Spreading Coefficient – Physical Pharmacy
- Define spreading coefficient.
- Give application of spreading coefficient.
- Give derivation of spreading coefficient.
- Why benzene spreads on water.
- What are two way by which you can increase spreading coefficient of given organic substance.
1. Define spreading coefficient.
2. Give application of spreading coefficient.
When a drop of oleic acid is placed on the surface of water, it will spread as a film. If the spreading is spontaneous, the surface free energy of the systems is reduced. The principles of spreading coefficient find applications in pharmacy.
- Absorption of medicament: When a cream or lotion is applied on the skin, it has to be spread on the affected area and permits absorption of medicament. Normally surfactants are added to the formulation to increase the polarity and the spreading coefficient.
- Stabilization of emulsions: A blend of surfactants are mixed with an oil and its spreading on the water surface layer is evaluated. Based on the results, a suitable blend of surfactants is selected for the preparation of a stable emulsion.
3. Give derivation of spreading coefficient.
- Calculate work of cohesion (Wc).
- Calculate work of adhesion. (Wa)
- Carry our subtraction of Wa – Wc to calculate S.
- Write inference of positive or negative value of S.
|Representation of the work of adhesion involving the separation of sublayer (S: water) and an overlaying liquid (L: oleic acid)|
Carry our subtraction of Wa – Wc to calculate S.
4. Why benzene spreads on water.
5. What are two way by which you can increase spreading coefficient of given organic substance.
- the presence of polar functional group such as -COOH or -OH in the structure. Examples are propanoic acid (S = 45.8); ethyl alcohol (S = 50.4).
- reducing the nonpolar chain length. Example are petrolatum (S = -13.4); oleic acid (S = 24.6).