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| Properties of Colloidal Sols |
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| Heterogenous nature |
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| Colloidal sols are biphasic in nature. It consists of the dispersed phase and the dispersion medium. In the colloidal solution, each particle is contained within its own surface boundary and therefore has a separate existence from the dispersion medium. |
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| Colligative properties |
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| Colloidal particles have high average molecular masses. Therefore the mole fraction of the dispersed phase is very low. Hence, in colloidal solutions the relative lowering of vapor pressure, elevation in boiling point, depression in freezing point and osmotic pressure is very low. However, osmotic pressure measurements are used in determining the molecular masses of the polymers. |
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| Optical properties |
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| Although a colloidal solution appears to be homogenous because the dispersed particles are too small too be seen, it can be distinguished from a true solution by its ability to scatter light. |
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| The scattering of light by colloidal sized particles is called the Tyndall effect. This effect was first observed by Tyndall in 1869. A strong beam of light was passed through a colloidal sol placed in a dark place. The path of the beam got illuminated. The illuminated path of the beam is called Tyndall cone. |
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| fig 7.14 - Demonstration of Tyndall effect |
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| a) Brownian movement |
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| Brownian movement was first discovered by Robert Brown, a botanist, in 1827. He observed that pollen grains in water do not remain at rest but move about continuously and randomly. This random continuous movement (Brownian motion) was observed in colloidal sol when the sol was viewed under a ultra microscope. Brownian motion in colloidal sols arises due to the impact of the molecules of the dispersion medium with the colloidal particles. It has been postulated that the impact of the molecules of dispersion medium on the colloidal particles are unequal. This leads to the zig-zag (random) motion of the colloidal particles. |
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| fig 7.15 - Zig-zag or Brownian motion |
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| This random movement decreases as the size of the particles increases because the effect of the impacts average out. When the size of the dispersed particles increases beyond the colloidal range, Brownian motion stops, i.e., no Brownian movement is observed. |
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| The significance of Brownian movement is that |
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| i) it provides a direct demonstration of ceaseless motion of molecules as postulated by kinetic theory. |
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| ii) It counters the force of gravity acting on colloidal particles and hence helps in providing stability to colloidal sols by not allowing them to settle down. |
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| b) Diffusion |
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| Colloidal particles like solutes in true solution diffuse from a region of higher concentration to that of lower concentration. Because of their bigger sizes colloidal particles move slowly and diffuse at a slower rate. |
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| c) Sedimentation |
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| Under the influence of gravity, colloids tend to settle down very slowly. This settling down rate is accelerated by ultra centrifugation. |
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| Electrophoresis |
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| The colloidal particles are electrically charged and they carry the same type of charge, that is, either the colloids are negatively charged or positively charged. The dispersion medium has an equal and opposite charge making the system neutral as a whole. Since the colloids carry the same type of charge, they repel each other and do not combine to form bigger aggregates. This is the reason why a sol is stable. The existence of charge on the colloidal particles is inferred from the observation that the colloidal particles move either towards the cathode or anode when the colloidal sol is placed in an electric field. This phenomenon of charged particles moving towards the oppositely charged electrodes in the presence of an electric field is called electrophoresis. |
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| Electrophoresis is carried out by placing the colloidal solution in a U tube which is fitted with platinum electrodes. |
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| fig 7.16 - Apparatus for electrophoresis |
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| When electric current is passed, the charged colloidal particles move towards the oppositely charged electrode. A colloidal sol of AS2S3, which is negatively charged will move towards the anode when placed in an electric field. |
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| Why colloidal particles get charged? |
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| There are various reasons: |
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| i) Due to friction between the colloidal particles and molecules of dispersion medium, the colloidal particles become charged. |
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| ii) Charges on colloids appear due to preferential adsorption of ions from solutions. An ionic colloid adsorbs ions common to its own lattice structure. For e.g., AgCl particles can adsorb Cl- ions if excess of KCl solutions is used for its preparation where as the same colloid can adsorb Ag+ ion if AgNO3 solution is used. Hence in the first case, AgCl colloid will be negatively charged while in the second case it will be positively charged. |
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| iii) Colloids can acquire charge by dissociation of surface molecules. For e.g., soaps expel alkali ions to acquire a negative charge. |
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| iv) Colloids can get charged by dissociation of molecular electrolytes adsorbed on the surface of particles. This happens in the case where H2S molecules get adsorbed on sulphides during precipitation. H2S undergoes dissociation and the hydrogen ions are lost. The particles become negatively charged due to (S2-) which are left on the colloidal particles. |
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| v) Colloidal particles get charged by the simple process of electron capture. The colloidal particles may acquire charge by capturing electrons from air or during electro-dispersion in Bredig's arc method. |
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| As mentioned above, colloidal particles can be negatively charged or positively charged. Some common positively charged and negatively charged colloids are listed below. |
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| Positively charged colloids: |
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| Fe(OH)3, Cu(OH)3, Al(OH)3, Ca(OH)2, TiO2, Methylene blue and Haemoglobin. |
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| Negatively charged colloids: |
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| AS2S3, Sb2S3, cds, metal sols of Cu, Au, Pt, Ag, starch, clay and silica acid. |
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