Which detergent is best?

Explore how detergent soap molecules have structures to attract and remove dirt during washing. 

Introduction

Teachers who have not used the problems before should read the section Using the problems before starting.

Prior knowledge

Cleansing action of soaps and detergents, including an appreciation that these molecules comprise both a hydrophillic ‘head’ and a hydrophobic ‘tail’ in these molecules. A detailed knowledge is unnecessary as students are encouraged to consult textbooks during the exercise.

Resources

Textbooks for reference.

Possible approaches

This is a short problem; if after 10 minutes or so of brainstorming, the idea that detergent molecules can be anionic or cationic has still not emerged, the students should be directed to an appropriate textbook.

There are two main types: anionic detergents contain molecules with a negative head, and cationic detergents contain molecules with a positive head. (See Background information below.)

The jelly-like solid is a precipitate produced when an anionic and cationic detergent are mixed together.

Suggested approach

During trialling the following instructions were given to students and proved to be extremely effective:

1. Working as a group, discuss the problem. Such discussion can play a vital role in working out possible solutions to open-ended problems like this. Several minds working on a problem together can stimulate ideas that one on its own could not manage. 
2. Write a brief account of your solution.
3. Working as a group, prepare a short (ca 5-minute maximum) presentation to give to the rest of the class. If possible all group members should take part: any method of presentation (such as a blackboard, overhead projector, etc) can be used.

Outline the problem, describe your explanation and explain how you arrived at it. After the presentation, be prepared to accept and answer questions and to discuss what you did with the rest of the class.

Background information

There are four types of detergent:

• Anionic – molecules with a negative head such as the alkylbenzene sulphonates. These are used extensively in shampoos because of their excellent foaming and cleansing properties. Soap is also anionic;
• Cationic – molecules with a positive head such as a quaternary ammonium group. Because of severe eye irritation they are rarely used in cosmetics and toiletries. Their largest use is in fabric conditioners such as Comfort™ or Lenor™, but they are also used for cleaning and disinfecting culinary equipment in restaurants and hospitals;
• Non-ionic – neutral molecules; cleansing action derives from hydrogen bonding to water molecules. They are used to modify the effect of the principal cleaner, eg as viscosity builders, foam modifiers, emulsifiers; and
• Amphoteric – contains both anionic and cationic groups; like amino acids, their electrical behaviour is strongly pH dependent (cationic at low pH, anionic at high pH). They are low foaming and have low irritability and are used in low eye-sting baby shampoos.