# Surface and Interfacial Phenomenon: Spreading coefficient and MCQs for GPAT, NIPER, Pharmacist and Drug Inspector exam

## Surface and Interfacial Phenomenon: Spreading coefficient and MCQs for GPAT, NIPER, Pharmacist and Drug Inspector exam

Spreading Coefficient (S): In general, spreading of a liquid occurs when the work of adhesion between two liquids exceeds the work of cohesion between the molecules of each liquid.

Work of adhesion (Wa)- Consider a liquid drop with surface tension ϒLV and a solid surface with surface tension ϒSV. When the liquid drop adheres to the solid surface, it forms a surface tension ϒSL. The work of adhesion is simply the difference between the surface tensions of the liquid/vapour and solid/vapour and that of the solid/liquid. The work of adhesion is given by the following equation:

Wa = ϒSV + ϒLV  – ϒSL

Work of cohesion (Wc)- The work of cohesion is the work of adhesion when the two phases are the same. Consider a liquid cylinder with unit cross-sectional area. When this liquid is subdivided into two cylinders, two new surfaces are formed. The two new areas will have a surface tension of 2ϒLV and the work of cohesion is expressed by the following equation:

Wc = 2ϒLV

The spreading coefficient (S) is the difference between the work of adhesion and the work of cohesion (Wa – Wc). This implies that if the work of adhesion is more than the work of cohesion, spreading will occur.

Then,

Wa – Wc = ϒL + ϒS – ϒLS – 2ϒL

S = ϒL + ϒS – ϒLS

S = ϒS  – (ϒL + ϒLS)

where ϒS  refers to the surface tension of the sublayer liquid, ϒL  refers to the surface tension of spreading liquid and ϒLS  refers to the interfacial tension between the two layers.

Spreading occurs when the surface tension of the sublayer liquid is greater than the sum of the surface tension of the spreading liquid and the interfacial tension between the sublayer and the spreading liquid.

1. If S is positive or zero, i.e. when ϒS is larger or equal to ϒL + ϒLS, spreading will take place.
2. If S is negative, i.e. when ϒL + ϒLS is larger than ϒs, the spreading liquid forms a globule or

a floating lens and spreading will not take place.

• Fatty alcohols and acids have high spreading coefficients because of the presence of polar groups such as OH and COOH, respectively (oleic acid spreads on the surface of water).
• As the nonpolar character of these molecules is increased by increasing the hydrocarbon chain, the spreading coefficient gradually decreases (liquid petroleum fails to spread on water).
• Benzene spreads on water not because of its polar nature but because of its cohesive forces, which are much weaker than the adhesive forces.

Table 1 – Spreading coefficient of some liquids at 20°C

 Liquid Spreading coefficient (S) (dyne cm -1) Benzene 8.8 Hexane 3.4 Octane 0.2 Toluene 6.8 Ethanol 50.4 Acetone 42.4 Oleic Acid 24.6 Chloroform 13.0 Hexadecane -9.3 Liquid parafłn -13.4

Multiple Choice Questions (MCQs)

1.In general, spreading of a liquid occurs when the work of adhesion between two liquids ____ the work of cohesion between the molecules of each liquid.

a)increases

b)decreases

c)exceeds

d)remains unchanged

a)S

b)S.Co

c)Sp

d)Wa

3.Work of adhesion is given by following equation

a)Wa = ϒSV + ϒLV  – ϒSL

b)S = ϒL + ϒS – ϒLS

c)Wa – Wc = ϒL + ϒS – ϒLS – 2ϒL

d)Wc = 2ϒLV

4.Work of cohesion (Wc) is given by following equation

a)Wa = ϒSV + ϒLV  – ϒSL

b)S = ϒL + ϒS – ϒLS

c)Wa – Wc = ϒL + ϒS – ϒLS – 2ϒL

d)Wc = 2ϒLV

5.If the work of adhesion is more than the work of cohesion, spreading will occur.

a)True

b)False

6.The spreading coefficient (S) is given by following equation

a)Wa – Wc = ϒL + ϒS – ϒLS – 2ϒL

b)S = ϒL + ϒS – ϒLS

c)S = ϒS  – (ϒL + ϒLS)

d)All of the above

7.On increasing the temperature, the kinetic energy of the liquid molecules

a)Lowered

b)Remains constant

c)Increases

d)Decreases

8.Spreading occurs when the surface tension of the sublayer liquid is _____ than the sum of the surface tension of the spreading liquid and the interfacial tension between the sublayer and the spreading liquid.

a)smaller

b)greater

c)equal

d)None of the above

9.If S is positive or zero, i.e. when ϒS is larger or equal to ϒL + ϒLS, spreading will take place.

a)True

b)False

10.If S is negative, i.e. when ϒL + ϒLS is larger than ϒs, the spreading liquid forms a globule or

a floating lens and spreading will not take place.

a)True

b)False

11.If adhesive force>cohesive force, then what occurs

a)Wetting

c)Capillary rise

d)All of above

12.Fatty alcohols and acids have ____ spreading coefficients

a)Lower

b)Higher

c)Optimum

13.As the nonpolar character of these molecules is increased by increasing the hydrocarbon chain, the spreading coefficient gradually

a)Increases

b)Decreases

c)Remains unchanged

d)Depends on the nature of hydrocarbon chain

14.The displacement of a solid-air interface with a solid-liquid interface is known as

a)Foaming

b)Antifoaming

c)Wetting

d)Wicking

15.Benzene spreads on water not because of its polar nature but because of its cohesive forces, which are much stronger than the adhesive forces.

a)True

b)False

Solutions:

1. c)exceeds
2. a)S
3. a)Wa = ϒSV + ϒLV  – ϒSL
4. )Wc = 2ϒLV
5. a)True
6. d)All of the above
7. c)Increases
8. b)greater
9. a)True
10. a)True
11. d)All of above
12. b)Higher
13. b)Decreases
14. c)Wetting
15. b)False

References:

1. GAURAV KUMAR JAIN – THEORY & PRACTICE OF PHYSICAL PHARMACY, 1st edition 2012 Elsevier, page no. 120-123.

2. Martins Physical Pharmacy, 6th edition 2011, page no. 665-669.

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