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SURFACE POTENTIAL

SURFACE POTENTIAL. Why should we be interested in colloidal system? 1. First reason – PURELY EPISTEMOLOGICAL If we consider the three states of matter- gas, liquid and solid- we can observe colloidal system in all possible combination. 2 . Second reason – ANTHROPOLOGICAL

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SURFACE POTENTIAL

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  1. SURFACE POTENTIAL

  2. Why should we be interested in colloidal system?1. First reason – PURELY EPISTEMOLOGICAL If we consider the three states of matter- gas, liquid and solid- we can observe colloidal system in all possible combination.

  3. 2. Second reason – ANTHROPOLOGICAL Life processes involve the control and transformation of colloidal assemblies and many diseases are associated with malfunction at the colloidal level

  4. DISPERSION COLLOIDS Dispersion colloidsmay be stabilized by charging the surfaces leading to electrostatic repulsion; particles then will keep distances of at least several nanometers. Charging occurs by adsorption of ions or surface hydrolysis. The counter ions are loosely bound in a diffuse cloud:

  5. The thickness of the counter ion cloud can be estimated by an adopted Debye-Hückel -theory. The ion clouds with their net charge stabilize the colloid against agglomeration; however, van-der-Waals forces always lead to an additional attractive interaction. The combination of both effects is basis of the DLVO -theory( Derjaguin, Landau, Verwey and Overbeek).

  6. ZETA POTENTIAL To use the DLVO theory we need to know the potential energy of attraction and repulsion between colloidal particles. In many practical situations, it is difficult to obtain a reliable estimate of a colloidal particles surface potential. An alternative strategy is to use electrokinetic measurement which we can interpret in terms of the zeta potential.

  7. ZETA POTENTIAL Experience has shown that we can correlate colloid stability with this readily accessible experimental quantity : table correlates critical coagulation concentrations with zeta potentials. Consequently, it is important to see how electrokinetic techniques can be used to determine the zeta potential.

  8. Elektrophoresis: Movement of charged particles in an electrical field. The background medium does not need to be a simple liquid, it may be also highly viscous: elelectrophoresis. The charged particles may be ordinary colloids or charged macromolecules (e.g. proteins, biochemistry !). From the observation of individual particle, the electrophoretic velocity v W can be measured. Under the assumption of non deformable, spherical and nonconducting particles, v W can be correlated to the surface charge z.

  9. Sedimentation potential: The effect is inverse toelectrophoresis: charged particles are moving in the gravity field. Since the mass of the particles is much larger than the mass of the individual ions in the surrounding ion cloud, the system of particle and ion is deformed, forms a dipole and thus gives rise to an electric potential difference along thesedimentation path.

  10. Electroosmosis: an electrolyte is moved relative to a charged surface. This applies to capillaries, membranes or powders. The effect relies on the fact, that the electrical field supplied exerts a force on the electrochemical double layer. The mobile layer drags on the electrolyte which results in a liquidstream through the apparatus.

  11. Streaming potential: the principle is comparable to electroosmosis, but now an external pressure difference drives an electrolyte through the bundle/aggregate of immobile charged surfaces. The effect is caused by the retardation of a part of the electrolyte in the double layer; the resulting charge separation sums up to a measurable potential difference.

  12. Researchwork

  13. Preparation and characterization of ZnS:Mn • At first Zn (Ac)2, Mn (Ac)2 and Cysteamin were dissolved in H2O under stirring. Then Na2S-solution was added slowly (drop wise). After this the solution was heated for 3 hours (100°C) under reflux. After that the crude solution was reduced to about 50ml, and the particles were precipitated by addition of ethanol. The particles were isolated by centrifugation and redissolved in a determined amount of water. The pH was adjusted to 4.8 by addition of acetic acid

  14. Characterization: • The particle size was measured in Malvern Instrument

  15. Results: • Influence of pH: pH = 5 small particles our sample about 5 nm pH › 5 bigger particles • Nanoparticles have good orange luminescence – dopping of Mn

  16. References • D.Fennell Evans, H.Wennerström: The Colloidal Domain, 1994 • J.Goodwin: Colloids and Interfaces with Surfactants and Polymers, 2004

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