Transcript Physical Pharmacy SURFACE TENSION

Physical Pharmacy
SURFACE TENSION
By
Abdul-Wahhab Khedr
Demonstrator of Pharmaceutics and Industrial
Pharmacy
Surface tension of liquids
• Why the free drops of water form spherical droplets?
Surface Tension:
• Equals the work (W) or energy necessary to
increase the surface area (A) by 1 cm2
W
 
A
dyne
dyne  cm
• Units:

 
cm
cm 2
• Is defined as “ The force in (dynes) acting along
the surface of a liquid at right angle to any line 1
cm in length”.
Experimental Determination of Surface Tension
• 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
Experiment
• 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:
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.
Results
Sample
Number of Drops
n1
n2
n3
Mean (n)
Water
Surface tension
(dyne/cm)
72.8
Sample 1
Sample 2
Sample 3
sample nH O

H O nsample
2
2
Comment
• 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
m.g  2 .r  
 m
 1 m1

 2 m2
Comment
• 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)
M   .V
• 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.
M  .V

• Here, the average mass (m) of one drop is m 
 1 m1

 2 m2
 1  1.V .n2

 2  2.V .n1
n
 1 n2

 2 n1
n