Soap





Soap 3339
Photo by: Vladimir Voronin

Soaps are cleaning agents that are usually made by reacting alkali (e.g., sodium hydroxide) with naturally occurring fat or fatty acids. The reaction produces sodium salts of these fatty acids, which improve the cleaning process by making water better able to lift away greasy stains from skin, hair, clothes, and just about anything else. As a substance that has helped clean bodies as well as possessions, soap has been remarkably useful.

History of Soap

The discovery of soap predates recorded history, going back perhaps as far as six thousand years. Excavations of ancient Babylon uncovered cylinders with inscriptions for making soap around 2800 B.C.E. Later records from ancient Egypt (c. 1500 B.C.E. ) describe how animal and vegetable oils were combined with alkaline salts to make soap.

According to Roman legend, soap got its name from Mount Sapo, where animals were sacrificed. Rain would wash the fat from the sacrificed animals along with alkaline wooden ashes from the sacrificial fires into the Tiber River, where people found the mixture helped clean clothes. This recipe for making soap was relatively unchanged for centuries, with American colonists collecting and cooking down animal tallow (rendered fat) and then mixing it with an alkali potash solution obtained from the accumulated hardwood ashes of their winter fires. Similarly, Europeans made something known as castile soap using olive oil. Only since the mid-nineteenth century has the process become commercialized and soap become widely available at the local market.

Chemistry of Soap

The basic structure of all soaps is essentially the same, consisting of a long hydrophobic (water-fearing) hydrocarbon "tail" and a hydrophilic (waterloving) anionic "head":

CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 COO or CH 3 (CH 2 ) n COO

The length of the hydrocarbon chain ("n") varies with the type of fat or oil but is usually quite long. The anionic charge on the carboxylate head is usually balanced by either a positively charged potassium (K + ) or sodium (Na + ) cation. In making soap, triglycerides in fat or oils are heated in the presence of a strong alkali base such as sodium hydroxide, producing three molecules of soap for every molecule of glycerol. This process is called saponification and is illustrated in Figure 1.

Like synthetic detergents, soaps are "surface active" substances ( surfactants ) and as such make water better at cleaning surfaces. Water, although a good general solvent, is unfortunately also a substance with a very high surface tension. Because of this, water molecules generally prefer to stay together rather than to wet other surfaces. Surfactants work by reducing the surface tension of water, allowing the water molecules to better wet the surface and thus increase water's ability to dissolve dirty, oily stains.

Figure 1.
Figure 1.

In studying how soap works, it is useful to consider a general rule of nature: "like dissolves like." The nonpolar hydrophobic tails of soap are lipophilic ("oil-loving") and so will embed into the grease and oils that help dirt and stains adhere to surfaces. The hydrophilic heads, however, remain surrounded by the water molecules to which they are attracted. As more and more soap molecules embed into a greasy stain, they eventually surround and isolate little particles of the grease and form structures called micelles that are lifted into solution. In a micelle, the tails of the soap molecules are oriented toward and into the grease, while the heads face outward into the water, resulting in an emulsion of soapy grease particles suspended in the water.

With agitation, the micelles are dispersed into the water and removed from the previously dirty surface. In essence, soap molecules partially dissolve the greasy stain to form the emulsion that is kept suspended in water until it can be rinsed away (see Figure 2).

As good as soaps are, they are not perfect. For example, they do not work well in hard water containing calcium and magnesium ions, because the calcium and magnesium salts of soap are insoluble; they tend to bind to the calcium and magnesium ions, eventually precipitating and falling out of solution. In doing so, soaps actually dirty the surfaces they were designed to clean. Thus soaps have been largely replaced in modern cleaning solutions by synthetic detergents that have a sulfonate (R-SO 3 ) group instead of the carboxylate head (R-COO ). Sulfonate detergents tend not to precipitate with calcium or magnesium ions and are generally more soluble in water.

Uses of Soap

Although the popularity of soap has declined due to superior detergents, one of the major uses of animal tallow is still for making soap, just as it was in years past. Beyond its cleaning ability, soap has been used in other applications. For example, certain soaps can be mixed with gasoline to produce gelatinous napalm, a substance that combusts more slowly than pure gasoline when ignited or exploded in warfare. Soaps are also used in "canned heat," a commercialized mixture of soap and alcohol that can be ignited and used to cook foods or provide warmth. Overall, soap is a remarkably useful substance, just as it has been for thousands of years.

David A. Dobberpuhl

Figure 2. How soap works: The hydrophobic tails of soap molecules embed in grease and oil, breaking it up into particles called micelles that lift off the surface and disperse into water.
Figure 2. How soap works: The hydrophobic tails of soap molecules embed in grease and oil, breaking it up into particles called micelles that lift off the surface and disperse into water.

Bibliography

Brady, James E.; Russell, Joel W.; and Holum, John R. (2000). Chemistry: Matter and Its Changes, 3rd edition. New York: Wiley.

Internet Resources

"The History and Chemistry of Soaps and Detergents." The Soap and Detergent Association. Available from the SDA Kids Corner at http://www.sdahq.org/ .



Also read article about Soap from Wikipedia

User Contributions:

leighton
Report this comment as inappropriate
Oct 12, 2008 @ 6:18 pm
thanks 4 puttin this bibliography on here it help w/ my science project =)
jesz flordeliz
Report this comment as inappropriate
Jan 11, 2010 @ 7:19 pm
.,!.thanks for putting bibliography in this article it was a very big help on my chemistry project!! GOD BLESS you all!!
Report this comment as inappropriate
Sep 23, 2010 @ 10:22 pm
please tell me about chemisrty(pH,stabilit,acidity,alkalanity etc.) of soaps like toilet soap,washing soap,liquid soap,face wash,shaving ceam
Report this comment as inappropriate
Nov 17, 2010 @ 4:04 am
thank u for putting this
i was very confused of how the soaps r made whn it was introduced now i thank u to give nice documentry.
Austin W.
Report this comment as inappropriate
Jan 13, 2011 @ 5:17 pm
Thanks so much for the bibliography I needed it for my essay! This is a well written article, good job.
Report this comment as inappropriate
Jul 21, 2011 @ 11:11 am
this article really helped. thanks a lot. please i need help on write ups on free caustic in soap. i would so much appreciate it.
Report this comment as inappropriate
Aug 11, 2011 @ 7:07 am
thank u for this .my chemistry file will be completed now.once again thank u.
Antara
Report this comment as inappropriate
May 12, 2012 @ 8:08 am
That's a great article. It does help to have some lucidly explained ones when the textbook aims at mind boggling.
Gauravjot
Report this comment as inappropriate
Jul 14, 2013 @ 1:01 am
Thumbs up to you man ! Thanks for this wonderful article. This will help in my science project.
Report this comment as inappropriate
Aug 16, 2013 @ 7:07 am
good and lucid explanation..bibliography part was really helpful
Nicolas Ramos
Report this comment as inappropriate
Nov 15, 2013 @ 2:14 pm
This really helped me on some research for science class. Thanks a lot!
sreelakshmi .s. nair
Report this comment as inappropriate
Jan 1, 2014 @ 8:08 am
Thanks for to give a good article.it
was helped me to do my chemistry project. thank you.thank you so much.
IORSUWE THEOPHILUS
Report this comment as inappropriate
Sep 7, 2014 @ 2:02 am
I want the difference between hydrophobic hydropholic head of a soap. Thanks alot for work.
tanvi
Report this comment as inappropriate
Oct 29, 2014 @ 4:04 am
thnx it helped me alot.a great research has been done.
Report this comment as inappropriate
Nov 25, 2014 @ 2:14 pm
THE LOCAL SOAPS ARE AGENT AND CLEANABLE IN WASHING CLOTHING

Comment about this article, ask questions, or add new information about this topic:

CAPTCHA


Soap - Chemistry Encyclopedia forum