Industrial chemistry Soap, Detergents and Surfactants Kazem.R.Abdollah

Soap 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. Soap is produced by a saponification or basic hydrolysis reaction of a fat or oil. Currently, sodium carbonate or sodium hydroxide is used to neutralize the fatty acid and convert it to the salt.

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.

General overall hydrolysis reaction:

The potassium soap formed from your fat is converted to a sodium soap by replacing the potassium ions with sodium ions. A large excess of sodium chloride supplies the sodium ion. You may also notice that the potassium soap is softer than the sodium soap. In addition there is a difference in the way the sodium and potassium soaps behave in water.

Types of Soap: The type of fatty acid and length of the carbon chain determines the unique properties of various soaps. Tallow or animal fats give primarily sodium stearate (18 carbons) a very hard, insoluble soap. Fatty acids with longer chains are even more insoluble. Coconut oil is a source of lauric acid (12 carbons) which can be made into sodium laurate. This soap is very soluble and will lather easily even in sea water. Fatty acids with only 10 or fewer carbons are not used in soaps because they irritate the skin and have objectionable odors.

Cleansing Action of Soap: The cleansing action of soap is determined by its polar and non-polar structures in conjunction with an application of solubility principles. The long hydrocarbon chain is of course non-polar and hydrophobic (repelled by water). The "salt" end of the soap molecule is ionic and hydrophilic (water soluble).

Monolayer: When soap is added to water, the ionic-salt end of the molecule is attracted to water and dissolved in it. The non-polar hydrocarbon end of the soap molecule is repelled by water. A drop or two of soap in water forms a monolayer on the water surface as shown in the graphics on the left. The soap molecules "stand up" on the surface as the polar carboxyl salt end is attracted to the polar water. The non-polar hydrocarbon tails are repelled by the water, which makes them appear to stand up.

Soap vs. oil vs. water: The oil is a pure hydrocarbon so it is non-polar. The non-polar hydrocarbon tail of the soap dissolves into the oil. That leaves the polar carboxylate ion of the soap molecules are sticking out of the oil droplets, the surface of each oil droplet is negatively charged. As a result, the oil droplets repel each other and remain suspended in solution (this is called an emulsion) to be washed away by a stream of water. The outside of the droplet is also coated with a layer of water molecules.

micelle A micelle is an aggregate of surfactant molecules dispersed in a liquid colloid.

Effect of Hard Water: If soap is used in "hard" water, the soap will be precipitated as "bath-tub ring" by calcium or magnesium ions present in "hard" water. The effects of "hard" water calcium or magnesium ions are minimized by the addition of "builders". The most common "builder" used to be sodium trimetaphosphate. The phosphates react with the calcium or magnesium ions and keeps them in solution but away from the soap molecule. The soap molecule can then do its job without interference from calcium or magnesium ions. Other "builders" include sodium carbonate, borax, and sodium silicate are currently in detergents.

Detergents and Surfactants Greasy stains do not mix with water because the main interactions between water molecules are hydrogen bonding and those between molecules of oils and fats (which constitute grease) are van der Waals forces. To get water and grease to mix we use molecules called surfactants or detergents.

Chemical classification of detergents: There are four main classes of detergents Anionic detergents Cationic detergents Non-ionic zwitterionic detergents (amphoteric)

Anionic Detergents Anionic means a negatively charged molecule. In the early days always remembered this by anionic (a negative). The detergency of the anionic detergent is vested in the anion. The anion is neutralised with an alkaline or basic material, to produce full detergency.

Cationic Detergents: Another class of detergents have a positive ionic charge and are called "cationic" detergents. In addition to being good cleansing agents, they also possess germicidal properties which makes them useful in hospitals. Most of these detergents are derivatives of ammonia.

Neutral or non-ionic detergents: Nonionic detergents are used in dish washing liquids. Since the detergent does not have any ionic groups, it does not react with hard water ions. In addition, nonionic detergents foam less than ionic detergents. The detergent molecules must have some polar parts to provide the necessary water solubility.

Amphoterics (zwitterionic detergents ) These have the characteristics of both anionic detergents and cationic fabric softeners. They tend to work best at neutral pH, and are found in shampoo’s, skin cleaners and carpet shampoo. They are very stable in strong acidic conditions and have found favour for use with hydrofluoric acid. 3-[(3-Cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS)

Bile Salts - Intestinal Natural Detergents: Bile acids are produced in the liver and secreted in the intestine via the gall bladder. Bile acids are oxidation products of cholesterol. First the cholesterol is converted to the trihydroxy derivative containing three alcohol groups. The end of the alkane chain at C # 17 is converted into an acid, and finally the amino acid, glycine is bonded through an amide bond. The acid group on the glycine is converted to a salt. The bile salt is called sodiumglycoholate. Another salt can be made with a chemical called taurine.

Bleach Bleach refers to a number of chemicals which remove color, whiten or disinfect, often by oxidation. Chlorine is the basis for the most commonly used bleaches, for example, the solution of sodium hypochlorite, which is so ubiquitous that most simply call it "bleach", and calcium hypochlorite, the major compound in "bleaching powder". Oxidizing bleaching agents that do not contain chlorine most often are based on peroxides, such as hydrogen peroxide, sodium percarbonate and sodium perborate.

Colors in most dye and pigments are produced by molecules which contain chromophores, such as beta carotene. Chemical bleaches work in one of two ways: An oxidizing bleach works by breaking the chemical bonds that make up the chromophore. This changes the molecule into a different substance that either does not contain a chromophore, or contains a chromophore that does not absorb visible light. A reducing bleach works by converting double bonds in the chromophore into single bonds. This eliminates the ability of the chromophore to absorb visible light.