The surface of a liquid has a tendency to contract to the minimum area possible. The contractile force is referred to as surface tension. Surfactants (surface active agents) are solutes that decrease the surface tension of solutions. Surfactants may be referred to as amphiphiles, they contain an appropriate balance of hydrophobic and hydrophilic regions, therefore have affinity for both polar and non-polar solvents. It is this property that allows them to be absorbed at interfaces. Surfactants are important in pharmaceutical science as they are used as emulsifying agents, solubilising agents, foaming or anti-foaming agents, flocculants, deflocculants and detergents. You will be given more details about these agents in your Physical Pharmaceutics lectures. There are a number of standard ways of measuring surface tension in this experiment you will investigate two standard methods. Students are to work in groups of two - four.
Measurement of Surface Tension - Surfactant Properties
Experiment 1
Drop Weight Method
If a drop of liquid forms slowly on the surface of a clean, vertical glass capillary, we can show that the mass of the drop is proportional to the surface tension. This is shown by Tates equation
mg 2prg
m mass of drop (kg) g gravitational constant (9.8ms-2) r radius of tube (m) g surface tension in Nm-1. The radius of the tube will influence the volume of the drop that forms. Unfortunately, ideal drops do not form hence a correction factor to account for the volume of the drop should be used.
The first experiment you will perform is a very simple assessment of how measurement of the mass of drops of liquid, can be used to investigate surface properties of drops. Although this is a very simple experiment, extreme care must be taken in measuring the correct number of drops and recording their masses accurately, otherwise meaningful results will not be obtained.
Method
You are supplied with a series of liquids, a glass beaker, a reservoir and tap arrangement that allows you to release solutions drop by drop. The liquids are distilled water and various concentrations of sodium chloride and cetrimide. You are required to measure the average mass of one drop of each of the liquids.
Make sure you are familiar with how to set the apparatus up to deliver one drop at a time.
1. Place the beaker on a clean, tarred balance and record the mass of the empty beaker.
2. Carefully place 20 drops of one solution into the beaker and re-weigh it.
3. Subtract the mass of the empty beaker to obtain the mass of the 20 drops of liquid.
4. Divide the mass obtained in part 3 by 20 to determine the average mass of 1 drop.
5. Without emptying the beaker, add 20 drops of a second liquid. Subtract from this, the mass recorded in part 2, to obtain the mass of 20 drops of this new solution.
6. Continue in this manner until you have recorded the average mass of 1 drop of each of the liquids supplied.
(NB. If you accidentally add more drops to the beaker this can be accounted for when you take the average mass of the drops ie if you have added 21 drops, divide the total mass by 21 instead of 20.)
Results
Mass of 20 drops distilled water 1.238 Average Mass of 1 drop 0.0619
Table Showing Mass of Drops of NaCl Solutions
Conc NaCl510152025Av. Mass of 1 drop0.072950.07570.078450.08100.08445Table Showing Mass of Drops of Cetrimide Solutions
Conc Cetrimide0.0050.0100.0150.020.0400.0600.080Av. Mass of 1 drop0.07070.06410.05420.0440.03580.03270.0296
Plot a graph of concentration NaCl and Cetrimide against mass of one drop. Ensure the graph is fully labelled and attach it to your report. The concentration of the solutions is plotted on the same x-axis, you will need to have 2 separate scales, 1 for concentration of cetrimide and 1 for concentration of NaCl. Mass of the drops in grams is plotted on the y-axis.
Clearly label the critical micelle concentration on the graph.
iii) Briefly explain here what a micelle is
Micelle is a globule of molecules mixed in water wherein the hydrophilic part will be buried in the surface of the water and the hydrophobic part will be pointed at the air above resulting in reduced surface tension. The water loving and the water hating parts of the molecule will orient themselves according to their favorite interaction with the surrounding solvent. A critical micelle concentration is a concentration at which the molecules of a surfactant will produce micelles there will an inflexion point on the graph as the concentration crosses the critical micelle concentration (White, 1972).
iv) Calculate the surface tension of i) 10 NaCl solution and ii) 0.02 cetrimide solution.
Use the equation mg 2prg Assume r 0.0022m
surface tension of 10 NaCl solution g 0.054 Nm-1
surface tension of 0.02 cetrimide solution g 0.031 Nm-1
v) Briefly explain what your results tell you about the nature of NaCl and cetrimide.
For a sodium chloride solution, the surface tension will increase linearly. For a concentration of 10 of NaCl, the average mass of one drop increases resulting in increase surface tension. Whereas, the addition of 0.02 of cetrimide lowers the average mass of one drop of water, resulting in reduced surface tension. Cetrimide here acts a surfactant and will bury their hydro-phillic part in the water and the hydro-phobic part will be pointed towards the air above. As the water-hating parts of the molecule will not bond together to each other strongly so the surface tension will be reduced (Gennes et al, 2003).
Experiment 2. Capillary Rise Method
This is an alternative method by which surface tension of liquids may be measured. The surface tension of a liquid is proportional to the height to which the liquid will rise in a narrow capillary, as shown in the equation below
g hrgr 2
Where g surface tension (Nm-1) h height of liquid (m) r density of liquid (kg m-3) g gravitational constant (9.8ms-2) r radius of curvature of meniscus (m).
NB. In this experiment you must be very careful to convert to the correct units throughout.
Method
1. Half fill 2 medium-sized weighing boats, 1 with distilled water and 1 with 0.08 cetrimide.
2. Take 1 capillary tube and place it vertically into the distilled water. Once the liquid has risen up the tube, place your finger over the end of the tube to seal it, remove it from the vessel and measure the height to which the liquid has risen.
3. Drain the contents of the tube, by placing the end of the tube onto tissue paper.
4. Place the same tube into the 0.08 cetrimide, repeating the above procedure, measure the height to which the cetrimide rises. Discard the tube once it has been used in the cetrimide. Note that you must take the measurement for distilled water first, take care not to get any cetrimide into the distilled water.
5. Repeat the entire procedure using 4 more capillary tubes.
Results
Heights of liquids in tubes in metres
Tube 1Tube 2Tube 3Tube 4Tube 5Average Height Distilled Water0.00270.00280.00260.00290.00260.002720.08 Cetrimide0.00240.00250.00260.00200.00240.00238
i) Calculate the surface tension of 0.08 cetrimide.
Use the equation g hrgr 2
Assume the density of the solution is 1g cm-3 and the radius r 0.002m.
( 1g cm-3 1000 Kg m-3
surface tension of 0.08 cetrimide g 0.00238 x 1000 x 9.8 x .0022 0.023 Nm-1
ii) On average, which of the two liquids rose to the greater height in the capillary tubes
Distilled Water rose to greater height.
iii) What is your explanation for this
The distilled water rose to a greater height than with cetrimide. This shows that the surface tension with cetrimide has dropped as the surface tension of a liquid is proportional to the height to which the liquid will rise in a narrow capillary.
iv)Why does placing the tip of the tube containing the liquid onto the tissue paper cause the liquid to drain out of the tube
Surface tension is in actual a property of liquids interface with another medium. Therefore, the surface tension between liquid - air and between liquid and any other material will be different. The surface tension between liquid and air will be greater than the surface tension between air and a material such as a tissue paper. Tissue papers will absorb through capillarity. As all forces are not going to balance for the liquid and tissue paper interface which will result in the flow of the liquid (Seddon and Templer, 1995).
Measurement of Surface Tension - Surfactant Properties
Experiment 1
Drop Weight Method
If a drop of liquid forms slowly on the surface of a clean, vertical glass capillary, we can show that the mass of the drop is proportional to the surface tension. This is shown by Tates equation
mg 2prg
m mass of drop (kg) g gravitational constant (9.8ms-2) r radius of tube (m) g surface tension in Nm-1. The radius of the tube will influence the volume of the drop that forms. Unfortunately, ideal drops do not form hence a correction factor to account for the volume of the drop should be used.
The first experiment you will perform is a very simple assessment of how measurement of the mass of drops of liquid, can be used to investigate surface properties of drops. Although this is a very simple experiment, extreme care must be taken in measuring the correct number of drops and recording their masses accurately, otherwise meaningful results will not be obtained.
Method
You are supplied with a series of liquids, a glass beaker, a reservoir and tap arrangement that allows you to release solutions drop by drop. The liquids are distilled water and various concentrations of sodium chloride and cetrimide. You are required to measure the average mass of one drop of each of the liquids.
Make sure you are familiar with how to set the apparatus up to deliver one drop at a time.
1. Place the beaker on a clean, tarred balance and record the mass of the empty beaker.
2. Carefully place 20 drops of one solution into the beaker and re-weigh it.
3. Subtract the mass of the empty beaker to obtain the mass of the 20 drops of liquid.
4. Divide the mass obtained in part 3 by 20 to determine the average mass of 1 drop.
5. Without emptying the beaker, add 20 drops of a second liquid. Subtract from this, the mass recorded in part 2, to obtain the mass of 20 drops of this new solution.
6. Continue in this manner until you have recorded the average mass of 1 drop of each of the liquids supplied.
(NB. If you accidentally add more drops to the beaker this can be accounted for when you take the average mass of the drops ie if you have added 21 drops, divide the total mass by 21 instead of 20.)
Results
Mass of 20 drops distilled water 1.238 Average Mass of 1 drop 0.0619
Table Showing Mass of Drops of NaCl Solutions
Conc NaCl510152025Av. Mass of 1 drop0.072950.07570.078450.08100.08445Table Showing Mass of Drops of Cetrimide Solutions
Conc Cetrimide0.0050.0100.0150.020.0400.0600.080Av. Mass of 1 drop0.07070.06410.05420.0440.03580.03270.0296
Plot a graph of concentration NaCl and Cetrimide against mass of one drop. Ensure the graph is fully labelled and attach it to your report. The concentration of the solutions is plotted on the same x-axis, you will need to have 2 separate scales, 1 for concentration of cetrimide and 1 for concentration of NaCl. Mass of the drops in grams is plotted on the y-axis.
Clearly label the critical micelle concentration on the graph.
iii) Briefly explain here what a micelle is
Micelle is a globule of molecules mixed in water wherein the hydrophilic part will be buried in the surface of the water and the hydrophobic part will be pointed at the air above resulting in reduced surface tension. The water loving and the water hating parts of the molecule will orient themselves according to their favorite interaction with the surrounding solvent. A critical micelle concentration is a concentration at which the molecules of a surfactant will produce micelles there will an inflexion point on the graph as the concentration crosses the critical micelle concentration (White, 1972).
iv) Calculate the surface tension of i) 10 NaCl solution and ii) 0.02 cetrimide solution.
Use the equation mg 2prg Assume r 0.0022m
surface tension of 10 NaCl solution g 0.054 Nm-1
surface tension of 0.02 cetrimide solution g 0.031 Nm-1
v) Briefly explain what your results tell you about the nature of NaCl and cetrimide.
For a sodium chloride solution, the surface tension will increase linearly. For a concentration of 10 of NaCl, the average mass of one drop increases resulting in increase surface tension. Whereas, the addition of 0.02 of cetrimide lowers the average mass of one drop of water, resulting in reduced surface tension. Cetrimide here acts a surfactant and will bury their hydro-phillic part in the water and the hydro-phobic part will be pointed towards the air above. As the water-hating parts of the molecule will not bond together to each other strongly so the surface tension will be reduced (Gennes et al, 2003).
Experiment 2. Capillary Rise Method
This is an alternative method by which surface tension of liquids may be measured. The surface tension of a liquid is proportional to the height to which the liquid will rise in a narrow capillary, as shown in the equation below
g hrgr 2
Where g surface tension (Nm-1) h height of liquid (m) r density of liquid (kg m-3) g gravitational constant (9.8ms-2) r radius of curvature of meniscus (m).
NB. In this experiment you must be very careful to convert to the correct units throughout.
Method
1. Half fill 2 medium-sized weighing boats, 1 with distilled water and 1 with 0.08 cetrimide.
2. Take 1 capillary tube and place it vertically into the distilled water. Once the liquid has risen up the tube, place your finger over the end of the tube to seal it, remove it from the vessel and measure the height to which the liquid has risen.
3. Drain the contents of the tube, by placing the end of the tube onto tissue paper.
4. Place the same tube into the 0.08 cetrimide, repeating the above procedure, measure the height to which the cetrimide rises. Discard the tube once it has been used in the cetrimide. Note that you must take the measurement for distilled water first, take care not to get any cetrimide into the distilled water.
5. Repeat the entire procedure using 4 more capillary tubes.
Results
Heights of liquids in tubes in metres
Tube 1Tube 2Tube 3Tube 4Tube 5Average Height Distilled Water0.00270.00280.00260.00290.00260.002720.08 Cetrimide0.00240.00250.00260.00200.00240.00238
i) Calculate the surface tension of 0.08 cetrimide.
Use the equation g hrgr 2
Assume the density of the solution is 1g cm-3 and the radius r 0.002m.
( 1g cm-3 1000 Kg m-3
surface tension of 0.08 cetrimide g 0.00238 x 1000 x 9.8 x .0022 0.023 Nm-1
ii) On average, which of the two liquids rose to the greater height in the capillary tubes
Distilled Water rose to greater height.
iii) What is your explanation for this
The distilled water rose to a greater height than with cetrimide. This shows that the surface tension with cetrimide has dropped as the surface tension of a liquid is proportional to the height to which the liquid will rise in a narrow capillary.
iv)Why does placing the tip of the tube containing the liquid onto the tissue paper cause the liquid to drain out of the tube
Surface tension is in actual a property of liquids interface with another medium. Therefore, the surface tension between liquid - air and between liquid and any other material will be different. The surface tension between liquid and air will be greater than the surface tension between air and a material such as a tissue paper. Tissue papers will absorb through capillarity. As all forces are not going to balance for the liquid and tissue paper interface which will result in the flow of the liquid (Seddon and Templer, 1995).
Quite informative post. Till now, I didn't know that Acetate could be useful for this too.
ReplyDelete