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Surface chemistry is the branch of science that deals with the nature of the surfaces and physical and chemical reactions between the two phases. A two-phase heterogeneous system in which one phase is the dispersed phase and the other is the dispersion medium, and the size of a particle is between 1-100 nm is a colloidal system. Dispersed phase: Component which is present in a small proportion. Dispersion medium: Component which is more in ratio.
For example, in a colloidal solution of mud in water, mud is the dispersed phase, and water is the dispersion medium. Based on the physical state of the dispersed phase and dispersion medium, the colloidal system can be classified as sols in which the dispersed phase is solid, and the dispersion medium is liquid. Sols can be classified into two types, as the lyophilic and lyophobic sols.
What are lyophilic sols?
As the word suggests, ‘lyophilic’ means ‘water loving’ or ‘solvent loving.’ There is a strong attraction between the dispersion medium and the dispersed phase in lyophilic sols. These sols are more stable and reversible. The dispersed phase in lyophilic sols does not readily precipitate. If water is used as the medium of dispersion, then such sols are also called hydrophilic sols. Examples of lyophilic sols can be the colloidal solution of starch, gum, gelatin, etc.
- Starch Sol: Starch forms a lyophilic sol with water as a dispersion medium. Starch sol is not affected by the presence of impurities in it. The formation of a sol is quicker if the water heats at 100°C.
- Gum Sol: Gum Sol is a lyophilic sol with water. Warm water is used to prepare the gum sol because gum is quite soluble in warm water.
What are lyophobic sols?
The word lyophobic means ‘liquid-repelling or hating.’ The dispersion medium has little or no interaction with the dispersed phase in these sols. These sols are relatively less stable than lyophilic sols. The addition of stabilizers is required in their preparation. Sols of various metals such as gold, silver, etc., are lyophobic sols examples, along with their insoluble compounds like sulphides and oxides such as arsenious sulphide, etc.
- Ferric Hydroxide Sol:
- Aluminium Hydroxide Sol:
- Arsenious Sulphide Sol:
Differentiation between lyophobic and lyophilic sols:
- The main lyophobic and lyophilic sols difference lies in the interaction between the two phases: Lyophilic sols have a strong force of interaction between the dispersion medium and the dispersed phase. In lyophobic sols, the interaction is weak, making them less stable.
- Reversible/Irreversible nature: Since there is less interaction between the two phases in lyophilic sols, they are easily reversible. Lyophobic sols are irreversible.
- Methods of preparation: The preparation of lyophilic and lyophobic sols is essentially different. Lyophilic sols can be prepared by directly shaking, mixing, or heating the two phases. Lyophobic sols require distinct methods such as chemical or non-chemical methods, disintegration or condensation methods, etc.
- Use of stabilisers: In the preparation of lyophilic sols, stabilisers are not required. However, in lyophobic sols, additional stabilisers are added.
- Organic substances like starch, gum, proteins, etc., form lyophilic sols, and inorganic materials like metals and their oxides, sulphides, etc., form lyophobic sols.
- Charge: The lyophilic sols can be positively, negatively, or neutrally charged, whereas the charge of lyophobic sols can be positive or negative.
Preparation of lyophilic and lyophobic sols:
The lyophobic and lyophilic sols’ difference in the interaction of the two phases also leads to varied preparation methods. Since the force of interaction between the two phases in lyophilic sols is strong, they can be prepared quickly, or we can say even simply by mixing the particles in the dispersed phase with the liquid or medium. However, there is less affinity between the two phases in lyophobic sols, and therefore, the preparation of the lyophobic colloids requires the distinct methods discussed below –
- Disintegration or dispersion methods: These methods include some physical techniques (to reduce particle size to the size of colloidal particles, i.e., between 1nm to 1000nm) such as:
- Electrical disintegration [Bredig’s arc method] used for preparing colloidal sols of metals
- Peptisation
- Mechanical dispersion method is used in preparing sols of indigo, sulphur, printer ink, toothpaste, ointments, etc.
- Condensation methods:
- Chemical methods:
- Non-chemical condensation methods include the exchange of solvent or condensing vapours (for example, a colloid of Sulphur NH4NO3.)
Purification of lyophilic and lyophobic sols
The colloidal sols which are prepared using various methods are in an impure form, with suspended particles and electrolytes present in them. A higher concentration of impurities affects the stability of colloidal sols. Therefore, the purification of sols is an essential step during their preparation. Various purification methods are possible such as dialysis, electro-dialysis, ultra-filtration, ultra-centrifugation, etc.
Conclusion
Sols are the heterogeneous mixture of two phases wherein one phase is dispersed phase (particles) and another phase is the dispersal medium (liquid). They are divided into two types depending upon the affinity of the interaction between the dispersed phase and dispersion medium. The sols which have a strong interaction between the two phases are lyophilic, and those which have a weaker interaction are the lyophobic sols.
The lyophilic sols are easier to prepare in comparison to lyophobic sols. Simply by shaking, heating or mixing can form lyophilic sols. Examples are starch, gelatin, gum sols, etc. Lyophobic sols require different methods of preparation, such as chemical methods, physical disintegration methods, condensation methods, etc. Lyophobic sols examples are metal sols such as gold, silver, etc. These sols are impure. Hence, the purification of sols is a critical step during their preparation process.
