Question

The critical surface tension for wetting skin is 28.5 dynes/cm. Surface tension of 3 popular oils in the cosmetic industry are: Coconut oil = 29.5, Isopropyl myristate IPM) = 28.4 dynes/cm and cyclomethacone (CM) = 18.5 dynes /cm. Which of the following statements are true for spreading of these oil on skin:

a. All three oils will spread on skin

b. Coconut oil will spread better than IPM and CM

c. IPM and CM will spread better than coconut oil

d. IPM will spread better than CM

Answer 1

Given, critical surface tension= 28.5 Dynes/cm

Surface tension of coconut oil= 29.5 Dynes/cm

Surface tension of IPM= 28.4 Dynes/cm

Surface tension of CM= 18.5 dynes/cm

For each liquid to be spreading upon the surface of the molecule, critical surface tension of wetting should be more than the surface tension of the liquid itself.

The increase in adhesive force between the liquid and the solid or a decrease in the cohesive force (surface tension) within the liquid, result in a greater wettability at smaller contact angle.

Therefore, the liquid with surface tension lesser than critical surface tension will spread better.

Thus, IPM and CM will spread better than coconut oil (as both have low surface tension than critical surface tension).

Option (C) is correct.

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