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.

Ag sol with different particle size shows different color!

mythical lighting of Beautiful water and golden sands

principle of ultramicroscope 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 For particles less than 0.1 m in diameter which are too small to be truly resolved by the light microscope, under 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 For two colloids with the same concentration: For two colloids with the same diameter: From: Noble Lecture, December, 11, 1926

3.2 Dynamic properties of colloids

1) Brownian Motion: Vitality? 1827, Robert Brown observed that pollen grains executed a ceaseless random motion and traveled a zig-zag 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. For particle with diameter > 5 m, no Brownian motion can be observed. Vitality?

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

x 2) Diffusion and osmotic pressure Fickian first law for diffusion Concentration gradient Diffusion coefficient

Einstein first law for diffusion 1905 Einstein proposed that: f = frictional coefficient For spheric colloidal particles, Stokes’ law Einstein first law for diffusion

A B C D E C1 C2 ½ x F Einstein-Brownian motion 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:

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

dh 3) Sedimentation and sedimentation equilibrium C Gravitational force Buoyant force a a’ b b’ C dh diffusion Mean concentration: (C - ½ dC) The number of colloidal particles:

Diffusion force: The diffusion force exerting on each colloidal particle The gravitational force exerting on each particle:

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: When the particle sediments at velocity v, the resistance force is: When the particle sediments at a constant velocity

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:

For sedimentation with constant velocity Therefore, ultracentrifuge can be used for determination of the molar weight of colloidal particle and macromolecules with aid of ultramicroscope.

rotor light Quartz window bearing To optical system balance cell Sample cell

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. Theodor Svedberg 1926 Noble Prize Sweden 1884-08-30 - 1971-02-26 Disperse systems (ultracentrifuge)