ie-Chemistry P6

Colloids

in development

Many substances on earth fit neatly into categories of solid, liquid, or gas.  But many of the foods and condiments we eat don't fit well.  They belong to a group of materials called colloids.  These generally contain two different materials which co-exist without complete mixing.  Often one exists as particles, fibers, or films surrounded by the second.  The first is separated or dispersed into portions not directly attached to each other.  The other portion is continuously connected.  We apply different names depending on the physical states of the dispersed and continuous phases.

For examples:

Tiny droplets of water surrounded by air composes a London fog.

Whipped cream has tiny bubbles of air surrounded by heavy cream forming a liquid foam.

Incomplete combustion often leaves specks of solid as smoke in the air.

A muddy river is a sol.

Styrofoam is a solid foam composed of solid polystyrene plastic surrounding bubbles of air.

These typically are all opaque to light, appearing cloudy because the dispersed phases are large enough to scatter light rays.  Two samples of gases will completely mix forming a gaseous solution so that a colloid composed of two gas phases is impossible.  The same is true of liquids which are soluble in each other.  But liquids which have low solubilities such as polar and non-polar liquids (such as water and oil), which would ordinarily separate with one floating atop the other can form a colloid (an emulsion) if a soapy substance called an emulsifier is used.  These emulsifiers are typically molecules with a polar end attached onto an otherwise longer non-polar molecule.

One might first think it would be impossible to have two continuous phases.  Indeed that would be true in a two-dimensional world.  A gel gets past this by weaving relatively continuous fibers amongst a continuous liquid.

Experiment

One common kind of gel is composed of protein fibers derived from connective tissue surrounded by a water solution with coloring, flavoring and sugar.  One commercial brand is Jello.  The gel is produced by dissolving the protein in boiling water.  As the mixture is allowed to cool the fibers entrap the liquid is a structure that increasingly becomes rigid.  This can be used to investigate the colloid called gel.  These gels have a wide variety of uses.  For example disposable diapers have a small amount of powder which is capable of capturing a large amount of urine in the form of a gel.

Procedure I: Desert Gelatin

1. Obtain a package of gelatin desert mix at a grociery store.  Caution: if you might wish to taste samples, clean all equipment and utensils and maintain cleanliness to assure the food remains uncontaminated.
2. Divide the contents into four equal portions.
3. Reducing all quantities in the package directions by four, use one of the package portions to make a sample of gelatin in the intended proportions.
4. With the remains of the package contents, systematically investigate what happens with greater and lesser concentrations of water.
5. How does the properties of the gelatin vary with the concentration of the ingredients?

Procedure II: Testing Water Absorption of Diapers

1. Take apart the diaper to extract (shake out?) the water absorbent (perhaps a powder).
2. Divide the absorbent into at least four equal amounts
3. Slowly add pure water to one of the portions of absorbent, stopping when excess water is not absorbed.  Determine the amount of water absorbed.
4. Repeat adding measured amounts of salt to the water before adding.  (Urine is about as salty as sea water.  Why?)
5. Determine how the salinity of the water influences the effectiveness of the diaper absorption.

Procedure III: Make an Emulsion, Mayonnaise

The Joy of Cooking describes making of a perfect mayonnaise as the Sunday job for Papa in France.  It was first called Mahonnaise after a French victory over the British at Port Mahon on the Island of Minorca.  It has been made by hand for over 300 years.  The combination of oil and water (lemon juice or vinegar), held together by egg yoke as its emulsifier, needs to be refrigerated to deter deadly and hard to detect bacteria growth.  Caution: if you wish to consume the mayonnaise, clean all equipment and utensils and maintain cleanliness to assure the food remains uncontaminated.

1. Two egg yolks, 1 cup of olive oil, and a medium bowl all need to be at room temperature or warmer before starting.
2. Beat two egg yolks in the bowl with a wire whisk or wooden spoon until achieving a lemon color.
3. Beat in 1/4 to 1/2 teaspoon dry mustard, 1/2 teaspoon salt, a few grains cayenne, 1/2 teaspoon lemon juice (or vinegar), and 1/2 teaspoon confectioners' sugar.
4. Beat in, very slowly adding 1/2 teaspoon at a time, 1/2 cup olive oil.  The mixture will begin to thicken and emulsify.
5. Separately combine 1 1/2 tablespoons vinegar and 2 tablespoons lemon juice.
6. Measure out 1/2 cup additional olive oil.
7. Add drop by drop the oil alternating with a couple drops of the vinegar and lemon juice mixture.  These must be added slowly with constant beating to avoid breaking the sauce.  Adding the oil too fast, or in too large of amounts, or when too cool can cause the emulsion to separate.
8. Add fresh herbs such as dill to taste.

Procedure IV: Determining Continuous Phase of Emulsions

1. Obtain an ordinary sheet of printer or notebook paper.
2. Place a small dab of the emulsion to be tested on the paper.
3. Inspect the back side of the paper for an oil mark.  If oil is the continuous phase it will soak through the paper leaving a darker transparent patch.  If water is the continuous phase there will be no effect on the back side of the paper.
4. Test a variety of household emulsions.  Is there any pattern?

Communicating technical information such as observations and findings is a skill used by scientists but useful for most others.  If you need course credit, use your observations in your journal to construct a formal report.

Reference