Series editors' preface - Wood Tools

Detergents
Commercially viable synthetic detergents were
developed in the 1940s, notably sodium lauryl
sulphate (CH 3(CH 2) 11OSO 3 – Na + ), a common
ingredient in shampoo formulations. Synthetic
detergents are in common use in a wide range
of industries, from cosmetics to foundry work,
and detergents are often custom-designed to
accomplish specific tasks in very specific
applications. As a result, there are thousands
of different formulations of detergents on the
market from which to choose. Detergents can
be broadly classified according to the ionization
of the hydrophilic end of the molecule
(anionic, cationic, non-ionic etc.), by their
hydrophile–lipophile balance number (HLB),
and their critical micelle concentration (CMC).
Cationic detergents have a positively charged
hydrophilic head group and include amine
salts, quaternary ammonium compounds and
amine oxides. They are commonly added to
commercial formulations for their antiseptic
properties. They are problematic for the cleaning
of varnished and decorated surfaces
because their strong attraction to aged coatings,
which are commonly acidic and therefore
negatively charged, can make removal of
residues after cleaning very difficult.
Anionic detergents have a negatively charged
hydrophilic portion and include alkali carboxylates
or soaps, sulphates, sulphonates and
phosphates. They work well in neutral or
alkaline solutions, but their cleaning properties
can be adversely affected by moderate changes
in pH and they have a limited electrolyte tolerance.
Anionic detergents may precipitate if the
solution environment becomes too acidic
(Rieger, 1985). Their negatively charged ‘head’
group means they are more effective than nonionic
detergents for lifting dirt, grime and oil
from a surface and keeping it in suspension.
The cleaning action of anionic detergents may
be inhibited by the presence of Ca 2+ and Mg 2+
ions (often present in tap water), which will
form insoluble salts with anionic detergent
molecules and precipitate out of solution.
Commercial detergents often contain chelating
agents to prevent this. It may be difficult to
completely remove residues of anionic detergents
from a surface after cleaning as their polar
characteristic makes them more likely to remain
fixed to a substrate that is basic (e.g. some
proteins) or that is slightly electro-positive.
Principles of cleaning 535
Non-ionic detergents do not ionize in water
and therefore do not have a charged ‘head’
group. The polar component is generally
larger than anionic or cationic detergents and
may be as large or even larger than the nonpolar
tail. Even though they do not ionize,
they are water-soluble because of the presence
of polar functional groups that are capable of
significant hydrogen bonding (Myers, 1992).
The absence of a net electrical charge can be
an advantage because they have a significantly
lower sensitivity to the presence of electrolytes
in the cleaning system, are less affected by
solution pH, and are unlikely to precipitate. As
a class, they are generally weaker detergents
than anionics, which can be an advantage
when less aggressive cleaning treatments are
required. They are equally good at penetrating
both polar and non-polar dirt because of
the overall structural balance. Branched nonpolar
tails will slow down or prevent penetration
into a surface. Non-ionics have been
widely used in conservation because their
properties tend to meet conservation cleaning
objectives (Moncrieff and Weaver, 1992).
Amphoteric surfactants can be either
cationic or anionic depending on the pH of
the solution and include zwitteronic surfactants,
which possess permanent positive and
negative charged groups. They are not in
common use in conservation.
Emulsions and hydrophilic lipophilic
balance (HLB) numbers
An emulsion is a liquid/liquid mixture in
which globules of one liquid are suspended in
another. They are generally found in two
forms, either oil-in-water or water-in-oil. Both
appear white or opaque and will separate if
left to stand. Emulsions can be stabilized by
the addition of a surfactant or detergent.
Emulsions can be used to build a cleaning
solution that combines polar and non-polar
elements. If a surface is sensitive to water, for
example, but the dirt would be best removed
by an aqueous solution, it is possible to use
an emulsion that suspends globules of water
(and additives such as pH buffers, chelators
etc.) in a non-polar hydrocarbon solvent.
Water suspended in a hydrocarbon solvent (an
example of a water-in-oil type emulsion) can
be used to clean water-sensitive materials such
as water gilded surfaces.