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LIQUID INTERFACES :

Surface and Interfacial Tensions

In the liquid state, the cohesive forces between adjacent molecules are well developed.

For the molecules in the bulk of a liquid – They are surrounded in all directions by other molecules for which they have an equal attraction.

For the molecules at the surface (at the liquid/air interface) –

• Only attractive cohesive forces with other liquid molecules which are situated below and adjacent to them.
• They can develop adhesive forces of attraction with the molecules of the other phase in the interface.
• The net effect is that the molecules at the surface of the liquid experience an inward force towards the bulk of the liquid and pull the molecules and contract the surface with a force F .

SURFACE AND INTERFACIAL TENSION:

SURFACE TENSION [γ ] is the force per unit length that must be applied parallel to the surface so as to counterbalance the net inward pull and has the units of dyne/cm.

INTERFACIAL TENSION is the force per unit length existing at the interface between two immiscible liquid phases and has the units of dyne/cm.

Invariably, interfacial tensions are less than surface tensions because an adhesive forces, between the two liquid phases forming the interface are greater than when a liquid and a gas phase exist together. If two liquids are completely miscible, no interfacial tension exists between them. Greater surface tension reflects higher intermolecular force of attraction, thus, increase in hydrogen bonds or molecular weight cause increase in ST.

SURFACE FREE ENERGY:

The work W required to create a unit area of surface is known as SURFACE FREE ENERGY/UNIT AREA (ergs/cm2) erg = dyne . cm. Its equivalent to the surface tension γ . Thus the greater the area A of interfacial contact between the phases, the greater the free energy.

W = γ ∆ A

For equilibrium, the surface free energy of a system must be at a minimum. Thus Liquid droplets tend to assume a spherical shape since a sphere has the smallest surface area per unit volume.

Multiple choice questions(MCQs)

1.The boundary between two or more phases is called

a)surface

b)interface

c)both of these

d)none of these

2.Several types of interface can exist depending on whether the two adjacent phases are in

a)solid phase

b)liquid phase

c)gaseous phase

d)all of the above

3.Rapid increase in solubility of a surfactant solution alcohol above a definite temperature is

a)Cloud point

b)Kraft point

c)CMS

d)Triple point

4.Surface tension of a liquid is at critical temperature

a)Zero

b)One

c)Negative

d)Maximum

5.For the molecules in the bulk of a liquid which of the following statement is correct?

a)They are surrounded in all directions by other molecules for which they have an equal attraction

b)Only attractive cohesive forces with other liquid molecules which are situated below and adjacent to them

c)They can develop adhesive forces of attraction with the molecules of the other phase in the interface

d)The net effect is that the molecules at the surface of the liquid experience an inward force towards the bulk of the liquid and pull the molecules and contract the surface with a force F

6.A water exposed to atmosphere is an example of

a)Solid surface

b)Liquid surface

c)Gas interface

d)Liquid interface

7.Interfacial tension are invariably ____ surface tension

a)Less than

b)Equal to

c)More than

d)Double than

8.For the molecules at the surface (at the liquid/air interface) which of the following statement is incorrect?

a)They are surrounded in all directions by other molecules for which they have an equal attraction

b)Only attractive cohesive forces with other liquid molecules which are situated below and adjacent to them

c)They can develop adhesive forces of attraction with the molecules of the other phase in the interface

d)The net effect is that the molecules at the surface of the liquid experience an inward force towards the bulk of the liquid and pull the molecules and contract the surface with a force F

9.The formula used for the determination of surface tension by capillary rise method

a)2ᵞ=πrcosⱷ

b)2ᵞ=πr2cosⱷ

c)2ᵞ=hrdg

d)2ᵞ=hr2Ƿg

10.With rise in temperature surface tension of a liquid

a)Increases

b)Decreases

c)Remains the same

d)None of the above

11.How do insects such as pond skaters stay afloat on water?

a)Because of high surface tension of water

b)As they can swim

c)Because they are less dense than water

d)None of the above reasons

12.Cohesive forces are the forces acting…

a)Between molecules of different materials

b)Between molecules of same material

c)Between water and glass capillary tube

d)Due to gravity

13.The contact angle forming between magnesium stearate and water is larger than that between lactose and water because:

a)Magnesium stearate is more hydrophilic

b)Magnesium stearate is more hydrophobic

c)Lactose has more surface energy

d)Both have equal hydrophilicity

14.Which of the following values for contact angle with water corresponds to a hydrophobic drug particle?

a)45°

b)120°

c)0°

d)None of the above

15.Surface tension is usually defined as the change in surface free energy per unit change in

a)Length

b)Area

c)Volume

d)Density

Solutions:

1. b)interface
2. d)all of the above
3. b) Kraft point
4. a) Zero
5. a)They are surrounded in all directions by other molecules for which they have an equal attraction
6. b) Liquid surface
7. a) Less than
8. a)They are surrounded in all directions by other molecules for which they have an equal attraction
9. c) 2ᵞ=hrdg
10. b) Decreases
11. a) Because of high surface tension of water
12. b) Between molecules of same material
13. b) Magnesium stearate is more hydrophobic
14. b) 120°
15. b)Area

References:

1. GAURAV KUMAR JAIN – THEORY & PRACTICE OF PHYSICAL PHARMACY, 1st edition 2012 Elsevier, page no. 109-122.

2. Martins Physical Pharmacy, 6th edition 2011, page no. 653-657.

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The post Surface and interfacial phenomenon: Liquid interface, surface and interfacial tensions, surface free energy and MCQs for GPAT, NIPER, Pharmacist and Drug Inspector exam appeared first on Gpatindia: Pharmacy Jobs, Admissions, Scholarships, Conference,Grants, Exam Alerts.