Type of Colloidal Solutions Chemistry Notes
Type of Colloidal Solutions :
On the basis of dispersed phase and dispersion medium, colloidal solutions are classified in various types which are given in table 5.4:
Types of Colloidal solutions
|Dispersed phase||Dispersion medium||Type of Colloid||Examples|
|Solid||Solid||Solid sol||Coloured gases, gemstones, ruby, glass, pearl, opal, alloys etc.|
|Solid||Liquid||Sol||Ag sol, Au sol, starch sol, As2S3 sol, paints, cell fluids etc.|
|Liquid||Liquid||Emulsion||Milk, hair cream, oil-water, water-oil mixtures.|
|Liquid||Gas||(Aerosol||Fog, mist, cloud, insecticide, sprays.|
|Gas||Liquid||(Foam||Froth, whipped cream, soap lather, shaving cream.|
|Gas||Solid||(Solid foam or solid (sol||Pumic stone, foam rubber, styrene foam.|
Classification of Colloids :
Colloids are classified as follows:
→ On the basis of affinity of dispersed phase for dispersion medium : According to this classification, colloidal solutions are of two types:
- Lyophilic colloid.
- Lyophobic colloid.
→ Lyophilic colloids or Reversible colloids or emulsified or Intrinsic colloids : The term ‘lyophilic’ means ‘liquid loving’. The colloidal solutions which are directly formed by mixing substances like gum, gelatin, starch, rubber etc. with a suitable dispersion medium are called lyophilic colloids. In this type of colloid, the dispersed phase and dispersion medium have high affinity for each other.
→ These sols are quite stable due to the presence of a layer of dispersion medium around the particles of dispersed phase. They cannot be separated easily. If the dispersion medium is separated from the dispersed phase by evaporation, then the sol can be easily brought back in the colloidal form by simply remixing with dispersion medium, So, such sols also are called as reversible sols. These sols are highly stable and cannot be coagulated easily.
→ Lyophobic colloids or Irreversible colloids or suspended or extrinsic colloids : The term ‘lyophobic mean ‘liquid hating’. The colloidal solutions which are not easily formed; are known as lyobhobic colloids. The colloidal solution of substances like metals, their sulphides, their hydroxides are prepared only by applying some special methods.
→ In lyophobic colloids, the dispersed phase and dispersion medium have very little affinity for each other. So these sols are not stable. They are readily precipitated or coagulated on addition of small amounts of electrolytes by heating or by shaking. If such colloidal sols are once precipitated they do not give back the colloidal sol by simple addition of the dispersion medium. Hence these sols are also known as irreversible sols. These sols require some stabilizing agents for their preservation.
→ The main points of differences between lyophilic and lyobhobic colloids are give in table 5.5
Differences between lyophobic and lyophilic colloids.
On the Basis of Particles of Dispersed Phase : On the basis of structure of colloidal particles, the colloids are classified into following types:
→ Micro-molecular or multi-molecular colloids : When a colloidal particle is made up of aggregates of atoms or molecules with molecular size less than 1 nm, then it is called multi-molecular colloid. For example, a gold sol may contain particles of various sizes having many atoms. Sulphur sol consists of particles containing a thousand or more of S, sulphur molecules. Moreover silver sol, albumin sol, silicone’s sol etc, are the examples of such class.
→ Macromolecular colloids : In macromolecular colloids, the colloidal particles are themselves large enough to have the colloidal dimensions i.e., 1 nm to 1000 nm. These colloids are quite stable and resembles true solutions in many respects. Their molecular masses are also very high. Examples : Naturally occurring macromolecules like starch, cellulose, proteins, gum arabic and those of synthetic macromolecules like polythene, nylon, polystyrene, synthetic rubber etc.
→ Associated colloids or micelles or micelle colloids : There are some substances which behave as strong electrolyte at low concentration but at higher concentration they show colloidal behaviour due to the formation of aggregates. These aggregate particles are called as micelles and they are also known as associated colloids. These particles are associated due to the presence of cohesive forces or van der Waal’s forces. Examples: soaps and detergents.
Some important terms related to associated colloids :
→ Critical micelles-concentration (CMC) : That concentration, below which substance behaves as electrolyte and above which behaves as colloids, is known as critical micelles concentration. In other words, we can say that minimum concentration, on which first micelle is formed, is called critical micelles concentration.
→ Example : CMC of soap is 10-4 to 10-3 mol L-1. It should be remembered that at CMC, several properties of solutions of surface active agents such as molar conductance, surface tension and osmotic pressure undergo a dramatic change.
→ Kraft temperature: The formation of micelles takes places only above a particular temperature called Kraft temperature.
→ Micelle : On increasing the concentration of solution of soaps and detergents from CMC then the molecules of soaps and detergents associated and form aggregate of colloidal size. The aggregates are called micelles. The number of particles or molecules present in aggregation of a micelle is known as aggregation number.
→ Example : Soaps like sodium stearate, sodium laurate etc. detergents like sodium dodecyl sulphate, alkyl ammonium chloride etc.
Micelle formation : Formation of micelle in a soap like sodium stearate takes place as follow: First of all sodium stearate molecules ionise in water.
C17H35COO–Na+ → C17H35COO–Na+
Anion (C17H35COO–) obtained from ionisation can be written as:
→ Here hydrocarbon part i.e., C17H–35 is called tail and polar part (COO) is called head. In associated colloids both type (i.e., lyophobic and lyophilic) of parts are present. For example, in stearate ion, the long hydrocarbon part ie, C17H35 is hydrophobic while -COO– part is hydrophilic where C17H35 parts is known as tail while polar part is known as he while polar part (-COO–) is known as head.
→ At critical micelle concentration, the anions (C17 H35COO–) are pulled into the bulk of the solution and aggregate to form a spherical shape. In this spherical shape the long hydrcarbon chain (tail) which is insoluble in water is directed towards the centre while the soluble polar head is on the surface in contact with water. This aggregate with spherical shape is known as ‘ionic micelle’. These micelles may contain as many as 100 such ions (C17H35COO–).
→ Similarly in CH3(CH2)11 SO4– Na+ the polar group is -SO42- along with the long hydrocarbon chain. The mechanism of micelle formation of detergent molecules is same as that of soaps.
→ Cleansing action of Soaps : As we have discussed earlier that a micelle consists of a hydrophobic hydrocarbon like central core. During cleaning, soap molecules form micelle around the oil droplet in such a way that hydrophobic part of the stearate ions is in the oil droplet and hydrophilic part projects out of the grease droplet like the bristles..
→ The polar part of stearate ion can interact with water so the oil droplet surrounded by stearate ions is now pulled in water and gets detached from the dirty surface of fabric. In this way soap helps in emuslification and washing away of oils and fats. The negatively charged sheath around the globules prevents them from coming together and forming aggregates.