1. Preparation of Colloidal sol. (1)Dispersion methods: – Mechanical dispersion – Electrical dispersion – Peptization (2) Condensation methods: – Oxidation and reduction – Hydrolysis – Solvent Exchange

2. Purification of Colloidal sol 1- Dialysis: This process is based on the fact that membranes contain very small pores through which only dissolved molecules and ions of the true solution can pass readily. 2- Ultrafiltration: Colloidal particles can pass through the pores larger than 1 micro along with solute particles of true solution. But the pores can be made smaller by soaking the filter paper in solution of gelatin or collodion subsequently hardening them by soaking in formaldehyde. This treated filters are known as ultrafilters. The process of separating colloids from solutes is known as Ultrafiltration.

5. A) Mechanical Properties (Brownian movement) When colloidal solutions have been observed through ultra microscope, the colloidal particles are seen in constant and rapid zigzag motion called Brownian movement. Sir Robert Brown first observed the phenomenon in 1827. Suspensions and true solutions do not exhibit Brownian movement.

6. Robert Brown, a botanist, discovered the zig-zag motion of colloidal particles like the movement of pollen grains in water. Hence it is termed as Brownian movement. Zig-zag motion of colloidal particles is due to impact of molecules of dispersion medium with colloidal particles, this impact on the particles of colloidal size is unequal leading to zig-zag motion. As the colloidal particles increases in size, these impact average out and Brownian movement becomes slow. When colloidal particles acquire dimension of suspension, no Brownian motion is observed. Brownian motion counters the force of gravity acting on colloidal particles and hence helps in providing stability to colloidal sol by preventing their settlement. Smaller the colloidal particles, more is the Brownian motion.

7. (area/unit time)

9. 4) Osmotic pressure  The most important colligative property from a pharmaceutical point of view is referred to as osmotic pressure.  The osmotic pressure of a solution is the external pressure that must be applied to the solution in order to prevent it being diluted by the entry of solvent via a process known as osmosis.  Osmosis: If two solutions of different concentrations are separated by a semi-permeable membrane (only permeable to the solvent) the solvent will move from the solution of lower solute concentration to that of higher solute concentration.

11. Osmosis In osmosis, the solvent water moves through a semipermeable membrane Water flows from the side with the lower solute concentration into the side with the higher solute concentration Eventually, the concentrations of the two solutions become equal.

12. Osmotic pressure Equal to the pressure that would prevent the flow of additional water into the more concentrated solution Increases as the number of dissolved particles increase

13. 5. Viscosity

14. (B) Optical properties or Tyndall effect: In 1869 Tyndall observed that when a strong beam of light is passed through colloidal sol placed in dark, the path of light is illuminated. This effect is called Tyndall effect, which is due to scattering of light by colloidal particles. The illuminated path of beam is called Tyndall cone.

15. In true solution the particles of solute are too small to scatter the wave to the sides because the particles are smaller than the wavelength of visible light. The light beam remains invisible. Light scattering can be used to study colloidal system by using of Dynamic light scattering (DLS) technique. DLS can determine size distribution profile of particles in suspension or polymer in solution. Careful studies of scattering as a function of direction can be used not only to determine size but also the shape of macromolecules. Thus albumin (mol. wt., 61 kDa) is ellipsoidal in shape, whereas fibrinogen, a blood clotting protein (mol. wt., 400 kDa) has an elongated, fabrillar shape. The importance of light scattering measurements: 1.Estimate the particle size 2. Estimate particle shape 3. Estimate particle interactions

16. (C) Electrical properties: In some colloids, the particles absorb ions and thereby acquire electrical charge. For example ferric oxide and aluminum hydroxide sol is positively charged due to absorption of H + ions. Arsenious sulphide sol forms a negative sol by absorbing S 2- ions. Gold, silver and platinum sols are also have negative charge. The mixing of positively charged ferric oxide sol and negatively charged Arsenious sulphide sol, coagulates both sols. The charge on colloidal particles is responsible for stability of colloids because the repulsion between similarly charged particles prevents them to settle down. The phenomenon of electrophoresis proves the existence of charge on colloidal particles.

18. Electrical Properties (Electrophoresis) Colloidal particles of a sol either carry positive or negative charge. Sols in, which the colloidal particles carry positive charge are called positive sols. When colloidal particles carry negative charge, the sols are called negative sols. The existence of charge on the colloidal particles can be demonstrated by a phenomenon called electrophoresis where the colloidal particles, when placed in an electric field, move towards either cathode or anode depending upon the charge on them. Sols of basic dyestuffs, ferric hydroxide, aluminium hydroxide etc., are some common examples of positive sols. Colloidal solutions of gums, starch, soap solution, metals (Ag, Cu, Au, Pt etc.), metal sulphides, and some acid dyestuffs are the examples of negative sols

19. Electrophoresis Involves the movement of a charged particle through a liquid under the influence of an applied potential difference. An electrophoresis cell, fitted with two electrodes, contains the dispersion. When a potential is applied across the electrodes, the particles migrate to the oppositely charged electrode. Electrophoresis: The movement of a charged particle relative to the liquid it suspended in under the influence of an applied electric field This technique finds application in - Measurements of zeta potentials of model systems (like polystyrene latex dispersion), - To test colloidal stability theory, - To asses the stability of coarse dispersion, - In identification of charge groups The particles move with a characteristic velocity which is dependent on the strength of the electric field (measured by the instrument), the dielectric constant and the viscosity of the medium. The velocity of a particle in a unit electric field is referred to as its electrophoretic mobility

20. Zeta potential  Is an abbreviation for electrokinetic potential in colloidal systems. Zeta potential is the potential difference between the dispersion medium and the stationary layer of fluid attached to dispersed particle. - If all the particles have a large negative or positive zeta potential they will Repel each other and there is dispersion stability. - If the particles have low zeta potential values then there is no force to prevent the Particles coming together and there is dispersion instability. Zeta potential is not measurable directly but it can be calculated using theoritical models and an experimently-determined electrophoretic mobility or dynamic electrophoretic mobility.

21. Adsorptive properties Colloids show high adsorptive properties for two reasons: 1- Colloids have a large surface area per gram as a result of their small size. 2- The particles of a colloid selectively adsorb ions and acquire an electric charge.

22. Coagulation of colloids The stability of colloids depend on the existence of charge on their particles. If this charge is neutralized, the colloidal sol is precipitated. This process of precipitation is called coagulation. At lower conc. of an electrolyte, the aggregation of colloidal particles is called flocculation. Flocculation can be reversed by shaking. At higher conc. of an electrolyte, coagulation takes place and cannot be reversed by shaking. Hydrophobic sol are easily coagulated compared to lyophilic sol. The coagulation of hydrophilic sols is not only due to neutralization of charge. The lightly bound water to the particles protect them from coagulation. Stronger the dehydrating power of an ion, more will be coagulating of particles.

23. Stability of Colloidal dispersions Stabilization serves to prevent colloids from aggregating. Steric stabilization and electrostatic stabilization are the two main mechanisms for colloid stabilization. Electrostatic stabilization is based on the mutual repulsion of like electrical charges. In general, different phases have different charge affinities, so that an electric double layer forms at any interface. Small particle sizes lead to increase surface areas, and this effect is greatly amplified in colloids. The charge on the dispersed particles can be observed by applying an electric field: All particles migrate to the same electrode and therefore must all have the same sign charge.

25. Stabilization of colloidal dispersion is accomplished by two means: providing the dispersed particles with an electric charge and surrounding each particle with a protective solvent.

26. Protection of colloids The lyophobic sols are very susceptible to the process of coagulation or precipitation. The lyophilic sols are enclosed in the sheath of solvent molecules. The cage serves as barrier preventing the particles from aggregating at low conc. of an electrolyte. The layer of lyophilic particles around the particles of lyophobic sol protect it from coagulation. The substances such as gum, gelatin, starch, tragacanth, etc. are known as protective colloids. Gold number is the number of (mg) of protective colloids which just prevents the coagulation of 10 ml of a standard red gold sol when 1 ml of 10% NaCl solution is added to it. Lower the gold number of a protective colloid, better is the protective action.

27. The addition of large amounts of hydrophilic colloid stabilizes the system. This concept is known as protection, and the added hydrophilic colloid is called a protective colloid. The protective property is expressed in terms of the gold number. The gold number is the minimum weight (measured in milligrams) of the protective colloid required to prevent a color change.  Examples Gold Numbers of Protective Colloids: Protective ColloidsGold Number Gelatin 0.005–0.01 Albumin 0.1 Acacia 0.1–0.2 Sodium Oleate1–5 Tragacanth2

28. Applications of colloids Many pharmaceutical preparations are in the form of colloidal dispersions and emulsions. Colloidal silver such as argyrol and protargol are used in eye lotions. Colloidal arsenic is used in treatment of eye diseases. Colloidal calcium and gold Colloidal sulphur is used as an important insecticide. In food, Milk Caseins are used as a protective colloid. Gelatin also is added to ice cream to preserve the smoothness and fine texture. In industry, colloids have many uses such as: Cleaning water, Sewage disposal, Rubber plating, Cottrell smoke precipitators, Chrome tanning and in building roads.

29. Summary One property that distinguishes colloid systems from true solutions is that colloidal particles scatter light. If a beam of light, such as that from a flashlight, passes through a colloid, the light is reflected (scattered) by the colloidal particles, and the path of the light can therefore be observed. When a beam of light passes through a true solution (e.g., salt in water) there is so little scattering of the light that the path of the light cannot be seen, and the small amount of scattered light cannot be detected except by very sensitive instruments. The scattering of light by colloids, known as the Tyndall effect, is important to determine particle size, shape and interactions. When an ultramicroscope is used to examine a colloid, the colloidal particles appear as tiny points of light in constant motion; this motion, called Brownian movement, helps keep the particles in suspension.

30. Adsorption is characteristic of colloids, since the finely divided colloidal particles have a large surface area exposed. The presence of colloidal particles has little effect on the colligative properties (boiling point, freezing point, etc.) of a solution. The particles of a colloid selectively adsorb ions and acquire an electric charge. All of the particles of a given colloid take on the same charge (either positive or negative) and thus are repelled by one another. If an electric potential is applied to a colloid, the charged colloidal particles move toward the oppositely charged electrode; this migration is called electrophoresis. If the charge on the particles is neutralized, they may precipitate out of the suspension. A colloid may be precipitated by adding another colloid with oppositely charged particles; the particles are attracted to one another, coagulate, and precipitate out. The presence and magnitude of a charge (Zeta potential) on a colloidal particle is an important factor in the stability of colloidal systems. Stabilization is accomplished by two means: 1- Providing the dispersed particles with an electric charge. 2- Surrounding each particle with a protective solvent.