The colloidal state

COLLOIDAL STATES

1. The C olloidal S tate

2. CONTENTS WHAT ARE COLLOIDS? LYOPHOBIC AND LYOPHILIC COLLOIDS? PREPERATION OF COLLOIDAL SOLUTION PURIFICATION OF COLLOIDAL SOLUION GENERAL PROPERTIES OF COLLOIDAL SYSTEM ELECTRIC DOUBLE LAYER

3. WHAT ARE COLLOIDS? A homogeneous non-crystalline substance consisting of large molecules or ultramicroscopic particles of one substance dispersed through a second substance. Colloids include gels, sols, and emulsions; the particles do not settle, and cannot be separated out by ordinary filtering or centrifuging like those in a suspension.

4. LYOPHOBIC AND LYOPHILIC COLLOIDS?

5. LYOPHOBIC AND LYOPHILIC COLLOIDS?

6. PREPERATION OF COLLOIDAL SOLUTION

7. PREPARATION OF LYOPHILIC COLLOIDS Lyophilic colloids  are  colloids  where the particles have a strong affinity for the liquid it is dispersed in. This makes these types of  colloids  very stable and easy to  prepare . Typically they can be  prepared  simply by mixing the particles with the liquid, sometime with the addition of heat.

8. PREPARATION OF LYOPHOBIC COLLOIDS Lyophobic colloids cannot be prepared just by heating, they need to be prepared by some special methods. The two methods by which the lyophobic colloids can be prepared are as follows: 1)Dispersion method 2) Condesation method

9. Dispersion Methods In dispersion methods, colloidal particles are obtained by breaking large particles of a substance in the presence of a dispersion medium. Since the solsformed are unstable, they are stabilized by adding mechanical energy input. Dispersion method in the preparation of colloids can be done by crushing the large suspension’s particles into small in size.

10. Dispersion Method In these methods, the bigger particles of a substances are broken down to form smaller particles of colloidal dimensions thus obtained are stabilized by the addition of certain stabilizing agents. Some important dispersion methods are as follows:

11. I . Mechanica l Dispersion Method A suspension of the substance in water is introduced into the mill. The coarse particles present in the suspension are grinded to the particles of colloidal dimensions and get dispersed in water to form a sol. Finer dispersion can be obtained by adding an inert diluents which prevents the colloidal particles to grow in size.

12. I . M e c han i c a l D i s pe r s i o n Method Mechanical dispersion is also called direct dispersion. It is a method of making a colloid by crushing or milling a given solid and the powder produced is dispersed into a given dispersing medium. Examples : Making of sulphur sol with the use of glucose as diluents.

13. I . Mechanica l Dispersion Method The diagram shows the mechanical dispersion method.

14. II . Electrica l Dispersion Method Also known as “Bredig arch method “ Is a method of preparing colloids, especially metallic sols. In this method, two metallic wires functioning as electrodes are immersed into water, then on both ends of wires is given a strong enough electric current to be evaporated and then it is dispersed into water to form a metallic sol.

15. II . Electrica l Dispersion Method In this method, an electric are is struck between the two electrodes of the metal (whose colloidal solution is to be prepared) immersed in the dispersion medium (say water). The dispersion medium is cooled by surrounding it with a freezing mixture. High temperature of the arc vaporizes some of the metal. The vapour condenses to the particles of colloidal size on cooling. The colloidal particles thus formed get dispersed in the medium to form a sol. of the metal.

16. II . Electrica l Dispersion Method This method is used for the preparation of sols metals such as gold, silver, platinum etc. Electtrical Dispersion Method

17. III. Peptization Dispersion T he process of converting aprecipitate into a colloidalsol by shaking it with thedispersion medium, in thepresence of a small amountof electrolyte Electrolyte = Peptizing agent This method is used to convert a freshly prepared precipitate into a colloidal sol

18. III . Peptizatio n dispersion In this method, a freshly prepared precipitate of the substance is made to pass into the colloidal state by the addition of a suitable electrolyte. The process of dispersing a freshly prepared precipitate into colloidal form by using a suitable electrolyte is called peptization. The electrolyte added is called peptizing agent.

19. III.Peptization Dispersion In peptization, the larger particles is dispersed into smaller particles in a colloidal size by adding a particular electrolyte which acts as a dispersing agent. For example, sediment of Al(OH)3 will change into a colloid by adding a solution of AlCl3 ; NiS will charge into sol when it is added into H2S; and sediment of AgCl will charge into a colloid by adding a solution of ammonia.

20. IV.Homogenization Homogenization is a process for preparing something to become homogeneous. In this preparation, a particular emulsifier is usually added to emulsify the fat particles in milk or cream so that the milk or cream form a stable colloid. In this process, skim milk powder is often used and it is conducted in a homogenization device.

21. Summary of Dispersion Method Dispersion method - larger particles of a substance (suspensions) are broken into smaller particles. The following methods are employed. There are 4 ways using dispersion method : Mechanical Dispersion Method - the substance is first ground to coarse particles. Electical Dispersion Method - is used to prepare sols of platinum, silver, copper or gold. Peptization Dispersion - The process of converting a freshly prepared precipitate into colloidal form by the addition of suitable electrolyte Homogenization - is any of several processes used to make a chemical mixture the same throughout.

22. CONDENSATION METHOD In this method, small particles are aggregated to form colloidal size particles. Double Decomposition: Example : Arsenious oxide is mixed with hydrogen sulfide to form arsenic sulfide sol. The excess amount of hydrogen sulfide is removed by passing stream of hydrogen. As 2 O 3  + 3H 2 S → As 2 S 3  (sol) + 3H 2 O Reduction: Example : Noble gases are reacted with organic reducing agents like  ethanol , tannic acid, formaldehyde to form their respective sol. AgNO 3  + tannic acid → Ag-sol Oxidation: When hydrogen sulfide is passed through a solution of sulfur dioxide it forms a sulfur sol. 2 H 2 S + SO 2  → 2H 2 O + S

23. PURIFICATION OF COLLOIDAL DISPERSION: Dialysis Electrodialysis Ultrafiltration Colloidal dispersions + electrolytes  Stable colloids Stable colloids have dispersed particles, electrolytes, dispersion medium. Purification is separation of dispersed particles only.

24. 1.Dialysis: Semi permeable membrane has fine pore. Ions/small molecules – pass Colloidal particles (large)- retained. Solution inside membrane – dialys er Solution outside membrane – diffusate

25. 2. Electrodialysis: This is similar to diffusion but enhanced by applying potential difference. Non-ionic impurities can not be separated.

26. 3. Ultrafiltration: Ordinary filter paper has large pore size – not useful Ordinary filter paper impregnated with collodion has small pores – separate colloid particles.

27. PROPERTIES OF COLLOIDS: 1. Optical properties 2. Kinetic properties 3. Electrical properties

28. 1. Optical properties: Useful to measure size, shape, structure & molecular weight of colloids. Includes light scattering & turbidity. Light scattering: Mechanism: Light + dispersed particle  polarize dipoles  Emmitt light in all directions  light scattering

29. Tyndall effect: Light scattering is clearly visible in dark back ground at perpendicular angle . Light scattering studied in light, ultra, electron microscopes. 1. Light microscope: Source of radiation – visible light 2 separate particles are visible if distance between them is 0.2µ. Not suitable for colloidal particles.

30. 2. Ultra microscope (dark-field microscope): Used to observe tyndall effect, Dispersed particles appear as bright spots in dark back ground. Used to determine zeta potential.

31. 3. Electron microscope: Used to measure particle size, shape, structure . Radiation source – high energy electrons (λ= 0.1A ) As wave length decreases resolution increases. Particle photographs can be taken .

32. 2. Kinetic properties: Used to detect stability of system, molecular weight of particles, transport kinetics. Includes Brownian motion, diffusion, sedimentation, viscosity, colligative properties. Brownian motion: Robert brown theory states colloidal particles (5µm) continuous random motion b/o thermal energy. In motion they collide with walls, other particles and change their direction, velocity. (light microscope) Particles move against gravitational force. Brownian motion stops with increase in size & viscosity.

33. Diffusion : Colloidal particles of small size pass through the porous plug b/o brownian motion. Sedimentation: This is influenced by gravitational force, applicable for particle size > 0.5 µm. Stokes law equation – velocity of sedimentation. Colloidal particles have brownian motion  No sedimentation Forced sedimentation – ultra centrifuge. Colligative properties: Only osmotic pressure is suitable for measurement of molecular weight of dispersed particles.

34. Sedimentation: This is influenced by gravitational force, applicable for particle size > 0.5 µm. Stokes law equation – velocity of sedimentation. Colloidal particles have brownian motion  No sedimentation Forced sedimentation – ultra centrifuge. Applications: Molecular weight estimation Study micellar properties of drug. Colligative properties: Only osmotic pressure is suitable for measurement of molecular weight of dispersed particles.

35. ELECTRIC DOUBLE LAYER Electrical double layer  is the structure of charge accumulation and charge separation that always occurs at the interface when an electrode is immersed into an electrolyte solution. The excess charge on the electrode surface is compensated by an accumulation of excess ions of the opposite charge in the solution . The   electrical double layer  ( EDL ) is the result of the variation of  electric potential  near a surface, and has a significant influence on the behaviour of  colloids  and other surfaces in contact with  solutions  or solid-state  fast ion conductors . The primary difference between a double layer on an electrode and one on an interface is the mechanisms of  surface charge  formation. With an electrode, it is possible to regulate the surface charge by applying an external electric potential. This application, however, is impossible in colloidal and porous double layers, because for colloidal particles, one does not have access to the interior of the particle to apply a potential difference.

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