Colloidal solution coagulation

Extremely small particle, typically 10-5 to 10 7 cm in diameter. Colloidal solutions or hydrosols contain colloidal particles that are electrically negatively charged, which contributes to their fine dispersion and the difficulty of sedimentation and clarification. Coagulation is usually carried out by causing the particles to adsorb positively charged ions, such as aluminum from alum. 

M. V. Smoluchowski, Mathematical theory of the kinetics of the coagulation of colloidal solutions, Z. Phys. Chem. 92, 129-168 (1917). 

H. W. Woodstra, Brownian movement and the coagulation of colloidal solutions, Chemisch Week-blad 15, 679-680(1918). 

Colloidal solutions are characterized by the degree of stability or instability. This is related to the fact that both kinds of properties in everyday phenomena need to be understood. The kinetics of coagulation is studied using different methods. The number of particles, Np, at a given time is dependent on the diffusion-controlled process. The rate is given by... 

Ellis and Powis (1912-15) introduced the concept of the critical zeta potential for the coagulation of colloidal solutions. 

The trivaleney of the complex ion, [Pt(NH3)5Cl] , is confirmed by conductivity measurements, the chloride having conductivity 404 at 1000 litres dilution, and by the power of the salts to coagulate colloidal solutions of arsenic trisulphide. 

II Figure 1). Adsorption continually occurs around the bubbles to replace protein in areas of the interface where coagulation or stretching of the film is occurring. The actual bubble size in the foam depends upon the rate of protein adsorption as well as upon the ease of film rupture. The protein films on adjacent bubbles come in contact and trap the liquid, preventing it from flowing freely. This restriction is governed by the viscosity of the colloidal solution. The polypeptides of denatured proteins situate to positions where their hydrophobic side chains are directed outward toward each other. Because liquid... 

Colloidal Solution of Gold (Red Gold Hydrosol). Pour 5 ml of distilled water and 0.3 ml of a 0.5% gold chloride solution neutralized with soda into a wide test tube. Heat the reaction mixture until it boils, next stop the heating, add in small portions 3 ml of a 96% solution of ethanol and resume heating up to the appearance of a cherry red colour of the solution. When the latter cools, pour in 0.3 ml of a 20% hydrochloric acid solution. What do you observe Does the sol coagulate ... 

When superheated selenium vapour is passed into air-free water, colloidal solutions are formed which are usually rose-coloured, but at first of a blue tint and cloudy. Under the most favourable conditions clear yellowish-red or deep red sols may be obtained,4 the former being the more highly dispersed. The blue sols after dialysis are extremely stable, but non-dialysed sols decompose after a few days, selenious acid being detected except in the yellowish-red sols. The dialysed sols may be frozen to an almost colourless ice which at the ordinary temperature thaws and decomposes. The sols are negative and are readily coagulated by the addition of chlorides. 

Very stable colloidal solutions of selenium may be prepared by the regulated action of concentrated hydrazine hydrate solution on selenium dioxide or grey crystalline selenium and subsequent dilution of the solutions with water and purification by dialysis. According to the degree of dispersion the colour of the solutions varies from intense yellow to blood red. These sols are completely irreversible. The dilute solutions are stable at the boiling-point, but are readily coagulated by barium sulphate. Sodium and potassium carbonates appear to increase the stability of the system. 

The colloidal silica dispersions are destabilized with hydrolyzed Al(III) primarily because of adsorption of polyhydroxo aluminum cations on the colloid surface which reduces the incipiently negative surface potential of Si02 colloids (adsorption coagulation). The pertinent solution variables describing the destabilization reaction are pH, total aluminum concentration Ct, and the ratio of aluminum dosage to the colloid surface concentration S. 

F. S. Brown and C. R. Bury 6 obtained colloidal solutions of phosphorus pentoxide in nitrobenzene by stirring the mixture in the presence of alcohols or organic acids. The hydroxy-compound is absorbed and peptizes the pentoxide. Traces of moisture cause coagulation. Cone. soln. set to gels on keeping. 

Preventing coagulation of a colloidal solution by any means is known as stabilising the colloid. Bancroft (1915) gave the position with regard to Stability. He said that any substance may be brought into a colloidal state, provided the particles of the dispersed phase are so small that the Brownian movement keeps the particles suspended and provided the coagulation of the particles is prevented by a suitable surface film... 

We have already studied the effect of temperature change on reversible reactions under Le-Chatelier s principle. Some catalysts are, however, physically altered by a rise in temperature and hence their catalytic activity may be decreased. This is particularly true with colloidal solutions like that of platinum, since a rise in temperature may cause their coagulation. In such a case, the rate of reaction increases up to a certain point and then gradually decreases. The rate of reaction is maximum at a particular temperature which is known as the optimum temperature. 

The addition of polymers can also destabilize a colloidal solution for two reasons. Firstly, when polymers adsorb on more than one colloid and this phenomenon is called bridging. Secondly, when polyelectrolytes neutralize oppositely charged colloids and thereby reduce the electrostatic repulsion between the colloids. In some cases both a destabilization and a stabilization of a solution is desired. One example is the manufacturing of paints, where it is required that the paint is easy to apply on the wall and that it stays on the wall. This can be achieved with a solvent which stabilizes the solution but rapidly evaporates when the paint is applied and makes it possible for the colloidal particles to coagulate. 

Colloidal rhodium may also be prepared by reduction of pure rhodium salts with hydrazine hydrate. Traces of impurity prevent the formation of the hydrosol, which is very unstable. Addition of a protective colloid, such as a 1 per cent, solution of gum acacia, renders the hvdrosol stable, so much so that it admits of concentration over sulphuric acid in vacuo, yielding a dark brown solid mass of colloidal metal, containing 99-4 per cent, of rhodium, and almost completely soluble in water.5 Shaking the colloidal solution with barium sulphate or animal charcoal serves to coagulate the metal. 

When evaporated to dryness over concentrated sulphuric acid in vacuo the solid hydrosol is obtained. This dissolves in warm water, yielding the colloidal solution again. The solution, when shaken with barium sulphate or animal charcoal, is decomposed, the metal being coagulated and thrown out as a precipitate. 

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