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Background Information

[Out! Darn Spot! Introduction
This introduction is a little more detailed for teachers and for students in upper grades and is intended to be used as a supplement to the first background page. The word "soap" means a cleaner made with naturally occurring substances. Original soaps were made using potash and tallow. Thus, for something to be truly soap there can be no human made substances in it.

The word "detergent," on the other hand, can be used for a variety of synthetic cleaners.  Ingredients in detergents that do the actual cleaning are called surfactants.  Surfactants are molecules that will attach themselves to and surround dirt molecules. In order to surround dirt, surfactants must first lower the surface tension of water. To understand this, we must first know a little bit about atoms and water molecules.

Atoms,  Molecules, and Water
Anything that exists is made up of matter.  All matter can be broken down into smaller parts and still be that type of matter.  (For example, you can tear a piece of paper into two pieces. Although each piece is smaller than the original, they are still paper.) The tiniest part of a kind of matter you can have is called an atom. An atom of oxygen is the smallest part of oxygen you can have and still have oxygen.

There are three parts of an atom protons, electrons, and neutrons.  Protons have a positive charge.  Neutrons have no charge.  Protons and neutrons are in the center of an atom called the nucleus. Electrons have a negative charge and travel in shells around the nucleus.

If the number of electrons equals the number of protons, the negative charges cancel the positive charges, thus the atom has no charge.  Atoms are considered stable if their electron shells are full.  The shell closest to the nucleus is stable with 2 electrons while the next shell needs 8.  If an atom is not stable, it looks for other atoms with which to share electrons to make it stable.

An oxygen atom has 6 electrons in its second shell which means it needs two more electrons to be stable.  A hydrogen atom has 1 electron in its first shell which means it needs one more electron to make it stable.  An oxygen atom will bond (join) with two hydrogen atoms, thus the two electrons from the hydrogen atoms will share with the six electrons in the outer shell of the oxygen atom.  When atoms share electrons, they form a covalent bond.  This bond of two hydrogen atoms and one oxygen atom forms a water molecule.  A shortcut for writing this formula is H2O.

Atoms may fill their outermost shell with electrons by losing or gaining electrons from another atom.  If an atom loses electrons, a molecule becomes positively charged while an atom that gains electrons becomes negatively charged.  These oppositely charged atoms are attracted to each other in the same way magnets are attracted to opposite poles. The bond formed in this manner is called an ionic bond.

Water molecules have a strong attraction to each other. The hydrogen atoms of one molecule are attracted to the oxygen atoms of other water molecules.  This forms strong connections between the molecules called hydrogen bonding.  The water molecules under the surface of the water are pulled in all directions.  However, the molecules on the surface are exposed to air.  Since the water molecules in the air (known as water vapor) are single molecules and very far apart, the hydrogen bonds with the surface molecules are extremely weak.  Thus, the surface water molecules are pulled only to the sides and down making a film or skin called surface tension.  (This can be seen by putting drops of water on a penny and looking at it from the side. This is the reason many insects can "run" across water.)

Soap Molecules:

 The formula for this molecule is C15H31 CO2Na.

The head of a surfactant molecule is hydrophilic, which means it loves water.  The tail of a surfactant molecule is hydrophobic which means it repels water.  In addition, the hydrophobic end of the soap molecule is attracted to dirt and grease.  Because the tail is hydrophobic, the soap molecule makes room for it by pushing the water molecules to the center of the surface film.  Soap molecules not on the surface meet below the surface in clusters with the hydrophobic tails clustered together.
 The following diagrams demonstrate how soaps and detergents clean dirt from fabrics:
A Soap Molecule 
The surfactant molecules push water molecules apart weakening the hydrogen bonds.  The push is so great that surfactant molecules lower the surface tension of water by 1/3.  This lowering of the surface tension allows the soapy solution of water to become "wetter" and penetrate deeper into the fibers of the fabric.
Since the hydrophobic tails are attracted to dirt, the molecules work their way between the dirt and the fabric.
The soap molecules hold the dirt in clusters surrounded by the tails.  These clusters suspend the dirt in the water and keep it from settling back on the fabric. 

Soap and Detergent Additives
The original soaps were harsh, irritated the skin, smelly and only did a passable job of cleaning. Another problem with soap was the hydrophilic part of the molecule bonded with minerals found in hard water forming a curd which could not be broken down by water. This problem in water causes "bathtub ring." To counteract these problems, detergents add builders which control minerals in hard water and prevent the buildup of soap scum. 

Another problem occurred when clothes were very dirty. The dirt would settle back on the clothes before it was rinsed away. Phosphates were added as a builder to keep the dirt suspended in water. However, it was discovered that the phosphates caused environmental problems and the use of phosphates was restricted in the 1960s.

Other builders added to detergent maintain alkalinity, aid in preventing dirt from redepositing on clothes, abrasives for scrubbing, suds controllers, perfumes, colorants and brightners, which are chemicals which make ultraviolet light visible (this is why your white clothes glow under ultraviolet light).

Additional Sites that might give you some useful tips for removing stains


Ansbro, Mary. (1997). Soap and Detergent. Academic American Encyclopedia, Volume 18, (4-6). Danbury, Connecticut: Grolier Incorporated.

Cleaning Tips. [Online] Available http://www.sanitarymaintenance.com/cleantp.htm, date unavailable.

Heaton, Linda M. and Hackler, Nadine. Stain Removal. [Online] Available http://hammock.ifas.ufl.edu/txt/fairs/7912, 05/91.

Toback, Bruce. Laundry Balls and Laundry Disk. [Online] Avaialable http://www.optc.com/~btoba ck/laundrystuff.html, 10/07/97.

Wiebe, Ann. (1990). Soap Films and Bubbles. Fresno, California: AIMS Education Foundation.

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