Surface tension of liquids

Objective:

1. To understand the basic concept of surface tension in liquids and how it affects the properties of liquids

2. To study the effects of detergent on the surface tension of liquids.

Introduction:

Water molecule consists on a big oxygen atom and two smaller hydrogen atoms. The hydrogen atoms hold the slightly negative charges which making the entire water molecule becomes polar. Eventually, the hydrogen bond is exists in between two neighboring water molecules. Each water molecule experiences a pull from other water molecules from every direction, but water molecules at the surface do not have molecules above the surface of the water to pull at them. These water molecules experience an inward pulling from the water molecules below than them. The difference in force draws the water molecules at the surface creating the surface tension.

Surface tension is the force acting at the angles of 90º to any line on the liquid surface. Surface tension is a property of a liquid surface that caused by the cohesive force between the same liquid molecules. In a liquid, each molecule within the body of the liquid tends to attracted equally at all directions by the cohesive force, so that it experiences no net force. However, those molecules on the surface of liquid have no neighboring molecules above which exhibits stronger attractive force upon on their nearest neighboring molecules on the surface. This results an inward force which pulling the molecules towards the interior of the liquid.

Due to the effect of surface tension, the surface of liquid will form a thin “film” which makes it more difficult to move an object floating on the surface than to move it when it is completely submersed in the liquid. Surface tension is typically measured in the unit of dyne/cm which is the force in the unit of dyne that required to breaks the surface film of water with the length of 1cm. The surface tension in room temperature is 72dyness/cm. It means 72 dynes of forces would be taken to break down a surface film of water 1cm long. The increase in temperature will affects the surface tension of the water dramatically. When the kinetic energy of water molecules increases, it will tend to break down the surface tension of water. Besides, the addition of some solute can influence the surface tension of the liquid but it is depends on the nature of the solutes added. However, some of the solute concentration may not have effect to the surface tension of liquid once the minimum is reached which is surfactant.

Surfactant (surface-active-agents) is a compound in which can lower the surface tension of a liquid or interface tension between two different liquids (or a liquid and a solid). Examples of surfactants are detergent, wetting agent, emulsifiers foaming agents and dispersants. Surfactants usually are amphiphilic, which means they contain both the hydrophobic groups and hydrophilic groups. Due to these properties, the surfactant molecules will migrate to the water surface, where the insoluble component will project out of the water surface and the hydrophilic water soluble component will remain in the water phase. Detergents are the chemicals which consist of hydrophobic (non-polar) hydrocarbon "tails" and a hydrophilic (polar) "head" group. Surfactants can interact with water in a variety of ways which able to disrupt the hydrogen bonding network between the water molecules. Since this will reduce the cohesive force between the molecules, so the surface tension of the water will be altered.

Surface tension is a term that used to describe the cause of the phenomena in which the difference between meniscus of water and mercury. The influence of intermolecular force and surface tension on the interfacial properties of liquid causes the meniscus formation. Stronger intermolecular force between water molecules and glass surface (adhesive force) compared to those between water molecules (cohesive force) cause the water to be drawn up onto the glass wall forming a concave meniscus. For the formation of mercury meniscus, a convex meniscus is formed due to the stronger intermolecular force between mercury atoms (cohesive force) compared to those between mercury atoms and glass surface which prevent mercury from wetting the glass surface.

Apparatus: Petri dish, Toothpick

Materials: Sulfur powder, liquid dish detergent, low fat milk, full cream milk, food colouring

Procedure:

Part A

1. Clean water was poured into a petri dish with depth of 1cm.

2. Powdered sulfur was dusted on the surface of clean water and observation was recorded.

3. The surface of water was touched by a toothpick and observation was recorded.

4. The tip of toothpick was dipped in detergent. The surface of water was touched and toothpick was hold in a place for a while. Observation was recorded.

Part B

1. Low fat milk was poured into a petri dish.

2. Four different coloured food colouring were added. The drops were kept close together in the centre of the petri dish and observation was recorded.

3. The surface of milk was touched with a toothpick and observation was recorded.

4. The tip of a toothpick was dipped in detergent. The surface of milk was touched and toothpick was hold in a place for a while. Observation was recorded.

5. The process was repeated by replacing low fat milk with full cream milk and water.

Results: Observation

Part A: Water with sulphur powder on surface

Before Touched by toothpick                 Touched by toothpick

Touched by toothpick with detergent             After 3 minutes

             After 5 minutes

Observation: The sulphur powder on the water surface where touched by toothpick with detergent was sank to the bottom.

Part B: Low fat milk with colouring

Before                                                    Touched by toothpick            

        

Touched by toothpick with detergent                   After 3 minutes

                After 5 minutes

Observation: The food colouring are being pushed away rapidly from the region where touched by the toothpick with detergent.

Part B: Full cream milk with colouring

Touched by toothpick                     Touched by toothpick with detergent

                After 1 minute                                After 3 minutes

               After 5 minutes

Observation: The food colouring are being pushed away slowly from the region where touched by the toothpick with detergent.

Part B: Water with food colouring

                Before                                    Touched by toothpick

Touched by toothpick with detergent               After 3 minutes

               After 5 minutes

Observation: The food colouring are not changed significantly from the region where touched by the toothpick with detergent.

Discussion:

In Part A of the experiment, the sulphur powder was sprinkled evenly on the water surface. The sulphur powder was floated on the surface because the size of sulphur is light and small which the surface tension of water can withstand and maintain them on the surface. When the surface was touched by the toothpick, the sulphur powder starts to float further from the centre where the place touched by the particular toothpick. However, the sulphur powder was started to sink into the bottom of water very quickly once it was touched by the toothpick with detergent. This is because the surface tension of water was disrupted by the detergent since detergent is a surfactant which acts to lower the surface tension of liquid. As a result, the weak surface tension of water no longer able to withstand the sulphur powder on its surface, so the sulphur sank into the bottom of water.

In part B, low fat milk is used as the liquid to show the property of surface tension when exposed to detergent. Low fat milk has a small amount of fat and the water is predominately in it. Due to the densities of colourigs are lower than milk, so they are floated on the surface when the food colourings were added on the surface of milk. When the toothpick was used to touch on the surface of milk, the food colourings were disrupted insignificantly due to the wave generated by the touch. However, the food colourings were dispersed rapidly and became faint in colours when the toothpick with detergent touched on the surface of milk. This is because the surface tension of the milk in the centre was lowered by the surfactant. The stronger surface tensions of the surrounding milk molecules pull the surface of milk away from the weak region where towards the edge of the plate. The detergent decreased the surface tension of milk by dissolving the fat molecules which caused the turbulence. This moment caused the food colourings to swirl, but the swirling of the colors continues for some times before stopping.

Full cream milk was used in the part C of the experiment to identify the difference in surface tension for two different milks. Full cream milk has a larger amount of fat compared to low fat milk. The toothpick without detergent did not affect the distribution of food colourings on the surface of milk. The fat globules in milk were steady and undisturbed. When the surface was touched by the toothpick with detergent, the food colourings started to disperse with a slower speed compared to the dispersion in low fat milk. This may be due to the full cream milk contains less water which limiting the movement of the milk so the colourings spread slower in full cream milk compared to low fat milk. The milk molecules with lower surface tension in the spot were pulled by the milk molecules with higher surface tension in the surrounding. Hence this caused the food colourings moved with the milk molecules streaming away from the detergent dropped. Comparing to the low fat milk, the colourings were scattered more in full cream milk. This may be due to the detergent weakens the milk's bonds more in the full cream milk because it had more fat. Hence, food coloring scattered more in the full cream milk.

In part D of the experiment, food colourings were added on the surface of water. The colourings were sunk to the bottom of water because they have higher density compared to water while there is some amount of colourings were floated on water surface. When the surface was touched by the toothpick, the distribution of the food colourings were not affected much. When the toothpick with detergent was used to touch on the water surface, the floated food colourings were pushed away from the centre where the toothpick had touched. The detergent reduced the surface tension of the water at middle and hence it caused food colourings to spread. This phenomenon was due to the water molecules in the edge with higher surface tension pulled the water molecules with lower surface tension from the centre.

Precaution steps:

1. Do not shake and stir the surface of liquid because it will influence its surface tension.

2. Make sure that all the fans have been switched off before carry out the experiment.

3. Make sure the milk is not expired because it will affect the property of the milk.

4. Do not exhale deeply which may affect the surface tension of liquid when carrying out the experiment.