Series editors' preface - Wood Tools

536 Conservation of Furniture
The effectiveness of surfactants as emulsifiers
can be described by their hydrophilic/
lipophilic balance number, or HLB. As previously
stated, detergents and surfactants are
characterized by a molecular structure that
contains both a polar end and a non-polar
carbon chain. The balance between the polar
(hydrophilic) and non-polar (lipophilic)
character of a surfactant molecule is described
by its hydrophilic–lipophilic balance number
or HLB. The HLB number, usually between 1
and 40, is experimentally determined and
measures the ability of a surfactant or detergent
to emulsify mineral oil in water. HLB
numbers are most successful at predicting the
behaviour of non-ionic surfactants but have
proven problematic for ionic surfactants.
HLB numbers provide information about the
properties of the detergent molecule and act
as an empirical guide to a surfactant’s wetting
and rinsing properties. The higher the HLB,
the more balanced its polar and non-polar
properties. Emulsions are most efficiently
stabilized when the surfactant is more soluble
in the continuous phase. As HLB is determined
in an oil-in-water system, the higher the HLB,
the more water-soluble (and therefore waterrinsable)
the detergent.
Detergents with an HLB number less than
10 are considered weak. They usually have a
long non-polar (lipophilic) ‘tail’, and as a result
are often soluble in hydrocarbon solvents, for
example, but insoluble in water. Thus residues
of such detergents will not be removed by an
aqueous rinse but will require a hydrocarbon
rinse. In contrast, detergents with a high HLB
(> 20) are strong surfactants. They are readily
soluble in water because the polar
(hydrophilic) and non-polar (lipophilic) parts
of the molecule are more balanced. Such a
detergent, e.g. sodium lauryl sulphate (HLB
40), would be readily water-soluble and very
effective for emulsifying oil in water.
Detergents with a high HLB (> 20) are very
effective for solubilizing oily materials but are
not considered appropriate for cleaning
decorative surfaces because they may swell or
soften surfaces with an oil component and are
more likely to result in over-cleaning.
As a general rule conservation cleaning
treatments utilize a detergent with a mid-range
HLB (c.13–20). These detergents have unbalanced
polar and non-polar properties but are
capable of detergency, that is they can
suspend oily dirt in solution. Non-ionics have
an upper HLB limit of about 20 and are often
used in conservation cleaning treatments
because they can effectively disperse most oily
dirt and can usually be cleared adequately
with water. They are less likely to bond to or
swell the substrate than the higher HLB anionics.
Even within this narrow mid-range HLB,
however, the lower the HLB number, the more
unbalanced (and usually non-polar) the surfactant.
A detergent such as Triton XL 80N (HLB
13.5) is still predominantly non-polar. It will
effectively emulsify water in oil, is sparingly
soluble in water but more readily soluble in
hydrocarbon solvents. Residue removal
requires that the surfactant be more attracted
to the rinse solution than to the surface. Thus
the lower the HLB, the more likely that a
hydrocarbon solvent rinse will be required to
remove detergent residues.
Critical micelle concentration (CMC)
The characteristic that distinguishes soaps and
detergents from other surfactants is the ability
to form micelles, which aid in detaching and
removing dirt from a surface. When a small
amount of detergent is added to water it
preferentially occupies the interface between
the water and other materials, in this case, the
interface between the water and its container,
and the water and the air. As more and more
detergent is added, there comes a point when
the detergent molecules will spontaneously reorient
themselves so that the hydrophilic head
is pointed outwards into the water and the
hydrophobic tail is pointed inwards towards
moieties of dirt, grime and other oily materials
(Figure 11.19a). The orientation of the
detergent molecules would be reversed if the
solution were predominantly oil-based, e.g. a
water-in-oil emulsion (Figure 11.19b). This
molecular configuration is called a micelle and
the concentration at which micelles form is
called the critical micelle concentration (CMC).
Micelles are aggregates of detergent molecules
that are directionally oriented according to the
polarity of the cleaning solution. The detergent
micelle allows a material to be suspended in
a liquid in which it would otherwise be insoluble.
Detergents are effective cleaning agents
because they penetrate areas of soiling, deflocculate
dirt into small particles that are then