Detergents and Surfactants
No matter how much fun we have squishing our toes in the mud,
we love to be clean. We want our clothes to be fresh, our dishes
to be spotless, and our cars to be shiny. We continue to invent
new ways to make things clean, and soap was probably discovered
only shortly after cooking, as the fats from the food hit the
ashes from the fire.
Detergents have molecules with one side that
prefers water (hydrophilic), and another side that prefers
oils and fats (hydrophobic).
The hydrophilic side attaches to water molecules, and the hydrophobic
side attaches to oil molecules. This action allows the oil droplets
to break up into smaller droplets, surrounded by water. These
smaller droplets are no longer stuck to the material to be cleaned,
and are washed away.
Detergents and soaps are surfactants
, short for
Surfactants have a hydrophilic
side of the molecule attaches to water, and a hydrophobic
side of the molecule that avoids water.
In the absence of oils, the hydrophobic side sticks out of
the surface of the water drop.
There is no longer any water at the surface to form a strong
surface tension, so the water no longer beads up, but spreads.
The hydrophobic end of the molecule is also free to attach to
grease, fat, or oil on the surface, aiding in the spreading.
Some detergents and surfactants are used as emulsifying agents
An emulsifier keeps oil droplets and water droplets from joining
together, so a thick mixture of oil and water will not separate.
Examples of emulsions are mayonnaise, butter, cream, homogenized
milk, and salad dressings.
Soap is made from a fatty acid that is reacted with an alkali.
The acid end of the fatty acid reacts with the alkali to form
a salt that is water-soluble. The other end is the fatty end,
which repels water, and is attracted to fats and oils. The
process of making soap is called saponification
kind of soap is sodium stearate
Beef fat reacting to alkaline wood ashes created the earliest
soap, and it was the cleaning product of choice for millenia.
Types of detergents
German scientists created the first detergents during the
shortages of World War II.
These were called
branched-chain alkyl benzene sulfonates
. Like soap,
they could take hard minerals out of water, leaving it soft.
As with soap, the minerals formed a scum, familiar to anyone
who has seen a bathtub ring.
Microbes could not break down branch-chain detergents,
so they left foam in river water. They were replaced by
straight-chain alkyl benzene sulfonates
, such as
Straight-chain detergents don't work in hard water. Phosphates
were added to detergents to soften the water, but phosphates
are excellent fertilizer for algae in rivers and oceans.
The algae blooms
deplete the oxygen in the water, killing fish. Phosphates
were replaced with other water softeners such as
Later, surface-acting polyglucosides were created. These
sugar-based detergents are easily broken down by microbes,
traces in the environment. They consist of a pair of glucose
molecules, with hydrocarbon side chains attached to act as the
hydrophobic ends. They are milder than soaps, and work
in hard water.
Another type of detergent is a group called the pyrrolidones
These are complex molecules that dissolve in both water and
Some laundry detergents contain "optical brighteners". These
are fluorescent dyes that glow blue-white in ultraviolet light.
The blue-white color makes yellowed fabrics appear white.
Laundry detergent may also contain polyethylene glycol
, a polymer that prevents dirt from
re-depositing on the clothes. This function used to be the job
of phosphates. Another polymer used for this purpose is
carboxy methyl cellulose
This is derived from natural cellulose, but is very soluble
Yet another ingredient in laundry detergents is
Diethyl Ester Dimethyl Ammonium Chloride (DEEDMAC)
It is a fabric softener. It is a cationic surfactant that
is rapidly biodegradable. It works by reducing the friction
between fibers, and between fibers and the skin. Cationic
surfactants are those where the hydrophilic part (in this case
the ammonium chloride) is positively charged, and is attracted
to substrates that are negatively charged, such as proteins and
many synthetic fabrics. Hair conditioners use this trick also.
You can think of a hair conditioner as fabric softener for your head.
A cationic surfactant will often have an ammonium group attached
to a halogen, as in the ammonium chloride mentioned above. Anionic
surfactants, such as soap, often have a sodium, potassium, or
ammonium group, as in
Non-ionic surfactants like polyethylene glycol esters (PEG)
are used as mild cleansers, or to add viscosity to a mixture like
Amphoteric surfactants are those that are an acid and a base at
the same time (like water is). Cocamidopropyl betaine
is an example, used in shampoos to
stabilize foam and thicken the mixture.
Some examples of detergents and surfactants are:
Classes of detergents:
- Alkyl benzene sulfonates (ABS).
Branched-chain, anionic surfactants.
Slow to biodegrade. Seldom used.
- Linear Alkyl benzene sulfonates (LAS).
Straight-chain, anionic surfactants.
Somewhat slow to biodegrade. Most common surfactants in use.
- Alkyl phenoxy polyethoxy ethanols (alcohol ethoxylates).
Also called nonyl phenoxy ethoxylate, or nonyl phenol.
Slow to biodegrade. Nonionic surfactant.
Used in dry detergents.
- Diethanolamine and Triethanolamine.
Commonly used to neutralize acids in shampoos, to reduce
irritation (pH balanced shampoos).
Slow to biodegrade.
- Alkyl ammonium chloride (Quaternium 15).
Acts as a surfactant, disinfectant, and deodorant.
- Alkyl glucosides
Quick to biodegrade. Made from oils and sugar.
- Mono Ethanol Amine (MEA).
A solvent used to dissolve other laundry detergent ingredients.
It also lowers the freezing point of liquid laundry products
to allow them to be transported in cold weather.
- Sodium carbonate peroxide.
Used as a bleach. It breaks down into sodium carbonate
which does the actual bleaching.
- Sodium sulfate.
Used to dilute powdered detergents.