Colloids

Dispersed system:

Dispersed system consist of particulate matter, known as the dispersed phase, distributed throughout a

continuous or dispersion medium. 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 pass 2. Can not pass

3.Semipermeable membrane

3. Can pass 3. Can not 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

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

Q: In certain cases, the same product may be a solution, emulsion or suspension-justify

Ans: 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)

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 itself produce

irritation. But it does not occur when colloidal preparations are used.

Stability: Colloidal preparation are stable than suspension and emulsion.

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.

Pharmaceutical application:

Certain medicinal have been found to possess unusual or increased therapeutic properties when formulated in the colloidal state. 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.

Classification:

On the basis of the interaction or attraction of the particle molecules, colloid are of three types-

Lyophilic colloidsLyophobic colloidsAssociation colloids

1. 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.

They can be classified in to two groups-HydrophilicLipophilic

2. Lyophobic colloids:

Lyophobic means solvent hating, where there is a little attraction between the dispersed phase and dispersion medium.

Ex: inorganic material in water.

3. 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.

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.

Preparation of lyophilic colloid:

It is very easy, simple and cheap method. It is prepared simply by warming the solid with liquid dispersion medium.

Ex: Insulin in water, gelatin in water etc.

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 methodAssociation/aggregation method

Disintegration method

In this method large macro sized particles are disintegrated into colloidal size and then dispersed in an appropriate medium in presence of suitable stabilizer.

1. Mechanical disintegration:

this include three process- Ball mill: It is based on breaking. It contains a vessel rotating

horizontally. 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.

Mortar and pestle

2. By peptization: Some freshly precipitated solids are dispersed into colloidal solution in water by the addition of small quantities of electrolytes. The precipitate adsorbs the common ion and electrically charged particles then split from the precipitate as colloidal particles.

3. By using ultrasonic wave: When ultrasonic waves are passed through a dispersion medium coarse particle are dissociated and dispersed in the medium.

Association method

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 + H2S = As2S3 + 3H2O Oxidation: Hydrosol of sulfur can be prepared by oxidation of H2S with SO2 or oxygen. 2H2S + SO = 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

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.

Purification of solutions

During the preparation of colloidal solution, the solution contains appreciable amounts of electrolytes. To obtain the pure solution, these electrolytes have to be removed. Purification of solution can be accomplished by three methods:

DialysisElectrodialysisUltrafiltration

a. Dialysis:

The process of removing ions (or molecules) from a solution by diffusion through a permeable membrane is called dialysis. When a solution containing dissolved ions (electrolyte) or molecules is placed in a bag of permeable membrane dipping in pure water, the ions diffuse through the membrane. By using a continuous flow of fresh water, the concentration of the electrolyte outside the membrane tends to be zero.

b. Electrodialysis:

In this process, dialysis is carried under the influence of electric field. Potential is applied between the metal screens supporting the membranes. This speeds up the migration of ions to the opposite electrode.

c. Ultrafiltration:

Normal filter paper is impregnated with regenerated cellulose (cellophane); the pore size is much reduced. Such a modifier filter paper is called an ultrafilter. Ultrafilter is a slow process. Gas pressure is applied here. The colloidal particles are left on the ultrafilter in the form of slime(A slurry containing very fine particulate matter).

Tyndall effect:

When a strong beam of light is passed through a solution and viewed at right angles, the path of light shows up as a hazy beam or cone. This is due to fact that solution particles absorb light energy and then emit it in all directions in space. The phenomenon of the scattering of light by the solution particles is called tyndall effect. The illuminated beam or cone formed by the scattering of light by the solution particles is often referred as tyndall beam or tyndall cone. True solutions do not show tyndall effect. Thus tyndall effect can be used to distinguish a colloidal solution from a true solution.

Brownian movement:

The continuous rapid zig-zag movement executed by a colloidal particle in the dispersion medium is called Brownian movement. Suspensions and true solutions do not exhibit Brownian movement. The movement of particle was caused by unequal number of molecules of the medium striking it from opposite directions. When more molecules struck the particle on one side than on another, the direction of movement changed.

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 counter ions from the medium, which form a second layer of negative charges. The layer of +ve charge on the particle and the –ve charges in the suspension medium constitute electrical double layer.

Zeta potential:

The potential difference between the surface layer and the solution is known as zeta potential.

Critical radius

Critical radius may be defined as the maximum allowable radius of the dispersed particles for the preparation of a stable colloidal system. If the particle size is above RC sedimentation occurs. It is denoted by RC.

Determination of critical radius by stoke’s equation:

A colloidal particle of the dispersed phase is under the influence of two forces-

a. Downward force due to gravity: This is exerted by the effective weight of the particle, i.e. the difference between the weight of the particle and the weight of the dispersion medium it displaces.

If the volume of a particle = V Density of a particle = ρ Density of a dispersion medium = ρ0 Then, the weight of a particle, W = mg W = ρVg (as m = ρV) When it is dispersed in the dispersion medium, the weight of the

dispersion medium displaced W' = m'g = ρ0Vg

According to Archimedis principle effective weight- We = W- W' = ρVg – ρ0Vg = V(ρ – ρ0)g =4/3×πr3(ρ – ρ0)g

[where, r = radius of the particle] Downward force due to gravity- Fg = 4/3×πr3(ρ – ρ0)g b. Retarding force due to viscosity: Retarding force = friction factor × velocity of sedimentation Fr = 6πηr × V Where, η = viscosity of the dispersion medium

When, r = RC, according to definition this two opposing forces become equal and counterbalanced by each others. So that-

Fr = Fg

6πη RC × V = 4/3×π RC3(ρ – ρ0)g 4× π RC

3(ρ – ρ0)g V = ------------------------------ 3×6πη RC

2 ρ – ρ0

V = (—— × ———) g RC2

9 η

For a particular colloidal preparation ρ, ρ0, g and η remain virtually constant at almost constant temperature. So, from equation we get-

V = K × RC2

2(ρ -ρ0)g RC = √ ( V/K), where K = 9 η Putting the value of V and K, RC is determined.

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