Soap & Detergent Chemistry
To understand what is needed to achieve effective cleaning, it is helpful to have a basic
knowledge of soap and detergent chemistry.
As you know water comprises approximately 95-99% of cleaning and sanitizing solutions.
Water functions to:
• carry the detergent or the sanitizer to the surface
• carry soils or contamination from the surface.
Water has a property called surface tension. In the body of the water, each molecule is
surrounded and attracted by other water molecules. However, at the surface, those molecules
are surrounded by other water molecules only on the water side. A tension is created as the
water molecules at the surface are pulled into the body of the water. This tension causes water
to bead up on surfaces, which slows wetting of the surface and inhibits the cleaning process.
Molecules inside a water drop are attracted in all directions. Drops on the surface are
attracted to the sides and inwards.
After raining you can see surface tension a drop of water onto a branch. The drop will hold its
shape and will not spread.
In the cleaning process, surface tension must be reduced so water can spread and wet
surfaces. Chemicals that are able to do this effectively are called surface active agents, or
surfactants. They are said to make water "wetter."
Consider a liquid droplet at rest on a flat, solid surface. The angle formed by the solid surface
and the tangent line to the upper surface at the end point is called the contact angle(θ).
The contact angle is a result of the interface/surface tensions (surface free energies) between
liquid and solid surrounded by vapor.
Measurement of the contact angle of a water droplet is a quick and simple way to evaluate
cleanliness of a solid surface as below:
From the contact angle, physical properties of interaction between solid and liquid like
wettability, affinity, adhesiveness and repellency can be studied. Typical evaluations are as
Different wetting states of a droplet
Soaps are produced during the chemical reaction known as saponification which is the
reaction between a fat or oil and a base (strong alkali), producing glycerol and a salt (soap)
fat or oil + base -----> glycerol + salt (soap)
Soaps are usually sodium or potassium salts of long-chain fatty acids.
Soaps are detergents in the sense that they help clean oily and greasy dirt from fabrics, metals,
our skin and hair. We restrict the term soap to the sodium salts of long-chain carboxylic acids.
A carboxylic acid is marked by the presence of a carboxyl group, -CO2H. With the anion of
the carboxyl group balanced by a sodium cation and tied by a covalent bond to a long chain of
-CH2- groups that terminate in a CH3- group, we have a soap molecule. We can generalize the
molecular structure of a soap molecule as
CH3 - (CH2)n - CO2 - Na+
CH3 - (CH2)n - resembles quite closely the long chains of the
hydrocarbon molecules. Like the molecules of gasoline and
mineral oil, this part of the soap molecule tends to dissolve
readily in materials that are or that resemble hydrocarbons, but
not in water. All these long chains of -CH2- groups of soaps and
of hydrocarbons and hydrocarbon-like materials intermingle
easily, but they don't mix readily with the H2O molecules of
water. The other end of the molecule, though is ionic:
-C - O - Na+
Like sodium chloride and other ionic compounds, that ionic end
tends to dissolve in water, but not in hydrocarbon solvents. As a
result, one molecule has two opposite and contradictory
tendencies. A hydrophilic structure is attracted towards water
molecules but shun hydrocarbons and other oily and greasy
substances. A hydrophobic structure shuns water but mixes easily
with those very oily, greasy substances that repel the hydrophilic,
or charged, part.
Fats and Oils
The fats and oils used in soapmaking come from animal or plant
sources. Each fat or oil is made up of a distinctive mixture of several
In a triglyceride molecule, three fatty acid molecules are attached to one molecule of
glycerine. There are many types of triglycerides; each type consists of its own particular
combination of fatty acids.
Fatty acids are the components of fats and oils that are used in making soap. They are weak
acids composed of two parts:
A carboxylic acid group consisting of one hydrogen (H) atom, two oxygen (O) atoms, and
one carbon (C) atom, plus a hydrocarbon chain attached to the carboxylic acid group.
Generally, it is made up of a long straight chain of carbon (C) atoms each carrying two
hydrogen (H) atoms.
An alkali is a soluble salt of an alkali metal like sodium or potassium. Originally, the alkalis
used in soapmaking were obtained from the ashes of plants, but they are now made
The common alkalis used in soapmaking are sodium hydroxide
(NaOH), also called caustic soda; and potassium hydroxide (KOH),
also called caustic potash.
How Soaps are Made
Saponification of fats and oils is the most widely used soapmaking process. This method
involves heating fats and oils and reacting them with a liquid alkali to produce soap and water
(neat soap) plus glycerine.
When the alkali is sodium hydroxide, a sodium soap is formed. Sodium soaps are "hard"
soaps. When the alkali is potassium hydroxide, a potassium soap is formed. Potassium soaps
are softer and are found in some liquid hand soaps and shaving creams.
The carboxylate end of the soap molecule is attracted to water. It
is called the hydrophilic (water-loving) end. The hydrocarbon
chain is attracted to oil and grease and repelled by water. It is
known as the hydrophobic (water-hating) end.
Cleaning Action of Soaps
Step 1 Decreases the water's surface tension, making it a better wetting agent.
Soap first lowers suface tension so that the water carrying the micelles, clusters of soap
molecules in which the hydrocarbon chains are attracted to each other by Van der Waals
forces (dispersion forces, London forces, weak intermolecular forces), can get to the dirt.
When the soap micelles reach the embedded dirt, the soap molecules that form these micelles
once again find themselves at a surface.
Step 2 Converts greasy and oily dirt into micelles that become dispersed in the soapy water
Now, as the hydrophilic heads of the soap molecules remain surrounded by water molecules,
the soap micelles break up and the hydrophobic hydrocarbon tails, which had remained in the
interior of the spherical micelles, become embedded in the grease. With this greasy dirt
providing as compatible a chemical environment for the hydrophobic tails as the water
provides for the hydrophilic heads, the tails are just as much at home in the grease as the
heads are in the water.
Step 3 Keeps the grease micelles in suspension, thereby preventing them from coalescing
back to large globules of grease that could be redeposited on a clean surface.
Agitation now breaks the grease into micelles whose surfaces are covered by the negatively
charged carboxylate groups, the hydrophilic -CO2- groups of the embedded soap molecules.
The grease droplets repel each other and remain suspended in the wash water instead of
coalescing and redepositing on the material being cleaned. In the end, the suspended droplets
go down the drain with the wash water. (While all this is going on the sodium ions move
about freely and independently in the wash water.)
As a cleaning agent, soap suffers from two main drawbacks:
1. It does not function well in acidic solutions because of the formation of insoluble fatty
CH3(CH2)16COOH(s) + Na+(aq) + Cl-(aq)
2. It forms insoluble precipitates with Ca2+and Mg2+ ions present in hard water, forming a
2CH3(CH2)16COO-Na+(aq) + Ca2+(aq)
[CH3(CH2)16COO-]2Ca2+(s) + 2Na+(aq)
Additives such as sodium carbonate and phosphates can help offset these effects.
How Water Hardness Affects Cleaning Action
Although soap is a good cleaning agent, its effectiveness is reduced when used in hard water.
Hardness in water is caused by the presence of mineral salts - mostly those of calcium (Ca)
and magnesium (Mg), but sometimes also iron (Fe) and manganese (Mn). The mineral salts
react with soap to form an insoluble precipitate known as soap film or scum.
Soap film does not rinse away easily. It tends to remain behind and produces visible deposits
on clothing and makes fabrics feel stiff. It also attaches to the insides of bathtubs, sinks and
Some soap is used up by reacting with hard water minerals to form
the film. This reduces the amount of soap available for cleaning. Even
when clothes are washed in soft water, some hardness minerals are
introduced by the soil on clothes. Soap molecules are not very
versatile and cannot be adapted to today's variety of fibers, washing
temperatures and water conditions.
Synthetic detergents are increasingly being used instead of soaps
because they do not suffer from these disadvantages to the same