8.8 Properties of colloids Optical property of colloids

Out-class reading: Levine pp colloidal systems lyophilic colloids lyophobic colloids sedimentation Emulsion Gels

1857, Faraday first observed the optical properties of Au sol 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.

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 Prize Germany, Austria, Colloid chemistry (ultramicroscope) Richard A. Zsigmondy (3) Ultramicroscope principle of ultramicroscope

1): Particle size 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 dark sky. Their differences in size are indicated by differences 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: For two colloids with the same diameter: 4) Concentration and size of the particles From: Nobel Lecture, December, 11, 1926

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 Concentration gradient Diffusion coefficient Concentration gradient

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

F A B C D E c1c1 c2c2 ½ x 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:

Perrin calculated Avgadro’s constant from the above equation using gamboge sol with diameter of  m,  = Pa  s. After 30 s of diffusion, the mean diffusion distance is 7.09 cm s -1 L = 6.5  Because of the Brownian motion, osmotic pressure also originates 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 dhdh 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: Altitude distribution

systemsParticle diameter / nm hh O2O km Highly dispersed Au sol m Micro-dispersed Au sol cm Coarsely dispersed Au sol  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.

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

radiustime 10  m 5.9 s 1  m 9.8 s 100 nm16 h 10 nm68 d 1 nm19 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.

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 10 6 g. Centrifuge acceleration: revolutions per minute

For sedimentation with constant velocity 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 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?