Unformatted text preview: f the molecule the more positive [S] as ethyl alcohol and propionic acid
o Non polar substances as Liquid petrolatum have negative [S] fail to spread on water
o For organic acids, as Oleic acid,
the longer the carbon chain decrease in polar character decrease [S] o Some oils can spread over water because they contain polar groups as COOH and OH Cohesive forces:
Benzene spreads on water not because it is polar but because the cohesive forces between its molecules are much weaker than the adhesion for water. 19 Application of Spreading coefficient in pharmacy The requirement of film coats to be spreaded over the tablet surfaces The requirement of lotions with mineral oils to spread on the skin by the addition of surfactants 20 Functional Classification
According to their pharmaceutical use, surfactants can be divided into the following groups: Wetting agents Solubilizing agents Emulsifying agents Dispersing, Suspending and Defloculating agents Foaming and antifoaming agents Detergents
21 Wetting agents Wetting agent is a surfactant that when dissolved in
water, lower the contact angle and aids in displacing the air phase at the surface and replacing it with a liquid phase. Solids will not be wetted if their critical surface tension is exceeded than the surface tension of the liquid. Thus water with a value of 72 dynes/cm will not wet polyethylene with a critical surface tension of 3 1 dynes/cm. Based on this concept we should expect a good wetting
agent to be one which reduces the surface tension of a liquid to a value below the solid critical surface tension. 22 According to the nature of the liquid and the solid, a drop of liquid placed on a solid surface will adhere to it or no. which is the wettability between liquids and solids. When the forces of adhesion are greater than the forces of cohesion, the liquid tends to wet the surface and vice versa.
Place a drop of a liquid on a smooth surface of a solid. According to the wettability, the drop will make a certain angle of contact with the solid. A contact angle is lower than 90°, the solid is called wettable
A contact angle is wider than 90°, the solid is named nonwettable. A contact angle equal to zero indicates complete wettability. 23 complete
wetting incomplete wetting no
wetting . Ө = 0°
γ s – γsL> 0 Ө Ө
90° > 90° =Ө γ s – γsL ≈ 0 90° < Ө 180° = γ s– γsL< 0 24 The surface of liquid water (meniscus) has a concave shape because water wets the surface and creeps up the side
The surface of Mercury has a convex shape it does not wet glass because the cohesive forces within the drops are stronger than the adhesive forces between the drops and glass.
25 Micellar Solubilization Surfactant molecules accumulate in the interfaces between water and water insoluble compound. Their hydrocarbon chains penetrate the outermost layer of insoluble compound which combine with the waterinsoluble molecules. Micelles form around the molecules of the waterinsoluble compound inside the micelles’ cores and bring them into solution in an aqueous medium. This phenomenon...
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- Spring '14
- Surface tension, CMC, surfactants, interfacial tension