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Factors Affecting Spreading:
1.If excess of spreading material is added, although initial spreading has occurred, saturation takes place and material may coalesce on the surface.
Thus when mutual saturation takes place, spreading coefficient is decreased or becomes negative. In case of organic liquid spread on water, the final spreading coefficient is always negative. Duplex films of this type are unsaturated and form monolayer with excess materials remaining as a less on the surface.
2.The types of molecular structure decide the spreading coefficient. An oil spread over water because it contain polar groups such as - COOH or - OH. Ethyl
alcohol and propionic acid have high value of S. As
number of polar groups increase, there is more spreading on water. As polar/ non-polar character increases, spreading coefficient increases. As the carbon chain of an acid increases, the ratio of polar/non-polar character decreases and spreading coefficient on water decreases.
3. If cohesive force between molecules is less than adhesive force between sublayer and spreading liquid, the liquid spreads e.g., benzene spreads on water not because it is polar but because the cohesive force between its molecules are much weaker than the adhesion for water.
Spreading coefficient has important application in pharmacy e.g., in order for lotion with a mineral oil base to spread freely and evenly on the skin its polarity and hence its spreading coefficient should be increased by addition of surfactant.
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