Physical PharmacyPhysical Pharmacy

SURFACE TENSIONSURFACE TENSION

ByAbdul-Wahhab KhedrAbdul-Wahhab Khedr

Demonstrator of Pharmaceutics and Industrial Pharmacy

Surface tension of liquidsSurface tension of liquids

• Why the free drops of water form spherical droplets?

Surface Tension:Surface Tension:

• Equals the work (W) or energy necessary to increase the surface area (A) by 1 cm2

• Units:Units:

• Is defined as “ The force in (dynes) acting along the surface of a liquid at right angle to any line 1 cm in length”.

A

W

2cm

cmdyne

cm

dyne

Experimental Determination of Surface TensionExperimental Determination of Surface Tension

• Drop weight and Drop number method: Drop weight and Drop number method: (Using Stalagmometer)

• In this method the surface tension of a liquid (γ1) can be determined in relation to another liquid of known surface tension (γ2).

a

b

ExperimentExperiment• By using Stalagmometer, determine the relative

surface tension of the given liquids with reference to distilled water (S.T=72.8 dyne/cm at 20⁰C)

• Procedure:Procedure:1. Wash the stalagmometer several times with water

before use.2. Introduce the flat tip into small beaker half-filled

with water, suck water till its level becomes above the upper mark (avoid entrance of air bubbles).

3. Adjust the level of water to the upper mark and remove any suspended drop.

4. Count the number of drops falling until the level of water reaches the lower mark (n1).

5. Repeat the procedure twice again and determine (n2) & (n3).

6. Repeat the procedure for the liquid to be measured after washing the apparatus.

ResultsResultsSample Number of Drops Surface tension

(dyne/cm)n1 n2 n3 Mean (n)

Water 72.8Sample 1 Sample 2

Sample 3

sample

OH

OH

sample

n

n 2

2

CommentComment• The drop of a mass (m) gets released

when its weight G=mg is equal or greater than the surface force at the end of tube

rgm .2.

m2

1

2

1

m

m

CommentComment

• There are two line marks on the stalagmometer: top line above the wide part and bottom line bellow it. The volume between these two lines is (V), and liquid with density (ρ) contained in this volume has a mass (M)

• Such a volume V corresponds to n drops, which are released from the stalgmometer upon the decrease of liquid level from top to bottom line mark.

• Here, the average mass (m) of one drop is

VM .

n

V

n

Mm

.

2

1

2

1

m

m

12

21

2

1

..

..

nV

nV

1

2

2

1

n

n