1. COLLOIDS

2. Dispersions dispersion medium dispersed phase
Dispersed systems consist of :
particulate matter (dispersed phase).
dispersion medium (continuous medium).
dispersion medium
dispersed phase

3. Dispersions
On the basis of diameter of particles, dispersed systems are classified into-
 Molecular dispersion/solution
Colloidal dispersion
coarse dispersion/suspension
Property
Molecular dispersion
Colloidal dispersion
Coarse dispersion
1. Particle size
1. Less than 1 nm
1. From 1 nm to 0.5 μm
1. Greater than 0.5 μm
2. Filter paper
2. Can pass
2. Can not pass
3.Semipermeable membrane
3. Can pass
3. Can not pass
4. Optical property
4. No tyndall effect
4. Tyndall effect is produced
4. Tyndall effect is observed
5. Visibility under microscope
5. Not visible
5. Visible under ultra microscope
5. Visible under normal and ultra microscope
6. Diffusion
6. Undergo rapid diffusion
6. Diffuse very slowly
6. Particles do not diffuse
7. Appearance seen by naked eye
7. Clear
7. Clear or may be turbid
7. Turbid
8. Effect of gravity
8. No effects
8. No appreciable effect
8. Particles are settle down

5. Colloids
The term ‘colloid’ has been derived from two Greek words ‘kolla’ and ‘eidos’.
Kolla means glue and eidos means like, so colloid means glue like.
A colloid may be defined as a heterogenous (two phase system consisting of minute particulate of 0.5μm-1nm) substance dispersed into a continuous phase or dispersion medium.
Example:
Natural colloids: fogs, moist, smoke
Ferric hydrosol

6. Colloidal dispersions
Colloidal matter commonly exists in the form of colloidal-sized phases of solids, liquids, or gases that are uniformly dispersed in a separate medium (sometimes called the  dispersions phase) which may itself be a solid, liquid, or gas.

7. In certain cases, the same product may be a solution, emulsion or suspension!!!!!!
The colloidal solution or colloidal dispersion is the intermediate between the true solution and suspension.
A colloidal system may contain more than one dispersed
phase. Ex: milk
Solution-dissolved sugar (lactose)
Suspension-albumin and casein as insoluble dispersed phase
Emulsion-fat globules (oil phase)
milk protein (emulgent)

8. Advantages of colloidal preparation:
Higher degree of catalytic activity: Due to increased surface area in colloidal preparation, the activity of a catalyst is generally accelerated.
Color: Colloidal preparations generally possess attractive color.
Taste: Colloidal preparation may also be used to pronounce the taste of a pharmaceutical preparation.
Better solubility, absorption and bioavailability
Compatibility with biological system: Ionic silver salt may itself produce toxicity-argyria and less bioavailability due to the formation of silver chloride which is insoluble and rapidly excreted from body. But it does not occur when colloidal preparations are used.
Stability: Colloidal preparation are stable than suspension and emulsion.

9. Disadvantages of colloidal preparation:
 As colloids are small in particle size so it is easily absorbed and gives extensive bioavailability which may support toxicity.
Preparation of lyophobic colloid is difficult
Stabilization of colloids is often difficult as it may be destabilized by a lot of factors (radiation, heat, drying etc.)
There is a great restriction on the particle size of the particles.

10. Pharmaceutical application:
Colloidal silver preparation are effective germicides and do not cause GI irritation that is the characteristic of ionic silver state.
Colloidal preparations are used in treatment and diagnosis of diseases.
Colloidal Hg (for syphilis)
Colloidal Cu (In the treatment of cancer)
Protein is a colloidal preparation. Plasma protein binds with certain drugs in our body, which affects the pharmacological activity of the drug.
Colloidal hydroxyethyl starch (HES) are used as plasma substitutes.
Colloidal macromolecules are used for coating purpose of the pharmaceutical products.
Colloidal electrolytes are sometimes used to increase the solubility, stability and taste of certain products.
Colloidal Al(OH)3 shows better rate of neutralization of stomach acid.
Dextran injection is a colloidal dispersion which acts plasma substitute.

11. Classification:
 On the basis of the interaction or attraction of the particle molecules, colloid are of three types-
 Lyophilic colloids
Lyophobic colloids
Association colloids

12. Lyophilic colloids:
The term lyophilic means ‘solvent loving’, where there is a considerable attraction between the disperse phase and disperse medium. Lyophilic colloids are more stable than lyophobic colloids. Lyophilic (hydrophilic) colloids are very common in biological systems and in foods. Ordinary gelatine is a common example of a lyophilic colloid.
 They can be classified in to two groups-
Hydrophilic colloid: These are water-loving colloids. The colloid particles are attracted toward water. Example: acacia in water
Lipophilic colloid: These are nonaqoues loving colloids. The colloid particles are attracted toward organic solvent. Example: rubber

13. Lyophobic colloids:
Lyophobic means solvent hating, where there is a little attraction between the dispersed phase and dispersion medium.
Lyophobic colloids are all inherently unstable; they will eventually coagulate. However, "eventually" can be a very long time (the settling time for some clay colloids in the ocean is years!).
Example: Gold sol

14. Association colloid:
Organic compounds which contain large hydrophobic moieties together with strongly hydrophilic groups in the same molecule are said to be amphiphilic.
The individual molecules are generally too small to bring their solution into colloidal size range; they tend to associate in aqueous or oil solution into micells. Such preparations are called association colloids.
Surface active molecules such as soaps and synthetic detergents form associated colloids in water.

15. Differences between Lyophilic & Lyophobic colloids
Lyophilic colloids
Lyophobic colloids
1. Prepared by direct mixing with dispersion medium
1. Not prepared by direct mixing with the medium
2. Little or no charge on particles
2. Particles carry positive or negative charge
3. Particles generally solvated
3. No salvation of particles
4. Viscosity higher than dispersion medium; set to a gel
4. Viscosity almost the same as of medium; do not set to a gel
5.Precipitated by high concentration of electrolytes
5. Precipitated by low concentration of electrolytes
6. Reversible
6. Irreversible
7. Do not exhibit Tyndall effect
7. Exhibit Tyndall effect
8. Particles migrate to anode or cathode or not at all
8. Particles migrate to either anode or cathode.

17. Preparation of lyophilic colloid:
It is very easy, simple and cheap method.
Lyophilic Sols may be prepared by Simply warming the solid with the liquid dispersion medium.
For example, Only heating with water is enough for preparing the sols of starch, gelatin, gum arabic etc.

18. Preparation of lyophobic colloid:
Lyophobic colloids are very difficult to prepare and requires the use of special technique. Basically the method of preparing lyophobic colloids fall into two categories-
Disintegration/dispersion method
Association/aggregation method

19. Dispersion Methods
In this method large particles of the substances are broken, into particles of colloidal dimensions in presence of dispersion medium. Since the sols formed are highly unstable. They are stabilized by adding some suitable stabilizer. Some of the methods employed for carrying out dispersion are as follows:
1. Mechanical disintegration: this include three process-
Ball mill:
Colloid mill:
Mortar and pestle
2. By peptization
3. By using ultrasonic wave
4. Bredig’s Arc Method:

21. Mechanical disintegration
Ball mill: It is based on breaking. It contains a rotating vessel. There are some metal balls within the vessel and arranged systemically. The balls continuously move up and down and thus strike the material taken in vessel. Ultimately the material is crushed. The material finally dispersed in a suitable medium to get the formulation.
Colloid mill: It contains two plates arranged at a particular distance from each other. It may be of two types-
One is fixed and other rotates
Both are rotating in opposite direction at high speed
 The solid along with dispersion medium is passed through the mill. So the particles are disintegrated and dispersed in the medium.
c) Mortar and pestle

22. Peptization, Ultrasonic wave, Bredig’s Arc Method:
2. By peptization: Some freshly precipitated solids are dispersed into colloidal solution in water by the addition of small quantities of electrolytes. During peptization, the precipitate adsorbs one of the ion of the electrolyte on its surface. The adsorbed ion is generally common with those of the precipitate. For example: When freshly precipitated Fe(OH)3 is shaken with aqueous solution of FeCl3 (Peptising agent) it adsorbs Fe3+ ions and thereby breaks up into small sized particles of type Fe(OH)3 / Fe3+.
3. By using ultrasonic wave: When ultrasonic waves are passed through a dispersion medium coarse particle are dissociated and dispersed in the medium.
4.Bredig’s Arc Method: This process involves dispersion as well as aggregation. Colloidal solutions of metals such as gold, silver, platinum etc. can be prepared by this method. In this method electric arc is struck between electrodes of metal immersed in the dispersion medium. The intense heat produced vapourises some of metal, which then condenses to form particles of colloidal size.

24. Association method 1. By chemical reaction:
1. By chemical reaction:
 Hydrolysis: Solutions of hydroxides of Fe, Cr, Al, Sn are prepared by hydrolysis of salts of respective metals.
2FeCl3 + 6H2O Fe2O3.3H2O + 6HCl
Double decomposition: An arsennous sulfide solution is prepared by passing slow steam of hydrogen sulphide through cold solution of arsenous oxide.
As2O3 As2S3 + 3H2O
H2S
Oxidation: Hydrosol of sulfur can be prepared by oxidation of H2S with SO2 or oxygen.
2H2S + SO2 3S + 2H2O
Reduction: The colloidal suspension of selenium is prepared by reduction of selenium oxide with SO2
SeO2 + 2SO2 Se + 2SO3
Oxidation-reduction: Hydrogeniodide and iodic acid interact to give blue suspension of iodine
5HI + HIO3 3I2 + 3H2O

25. Association method
2. Thermal condensation: This process involves the passing of hot vapor through water, which condenses releasing heat and form precipitate.
3. By reducing solubility: If a concentrated solution of a substance is poured in another liquid in which the substance is insoluble, it undergoes precipitation due to super saturation.

26. *****The important physical methods for preparing lyophobic sols are:
By Exchange of Solvent:
When a true solution is mixed with an excess of the other solvent in which the solute is insoluble but solvent is soluble, a colloidal solution is obtained. For Example, when a solution of sulphur in alcohol (ethanol) is added to an excess of water, a colloidal solution of sulphur is obtained due to decrease in solubility.
By Excessive Cooling:
The colloidal solution of ice in an organic solvent such as CHCl3 or ether can be obtained by freezing a solution of water in the solvent. The molecules of water which can no longer be held in solution separately combines to form particles of colloidal size.

27. Purification of colloidal solutions:
When a colloidal solution is prepared is often contains certain electrolytes which tend to destabilize it. The following methods are used for purification:
(1) Dialysis
(i) The process of separating the particles of colloid from those of crystalloid, by means of diffusion through a suitable membrane is called dialysis.
(ii) It’s principle is based upon the fact that colloidal particles can not pass through a parchment or cellophane membrane while the ions of the electrolyte can pass through it.
(iii) The impurities slowly diffused out of the bag leaving behind pure colloidal solution
(iv) The distilled water is changed frequently to avoid accumulation of the crystalloids otherwise they may start diffusing back into the bag.
(v) Dialysis can be used for removing HCl from the ferric hydroxide sol.

28. Purification of colloidal solutions:
(2) Electrodialysis
(i) To increase the process of purification, the ordinary dialysis is carried out by applying electric field. This process is called electrodialysis.
(ii) The important application of electrodialysis process in the artificial kidney machine used for the purification of blood of the patients whose kidneys have failed to work.
(3) Ultra – filtration
If the pores of the ordinary filter paper are made smaller by soaking the filter paper in a solution of gelatin and subsequently hardened by soaking in formaldehyde, the treated filter paper may retain colloidal particles and allow the true solution particles to escape. Such filter paper is known as ultra - filter and the process of separating colloids by using ultra – filters is known as ultra – filtration.

29. (4) Ultra – centrifugation
Ultracentrifugation involves the separation of colloidal particles from the impurities by centrifugal force.
The impure sol is taken in a tube and the tube is placed in an ultra-centrifuge. The tube is rotated at high speeds.
On account of this, the colloidal particles settle down at the bottom of the tube and the impurities remain in the solution.
This solution is termed as centrifugate. The settled colloidal particles are removed from the tube and are mixed with an appropriate dispersing medium. Thus, the pure sol is obtained.
by using high speed centrifugal machines having 15,000 or more revolutions per minute. Such machines are known as ultra–centrifuges.

31. Properties of colloids
Physical properties
(i) Heterogeneous nature: Colloids are heterogeneous in nature. They consist of two phases; the dispersed phase and the dispersion medium.
(ii) Stable nature: The colloidal solutions are quite stable. Their particles are in a state of motion and do not settle down at the bottom of the container.
(iii) Filterability: Colloidal particles are readily passed through the ordinary filter papers. However they can be retained by special filters known as ultrafilters (parchment paper).
(iv)Particle size: The particle size of colloids generally varies from 1 nm to 1 um.
(v) Colour: The colour of a hydrophobic sol depends on the wavelength of the light scattered by the dispersed particles. The wavelength of the scattered light again depends on the size and the nature of the particles.

32. Properties of colloids
C. Kinetic properties: Which relate to the motion of the particles within the dispersion medium as following:
a. Brownian motion, b. Diffusion, c. Sedimentation, d.Osmotic pressure,
e. Viscosity.
D. Optical properties:
Light scattering (Tyndall effect).
Ultra microscope.
Electron microscope.
E. Electrical properties:
The sol particles carry an electric charge
Electrophoresis
Electro-osmosis
Streaming potential
The Donnan membrane effect

33. Kinetic properties: V = D2 (i-e)g/18η
(i)Brownian movement: The continuous rapid zig-zag movement executed by a colloidal particle in the dispersion medium is called Brownian movement.
(ii) Sedimentation (Stoke’s law) : - The velocity of sedimentation is given by Stokes‘ Law:
V = D2 (i-e)g/18η
V = rate of sedimentation
D = diameter of particles
 = density of internal phase and external phase
g = gravitational constant
η = viscosity of medium
At small particle size (less than 0.5 um) Brownian motion is significant & tend to prevent sedimentation due to gravity & promote mixing instead.
The colloidal particles settle down under the influence of gravity at a very slow rate. This phenomenon is used for determining the molecular mass of the macromolecules.

34. Kinetic properties:
(iii) Diffusion: The sol particles diffuse from higher concentration to lower concentration region.
(iv)Viscosity: The viscosity of colloids depends upon the shape of colloidal particle. Spherical colloid show low where as linear shows more. Viscosity increases due to solvation.
(v) Osmotic pressure: It depends on the number of particles in dispersion. In associated colloids each aggregate acts as one particle and osmotic pressure is small. This can be used to calculate the MW of colloidal material (π=CRT)

35. sedimentation

36. Optical properties: Light scattering (Tyndall effect).
When a strong beam of light is passed through a sol and viewed at right angles, the path of light shows up as a hazy beam or cone. This is due to the fact that sol particles absorb light energy and then emit it in all directions in space. This ‘scattering of light’, as it is called, illuminates the path of the beam in the colloidal dispersion.
The phenomenon of the scattering of light by the sol particles is called Tyndall effect.
The illuminated beam or cone formed by the scattering of light by the sol par ticles is often referred as Tyndall beam or Tyndall cone.
True solutions do not show Tyndall effect. Since ions or solute molecules are too small to scatter light, the beam of light passing through a true solution is not visible when viewed from the side. Thus Tyndall effect can be used to distinguish a colloidal solution from a true solution.

37. Ultramicroscope
Sol particles cannot be seen with a microscope. Zsigmondy (1903) used the Tyndall phenomenon to set up an apparatus named as the ultramicroscope. An intense beam of light is focussed on a sol contained in a glass vessel.
The focus of light is then observed with a microscope at right angles to the beam. Individual sol particles appear as bright specks of light against a dark background (dispersion
medium).
An ultramicroscope does not give any information regarding the shape and size of the sol particles.

38. Electron microscope
In an electron microscope, beam of electrons is focused by electric and magnetic fields. This focused beam is allowed to pass through a film of sol particles on to a photographic plate. Thus it is possible to get a picture of the individual particles showing a magnification of the order of 10,000.
With the help of this instrument, we can have an idea of the size and shape of several sol particles including paint pigments, viruses, and bacteria. These particles have been found to be spheriod, rod-like, disc-like, or long filaments.

39. ELECTRICAL PROPERTIES OF SOLS :The sol particles carry an electric charge
The most important property of colloidal dispersions is that all the suspended particles posses either a positive or a negative charge. The mutual forces of repulsion between similarly charged particles prevent them from aggregating and settling under the action of gravity. This gives stability to the sol.

40. Electrical Double layer
The surface of colloidal particle acquires a positive charge by selective adsorption of a layer of positive ions around it. This layer attracts counterions from the medium which form a second layer of negative charges. The combination of the two layer of +ve and –ve charges around the sol particle was called Helmholtz Double layer.
More recent considerations have shown that the double layer is made of :
(a) a Compact layer of positive and negative charges which are fixed firmly on the particle surface.
(b) a Diffuse layer of counterions (negative ions) diffused into the medium containing positive ions.
The combination of the compact and diffuse layer is referred to as the Stern Double layer.
Because of the distribution of the charge around the particle, there is a difference in potential between the compact layer and the bulk of solution across the diffuse layer. This is called by Electrokinetic or Zeta potential.

42. Electrical properties:
Electrophoresis: The movement of sol particles under an applied electric potential is called electrophoresis or cataphoresis.
Thus by noting the direction of movement of the sol particles, we can determine whether they carry a positive or negative charge.
Electro-osmosis: A sol is electrically neutral. Therefore, the dispersion medium carries an equal but opposite charge to that of the dispersed particles, whether they carry a positive or negative charge. The movement of the dispersion medium under the influence of applied potential is known as electroosmosis
Streaming Potential: Differs from electro-osmosis in that the potential is created by forcing a liquid to flow through a bed or plug of particles.
Donnan membrane effect: The presence of charged macromolecules (colloid) on one side of a semipermeable membrane affects the diffusion of small ions such as drug ions through the semipermeable membrane. This effect is due to the electrical gradient across the membrane and as a consequence, the charged drug ions of the same charge as the macromolecules are driven to the opposite side of the membrane altering the concentration of the drug ions. This is termed as Donnan membrane equilibium or simply Donnan effect.

44. The Gibbs–Donnan effect (also known as the Donnan's effect, Donnan law, Donnan equilibrium, or Gibbs–Donnan equilibrium) is a name for the behavior of charged particles near a semi-permeable membrane that sometimes fail to distribute evenly across the two sides of the membrane.[1] The usual cause is the presence of a different charged substance that is unable to pass through the membrane and thus creates an uneven electrical charge.[2] For example, the large anionic proteins in blood plasma are not permeable to capillary walls. Because small cations are attracted, but are not bound to the proteins, small anions will cross capillary walls away from the anionic proteins more readily than small cations.

45. Some ionic species can pass through the barrier while others cannot
Some ionic species can pass through the barrier while others cannot. The solutions may be gels or colloids as well as solutions of electrolytes, and as such the phase boundary between gels, or a gel and a liquid, can also act as a selective barrier. The electric potential arising between two such solutions is called the Donnan potential.

46. Protective action of sols
Lyophobic sols are readily precipitated by small amounts of electrolytes. However these sols are often stabilized by the addition of lyophilic sols.
The property of lyophilic sols to prevent the precipitation of a lyophobic sol is called protection.
The lyophilic sol used to protect a lyophobic sol from precipitation is referred to as a Protective colloid.
Example. If a little gelatin (hydrophilic colloid) is added to a gold sol (hydrophobic sol), the latter is protected. The ‘protected gold sol’ is no longer precipitated on the addition of sodium chloride.
Explanation. The particles of the hydrophobic sol adsorb the particles of the lyophilic sol. Thus the lyophilic colloid forms a coating around the lyophobic sol particles. The hydrophobic colloid, therefore, behaves as a hydrophilic sol and is precipitated less easily by electrolytes.

47. Gold number
The protective action of different colloids is measured in terms of the ‘Gold number’ introduced by Zsigmondy. The gold number is defined as :the number of milligrams of a hydrophilic colloid that will just prevent the precipitation of 10 ml of a gold sol on the addition of 1 ml of 10 per cent sodium chloride solution.
The smaller the gold number of a hydrophilic colloid, the greater is its protective power. Gelatin has a small gold number and is an effective protective colloid. Starch has a very high value, which shows that it is an ineffective protective colloid. The use of protective colloids to stabilize colloidal systems is widespread. Argyrol, used in eye drops,
is a sol of silver protected by organic material.

48. STABILITY OF SOLS
A true colloidal solution is stable. Its particles do not ever coalesce and separate out. The stability of sols is mainly due to two factors :
(1) Presence of like charge on sol particles
The dispersed particles of a hydrophobic sol posses a like electrical charge (all positive or all negative) on their surface. Since like charges repel one another, the particles push away from one another and resist joining together. However, when an electrolyte is added to a hydrophobic sol, the particles are discharged and precipitated.
(2) Presence of Solvent layer around sol particle
The lyophilic sols are stable for two reasons. Their particles possess a charge and in addition have a layer of the solvent bound on the surface. For example, a sol particle of gelatin has a negative charge and a water layer envelopes it. When sodium chloride is added to colloidal solution of gelatin, its particles are not precipitated. The water layer around the gelatin particle does not allow the Na+ ions to penetrate it and discharge the particle. The gelatin sol is not precipitated by addition of sodium chloride solution. Evidently, lyophilic sols are more stable than lyophobic sols.

50. Coagulation or Precipitation
We know that the stability of a lyophobic sol is due to the adsorption of positive or negative ions by the dispersed particles. The repulsive forces between the charged particles do not allow them to settle. If, some how, the charge is removed, there is nothing to keep the particles apart from each other. They aggregate (or flocculate) and settle down under the action of gravity.
The flocculation and settling down of the discharged sol particles is called coagulation or precipitation of the sol. The coagulation or precipitation of a given sol can be brought about in four ways :
(a) By addition of electrolytes
(b) By electrophoresis
(c) By mixing two oppositely charged sols
(d) By boiling

51. Coagulation or Precipitation
(a) By addition of Electrolytes. When excess of an electrolyte is added to a sol, the dispersed particles are precipitated. The electrolyte furnishes both positive and negative ions in the medium.The sol particles adsorb the oppositely charged ions and get discharged. The electrically neutral particles then aggregate and settle down as precipitate. From a study of the precipitating action of various electrolytes on particular sol, Hardy and Schulze gave a general rule.
Hardy-Schulze Rule states that the precipitating effect of an ion on dispersed phase of opposite charge increases with the valence of the ion. The higher the valency of the effective ion, the greater is its precipitating power. Thus for precipitating an As2S3 sol (negative), the precipitating power of Al 3+ , Ba 2+, Na+ ions is in the order
Al3+ > Ba 2+ > Na+

52. Coagulation or Precipitation
(b) By Electrophoresis. In electrophoresis the charged sol particles migrate to the electrode of opposite sign. As they come in contact with the electrode, the particles are discharged and precipitated.
(c) By mixing two oppositely charged sols. The mutual coagulation of two sols of opposite charge can be effected by mixing them. The positive particles of one sol are attracted by the negative particles of the second sol. This is followed by mutual adsorption and precipitation of both the sols. Ferric hydroxide (+ve sol) and arsenious sulphide (–ve sol) form such a pair.
(d) By boiling. Sols such as sulphur and silver halides dispersed in water, may be coagulated by boiling. Increased collisions between the sol particles and water molecules remove the adsorbed electrolyte. This takes away the charge from the particles which settle down.

53. Sensitization of colloid
Sensitization: the addition of small amount of hydrophilic or hydrophobic colloid to a hydrophobic colloid of opposite charge tend to sensitize (coagulate) the particles.
Polymer flocculants can bridge individual colloidal particles by attractive electrostatic interactions.
For example, negatively-charged colloidal silica particles can be flocculated by the addition of a positively-charged polymer.