A surface-active molecule, also called a surface active agent or surfactant, possesses approximately an equal ratio between the polar and nonpolar portions of the molecule.  When such a molecule is placed in an oil-water system, the polar group(s) are attracted to or oriented toward the water, and the nonpolar group(s) are oriented toward the oil. The surfactant is adsorbed or oriented in this manner, consequently lowering interfacial tension between the oil and water phase.  When a surfactant is placed in a water system it adsorbs at the surface and lowers the surface tension between the water and air.  When it is place in a mixture of solid and liquid it adsorbs on the surface of the solid and lowers the interfacial tension between the solid and the water.  Since the surfactant is adsorbed at the surface it is logical that the concentartion of surfactant at the surface would be greater than the concentration in the bulk solution.  Mathematically such a relationship has been derived by Gibbs.  The Gibbs adsorption Isotherm will be discussed in class.  It relates lowering of surface tension to excess concentration of surfactant at the surface.

Surfactants have many uses.  Depending on the use the surfactant can be called by many names such as - Wetting Agents, Emulsifying Agents, Solubilizing  Agents, and Detergents. The body is filled with naturally occuring surfactants called phospholipids which are a key component in cell membranes.

When two immiscible liquids are in contact, they tend to maintain as small a surface as possible.  It is therefore difficult to mix these two liquids and have them stay mixed.  A good example of this is an oil and vinegar salad dressing.  The addition of a proper surfactant will lower the interfacial tension between the two liquids and allow them to mix.  Creamy salad dressing contains egg yoke or an artificial surfactant to do just that.

Surfactants are widely used in pharmacy.  Wetting agents are added to suspensions, both oral and parenteral, to hinder caking of particles during storage.  Powdered suspensions for reconstitution contain wetting agents to facilitate rapid suspension of the particles upon the addition of a vehicle such as water.  Surfactants have even been added to tablets to aid in the penetration of moisture into the tablet to hasten its disintegration.

PROCEDURE:

There are many methods for determining the surface or interfacial tension of a liquid.  We will use the du Nouy tensiometer in all of our measurements.  This is a delicate instrument and you should use care when working with it.  There are only three du Nouy tensiometers so you must share the instruments.  A video will be shown before each laboratory that will demonstrate the correct use the instrument.

The du Mouy tensiometer consists of  a platinum-iridium ring supported by a stirrup attached to the beam of a torsion balance. The ring is placed at the interface of two liquids or at the surface of a liquid with air.  It is than pulled upward until it breaks free of the liquid and moves into the second liquid or into the air.  The force that is just require to break the ring free of the liquid/liquid or liquid/air interface is proportional to the surface tension.  The instrument that we will use in class has been designed by the manufacturer so the dial reading is the apparent interfacial or surface tension.  You can correct this apparent surface tension to obtain the true surface tension by multiplying by a correction factor(F) based on the geometry of the instrument.  The correction factor is normally very close to one so we will ignore it during our class activity.  However, you should know that the answers you are getting are not the true values of the surface tension.

When making measurements with the du Nouy tesiometer place the liquid to be measure in a small beaker.  Suspend the ring in the liquid and zero the instrument with the ring below the surface of the liquid. Be sure the ring is clear of the sides of the container throughtout the measurement.  If you are making a surface tension measurement begin to lower the platform that the beaker is on and add tension to the ring to maintain the ring in position until the ring breaks free of the liquid.  If you are measuring the interfacial tension be sure the ring is immersed in the denser liquid before adding the second liquid to the surface of the first.  Then make the measurement as before.  The ring can be cleaned with distilled water and occasionally passed through a flame.  Care must be taken that the ring is not touched with the fingers or bent.

Your group will complete all seven parts to the laboratory- Divide the work so that two students work on A,B, & C while one student completes parts D & E and the last student completes parts F & G. Share your findings with each other and include them in the report due the following week.

The surface tension mesurements should be taken using the small petri dish or the 50 ml beaker, while the interfacial tension measurements should be taken using a 50 ml beaker. Surfactants are substances which tend to concentrate at the interface and lower the surfcae or interfacial tension.  In this laboratory we will use two types of surfactant.  The sodium lauryl sulfate is an ionic surfactant.  Tween 80 (polysorbate 80) is a nonionic surfactant.

Part A - A 1% solution of Sodium Lauryl Sulfate will be available for you.  Using it prepare solutions of the following concentrations. - 0.05, 0.01, 0.001 and 0.1 %.  Please make 100 ml of each solution and be sure and save them after doing part A to use in part C.   Measure the Surface tension of pure water and the surface tension of the four concentrations of surfactant.  Tabulate your data and plot the surface tension versus concentration.  On the graph select the point were the graph levels off.  This is the critical micelle concentration.

Part B - Determine the surface tension of the oil.  Be sure and flame the ring after each measurement made with the oil.  Now measeure the interfacial tension between mineral oil and water.

Part C - Using the four solutions made in part A determine the interfacial tension between mineral oil and each of these solutions.  Tabulate your results and plot the interfacial tension against concentration of surfactant.  Include the data from part B in the plot.

Wetting agents are surfactants which are capable of lowering the contact angle between a liquid and the solid surface over which the liquid spreads.  This helps remove air at the solid surface and replaces it with the liquid.  They hinder caking of the particles in oral and parenteral suspensions during storage, reduce the amount of work required to reduce particle size and facilitate rapid dispersion of the solid in the liquid.

Part D - You will prepare three sulfur suspensions using different procedures and compare the results.  1) Using a glass mortar and pestle, prepare 100 ml of a 2 % suspension of sulfur in water.  After a brief trituration of the sulfur with a small amount of water, transfer the contents to a 100 ml graduate and add sufficient water to make 100 ml.  2) Using a glass mortar and pestle, prepare 100 ml of a 2 % suspension of sulfur.  This time triturate the sulfur with 2 ml of alcohol first than add water and transfer to a 100 ml graduate and qs to 100 mls.  3) Again prepare 100 ml of a 2% sulfur suspension but this time use 10 ml of the 1% Sodium Lauryl Sulfate solution to triturate the powder before transfer to the 100 graduate and qs.  Now observe all three mixtures.

Part E - In adjacent beakers place purified water and 0.1 % Sodiun Lauryl Sulfate solution.  Simultaneously add approximately 1 g of powdered sulfur to the two beakers and observe.

Spreading - When a drop of liquid is placed on the surface of a solid or a liquid with which it is immiscible, it may spread to form a film or remain as a drop.  The surface tension of the two liquids and the interfacial tension between them determines whether or not the liquid will spread.  The spreading of a liquid on a solid is controled by the same factors.  The work of cohesion is the work required to separate the molecules of the spreading liquid so it can flow over the sublayer.  Spreading occurs if the attraction between two immiscible liquids or the work of adhesion is greater than the work of cohesion.  The difference between the work of adhesion and the work of cohesion is known as the spreading coefficient.

Part F - Place purified water in three adjacent petri dishes.  Add a drop of castor oil to one a drop of mineral oil to the second and a drop of oleic acid to the third.  Observe what happens.  To the petri dish with the mineral oil add several drops of the 0.1% Sodium Lauryl Sulfate solution.  Observe the results.

Part G - Prepare TWO petri dishes by shaking a fine layer of talcum powder on the surface of water in a petri dish.  Be sure not to use too much talcum.  It should only be a light dusting. Add one drop of mineral oil near the center of the floating talc to one of the petri dishes.  Observe the results.
Prepare a 1% (v/v) mixture of Tween 80 (polysorbate 80) in mineral oil.  Add one drop of the Mineral oil Tween mixture to the second petri dish.  Observe the results. (Please make a small amount.  Share with your friends)

Your report should attempt to explain your observation in terms of the theories we have discussed with respect to surfactants.