Aug. 24, 2005

Wetting Agents

In order for any cleaning solution to have an effect, it must come into intimate contact with the surface it is applied to. It must wet the surface. Water molecules wet a surface when they are more attracted to the surface molecules than they are to each other. The attraction water molecules have for each other is easiest to observe where water molecules pull together at the surface to form a kind of skin. You can see such a skin of surface tension as a drop forms at the end of a faucet. All liquids have surface tension, and you can easily compare different liquids by putting equal-sized drops on waxed paper and comparing the height of the drops. Some will be flatter drops than others.

Hard-surface cleaners all contain wetting agents called surfactants. The cleaning solution industry has developed a variety of surfactants. Do an experiment to compare their ability as wetting agents.

Materials and Equipment

• brown paper bag
• scissors
• pencil
• salad oil
• paper towel
• hair dryer (optional)
• a selection of hard-surface cleaners from under your sink: for general cleaning, glass, woodwork, floors, etc.
• shallow dishes
• straws, one for each kind of cleaner
• measuring spoons

Procedure

Cut pieces of brown paper bag about two inches square. With the pencil, label each one for a cleaning solution you intend to test. Label one "control." Spread about half a teaspoon of salad oil on each piece of paper. The salad oil will sink into the paper. Spread it evenly, and wipe up any excess with paper toweling.

Pour a sample of the cleaning solutions, each in its own shallow dish. Dip the oil-coated paper labeled for a cleaner in that solution. Let the cleaning solution dry on the paper. Use a hair dryer to speed up the drying process if you wish. After all the pieces of oiled paper have been coated with a cleaning solution and dried, you are ready to do your experiment. Note: do not treat the paper labeled "control" with any cleaning solution.

Put a drop of water on each piece of paper. To make sure that each piece of paper gets the same size drop, dip a straw into a glass of water. A small amount will remain in the end of the straw when you lift it out. Gently blow into the straw so that the drop lands on the paper. You can compare the wetting ability of the different surfactants by the curve of the drop of water. Water will not wet an oiled surface, and a drop pulls together to create a high, rounded shape. The rounder the drop, the less the surface is being wet. The best surfactants will cause the drop of water to spread through the paper.

Observations and Suggestions

In my experiments, I found that a window cleaner had the least wetting ability for this particular test, while a household general cleaner was the best. Water was the roundest on the oiled but untreated control.

Reprinted with permission from The Secret Life of Hardware: A Science Experiment Book by Vicki Cobb. Text © 1982 by Vicki Cobb (www.vickicobb.com). Published by J.B. Lippincott.

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