
Diffusion coefficient
Concentration gradient
1905 Einstein proposed that:
Lf
RT
f
TkD B
For spheric colloidal particles,
rf 6 Stokes’ law
f = frictional coefficient
rL
RTD
6
1 Einstein first law for diffusion
F
A
B
C
D
Ec1 c2
½ x ½ x
x
cc
dx
dc )( 21
)(
2
1
2
1
2
12121 ccxcxcxm
x
ccD
dx
dcD
)( 21
)(
2
1)(21
21 ccxtx
ccD
Dtx 2
r
t
L
RTx
3 Einstein-Brownian motion equation
The above equation 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:
LrM 3
3
4
r
t
L
RTx
3
Perrin calculated Avgadro’s constant from the above equ
ation using gamboge sol with diameter of 0.212 m, = 0.0
011 Pas. After 30 s of diffusion, the mean diffusion distanc
e is 7.09 cm s-1
L = 6.5 1023
Because of the Brownian motion, osmotic pressure also originates
RTV
n
Which confirm the validity of Einstein-Brownian motion equation
(3) Sedimentation and sedimentation equilibrium
diffusion
1) sedimentation equilibrium
Gravitational force
Buoyant force
a a’
b b’
c
dh
Mean concentration:
(c - ½ dc)
The number of colloidal particles:
AdhLdc
c )2
(
Diffusion force: cRT RTdcd The diffusion force exerting on each colloidal particle
cdhL
RTdc
AdhLdc
c
Adfd
)2
(
The gravitational force exerting on each particle:
grf g )(3
40
3
dg ff
ghhRT
LV
c
c))((ln 12
0
2
1 Altitude distribution
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
Heights needed for half-change of concentration
This suggests that Brownian motion is one of the important reasons for the stability of colloidal system.
ghhRT
LV
c
c))((ln 12
0
2
1
2) Velocity of sedimentation
Gravitational force exerting on a particle:
grf g )(3
40
3
When the particle sediments at velocity v, the resistance force is:
rvfvfF 6
When the particle sediments at a constant velocitygF ff
gr
v)(
9
2 02
radius time
10 m 5.9 s
1 m 9.8 s
100 nm 16 h
10 nm 68 d
1 nm 19 y
Times needed for particles to settle 1 cm
For particles with radius less than 100 nm, sedimentation is
impossible due to convection and vibration of the medium.
gr
v)(
9
2 02
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: xa 2
revolutions per minute
r2xMFc
r2xMFc
xvMxMFb2
0r02
dt
dxLfFd
For sedimentation with constant velocity
dxvRT
xM
c
dc)1( 0
2r
)()1(
ln2
21
22
20
1
2
r xxv
cc
RT
M
Therefore, ultracentrifuge can be used for determination of the molar
weight of colloidal particle and macromolecules and for separation
of proteins with different molecular weights.
light
Quartz window
balance cell
bearing To optical system
rotor
Sample cell
1926 Noble Prize
Sweden
1884-08-30 - 1971-02-26
Disperse systems (ultracentrifuge)
Theodor Svedberg
The first ultracentrifuge, completed in 1924, was capable of generating a centrifugal force up to 5,000 times the force of gravity.
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 (血色素 ).
Why does Ag sol with different particle sizes show different color?