What are the conditions that favor the use of soap over detergent?

Answer 1

Soaps and detergents behave differently in hard water. Soaps form a scum in hard water and this scum will not rinse away easily and is known to turn laundry a grayish hue. The insoluble film it leaves can leave a residue on your laundry much like you would see in a shower stall where hard water is present. Detergents react less to minerals in water and do not leave this residue. (3) If you live in an area where the water is soft, you will have more success with soaps, but even then a gradual build-up of calcium and magnesium ions (also called 'curd') will be left in the fabric of your diapers or any of your family's laundry over time. Another disadvantage cited with soaps is that they " . . .will deteriorate in storage and lack the cleaning power when compared with the modern synthetic surfactants."(4) Because of this, detergents have steadily become the cleaning agent of choice since World War II, because for all practical purposes - synthetic or not, they will leave your laundry cleaner.....

Answer 2

Soap is less toxic than many detergents and often causes less environmental harm. The terms aren't very precise in common usage so I will define soap to be the fatty acids removed from the glycerin from fats. Detergents often contain phosphates that cause algae blooms.

Answer 3

Use on people or animals.

Soap is milder than detergent and most don't have caustic substances or abrasives that can injure the skin. LAVA brand is made with pumic and has some abrasives, but it is one of the few.

Soap has lye in it, but in small doses. Most of the time it is made from rendered animal fat.

Shampoo only works on hair since it doesn't have any lye in it. The lye would ruin the hair. So if you only use shampoo to wash yourself you are not cleaning yourself very well.

According to Wikipedia: http://en.wikipedia.org/wiki/Detergent
Detergent has:
"- Surfactants to 'cut' grease and to wet surfaces
- Abrasive to scour
- Substances to modify pH or to affect performance or stability of other ingredients, acids for descaling or caustics to breakdown organic compounds
- Water softeners to counteract the effect of "hardness" ions on other ingredients
- oxidants (oxidizers) for bleaching, disinfection, and breaking down organic compounds
- Non-surfactant materials that keep dirt in suspension
Enzymes to digest proteins, fats, or carbohydrates in stains or to modify fabric feel
- Ingredients that modify the foaming properties of the cleaning surfactants, to either stabilize or counteract foam
- Ingredients that affect the aesthetic properties, such as optical brighteners, fabric softeners, colors, perfumes, etc.
- Washing agents may contain soap for the purpose of reducing foam rather than cleaning fabric. "

According to Wikpedia: http://en.wikipedia.org/wiki/Soap
"Soap is a surfactant used in conjunction with water for washing and cleaning that historically comes in solid bars but also in the form of a thick liquid, especially from soap dispensers in public washrooms.

Historically, soap has been composed of sodium (soda ash) or potassium (potash) salts of fatty acids derived by reacting fat with lye in a process known as saponification. The fats are hydrolyzed by the base, yielding glycerol and crude soap.

Many cleaning agents today are technically not soaps, but detergents, which are less expensive and easier to manufacture."

According to Wikipedia: http://en.wikipedia.org/wiki/Surfactant
"Surfactants, also known as tensides, are wetting agents that lower the surface tension of a liquid, allowing easier spreading, and lower the interfacial tension between two liquids."

According to Wikipedia: http://en.wikipedia.org/wiki/Shampoo
"Shampoo is a common hair care product used for the removal of oils, dirt, skin particles, dandruff, environmental pollutants and other contaminant particles that gradually build up in hair. The goal is to remove the unwanted build-up without stripping out so much as to make hair unmanageable.

Shampoo, when lathered with water, is a surfactant, which, while cleaning the hair and scalp, can remove the natural oils (sebum) which lubricate the hair shaft."