Surfactants: the ubiquitous amphiphiles
The surfactant industry is a huge and dynamic business, and soap
is just the start, says Tony Hargreaves
Most familiar of all surfactants is soap, a simple substance
which, in water, clearly demonstrates two effects. It produces foam
due to its action at the air–water interface, and it makes the
grease transfer from grubby hands into the soapy water as a result
of its activity at the water–oil (grease) interface. However, soap
was probably not the first surfactant in the service of humankind.
Many plants produce significant quantities of saponins (steroid
or triterpenoid glycosides) which have surfactant properties. One
such plant is the soapwort Saponaria officianalis whose
foliage yields a glycoside capable of wetting, foaming and grease
dispersion – the very qualities that we recognise in a modern
detergent. It is likely that the saponins provided our ancestors
with our first useful surfactants. These natural glycosides are
still in use today for specialised processes such as washing
1. The global surfactant market |
Modern surfactants, however, are of many different chemical types
and do far more than produce foams and disperse grease. The global
surfactant industry is a multi-billion pound business (Fig
1), with markets everywhere from household detergents to
explosives (Table 1). Surfactants (surface active agents)
can be broadly defined as compounds which, when dissolved in water,
concentrate at surfaces (interfaces) such as water–air or water–
oil. The interfacial activity of these substances, which can be
explained in terms of their molecular structure, gives rise to a
wide range of surface chemistry functions: wetting, emulsifying,
solubilising, foaming/defoaming, rheology-modifying, antistatic,
‘glossing’, lubricity and surface conditioning.
The major surfactant markets |
|Industrial and institutional
|Paints and coatings
|Textile spin finish
|Pulp and paper
Seldom are surfactants on their own put directly into use. In the
area of household cleaning preparations, the surfactant is normally
blended with a range of ingredients such as other surfactants,
thickeners, foaming or defoaming agents, alkalis/salts, chelating
agents and so on. This is the province of specialist formulators
such as Colgate Palmolive, Unilever, and Procter & Gamble.
Generally, these modern formulated products rely on man-made
surfactants but there are exceptions. Certain foods, in particular,
include natural surfactants such as lecithin, an emulsifier in
chocolate and ice cream manufacture. Nature also relies heavily
heavily on surfactant chemistry: our liver produces surfactants, the
bile acids; our lungs use surfactants to maximise the efficiency of
gas exchange across the air–water interface; and in every cell in
every organ in our bodies there is a complex membrane that functions
due to surfactant chemistry.
|1. Surfactants explained
have low solubility in water. A typical surfactant has a large
lipophile (hydrophobe) which restricts its aqueous solubility.
For example, sodium dodecylbenzene sulphonate has a solubility
maximum of 0.04.mol l–1.
Beyond this concentration the molecules associate to form
colloidal aggregates known as micelles. This concentration is
the critical micelle concentration (CMC). Different
surfactants have different CMC values.
In a micelle
the surfactant orients itself with its lipophiles towards the
interior, thus presenting a hydrophilic surface to the water.
The simplest micelles are spheres but as surfactant
concentration increases the micelles grow and form rods.
At high surfactant concentrations the rods form larger
structures such as hexagonally packed rods and palisade
arrangements. As these structures increase in size they take
on a greater degree of order until, for the biggest
structures, they occur as liquid crystals. These structural
changes are reflected in the viscosity of the surfactant
characteristics of the surfactant in water is an important
feature of many surfactant formulations. For surfactant
systems these characteristics are explained in terms of the
micelles. Where unassociated molecules are present the
viscosity is virtually that of water.
The formation of
spherical micelles has little effect, but where the
sphere-to-rod changes occur there is a marked increase in
viscosity. The hexagonal packing of rods causes viscosity to
climb further but, surprisingly, the formation of the largest
palisade structures results in a dramatic fall in viscosity.
The latter effect is due to the large layer-like
structures being able to slide over each other so that there
is little intermolecular friction. In surfactant technology
the micelle structure can be manipulated to give a product
with the desired properties such as a shampoo gel rather than
a runny liquid.
Micelles can act as solubilisers in which oily molecules
of oil/grease/hydrocarbon are taken into the lipophilic core
of the micelle and retained by the lipophile-to-lipophile
attractive forces. By this means it is possible to make
colloidal emulsions or microemulsions –¨sometimes called
swollen micelles. An important application is to make a
solvent such as a hydrocarbon ‘dissolve’ in water and
dirty dishes or clothes involves a complex mechanism
comprising many physical and chemical effects resulting from a
variety of soil types and a range of substrate materials.
However, the most important cleaning action is a result of
surface chemistry and surfactants. Detergent action to remove
oily/greasy soiling involves wetting, emulsification,
solubilisation and micelles.
Adsorption at the
water–oil interface results in dispersion of one phase into
the other depending on the properties of the system. These
dispersions are emulsions and of two types: oil-in-water (o/w)
or water-in-oil (w/o).
Water is not attracted on an oily/greasy surface, but with
a surfactant present wetting occurs because the molecules
adsorb at the oil–water interface in a manner similar to that
seen for emulsification.
A foam is a dispersion of gas in liquid, generally air in
water, where there is only a small volume of liquid compared
with the large volume of gas. Each gas space, or cell, has
walls made up of a thin layer of water with surfactant
molecules adsorbed at the surfaces. Adsorption of a suitable
surfactant creates a foam that gets its mechanical stability
from surface elasticity and just the right amount of drainage
in the water between the surfaces of the film.
Surfactants in antiquity
People first began to make surfactants, namely soap, in about
1500 BC, but soap-like substances have been found dating back to
2800 BC. Soap was, and still is, made by the alkaline hydrolysis of
animal fats or vegetable oils – a process known as saponification.
Soap is the most widely used single surfactant, accounting for
around 30 per cent of the current surfactant market.
Moving on from soaps – and into the 19th century – the next
surfactants to be developed were the sulphates and sulphonates of
vegetable oils. The reaction of castor oil with sulphuric acid is a
classic example from the late 1800s. In this reaction the product is
a mixture of sulphates and sulphonates which, after neutralisation
with sodium hydroxide, give a product known as Turkey Red oil useful
in the dyeing of linen.
going full circle? |
Later, the development of sulphonation and sulphation processes
using other oils as reactants led to a move away from natural and
renewable plant oils and animal fats to the sulphonation of
petroleum products. The introduction of alkyl benzene sulphonatis
(ABSs), for example, was brought about by nucleophilic substitution
in the benzene ring using oleum (H2S2O7)
or sulphur trioxide. The ABSs made a major contribution in changing
the traditional soap powders to detergent powders for household
laundry. One well-known example in the 1950s was the change in the
composition of Lever Brothers’ Persil, where the soap in the
original formulation (PERborate+SILicate+soap) was replaced by ABS.
Other brands of washing powders followed suit and the word
detergent is now a part of everyday vocabulary. Modern detergent
powders now typically contain linear alkylbenzene sulphonates
(LABSs), which biodegrade more quickly than the original ABSs.
Progress was not confined to the sulphonation of different oils,
but was soon accompanied by ethoxylation, in which a few or many
ethylene oxide (EO) molecules react with a fatty alcohol – which may
be synthetic or plant derived – to make the surfactant molecule.
Thus, alcohol ethoxylates, alcohol ether sulphates and alkyl phenol
ethoxylates became available.
Along with the development of ether technology came the
polymerisation of ethylene oxide with propylene oxide (PO) to give
EO–PO copolymers – surfactants that are totally reliant on
petrochemicals as raw materials.
But things may slowly be beginning to turn back around. Current
pressure to move away from non-renewable petroleum feedstocks and
towards plants as sources of raw materials has led to a lot of
effort on developing surfactants from oleochemical feedstocks. Many
recently-developed surfactants are an attempt to satisfy the modern
consumers’ desire for products to be ‘more natural’.
Most important amongst these are surfactants derived from the
carbohydrates sorbitol, sucrose, glucose and from plant oils such as
coconut or palm kernel. Thus we have: sorbitan esters, sucrose
esters, alkyl polyglucosides (AGs), alkyl glucamides (Box
2). Sorbitan esters are used as emulsifiers in cosmetics and
the sucrose esters in food manufacture. The alkyl polyglucosides
find application as detergents rather than as emulsifiers and are
making inroads into some everyday products.
|2. 'Natural surfactants'
|A few of the currently available
carbohydrate-based surfactants include:
Carbohydrate-based surfactants, being based
on plant-derived chemicals, use renewable resources, are
readily biodegradable, non toxic and do not add to the Earth’s
three common carbohydrate-based surfactants are shown below:
- Alkyl polyglucosides – Triton APGs (Union Carbide),
Plantcare (Cognis/Henkel), Lauryl glucoside, Monatrope
- Sorbitan esters – Crills (Croda) and Spans (ICI/Uniqema)
- Sucrose esters – Crodestas (Croda)
How surfactants work
A look through the surfactant literature presents us with a huge
number of different surfactants, along with an even greater number
of names. At the core of all this is some relatively simple
All these surfactant molecules have in common the same basic
molecular structure – a hydrophile attached to a lipophile – and it
is this nature that makes them adsorb at surfaces. The hydrophile is
attracted to water in preference to lipid-like substanoes
(hydrocarbons such as oil and grease) whereas the lipophile is
attracted to these in preference to water.
Other terms are sometimes used, for example, a hydrophile is a
lipophobe and a lipophile is a hydrophobe. However, the use of
‘phobe’ is unfortunate because it implies repulsion whereas the true
situation is one of only feeble attraction. Paraffin wax is
lipophilic but it does not repel water, as is demonstrated by the
fact that droplets of the liquid cling, somewhat precariously, to
the underside of a wax block – the water must be attracted to the
It is the amphiphilic nature of surfactant molecules that makes
them bifunctional. This can be seen when oil/grease and water come
together. No matter how much energy is expended in getting them to
mix, the oil and water will always separate into two distinct
phases. The intermolecular forces between water molecules and
between oil molecules are stronger than the forces between water and
Added surfactant molecules adsorb at the oil–water interface,
where they orient themselves such that the hydrophile is in the
water and the lipophile is in the oil. With a little agitation the
oil becomes dispersed in the water and the surfactant acts as an
Types of surfactants
Lipophiles are usually similar from one surfactant to another but
hydrophiles show a range of chemical types and this is the basis for
surfactant classification: anionic, cationic, non-ionic and
amphoteric (Fig 2).
2. Structures of some common surfactants |
Anionic surfactants, which include soap, are the most widely used
for cleaning processes because many are excellent detergents. In
anionic surfactants the hydrophile comprises some highly
electronegative atoms, making these molecules strongly polar. The
counterion is usually a small cation such as sodium but occasionally
may be a larger cation such as ammonia or amines.
Cationic surfactants, in contrast, comprise a long chain
hydrocarbon as the lipophile with a quaternary amine nitrogen as
hydrophile, and a halide ion as counterion. An important property of
cationics is that they are attracted to surfaces carrying a negative
charge, upon which they adsorb strongly. Proteins and synthetic
polymers can thus be treated with cationics to provide desirable
surface characteristics. For example, hair conditioners and fabric
softeners are cationic surfactants.