Properties of colloids ( praveen suthar )

Small effort to sum up the properties of colloids.

1. PRESENTATION ON PROPERTIES OF COLLOIDS Praveen Suthar & naage ndra dewasi B . p h a r m student E mail : praveensuthar123 @ g m a i l . c o m

2. TYNDALL EFFECT When a beam of light falls at right angles to the line of view through a solution, the solution appears to be luminescent and due to scattering of light the path becomes visible strong in lyphobic colloids while in lyophilic colloids it is quite weak OPTICAL PROPERTIES

3. Explanation   The Tyndall Effect is due to scattering of light by colloidal particles. The colloidal particles first absorb light and then a part of absorbed light is scattered from the surface of the colloidal particles. Maximum scattering intensity being in a plane at right angles to plane of incident light, the path becomes visible when seen from that direction. The particles of pure solvents or true solution are too small to scatter light.

4. Light scattering When a beam of light is directed at a colloidal solution or dispersion, some of the light may be absorbed (colour is produced when light of certain wavelengths is selectively absorbed), some is scattered and the remainder is transmitted undisturbed through the sample.

5. Dark-field microscopy-the ultramicroscope Dark-field illumination is a particularly useful technique for detecting the presence of, counting and investigating the motion of suspended colloidal particles. It is obtained by arranging the illumination system of an ordinary microscope so that light does not enter the objective unless scattered by the sample under investigation. Lyophobic particles as small as 5-10 nm can be made indirectly visible in this way. The two principal techniques of dark-field illumination are the slit and the cardioid methods. 1) In the slit ultramicroscope of Siedentopf and Zsigmondy (1903) the sample is illuminated from the side by an intense narrow beam of light from a carbon-arc source

6. The transmission electron microscope To increase the resolving power of a microscope so that matter of colloidal (and smaller) dimensions may be observed directly, the wavelength of the radiation used must be reduced considerably below that of visible light. Electron beams can be produced with wavelengths of the order of 0.01 nm and focused by electric or magnetic fields, which act as the equivalent of lenses. The resolution of an electron microscope is limited not so much by wavelength as by the technical difficulties of stabilising high-tension supplies and correcting lens aberrations. The useful range of the transmission electron microscope forparticle size measurement is c. 1 nm-5 μ m diameter.

7. KINETIC PROPERTIES

8. There are continuous collisions between the colloidal particles and molecules of dispersion medium which are in constant motion and passes kinetic energy to colloidal particles by striking it from all sides. This results into zigzag movement of colloidal particles. This zigzag movement of colloidal particles is known as Brownian movement BROWNIAN MOTION

9. DIFFUSION

10. SEDIMENTATION

11. VISCOSITY

12. Electrical properties of colloids

13. Zeta potential ( ζ- potential ) Zeta potential is the electrokinetic   potential difference between the dispersion medium and the stationary layer of fluid attached to the dispersed particle. Diagram showing the ionic concentration and potential difference as a function of distance from the charged surface of a particle suspended in a dispersion medium.

14. The significance of zeta potential is that its value can be related to the stability of colloidal dispersions (e.g., a multivitamin syrup). The zeta potential indicates the degree of repulsion between adjacent, similarly charged particles (the vitamins) in a dispersion . For molecules and particles that are small enough, a high zeta potential will confer stability , i.e., the solution or dispersion will resist aggregation. When the potential is low, attraction exceeds repulsion and the dispersion will break and flocculate.

15. Electrophoresis (or Cataphoresis ):   Since the colloidal particles are electrically charged (+ or -)with respect to the dispersion medium, hence on passing electric current through colloidal solution the charged particles move towards oppositely charged electrodes and get discharged to give precipitate. So, this migration of colloidal particles under the influence of electric field is called electrophoresis.

16. Electro-osmosis: molecules of dispersion medium are allowed to move under influence of electric field Coagulation or flocculation: which involves coming together of colloidal particles so as to change into large sized particles which ultimately settle as a precipitate or float on surface.it is generally brought about by addition of electrolytes GOLD NUMBER: Covering up of lyophobic particles by lyophilic particles is known as its protective action and such colloids are called protective colloids. Gold number is defined as amount of protective sol that will prevent the coagulation of 10 ml of a gold solution on the addition of 1 ml of 10% NaCl solution.

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