As components in soaps and washing powders, surfactants are very familiar washing aids.
How do they work?
The simplest description of a surfactant is a long molecule chain – one end of which carries a degree of charge, with good compatibility with water, and the other end of which is non-polar, with good compatibility with oils and non-polar solvents.
This means that one end of the molecule prefers to be surrounded by water and avoid oils, while the other end will avoid water and be attracted to oils. Surfactants act as an interface helping substances to mix that would otherwise remain separate.
Surfactants can be one of four types.
- Anionic – where the charged end is an anionic functional group that carries a negative charge, making it good at attracting positively charged dirt like metal salts. They tend to work better in mildly acidic conditions, and worse in alkaline conditions.
- Cationic – where the charged end is a cationic functional group that carries a positive charge.
- Amphoteric – where a molecule has both positively and negatively charged regions.
- Non-ionic – where the charged end is polar not ionic.
Anionic and non-ionic are the most widely used types across several industries. Non-ionic surfactants are often preferred for heritage conservation work because of their lower HLB values.
Enough surfactant added to just water will form small spherical clumps, with the water-attracted ends pointing outwards, and the oil-attracted ends grouped in the middle away from the water.
Similarly, enough surfactant added to a non-polar solvent will also form these clumps, but with the oil-attracted ends pointing outwards and the water-attracted ends grouped together in the middle.
These clumps are called micelles. Their formation is a crucial step in how surfactants carry out cleaning. These micelles, subject to gentle agitation, can open up to swallow and form around other particles. This can, for example, help keep dirt suspended in water rather than being re-deposited, and even more usefully, can lift greasy dirt from a surface.
All surfactants have a number supplied by the manufacturer that describes how much surfactant is required for these micelle clumps to form in water. This CMC number, standing for Critical Micelle Concentration, is currently expressed by manufacturers in a range of units that makes comparisons between brands confusing.
Below this concentration, if not enough surfactant is used or if the surfactant is used (familiar to us as the point when the bubbles disappear when washing-up dishes). Such surfactant that is present will congregate ineffectually out of solution – such as at the air-water interface
Conditions can be adjusted to promote micelle clump formation.
A common approach, often taken by industry, is to add soluble salts.
An example of this is the addition of common salt to dishwashers, either within the detergent or by the customer upon use.
Surfactants also have optimal temperatures at which they work best, such as around 25°C for sodium lauryl sulphate.
Conversely, hard water containing calcium or magnesium ions will require more surfactant due to the formation of insoluble scum.
CMC numbers can also tell us about the nature of the surfactant.
Those with a low CMC below 2mM are very hydrophobic in character, with the oil-attracted region most dominant.
A medium CMC between 2-20mM suggests the water-attracted and oil-attracted regions are roughly equivalent.
A high CMC over 20mM shows the surfactant is very hydrophilic in character, with the water-attracted region most dominant.
A more oil-attracted surfactant with a low CMC will be more likely to leave residues on greasy surfaces like oil-based inks, and with residues more easily cleared using solvents than water.
A more water-attracted surfactant with a high CMC will be less likely to leave residues on oil-based inks, with residues best cleared using water.
Calculations of how much surfactant is required by using CMC will need to be adjusted upwards if it is supplied in an already partially-diluted form.
Under some conditions mixtures of surfactant in water will separate into surfactant rich and surfactant poor regions – turning cloudy.
Cloud point can be a function of temperature, and becomes more likely the more soluble salt is added. Cloud point figures should be available for each surfactant. Clouding is also made more likely the more soluble salt is added (different salts affect this to varying degrees, with the type of anion having greatest influence).
HLB and associated uses
HLB stands for Hydrophile-Liphophile Balance, and is an assigned scale running from 1-40 describing how strong a surfactant is. Wolbers lists HLB values and associated treatment uses:
4-6 Water-in-oil emulsifiers
7-9 Wetting agents
8-18 Oil-in-water emulsifiers
Dow Chemical Company suggest <10 w/o emulsifier, > 10 o/w emulsifier, 10-15 good wetting, 12-15 detergents.
A surfactant matching the usual requirements for a conservation grade detergent might be suggested to have a HLB of 12-13, a cloud point above 60degC, and also to be non-ionic in character.
Mixing two surfactants with different HLB values will give a working HLB value in between the original values.
Some surfactants are formulated to produce fewer bubbles as a specific working quality required by users. Within a specific brand of surfactant, e.g. Dehypon, there is a tendency for the lower HLB values to also be associated with lower levels of foaming.
Health and the environment
It is inherent to their nature and use that surfactants will tend to be hazardous irritants, with effects including the stripping of protective oils from the skin. Many are very chemically stable and are very poor at breaking down in the environment. There has been a move away from surfactants with dangerous bio-accumulative and hormone-mimicking properties, but a general shift towards better bio-degradability is slow.
Dow Chemical company’s Ecosurf range are some examples of those with improved biodegradability.
Surfactants in artworks.
Surfactants are present in very high amounts in some modern paints e.g. acrylics paints and water-soluble oil paints. These have a big influence on how the paints age, react to changing conditions, and respond to treatment:
Surfactants move deeper into paint films at higher temperatures and lower humidity levels, and towards the paint films surface at lower temperatures and higher humidity levels.
Migrating surfactants can carry dirt with them Surfactants on the surfactants on the surface can give a bloom-like appearance Different surfactants can be mobilised by a range of solvents from water to siloxanes.
Accumulations of surfactants can appear as tidelines.
Surfactant loss during swabbing can be suppressed by taking advantage of conditions that lower cloud point. Aggressive reactions from liberated high HLB paint surfactants can be mitigated by lowering the HLB of mixtures formed during cleaning by making up cleaning solutions that include very low HLB surfactants.
It’s also worth noting that the breakdown products of the surfactants in paints and inks could be toxic so avoid accidental contamination
Uses in conservation
The requirement for gentle agitation explains why surfactants receive limited use in paper conservation washing treatments, alongside worries about effective rinsing.
Where agitation can be used during immersion and surface swabbing treatments, surfactants are more successfully used. Examples include washing some textiles, cleaning some plastics, and swabbing painted surfaces.
While often used for their direct cleaning effect, other uses include as emulsifiers for solvent and water mixtures.
The development of new surfactants and changes in regulation affecting older surfactants mean that more recent articles are likely to be more relevant than older articles.
The best places to look for more information about surfactants are the details provided by manufacturers and suppliers, and textile and paintings conservation articles, with these disciplines being among the most regular users of surfactants with heritage artefacts.