9.5.E - Saponification


Soap making is a chemical process where animal fats are hydrolysed in an alkaline solution to produce glycerol and salts of fatty acids (soap). In industrial soap making, soaps are usually made in a two-step process. In the first step, fatty acids are neutralised using soda ash (sodium carbonate) and in the second step unreacted fatty acids are hydrolysed using caustic soda (sodium hydroxide) to salts of fatty acids.

The cleaning action of soap can be described in terms of the bipolar nature of the soap molecule. One end is polar while the other is non-polar. This makes soap a surfactant that can remove oils in a solution of water.

There are three types of detergents: anionic, cation and non-ionic. The properties of each detergent determine their particular uses.

In this topic students:

  • Describe the chemistry of soap making in the laboratory and on an industrial scale
  • Account for the cleaning action of soaps and detergents
  • Describe the properties of an emulsion and relate them to their uses
  • Distinguish between cationic, anionic and nonionic detergents in terms of their chemical composition and uses
  • Discuss the environmental implications of using soaps and detergents


Fats and Oils

Chemically speaking, soap is made of salts of fatty acids. It is the product that results from the reaction of a fatty acid and a strong base (alkali). In cleansing soaps, the fatty acids come from oils and fats; the strong alkali base is sodium hydroxide, also known as lye, (for hard soaps) or potassium hydroxide, also known as potash, (for soft soaps). As such, it would be useful to first examine the structure of some fats and oils.

Fatty Acids

The structure the saturated fatty acid, palmitic acid,
and the unsaturated fatty acid, oleic acid.

Fatty acids are found in fats and oils. A fatty acid is made up of a long chain of hydrogen and carbon atoms, with an extra hydrogen atom at one end and a carboxyl (alkanoic acid) group on the other end. They are called fatty acids because they are actually very long chain alkanoic acids.

A fatty acid can be saturated or unsaturated. In a saturated fatty acid, the carbon atoms are bonded with single bonds; they share one set of electrons. As a result, saturated fatty acids have two hydrogen atoms for each carbon atom. Palmitic acid, pictured above, is a saturated fatty acid.

In an unsaturated fatty acid, there is at least one double bond where one set of carbon atoms is bonded by sharing two sets of electrons, instead of each being connected to a hydrogen atom. Oleic acid, below, is an unsaturated fatty acid.

Fats and Oils

There is no sharp distinction between a fat and an oil. Oil commonly means a liquid which at ordinary temperature will flow as a slippery, lubricating, fairly thick fluid. Fat normally implies a greasy, solid substance slippery to the touch. It is necessary to differentiate the oils and fats used in the manufacture of soap.

Hydrocarbon (petroleum-based) oils or paraffins, while included in the general term oil, do not contain fatty acids and cannot be used to make traditional soap. Animal and vegetable-based oils and fats do contain the necessary fatty acids, in the form of triglycerides.

Triglycerides

The structure of a triglyceride made from three idetnical fatty
acids. In practice, the type of fat will be characterised by
different combinations of fatty acids.

When the carboxyl group ends of three fatty acid molecules combine with one molecule of glycerol it produces a triglyceride containing three ester bonds. This is what we usually think of as oil or fat. The actual physical characteristics of the oil depend upon which fatty acids have attached to the glycerol and whether they are connected to the top, middle or bottom of the glycerol molecule. If primarily unsaturated fatty acids are contained in the triglyceride, then the oil is considered to be an unsaturated fat. The type of fatty acids also determines whether the triglyceride is solid or liquid at room temperature, how thick it is, the nutritional value and - for soapmakers - the qualities the oil will impart to the soap and its lather.

Tallow is rendered beef or mutton fat, processed from suet. Suet is raw beef or mutton fat, especially the hard fat found around the loins and kidneys. It melts at about 21°C and is a saturated fat. The tallow derived from beef is called stearin (a triglyceride of stearic acid). Unlike suet, tallow can be stored for extended periods without the need for refrigeration to prevent decomposition, provided it is kept in an airtight container to prevent oxidation.

It is used in animal feed, to make soap, for cooking, as a bird food, and was once used for making candles. It can be used as a raw material for the production of biodiesel and other oleochemicals.

Industrially, tallow is not strictly defined as beef or mutton fat. In this context, tallow is animal fat that conforms to certain technical criteria, including its melting point, which is also known as titre. It is not uncommon for commercial tallow to contain fat derived from other animals, such as swine.

Lard is an animal fat produced from rendering the fat portions of the pig. Lard was commonly used cooking oil though its use in contemporary cuisine has been diminished due to the health concerns posed by saturated fat and cholesterol. Lard is still commonly used to manufacture soap by small-scale artisanal soap crafters and large industries alike.

Like all fats, plant oils are tri-esters of glycerin. Coconut oil and palm oil contain carrying amounts of saturated and unsaturated fatty acids, however, plant oils usually contain more unsaturated than saturated fatty acids. Common plant oils used for making soap include palm oil, coconut oil and olive oil.

 

Saponification

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. Later records from ancient Egypt (c. 1500 B.C.) 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. Mount Sapo is a fictional mountain supposed to exist somewhere near Rome. It appears in a fanciful rewriting of the history of soap, and it is often claimed to explain the origins of the name. The tale occurs in a number of online sources, including the website of the Soap and Detergent Association.

The story about Mount Sapo explains that upon its slopes, ancient Romans used to sacrifice animals as burnt offerings. Wood ash from the fires of their altars mingled with the grease from the animal sacrifices, forming a primitive kind of soap. This soap found its way to the clays of a nearby stream, where local people found that it helped them get their laundry cleaner. Soap gets its Latin name, sapo, from the name of the mountain.

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.

Saponification is the hydrolysis of an ester under basic conditions to form an alcohol and the salt of the acid. Saponification is commonly used to refer to the reaction of a metallic alkali (base) with a fat or oil to form soap. Said another way, saponification is the alkaline hydrolysis of a triglyceride. Hydrolysis is a chemical reaction or process in which a molecule is more usually split into two parts by reacting it with a molecule of water. One of the parts gets an OH - from the water molecule and the other part gets an H+. In the case of saponification, the triglyceride (tri-ester) is split into three fatty acids and glycerol using NaOH instead of water.

CH2(-O-CO-R)-CH(-O-CO-R)-CH2(-O-CO-R) + 3NaOH →

CH2(-OH)-CH(-OH)-CH2(-OH) + 3 Na(R-CO-O)


The formation of sodium palmitate (a soap) from a tryglyceride. This is the reverse of esterification and
but produces glycerol instead of three molecules of water.

Lye is a form of sodium hydroxide (NaOH) which is a caustic base. If NaOH is used a hard soap is formed, whereas a soft soap is formed when potassium hydroxide (KOH) is used. Vegetable oils and animal fats are fatty esters in the form of triglycerides. The alkali breaks the ester bond and releases the fatty acid and glycerol which is illustrated in the diagram above. Saponification is, therefore, the reverse of esterification and is referred to as the alkaline hydrolysis of fatty acids. Soap can also be made in a neutralisation reaction between a single chain fatty acid and sodium hydroxide, although this procedure is less common.

You might also be interested to know that saponification can also refer to the conversion of fat and other soft tissue in a corpse into adipocere, often called "grave wax." Adipocere, grave wax or mortuary wax is the insoluble fatty acids left as residue from pre-existing fats from decomposing material such as a human cadaver. It is formed by the slow hydrolysis of fats in wet ground and can occur in both embalmed and untreated bodies.This process is more common where the amount of fatty tissue is high, the agents of decomposition absent or only minutely present and the burial ground is particularly alkaline.

Industrial Production of Soaps

The continuous batch process manufacturing of soap.

Traditional bar soaps are made from fats and oils or their fatty acids which are reacted with inorganic water-soluble bases. The main sources of fats are beef and mutton tallow (from abattoirs), while palm, coconut and palm kernel oils are the principal oils used in soap making. The raw materials may be pretreated to remove impurities and to achieve the color, odor and performance features desired in the finished bar.

Soap was made by the batch kettle boiling method until shortly after World War II, when continuous processes were developed. Continuous processes are preferred today because of their flexibility, speed and economics.

Both continuous and batch processes produce soap in liquid form, called neat soap, and a valuable by-product, glycerin shown in the diagram at (1). The glycerin is recovered by chemical treatment, followed by evaporation and refining. Refined glycerin is an important industrial material used in foods, cosmetics, drugs and many other products.

The next processing step after saponification or neutralization is drying. Vacuum spray drying is used to convert the neat soap into dry soap pellets (2). The moisture content of the pellets will vary depending on the desired properties of the soap bar.

In the final processing step, the dry soap pellets pass through a bar soap finishing line. The first unit in the line is a mixer, called an amalgamator, in which the soap pellets are blended together with fragrance, colorants and all other ingredients (3). The mixture is then homogenized and refined through rolling mills and refining plodders to achieve thorough blending and a uniform texture (4). Finally, the mixture is continuously extruded from the plodder, cut into bar-size units and stamped into its final shape in a soap press (5).



A short video showing Vermont Soap Organics manufacturing process for soaps.

 

Cleaning Action of Soap

The structure of soap with a hydrophillic, polar head
and a hydrophobic non-polar tail.

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

The length of the hydrocarbon chain 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.

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.

The diagram showsing the cleaning action of soap with the electrstatic
interaction between the negative head of the soap and the water
molecule and the non-polar tail of the molecule and the dirt/grease.

In studying how soap works, it is useful to consider a general rule of nature: "like dissolves like." The non-polar 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.



A simple video showing the cleaning action of soap. Note that the video refers to the soap ion as a triglyceride when
it is better known as a fatty acid. Remember that a tryglyceride molecule is made up of three fatty acids and glycerol.


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-SO3-) 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.

Emulsifiers

There is a dot-point in the syllabus regarding an investigation where you are to analyse the properties of an emulsion. Emulsions are not referred to anywhere else in this section of the syllabus and this reference to them seems vague and out of context. However, on closer examination emulsifers can act as surfactants and, like soaps and detergents, bind immisicble substances together. Some information on emulsions appears below.

An emulsion is a mixture of two immiscible (unblendable) substances. One substance (the dispersed phase) is dispersed in the other (the continuous phase). Examples of emulsions include butter and margarine, espresso, mayonnaise, the photo-sensitive side of Photographic film, and cutting fluid for metalworking. In butter and margarine, a continuous lipid phase surrounds droplets of water (water-in-oil emulsion). Emulsification is the process by which emulsions are prepared.

An emulsifier (also known as a surfactant' from surface active material or emulgent) is a substance which stabilizes an emulsion. Examples of food emulsifiers are egg yolk (where the main emulsifying chemical is the phospholipid lecithin), and mustard, where a variety of chemicals in the mucilage surrounding the seed hull act as emulsifiers; proteins and low-molecular weight emulsifiers are common as well. In some cases, particles can stabilise emulsions as well through a mechanism called Pickering stabilization. Both mayonnaise and Hollandaise sauce are oil-in-water emulsions stabilized with egg yolk lecithin. Detergents are another class of surfactant, and will chemically interact with both oil and water, thus stabilising the interface between oil or water droplets in suspension. This principle is exploited in soap to remove grease for the purpose of cleaning. A wide variety of emulsifiers are used in pharmacy to prepare emulsions such as creams and lotions.

 

Detergents

The molecular structure of two common
detergents belonging to the alkyl benzene
sulfonate family.

Soaps are problematic for use in hard water. Hard water has calcium and magnesium ions, which make it difficult for soap to lather. Synthetic detergents overcome this problem. Synthetic detergents do not have sodium salts of fatty acids. In spite of this the detergents have all the properties of soap. Synthetic detergents have long chain molecules such as sodium n-dodecyl benzene sulfonate and sodium n-dodecyl sulfate. These detergent molecules belong to a family of molecules known as the alkyl benzene sulfonates.

You can see from the molecular structure of detergent molecules that they are similar to the soap molecules. It has a long hydrocarbon chain (or tail) and a short ionic head. Like soap, these two parts are hydrophobic and hydrophilic, respectively. The cleansing action of a synthetic detergent molecule is similar to that of the soap. This type of detergent is known as an anionic detergent because it has a negatively charged head.

Synthetic detergents are manufactured from long chain hydrocarbons obtained as a byproduct of the petroleum industry. The hydrocarbons are treated with concentrated sulfuric acid and sodium hydroxide. A neutral sodium salt is obtained, which is the synthetic detergent.

Typical examples of the three types of detergents.

Anionic Detergents

Most detergents have a negative ionic group (like soap) and are called anionic detergents, like the ones described above. The majority are alkyl sulfates. Others are "surfactants" (from surface active agents) which are generally known as alkyl benzene sulfonates.

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. They have the ability to disrupt the cell membrane of bacteria. Most of these detergents are derivatives of ammonia.

A cationic detergent is most likely to be found in a shampoo or fabric softener. The purpose is to neutralize the static electrical charges from residual anionic (negative ions) detergent molecules. Since the negative charges repel each other, the positive cationic detergent neutralizes this charge.

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.

In the graphic above-right, the polar part of the molecule consists of three alcohol groups and an ester group. The non-polar part is the usual long hydrocarbon chain.

A comparison of the properties and uses of the three types of detergents are shown in the table below.

Class

Properties

Uses

Anionic

Strongly lathering.

More effective surfactant than soap.

Not used in shampoos as it strips too much oil from the hair.

Dishwashing liquids
Toothpaste
Laundry detergent
Oven cleaners

Cationic

Strongly lathering.
Binds strongly to the negative surfaces of fabric.
Good for cleaning plastic products.
Have mild antiseptic properties.

Fabric softeners
Hair shampoos
Antiseptics
Nappy washes

Non-ionic

Weakly lathering.
Prevents the foam clogging the water jets in dishwashers.
Strong alkalis in dishwashing powders dissolve grease.

Dishwashers
Car shampoos
Front loading washing machines
Paints
Cosmetics


Environmental Issues

Phosphates are primarily used in detergents as 'builders', which soften water by stoping the formation of insoluble calcium and magnesium salts and also provide alkaline conditions to allow surfactants (the cleaning agents) to work more effectively. When phosphate detergents are used, disposal of the wash wastewater is an environmental issue. The breakdown of the phosphorus complexes in phosphate detergent wastewater (and in other household products, and in human and industrial wastes containing phosphates) creates biologically available phosphates. In waterways these can contribute to an oversupply of phosphate. Low concentrations of plant nutrients, such as nitrogen and phosphorus, can limit plant and algae growth. When phosphates from the sources mentioned above are introduced to waterways (eutrophication) any phosphorus-limitation may be removed and may cause excessive algae growth which, in turn, can lead to:

  • increased numbers of insects, crustaceans and fish in the waterway which, when they and the algae die and decompose, can reduce oxygen in the water to such low levels that other aquatic organisms are killed.

  • the release of toxins by some cyanobacteria (blue-green algae) that can kill other organisms and make the water unsuitable for humans, livestock and wildlife.

For these reasons wastewater containing phosphate detergents should be directed to the sewerage system and not be allowed to wash into the storm water drain.

Many phosphate free detergents are made from non-ionic detergents as they do not form insoluble precipitates with calcium and magnesium ions (they are not ionic). Disposal of phosphate-free detergent wastewater can also be an environmental issue. In phosphate-free detergents the phosphates are replaced by alternative builders such as zeolites (complex compounds of aluminium, silicon and oxygen). While this reduces the problem associated with eutrophication of waterways it often means that more surfactant is added to compensate for the less effective builder. Surfactants are among the most toxic compounds in detergents and have been implicated in decreasing the ability of aquatic organisms to breed. To minimise the environmental harm caused by phosphate-free detergents they must be treated in sewage treatment plants as well and not allowed to wash into waterways from overflowing storm water drains. Unfortunately, even after sewerage treatment the effects of some alternative builders remain.