…And Soap It Goes

Cleaning Feathers with Surfactants

Concentrated surfactants tested in the cleaning study. @AMNH/M.Paulson

What Is a Surfactant?

Surfactants (or surface-active agents) reduce the surface tension of a solvent (usually water) so that the surface is more easily wetted[1]. Many surfactants also have the power to dislodge, disperse, and hold soil in suspension until it is rinsed off. Surfactants thus include wetting agents, emulsifiers, dispersants and foaming agents, and well as detergents and soaps that are used in cleaning, particularly for the removal of dried or greasy soiling. A surfactant has a non-polar (hydrophobic and lipophilic) tail of long chain hydrocarbon groups and a polar (hydrophilic) head. The hydrophilic head dissolves in solvent (usually water) while the hydrophobic tail attaches to dirt.

Surfactants in Our Community Survey

How extensively are detergents and soaps used in the conservation of feathers? According to the majority of our survey respondents, rarely or never. Surfactants are mostly reserved for the preparation of fresh, new feathers (as in the production of new taxidermy mounts or contemporary art). When they are used to clean existing feather objects, people described using low concentrations, applying cautious amounts by sponge or brush, supporting feathers with net or Mylar sheets, and immersing them in an aqueous bath[2] or using an ultrasonic bath (which requires that the feathers can be detached).

Cleaning feathers with surfactants raises concerns beyond wet cleaning in general because of the need to follow with rinsing to remove soil in suspension, as well as any residual surfactant. Survey respondents relayed several specific observations about rinsing feathers: it requires increased manipulation; rinsing can draw debris into inaccessible feather interstices as it wicks down the vane towards the rachis; and if rinsing is ineffective, residues may be left behind that can alter the arrangement of feather microstructures or trap dust and pollutants.

Fresh feather treated with Surfonic JL-80X 1% in solution. After drying and before rinsing, distal barbules are clumped together. @AMNH/L.Vilicich

Fresh feather treated with Surfonic JL-80X 1% in solution. After rinsing, distal barbules realigned. @AMNH/L.Vilicich

Key Criteria in Differentiating Surfactants

Solvents and Solvent Mixtures

Most familiar detergents and soaps are soluble in water. Aqueous solutions described in the survey include a variety of percentages (generally between 0.5 and 3%) of the surfactants Dawn dish soap (ionic and nonionic components), Orvus WA paste (anionic), Synperonic N (nonionic and banned in the EU for environmental reasons) and Synperonic A7 (nonionic), Marlipal® 1618/25 (nonionic), Detertec (nonionic), and Alconox (nonionic).

Considerations for cleaning with water-soluble surfactants overlap with those of aqueous treatments in general, which increase the risk of damage when used on water-sensitive degraded feathers. The need for rinsing lengthens the necessary exposure to liquid water. Similarly, materials like skin and hide that are often associated with feathers may react poorly to water used in cleaning and rinsing.

Only a few survey respondents report using surfactants in non-aqueous solutions. These included Triton XL-80 and Triton-100 (now both replaced by the nonionic Surfonic JL-80X) in petroleum benzine applied in solution and as a poultice; Synperonic A7 in either ethanol or water; Vulpex in stoddard solvent (nonionic, however, some respondents specifically advised against using Vulpex on feathers[3]); Knobloch’s Kemal-4 (a detergent-type degreaser used in taxidermy preparation); and others. Not mentioned in the survey but of interest is Surfynol 61, a multifunctional acetylenic wetting agent that is miscible with various solvents and is itself volatile.

If otherwise safe and effective, these non-aqueous surfactants may represent alternatives conservators can use to avoid some of the risks of aqueous systems on deteriorated feathers, and furthermore, in the case of volatile surfactants, to mitigate the risks of persistent residues and unnecessary physical manipulation in rinsing.

Still from a video of cleaning an aged swan feather with Synperonic A7 in deionized water. @AMNH/M.Paulson
An ultraviolet luminescence image of blotter paper after cleaning a fresh swan feather with Dawn Ultra in deionized water. @AMNH/M.Paulson

Anionic and Non-ionic Surfactants

Surfactants are classified by their electrical charge in water: anionic (negative), cationic (positive), non-ionic (no charge), and amphoteric (capable of carrying a both positive and a negative charge). Surfactants used in conservation are generally anionic or non-ionic.

Anionic surfactants are able to attach to a broad range of soils because of their attraction to metallic cations and are thus widely used as soaps and detergents. They are useful for lifting and suspending particulates and rinse more easily than non-ionic surfactants, but can be more foamy.

Non-ionic surfactants are better wetting agents and emulsifiers and tend to foam less dramatically. They are useful for the removal of organic soils and can be pH adjusted. Over time, non-ionic surfactants are reported to degrade when exposed to oxygen and light, which may further mitigate any residues left after rinsing[4]. Due to their lower critical micelle concentration, they also remain effective at lower concentrations[5].

Critical Micelle Concentration (CMC)

Micelles are clusters of surfactant molecules dispersed in solution. Each surfactant has a specific critical micelle concentration at which the surfactant has the power to change the solution’s properties. An optimally effective cleaning solution has a surfactant concentration of 3–10 times the CMC. More than 10 times the CMC does not improve efficacy but will likely result in excess surfactant being left behind on the surface. Since manufacturers often do not report CMC, and frequently adjust the concentration of their product, controlling CMC closely may require measuring[6] it each time you acquire a new batch of product.

Hydrophilic-lipophilic Balance (HLB)

The HLB number represents the relative molecular weight of the hydrophilic head to the hydrophobic/lipophilic tail of the molecule. The higher the number, the more soluble and stronger the surfactant. Weak surfactants (HLB <10) have a long non-polar tail and can be insoluble in water but soluble in hydrocarbons. Some conservators recommend using surfactants with mid-range HLBs (13-20)[7].

Table 1 created from information found in Cheng et al. 2020, Kanicky et al. 2001, and “Hydrophilic-lipophilic balance” page on WikiPedia.

Health and Safety Surfactants can present both physical hazards (e.g., flammability), and health hazards such as respiratory, skin, or eye irritation, or oral toxicity. A helpful list of health and safety concerns associated with common conservation surfactants and soaps is available in the AIC Wiki.

Environmental Impact

Because of their ability to disrupt lipids in aqueous environments, surfactants disposed of incorrectly can be toxic to living organisms and damage ecosystems. Some popular surfactants used in conservation have been phased out due to their poor biodegradability, and/or their negative effect on aquatic organisms.

Surfactants Selected for Study

After an open-ended exploration of many products named in the survey, we selected representative set to investigate more in-depth. We prioritized those that were prevalent in the survey data and sought to include products that differed both in charge and solubility.

In our experimental work, we will test each of these cleaning solutions more systematically, assessing short-term outcomes at the macroscopic and microscopic levels. We will also take a longer view, placing feathers cleaned with these detergents into the accelerated aging chamber and looking for persistent residues, as well as any change in the rate of discoloration or oxidation relative to untreated feathers.


Footnotes

[1] The surface tension of water (0.073 N/m) is reduced to about 0.030 N/m (Newton/meter) in the presence of a surface-active agent.

[2] Method used in a feather conservation workshop (attended by Renee Reidler in 2007 with Allison Rae): A feather is put between two layers of Mylar and the sandwich is immersed into a container with water (+/- surfactant). Mylar reduces the stress on a degraded feather while it is moved around.

[3] Concentrated Vulpex has a pH of 13±1, so it is quite strongly alkaline. Ross and Phoenix recommend caution and the use of low concentrations (in water or mineral spirits) for cleaning oil paintings.

[4] In addition to being mentioned in some of the references below, Stavroudis discusses this in the Modular Cleaning Program

[5] Anionic surfactants are used in a concentration of 0.2%-0.5% and non-ionics in a conc. of 0.05-0.02%.

[6] A guideline how to measure the CMC is provided by textile conservator Karin von Lerber

https://cool.culturalheritage.org/byform/mailing-lists/texcons/2005/05/msg00013.html

[7] Rivers, S and Umney, N. 2003. Conservation of furniture. p. 536 “these detergents have unbalanced polar and non-polar properties but are capable of detergency, that is, they can suspend oily dirt in water”. This property is likely equally good for furniture and feathers.


References

Cheng, K.C., Khoo, Z.S., Lo, N.W., Tan, W.J., and Chemmangattuvalappil, N.G., 2020. Design and performance optimisation of detergent product containing binary mixture of anionic-nonionic surfactants. Heliyon. Vol. 6. https://doi.org/10.1016/j.heliyon.2020.e03861

Currie, F., Andersson, M., & Holmberg, K. (2004). Oxidation of Self-Organized Nonionic Surfactants. Langmuir, 20(10), 3835–3837. doi:10.1021/la0499665

Gotoh, K., Horibe, K., Mei, Y. and Tsujisaka, T. 2016. Effects of water hardness on textile detergency performance in aqueous cleaning systems. Journal of Oleo Science 65 (2): 123-133.

Hydrophilic-lipophilic balance. https://en.wikipedia.org/wiki/Hydrophilic-lipophilic_balance

Kanicky, J., Lopez-Montilla, J-C., Pandey, S. and Shah, D. 2001. Surface Chemistry in the Petroleum Industry. Handbook of Applied Surface and Colloid Chemistry.

Leung, Y-M.B., Morgan K.J., White B.J., Ward J.M. and Chilvers, B.L.. 2015. Washing oiled marine birds in New Zealand—do we have a detergent that can be used? New Zealand Journal of Marine and Freshwater Research: 1-5

Lewis, J. and Eastop, D. 2001. Mixtures of anionic and non‐ionic surfactants for wet‐cleaning historic textiles: A preliminary evaluation with standard soiled wool and cotton test fabrics. The Conservator 25 (1): 73-89.

Mürer, Kerstin. 2006. Surfynol 61 Lösungsmitteleigenschaften eines Tensids. Pp.101-114 in Oberflächenreinigung. Material und Methoden. VdR Schriftenreihe 2 (C. Weyer, ed.). Theiss Verlag, Stuttgart.

Ross, S. and Phenix, A. 2005. Vulpex spirit soap as a cleaning agent for painted surfaces. WAAC newsletter 27 (1): 15-22

Silva D, Syllos S, Chiavone-Filho O, de Barros Neto EL, Mota ALN, Foletto EL, Nascimento CAO. 2014. Photodegradation of non-ionic surfactant with different ethoxy groups in aqueous effluents by the photo-Fenton process. Environmental Technology 35 (12): 1556–1564. doi: 10.1080/09593330.2013.873485

Stavroudis, C. 2009. Sorting Out Surfactants. WAAC newsletter 31 (1): 18-21.

Tímár-Balázsy, Ágnes. 1999. Wet cleaning of historical textiles: surfactants and other wash bath additives. Studies in Conservation 45 (Supplement-3): 46-64.


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