PHM2213 Practical

interfacial tension

PHM2213 Physical Pharmacy 2

Determination of interfacial tension

Objectives

To determine the interfacial tension of oil/water with and without surfactants.

Theory

Molecules on the surface of a liquid or at the interface are subjected to an unbalanced force of molecular attraction as the other molecules of the liquid tend to pull those at the surface inward. This phenomenon enables the liquid to maintain the minimum surface area. The magnitude of this force acting perpendicular to a unit length of a line in the surface is called the surface tension (γ) in dynes cm-1 or Newton meter-1.

Surface tension can be defined as a tension that exists between a liquid and the atmosphere. On the other hand, interfacial tension exists at the interface between two immiscible liquids.

Experimental

Apparatus

Du Nouy Tensionmeter

Volumetric flask and beakers.

Chemicals

vegetable oil

anionic surfactant

non-ionic surfactant

Sample preparation – Surfactant in oil only

  1. Weigh 20 g of oil into a 50 ml beaker.
  2. Prepare a total of 8 beakers.
  3. Add non-ionic surfactants at the following concentration to the oil:
    0, 0.005, 0.01, 0.02, 0.03, 0.04, 0.05 and 0.10% w/w (e.g. at 0.02%w/w, surfactant is 0.004g. This amount is very difficult to handle. To facilitate the weighing, prepare 1%w/w of surfactant in oil. Therefore to prepare a 0.02% w/w surfactant in oil, you need to add (20g x 0.02% / 1%) 0.4 g of 1% surfactant solution.

Sample preparation – Surfactant in water only

  1. Weigh 20 g of water into a 50 ml beaker.
  2. Prepare a total of 8 beakers.
  3. Add anionic surfactants at the following concentration to the water:
    0, 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, and 0.10% w/w.

Sample preparation – Surfactant in both water and oil

  1. Repeat step 1 to 3 BUT use only half the concentration: 0, 0.0025,0.005,0.01,0.015, 0.02, 0.025 and 0.05% w/w.
  2. Repeat step 4 to 6 BUT use only half the concentration: 0, 0.0025,0.005,0.01,0.015, 0.02, 0.025 and 0.05% w/w.

Measure the interfacial tension of samples prepared:

a)surfactant in oil (step 3) – water

b)surfactant in water (step 6) – oil

c)surfactant in oil (step 7) – surfactant in water (step 8)

Using the tensiometer – methods

Step 1 Cleaning of platinum ring and zeroing tensiometer

  1. Clean the platinum ring by dipping it chromic acid for 2-3 minutes.
  2. Wash it in six changes of water in beaker.
  3. Dry the plate with filter paper and avoid touching it with fingers.
  4. Attach the suspension to the arm of the tension balance and unclamp the beam.
  5. Set the pointer to the reading of zero until the pointer comes to zero mark.
  6. Clamp the beam and make sure that it remains at the zero position.

Step 2 Measuring surface tension of water

  1. Clean the beaker provided with chromic acid.
  2. Wash well with water and place about 20ml of water in the beaker.
  3. Place the beaker in the platform below the platinum ring.
  4. Lower the platform and beaker to the minimum extent by means of the adjusting screw.
  5. Adjust the position of the beaker and platform so that the surface of the liquid is about 1 cm below the platinum ring.
  6. Adjust the beaker until the ring is at the interface and the lever arm is in the zero position.
  7. Increase the tension of the wire by lowering the beaker, while keeping the lever arm at zero.
  8. The reading of the tensiometer when the film breaks, is the apparent surface tension.

Step 3 Measuring interfacial tension

  1. Place water in a beaker and raise the platform until the ring is immersed approximately 10mm in the water.
  2. Carefully add oil until the layer is approximately 10mm,or deep enough to prevent the ring from entering the upper face before the film breaks.
  3. Adjust the beaker until the ring is at the interface and the lever arm is in the zero position.
  4. Increase the tension of the wire by lowering the beaker, while keeping the lever arm at zero.
  5. The readingof the tensiometer when the film at the interface breaks, is the apparent interfacial tension.

Results and discussion

Tabulate your results.

Table 1 Surface tension of water

Tensiometer model / Surface tension (dyne/cm)
Tensiometer no. / 1
Ring no. / 2
Temperature (C) / Average

Table 2 Interfacial tension at oil-water interface

Type of oil / Type of surfactant
concentration of surfactant (% w/w), c / log c / a) / b) / c)
0
0.005
0.01
0.02
0.03
0.04
0.05
0.10

Plot interfacial tension in dyne/cm, , against log (c). Label your graph properly.

Write a report within one week after the practical.

Include the following in your discussion:

  1. Is there any difference in the drop in interfacial tension as concentration increased? Why?
  2. Predict the emulsion stability based on the interfacial tension results.

Prepared by: Kausar Ahmad

Date created: 20-Nov-2003

Date updated: 31-Dec-04

Page 1 of 3