Laboratory 6 - Characteristics of Surfactants and Emulsions

A surface active agent (surfactant) possesses approximately an equal ration between the polar and nonpolar portions of each molecule.  When placed in an oil-water system, the polar groups are attracted to or orient toward the water, and the nonpolar groups are oriented toward the oil.  The surfactant molecule lowers the interfacial tension between the oil and water phases.

Realizing that the surfactant is adsorbed at the interface and by assuming that it is adsorbed in a nonomolecular layer, the amount of emulsifing agent required to emulsify a given volume of a liquid to a certain globular size can be calculated.  The molecular weight and the cross-sectional area occupied by a molecule of the surfactant must be known.

Sodium Lauryl Sulfate has a molecular weight of 288 g/mole and a cross-sectional area of 22 square angstroms ( 22 X 10-16 cm2).  If 100 ml of oil is emulsified to an average globular diameter of 1 micron, the amount of sodium lauryl sulfate required can be calculated .  The volume of a sphere is given by the equation:

V = 4/3 times Pi time r3

The area of the same sphere can also be calculated using the equation:

A = 4 times Pi times r2

Calculate the amount of Sodium Lauryl Sulfate needed. This Calculation should be a part of your report

Surfactants are classified as cationic (Zephiran), anionic (Aerosol OT) and nonionic (Tween) based on the type of polar group on the surfactant.  Cationic surfactants are often used as antibacterial agents because of their ability to disrupt the cell membrane of the microorganism.  The ionized surfactants have a relatively high water solubility and thus generally make oil in water emulsions.  The nonionic surfactants, however, can be used to make either type of emulsion.  To assist formulators in the selection of an appropriate surfactant, the HLB score was developed.

HLB stands for hydrophile-lipophile balance.  Surfactants with a low HLB are more lipid loving and thus tend to make a water in oil emulsion while those with a high HLB are more hydrophilic and tend to make an oil in water emulsion.  The HLB value of each surfactant is determined by an analysis of the characteristics of the surfactant.  A list of HLB values for various surfactants is available in many references such as the Handbook of Pharmaceutical Excipients, 3rd Edition.

In the formation of a stable emulsion, it is advisable to have a blend of two or more nonionic surfactants rather than a single surfactant molecule.  The HLB values of the surfactants are additive and the HLB value of the blend can be determined .  For example, the HLB value of a 60% Tween 80(HLB of 15) and 40 % Aracel 80 (HLB of 4.3) is

Arlacel 80               4.3  X  0.4  =  1.7
Tween 80             15.0   X   0.6 =  9.0
                                                  10.7

Some oils require different HLB surfactants to form the most stable emulsion.  The required HLB value for some of the most common oils used in pharmaceutical emulsions have been determined experimentally.  Mixtures of these material will need a surfactant that matches the average HLB requirement of the oil components of the emulsion.  For example, to form an o/w emulsion of stearyl alcohol you need an surfactant with an HLB value of 14 while to form an o/w emulsion of white petrolatum you need a surfactant with an HLB value of 10.5.  If you wished to prepare an emulsion that contained an equal amount of the two substances, the HLB value of the surfactant used would be the average of the two or 12.25.  You could use alligation to determine the amount of each of the two surfactants need to have a blend with the desired HLB.

        Surfactant     HLB          Needed HLB             Parts Required

Arlacel 80            4.3                                                   2.75

                                                12.25

Tween 80             15                                                   7.95

Total Parts                                                                   10.7
 
 

- PROCEDURE.  There are three parts to this laboratory.  One person should do part A , one person should do part B, and two people should work on part C.  The surface orientation of molecules can be demonstrated by simple procedures.  In fact, by employing careful technique, the area occupied by the surfactant may be measured with a minimum of equipment.

. Part A.  Place approximately 5 g of pure stearic acid on the surface of hot water in a beaker.  The fatty acid will melt to form a lens-shaped drop.  Be sure it does not touch the sides of the beaker.  You can use a glass rod to help keep the lens in the center of the beaker while it is cooling.  Remove the heat and allow to cool.  When it has solidified, remove it without disturbing the surfaces and allow it to dry.

With a few drops of water attempt to wet the bottom of the cake which solidified in contact with water.  Record your observations.  Add a few drops of water to the top of the cake which solidified in contact with the air and attempt to wet the surface.  Record your observations.

. Part B. Using equal volumes of heavy mineral oil and distilled water, prepare 100 ml of an o/w emulsion using a blend of nonionic surfactants (Spans & Tweens).  The average HLB required to make an oil in water emulsion of mineral oil is 12.  Record the proportions and amounts of each surfactant you used.  There is a list of the HLB values for a number of nonionic surfactants in the laboratory and on reserve in the PIC for your use.  The total weight of the surfactant blend should be 10% w/w of the final mixture.

Now using the same surfactants but in different combination make a water in oil emulsion.  The HLB needed to make a water in oil emulsion with mineral oil is 5.
Record you observations about the quality of the emulsion and its overall appearance.  Determine if you indeed made the desired emulsion type using the drop dilution or dye method.

While the HLB system allows us to calculate the appropriate ratio of surfactants to get a specific result the amount of surfactant blend can only be determined through trial and error.

. Part C.  By various procedures given, prepare emulsions using the formula

              Mineral Oil                                   35 ml
             Sorbitan Monooleate HLB 4.3       2.5 ml
             Purified water                              12.5 ml

1.  Place the oil and the emulsifier into a mortar.  While triturating with a pestle, add the water.  Now expose the emulsion to the hand homogenizer for 2 minutes.  Determine the type of emulsion formed.  Set aside to compare to the other emulsions.

2.  Without prior mixing add all ingredients and use the hand homogenizer for two minutes.  Determine the type of emulsion and set aside to compare to the other emulsions.

3.  Place the oil and emulsifier in a Waring blender.  With the blender operating gradually add the water.  Determine the type of the emulsion formed and set aside to compare to the other emulsions.

4.  Place the oil, emulsifier and water into the Waring blender.  Turn the blender on for 30 seconds.  Determine the type of the emulsion formed and set aside to compare to the other emulsions.

5.  Decrease the concentration of oil to 25 ml and increase the amount of water to 22.5 ml.  Make an emulsion by one of the four methods previously described. Determine the type of the emulsion formed and set aside to compare to the other emulsions.

Now examine all five emulsions and record your observations.
Did the mixtures make the type of emulsion you expected?