8.8 properties of colloids 8.8.1 optical property of colloids
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8.8 Properties of colloids8.8.1 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 sol8.8.1 Tyndall effect and its applications Dyndall Effect: particles of the colloidal size can scatter light.(1) Tyndall effect1871, 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 Colors of scattering light and transition light: blue sky and colorful sunset Intensity of scattering light: wavelength, particle size. Homogeneous solution? Scattering light of macromolecular solution?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 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, ellipsoid2) 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 pressureFickian first law for diffusionConcentration gradientDiffusion coefficientConcentration gradient
1905 Einstein proposed that:For spheric colloidal particles,Stokes lawf = frictional coefficientEinstein first law for diffusion
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 Avgadros constant from the above equation using gamboge sol with diameter of 0.212 m, = 0.0011 Pas. After 30 s of diffusion, the mean diffusion distance is 7.09 cm s-1 L = 6.5 1023 Because of the Brownian motion, osmotic pressure also originatesWhich confirm the validity of Einstein-Brownian motion equation
(3) Sedimentation and sedimentation equilibriumdiffusion1) sedimentation equilibriumMean concentration: (c - dc)The number of colloidal particles:
Diffusion force: The diffusion force exerting on each colloidal particleThe gravitational force exerting on each particle:Altitude distribution
Heights needed for half-change of concentration This suggests that Brownian motion is one of the important reasons for the stability of colloidal system.
systemsParticle diameter / nmhO20.275 kmHighly dispersed Au sol1.862.15 mMicro-dispersed Au sol8.532.5 cmCoarsely dispersed Au sol1860.2 m
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 particles to settle 1 cm For particles with radius less than 100 nm, sedimentation is impossible due to convection and vibration of the medium.
radius time10 m5.9 s1 m9.8 s100 nm16 h10 nm68 d1 nm19 y
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:revolutions per minute
For sedimentation with constant velocityTherefore, ultracentrifuge can be used for determination of the molar weight of colloidal particle and macromolecules and for separation of proteins with different molecular weights.
1926 Noble PrizeSweden 1884-08-30 - 1971-02-26 Disperse systems (ultracentrifuge) 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 ().
Why does Ag sol with different particle sizes show different color?