Hemin J Majeed MSc. Pharmaceutical sciences Physical Pharmacy Lab 3- Surface tension Hemin J Majeed MSc. Pharmaceutical sciences

SURFACE TENSION   Surface tension: is the force per unit length (dyne/cm) on the surface of liquid which opposes expansion of the surface area. Interface: The boundary between two or more phases exist together. Cohesion force: attractive force existing or develop between like molecules e.g. liquid and liquid. adhesion force: attractive force existing between unlike molecules e.g. liquid and air.

Methods: 1. Capillary rise method: If a capillary tube is immersed in a liquid such as water contained in beaker the liquid immediately rises up the tube to a certain height , this rise of a liquid in the tube occurs because the force of adhesion between water molecule and the capillary wall is greater than the force of cohesion between water molecules. By measuring the rise in the capillary, it possible to determine the surface tension of the liquid.

The liquid in the capillary tube continuous to rise till the up word movement is just balanced by the down word force of gravity due to the weight of the liquid. So we have two forces opposing each other the up word force due to the surface tension and the counteracting force due to the weight of the column of liquid in the tube.

Force due to the surface tension a = 2π r У Force due to the weight of the liquid column d = π r²h (P P˚) g π r² : is the cross sectional area of the capillary tube h: is the height of liquid column in the capillary tube up to the lowest point of the meniscus. P and p˚: are the densities of the liquid and its vapor. g: is the acceleration due to gravity.

The density of liquid is being much greater than the density of its vapor , the weight of the liquid in the column being much greater than the weight of the liquid above h in the meniscus. So P˚ will be disregarded So d = π r²hP g at equilibrium the up word force is equal to the down word force thus 2 π r У = π r²hP g У = π r²hP g = 1 r h p g 2π r 2 Aim: to measure the surface tension of different liquids and to compare between them.

Procedure : 1. Different liquids will be used in this experiment , water(1 g/ml), orange oil(0.84g/ml), , castor oil(0.97g/ml), and ethanol(0.789g/ml). 2.Full 5 beakers with the 5 liquids. 3. Take a 5 capillary tubes of a known radius and immerse it in the beakers , the liquids will start to rising up in the tubes, wait until the rising of the liquids will stop and measure the height of the liquids in each tube and use it in the calculation of the surface tension.

Drop weight method The surface tension of liquid is related to the weight of a drop of that liquid which falls freely from the end of a tube by the expression: У = w x g / 2 π r W : is the weight of one drop of liquid

Procedure: 1. Different liquids will be used in this experiment , water (1g/ml), orange oil(0.84g/ml), castor oil(0.97g/ml), and ethanol(0.789g/ml). 2. Check that the glass tips are clean and free from defects particularly around the edges. 3. Each liquid will be sucked to a certain height in the capillary tube. 4. Allow the liquid drops to detach slowly from the tip and collect 10-15 drops in a beaker on a balance under constant temp condition.

Notes: a. It is important that the drops has been correctly formed and detached. b- Check that the end of the tip is horizontal.   5. calculate the weight of one drop of the liquid and use it in the calculation 6. When using volatile liquids take special care to prevent evaporation of the liquid from the tip and from the weighing during the determination.