Paints and Detergents
Introduction to surface chemistry
We normally think of three states of matter (solids, liquids and gases), however looking
at the world around us it is apparent that this is too simple a model for many everyday
substances. For example, a pot of paint consists of solid particles (e.g. pigments
which are responsible for the colour) finely dispersed in a liquid medium. Thus we
cannot describe paint as being purely solid or liquid.
These fine dispersions of one state of matter (or phase) in a second continuous phase
colloids. There are eight possible combinations of two such phases:
S/L L/L G/L S/G L/G L/S G/S S/S
In colloidal dispersions the dispersed units generally lie in the size range of a few
nanometres to a few micrometres. They do not normally settle out under gravity and
often require an electron microscope to observe them.
Dispersed units in the size range of a few micrometres to a few hundred micrometres
are generally referred to as suspensions rather than colloidal dispersions and settle
quickly under gravity and may be observed with an optical microscope.
Whether dealing with suspensions or colloidal dispersions, it is important to consider
the interfacial region, or the interface between the two phases. Molecules at, or near,
an interfacial region experience a different environment from those in either of the bulk
phases. In particular, they experience a different balance of intermolecular forces,
which leads to a basic physical property associated with all surfaces, or interfaces.
This property is the surface tension, or interfacial tension.
Surfaces are involved whenever two bulk phases come into contact; this is true not
only for microscopic dispersed units such as colloidal dispersions, but also for
macroscopic objects. For example if we cut up a block of solid metal, thus forming a S
/ G interface in the process, we cannot reform the original block by simply putting the
two pieces together again. However 2 small pools of liquid mercury will form into one
pool if they are allowed to touch each other. Clearly, S / G surfaces differ from L / G
surfaces. This is further complicated by the fact that sometimes 3 or more phases are
This subject has wide applications in a number of industries photography, foodstuffs,
paints, cosmetics, pharmaceuticals, pesticides, detergents, mineral processing, coal,
and oil. In biology and the environment it also plays a vital part; for example tissues
and organs are complex structures, consisting of cell which themselves contain other
discrete units. Blood is a dispersion of cells in an aqueous solution (plasma).
Detergency may be defined as the process of removing unwanted materials from
solid surfaces by surface chemical means. The agents used are members of a large
class of organic chemicals that possess the special property of surface activity, and
are thus known as surface-active agents. This is usually abbreviated to surfactants.
By surface activity we mean that chemicals seek out and modify the properties of
interfaces such as those formed between a liquid and a gas, or a liquid and a solid.
Structure of soaps
The special properties of surfactants are due to the structures of their molecules. The
simplest surfactant, soap, is made by reacting triglycerides with NaOH.
CH2O C R CH2OH
CH2O C R + 3NaOH CH2OH + 3R C
CH2O C R CH2OH
triglyceride glycerol Soap (sodium salt of
a fatty acid)
The reaction gives the sodium salt of the fatty acid (soap) and glycerol. The soap
molecule has a carboxylate head group ( -CO2 ) attached to a hydrocarbon chain. The
carboxylate head is strongly solvated by water and this is hydrophilic while the long
hydrocarbon chain is rejected by water and is hydrophobic. This structure gives the
molecule a dual nature, and is characteristic of all surfactants, and is responsible for
many of their properties. In solution the soap molecule is ionised, and since the
carboxylate ion is negatively charged, soap is known as an anionic surfactant.
“Soapless detergents” also have this structure, but the hydrocarbons are derived from
petroleum. They may be anionic, cationic or non-ionic.
Anionic: The hydrophilic group of a typical anionic surfactant is either a sulphate or a
sulphonate. Commercial surfactants are commonly made by sulphonating an
alkylbenzene, because the benzene ring is usually easier to sulphonate.
e.g.: CH3(CH2)11OSO3 Na sodium dodecyl-sulphate
Cationic: These include a variety of long chain quaternary amines and amine salt.
These are used in fabric and hair conditioners and as dispersion agents.
e.g.: CH3(CH2)11N (CH3)3Br dodecyl-trimethyl-ammonium bromide
Non-ionic: The hydrophilic component is usually a poly(ethylene oxide) chain
terminated by a hydroxyl group. Non-ionic surfactants are used in many household
products, especially for window cleaning and car washing, as they allow for smooth
draining without deposit, even when rinsing is incomplete. Non-ionic surfactants tend
to produce less stable foams than ionic surfactants, and are therefore more useful in
washing machines and dishwashers.
e.g.: CH3(CH2)11(OCH2CH2)6OH dodecyl-hexa-ethoxylate
Normally, surfactants with straight chains are preferred to those with branched chains,
since the former are more readily broken down biologically (biodegradable). Thus
straight chains are much less of a pollution hazard in rivers and streams since they are
much more readily broken down by bacterium in sewage treatment plants.
The surface tension of a liquid is defined as the work required to create a unit area of
To measure the surface tension using a capillary tube, the height h to which the liquid
rises in the tube must be measured, and the radius R of the rube determined.
The surface tension is given by the formula:
where is the density of the liquid and g the acceleration due to
Critical micelle formation
It may be experimentally determined that on the addition of quite small amounts of
surfactant, there is a very large decrease in the surface tension of pure water. This
supports the idea that the surfactant molecules rise to the surface of the water to the
interface. There they orient with the hydrophobia tail pointing away from the water
while the head is immersed in the water. Also, above a certain concentration of the
surfactant molecules in solution, the surface tension appears to be constant. This can
be explained in terms of the surfactant structures. As explained earlier, the
hydrophobic tails tend to be rejected by the water while the heads remain in the water.
One result of this is the strong adsorption of the surfactant molecules into the water
surface. Above a certain concentration however, the molecules will associate in the
water to form structures called micelles.
Micelles are generally spherical, but ellipsoidal or even cylindrical ones can form at
higher concentrations. The interior of the micelle consists of fifty to a hundred
hydrocarbon tails, while at the surface all the hydrophilic heads are exposed to the
aqueous environment. The minimum surfactant concentration at which micelles first
form is called the critical micelle concentration ( c.m.c. ). So with increasing
concentration past the c.m.c., further micelles form, but the concentration of free
monomeric surfactant molecules remains constant.
Effect of a detergent on an oil film on a fibre
The two main processes involved in the removal process are solubilization and
The hydrophobic dirt diffuses into the interior of micelles, as this interior is a more
favourable environment than water for hydrophobic dirt, and so the system is stable.
Since micelles are quite small ( 5 - 50 nm diameter ), the solution may appear quite
This process involves breaking the liquid oil film’s contact with the solid surface to form
easily detachable droplets.
Adsorption of the surfactant at the oil / water (O/W) and solid / water (S/W) interfaces
lowers the interfacial energies. As the adsorption proceeds, a good detergent will
favour the S/W interface at the expense of the O/W interface. This cause the contact
angle of the O/W interface to the solid to change such that it would cause the
formation of droplet of oil, almost detached from the solid surface. A slight agitation of
the system will then result in the drop detaching from the surface and forming a
droplet. Solid dirt can also be removed in this manner.
To prevent the dirt from being redeposited once it has been removed, which is likely to
happen if the detergent solution becomes depleted, steps must be taken to ensure that
detached droplets form stable dispersions in the water, or are collected at the surface
of the water. Foams can assist in the latter process if the droplets or particles are
incorporated in the foam structure which rises to the structure of the wash.
Stabilisation of the dirt in micelles also prevent redeposition. Certain chemicals, known
as builders, also prevent redeposition when added to detergents. One common
builder is carboxy-methyl-cellulose.
The two primary functions of paint are to decorate and protect a given surface. In all
cases, at least two ingredients are needed: one giving rise to colour, and the other
leading to the formation of a protective film. A number of properties are essential both
to aid its application and to give the final characteristic required. In addition, it may be
necessary to sustain a long shelf-life for the paint in the tin.
Nearly all paints are liquid based, the liquid commonly being referred to as the vehicle.
It may be a hydrocarbon mixture (white spirits for example) or water. There are clear
environmental and economic advantages from using water; however water has a high
energy of vapourisation compared with most hydrocarbons and this makes water
expensive where heating is an integral part of the drying and film-forming process, as it
is for car paints.
For a molecule to act as a dye it must absorb light in the visible region of the spectrum.
Photon absorption in the visible region is related to electronic excitations in the
With organic molecules colour is often associated with the possibility of extensive
electron delocalisation when certain groups called chromophores (colour bearers)
are attached to chromatic rings.
With inorganic molecules, colour is commonly associated with transition metal
complex ions. In other cases, a specific charge transfer process may take place, as in
lead iodide crystals which are bright yellow.
2+ - + -
Pb (I )2 --- (light) ----> Pb I + I
Most colloidal dispersions scatter light, regardless of whether any light is absorbed.
Unlike absorption, scattering occurs at all wavelengths, although scatter is greater at
shorter wavelengths due to the higher frequencies.
The storage condition of the paint in the tin, the application of the paint, and the final
properties of the paint film all require that the paint is stable. This may be used with
reference to sedimentation, or it may refer to aggregation (particle “sticking” together
as a result of intermolecular attraction). The aggregation of colloidal particles is known
Let us investigate the stability of the most commonly used white pigment, TiO2, both in
water and white spirit. TiO2 is hydrophilic due to the presence of -OH groups on its
surface. In white spirit the powder is not totally dispersed into individual particles (as in
water) because the powder is not readily wetted by the hydrocarbons. Moreover, as
soon as energy input into the system is stopped (by stopping shaking), flocculation
occurs rapidly. The TiO2 particles in water do not flocculate, but sediment slowly to
give a much more closely packed sediment. The reason the particles are more stable
in water is because their hydrated surfaces undergo proton transfer reactions with the
solvent. Whether the TiO2 particles have a net positive or negative charge depends on
the pH of the water. In hydrocarbon solvents, the proton exchange reactions do not
occur, and so the particles are uncharged and mutually repel each other.
To stabilise the TiO2 particles in white spirits, a method known as steric stabilisation
may be used. It involves adding polymeric additives (such as starch); the adsorbed
layer of polymer prevents the particles from coming into contact with each other and
Gloss and matt finishes
However it sometimes desirable to have weak flocculation in paint. It would increase
shelf life as the weakly aggregated particles would result in an open network
sedimentation instead of the denser cake which would result if there was no
flocculation. With the more open structure, the particles can be redispersed simply by
The degree of aggregation of the particles at the end determines whether the paint has
a gloss or a matt finish. If incident light is reflected evenly from a flat surface (from
deflocculated particles) it will appear to have a glossy appearance, and vice-versa as if
light is dispersed from a bumpy surface (flocculated particles) it will appear dull and
have a matt finish.
Protective film formation
Alkyd resins are condensation polymers of dibasic acids and dihydric alcohols. A
simple example would be that made from ethane-1,2-diol (ethylene glycol) and
benzene-1,2-dicarboxylic anhydride (phthalic anhydride). This reaction gives a linear
polymer chain, that is one which is not branched or cross-linked. This by itself would
not constitute a useful film-forming polymer on drying in air. In order to achieve this, it
is necessary to incorporate a so-called drying oil, of which linseed oil is a typical
example. These are long-chain hydrocarbons containing unsaturated double bonds,
and are included in the condensation reaction by replacing ethane-1,2-diol with a
monoglyceride such as linseed oil monoglyceride which contains 9,12,15-
octadecatrienoic acid (linolenic acid). The molecule has two -OH groups so it will react
with benzene-1,2-dicarboxylic anhydride to give a polymer containing a repeat unit.
We now have a branched-chain polymer; on exposure to oxygen, the unsaturated
hydrocarbon side chains on different polymer backbones react together so that the
chains become chemically cross-linked by oxygen. A network structure results, which
constitutes the polymer film.
Applications of surface chemistry
Wetting and spreading of agricultural sprays
In the application of pesticides to leaf surfaces it is usually essential for the droplets to
adhere to the surface of the leaf and then to spread over it giving maximum coverage.
This requires the addition of surface-active agents to the spray to lower its surface
tension. As a result the contact angle to the leaf and pesticide is reduce which
ensures maximum coverage.
Mineral recovery by froth flotation
Many of the valuable rock strata in the earth consist of a complex mixture of minerals.
To extract the various minerals it is necessary to crush the rocks and then to separate
the constituents. One way of doing this is by froth flotation. By careful choice of
surfactants, certain fractions of the ground-up rock particles can be selectively made
hydrophobic. This is achieved by making up a slurry of the particles in a solution of the
surfactant. A stream of air bubbles is then passed through the slurry to produce a
froth. Suppose the slurry consists of two different types of particles. One of these
particles is made hydrophobic, and is carried to the surface by the air bubble forming
the froth, while the other particles stay as sediment.
To look at a specific example, suppose one of the particles was silica (SiO2 -
hydrophilic and negatively charged) and the other type was iron(III) oxide (Fe2O3 - also
hydrophilic but positively charged). Now the addition of a cationic surfactant would
result in the SiO2 becoming hydrophobic due to its adsorption by the surfactant of
opposite charge. The Fe2O3 would remain unchanged however due to its charge
being similar to the surfactant, and thus it would stay hydrophilic. Thus the
hydrophobic particles are carried upwards by air bubbles while the hydrophilic particles
Large amounts of the world’s oil reserves are contained in fine pores and capillaries in
the oil-bearing strata. This oil is not released using convention methods. One
suggested method of “enhanced” oil-recovery is to pump aqueous surfactant solutions
instead of water into the oil-bearing seams. This would result in the naturally oil-wetted
capillaries being water-wetted, by adsorption of the surfactant molecules, and also
would reduce the oil/water interfacial tensions. Both these effects would greatly reduce
the pressure required to push oil through the fine capillaries in the rock structure.
Treatment of oil pollution at sea
The spillage of oil at sea is a common occurrence and can happen in various ways.
The most serious of these is accidental spillage’s from large tankers running aground.
The most effective way of dealing with oil on sea water is to disperse it by chemical
agents before it reaches the shore. A fast craft carries the detergent (or dispersant)
to the slick where it is sprayed evenly over the oil layer. Then the oil and dispersant
are mixed by means of wooden boards pulled through the mixture, resulting in them
forming a stable oil-in-water emulsion. The oil in the form of droplets has a far greater
surface area and is thus more readily degraded by biological action.
Most porous materials contain a highly complex and tortuous network of irregularly
shaped objects. A partial understanding of how liquids are absorbed in such a network
can be gained from our experience with single capillary tubes.
The pressure drop across the meniscus of a liquid contained within a narrow cylindrical
tube is given by equation. If the liquid wets the walls, the pressure drop acts to drive
the liquid more deeply into the tube. The narrower the tube the greater the pressure.
Thus liquid will tend to migrate from larger pores to smaller ones.
The most common building materials are highly porous, and capillary rise of ground-
water is a frequent cause of damp in older houses without waterproofing membranes
or damp courses. The problem is also found in more modern buildings with damaged
damp courses or building debris.
One solution is to make the water conducting capillaries hydrophobic by injecting an
aqueous emulsion of wax or silicone material. One this has dried and been deposited
on the capillary walls, the capillary angle increases and a positive pressure is required
to drive the water into the pore system. Fabrics for raincoats are waterproofed in a
similar fashion by making the fibres hydrophobic.
Emulsification is one of the most useful tools in the manufacture of cosmetics. It
allows otherwise impractical combinations of cosmetic and permits proper application.
Some emulsions include cold creams, cleansing creams, lotions, hair creams and
Non-ionic surfactants are primarily used as emulsifying agents in making products
such as creams and lotions. They are also used to solubilise essential oils (perfumes)
to produce transparent water-miscible bath oils. Certain oil-soluble, non-ionic
surfactants such as lecithin are used in conjunction with beeswax as dispersing agents
for the pigments in lipsticks. These also make the application of the lipstick easier and
improves its adhesion on the lips.
Anionic surfactants are usually readily soluble in water and have good foam and
detergency properties. They are used in shampoos, hair dyes, permanent waving
products, bath foams, and shaving preparations. Practically all toothpastes contain a
detergent with the abrasive. Sodium dodecyl sulphate is the most widely used dental
Cationic surfactants are less good at foaming and cleaning than anionic surfactants.
However, they are strongly adsorbed at protein and other negatively charged
considerably modifying the surface properties. For example they improve the feel of
hair, and so they used in hair conditioners as well as fabric softeners. They also have
germicidal properties and are used in some anti-dandruff preparations.
Weather evaporation control
Loss of water by evaporation is a major problem in arid regions of the world.
Considerable success in reducing evaporation losses has been achieved by use of
substances forming insoluble monolayers on the surface of reservoirs. The substance
used must form an effective barrier to evaporation and, of course, it must not be
harmful to aquatic life or humans.
The best substances for this are straight-chain compounds which form condensed
monomers at ordinary temperatures. The best protection against evaporation appears
to be provided by fatty alcohols. Hexadecan-1-ol is particularly efficient; it has a high
equilibrium pressure which prevents disruption and inhibits the entry of impurities
which can reduce the evaporation resistance of the film. Hexadecan-1-ol spreads
rapidly and retards evaporation by about 50%.
Weather modification by nucleation of clouds
Clouds are simply aerosols with small water droplets. In the higher regions, the
atmosphere may be super-cooled but, in the absence of nuclei, freezing and snow
formation do not occur. However in the presence of nuclei, rain droplets or snow
crystals can be formed. Nuclei are simply dust particles, smokes or salt particles from
marine sources. They are efficient condensation centres for water. Apart from micro-
meteoric dust particles, there are very few natural nuclei which can induce the growth
of snow and ice at high altitudes.
It is now well established that silver iodide, lead iodide, zinc sulphide, and cadmium
sulphide can serve as nuclei for ice formation. Various experiments have resulted in
successful formation of ice and rain. In the US and the Soviet Union experiments are
under way to suppress hail formation and thunderstorms, which cause fatalities and
damage to crops.
In the Soviet Union, the seeding material has been introduced by artillery or rockets
launched from the ground, with successes claimed.