We’ve all seen Fight Club and learnt that the crucial ingredient in soap making is fats – but hopefully not from vacuum-sealed bags of liposuction remains. Speaking of “lipo” as a prefix, we most often use the term “lipids” to describe fats and oils when having learnt some biology beyond the age of 13, and apply it to describe food groups and balanced diets. We then further our understanding of the role of lipids in insulation, both mechanical and thermal, whereby many organs of large animals are lined with a cushioning layer for protection against physical damage and thermal dissipation. But only when we encounter lipids as biological molecules, does it all start to make sense.

Do you remember the viral video during the pandemic, when people were gawking over the wondrous effects of washing hands with soap rather than just splashing them off with water? They sprinkle some black pepper all over the water, dip their finger in a liquid detergent, and then place it in the bowl of water and pepper and it magically pushes the black pepper off to the sides. This didn’t necessarily prove the vital role of soap in hand-washing, but more so the role of charge and no charge that determines the properties of molecules.

What charge?

My a-ha moment from teaching as well as cleaning greasy dishes stemmed from the time I taught lipid cell membranes and their biochemistry to my advanced biology class. Now, bear with me, if you’re doubting my skillset as a teacher for not connecting the dots as quickly as you have,I was very pleased with myself once I started to draw the delta + and delta – signs around a water molecule and explained that slightly electronegatively charged molecules will never respond to the flirtations of the uncharged, non-polar, molecules – like lipids. We have been taught that opposites attract, but the new law that I am obsessed with (and something I claim to be a law as simple as what I’m about to describe it as) is that “like dissolves with like.”

“Like dissolves with like“

Water-repelling molecules are termed hydrophobic due to their large atomic mass, no charged particles, nor any slight electronegative charges either, an example is the the type of lipid we store as fats – triglycerides. They are often stable and unreactive molecules when found among other like molecules. Therefore, if one places a drop of this molecule into a vessel of water, which is in itself a water-attracting molecule, hydrophilic, and does have slight electronegative charges (long story short – oxygen in a water molecule will cause electrons to shift disproportionately and therefore cause slight shifts in overall charge of the molecule) it will force the lipid to clump up, aggregate, or move as far away from the hydrophilic source as possible. The soap and water analogy therefore explains that soaps are hydrophobic lipids that are scrambling as fast as they can to move away from the hydrophilic source, water. But don’t fall into the classic trap that molecules are sentient beings, these interactions are simply based on force, the electrostatic force, specifically a subcategory of forces termed Van der Waals (VdW) force. The VdW forces are a niche but vital category of forces that explain the interaction between the soap, water, and black pepper. The black pepper plays no role but to show the detergent, lipid, can move as it is repelled by the hydrophilic source, the water. Without it, we wouldn’t be able to see the movement of the detergent as it appears as a clear film or ring around our fingers. We can now see that opposites do NOT indeed attract when it comes to Van der Waals, and instead “like dissolves with like”.

What should I use to wash off a tacky glue stain?

Okay, first, I want to apologise to any of the DIYers that have stumbled across this page looking for a cut-to-the chase answer and got this mumbo-jumbo instead. But! if you’ve made it this far, you can thank me later as you’ll never have to ask this question again because … Science!

Most glues and stick-on items are made with lipid-based molecules, this is often why you will see some stains on chalk-painted walls after sticking something on with tape/sticky tack/double-sided gooey circles. These therefore can easily be removed by applying the “like dissolves with like” principle, if you have something stuck on fairly tight to your dashboard/window/device and need it completely off or don’t want to damage the surface with scratches, you can try some vegetable oil to get it loose. If it’s a stubborn little gadget with superb tacky gluey circles (you know what I’m taking about) then work your way up to a more penetrating oil – look this term up on the web as I would like to avoid any copyrighting of brand names, but let’s just say that the products’ smell is quite addictive and borders the smell of petrol.

You will need to use a plastic tool or something tough to scrape the excess off*, however, the glue should be more malleable now and will enable you to move whatever is stuck off. You should also aim to leave the oil on for a while to let it diffuse through the substance and let it loosen and dissolve the glue, almost like adding water to cubes of sugar.

*This is another a-ha moment from teaching and completing chores, click here to read why some tools scratch some surfaces and others don’t, and which tool is best for which job.

It’s off, but now I’m covered in toasted sesame seed oil because I ran out of vegetable oil…

Now to get yourself out of the oily slippery mess, the best mop-up chemical is alcohol-based, specifically ethanol. This does throw a spanner in the works because alcohols have a hydroxyl, -OH, group which would imply that alcohols are hydrophilic, but due to the powers mentioned above and the uncharged hydrocarbon chain and the slightly negatively charged hydroxyl group structure of alcohols, ethanol instead acts as both a hydrophobic and a hydrophilic molecule, giving rise to the new term – “amphipathic”. This enables ethanol to be the perfect bridging molecule between an oily and slimey mess of fats, to a dry and clean space. Ethanol can be found in many alcoholic beverages, but in it’s purer form in any type of vodka, and at higher concentrations (volumes as referred to in alcohol scaling) in rubbing alcohol. Use some alcohol to remove excess oil, and wipe up the surfaces, and then wipe over with some water to get it back to a dry clean area. If your item is still stuck, repeat the oiling process again, as the glue can harden if exposed to oxygen and becomes dehydrated, meaning that the first attempt of oiling only removed the superficial tough rind of the gluey tacky circles and some more forceful scraping will help.

Alternatively, you can buy some glue stripping products which are just as effective and perhaps even more so, but if you’re in a hydrophilic pickle and have just realised you urgently need something to come off, you can now apply this science life hack and use some household items!

Is there more to this A-ha?

Essentially, yes, any oils that we encounter are lipids, and any products used to wash off these oils, are also lipids. From shampoos that help wash the grease off our hair, to the shower gels that help scrape off our sweaty fatty goodness from our bodies, lipid-lipid interactions are key and the same can be applied to water-based products. Why should the lubricant used in the bedroom be water based rather than oil based? It’s not only to protect your lovely satin bedding, but to protect the contraceptive hydrocarbon hydrophobic glove that you wouldn’t want to melt over an appendage.

References

• Alberts, B., Johnson, A. and Lewis, J. et al. (2002) Molecular Biology of the Cell. 4th edn. New York: Garland Science. Available at: https://www.ncbi.nlm.nih.gov/books/NBK26871/ (Accessed: 16 July 2024).
• Kendall, K. and Roberts, A. D. (2015) ‘Van der Waals forces influencing adhesion of cells’, Philosophical Transactions of the Royal Society B: Biological Sciences, 370(1661), p. 20140078. doi: 10.1098/rstb.2014.0078.
• Lichtenberg, D., Ahyayauch, H. and Goñi, F. M. (2013) ‘The mechanism of detergent solubilization of lipid bilayers’, Biophysical Journal, 105(2), pp. 289-299. doi: 10.1016/j.bpj.2013.06.007.
• Patra, M., Salonen, E., Terama, E., Vattulainen, I., Faller, R., Lee, B. W., Holopainen, J. and Karttunen, M. (2006) ‘Under the influence of alcohol: the effect of ethanol and methanol on lipid bilayers’, Biophysical Journal, 90(4), pp. 1121-1135. doi: 10.1529/biophysj.105.062364.
• Thompson, T. E. (2024) ‘Lipid’, Encyclopedia Britannica. Available at: https://www.britannica.com/science/lipid (Accessed: 16 July 2024).
• Thompson, C. J., Ainger, N., Starck, P., Mykhaylyk, O. O. and Ryan, A. J. (2023) ‘Shampoo Science: A Review of the Physiochemical Processes behind the Function of a Shampoo’, Macromolecular Chemistry and Physics, 224, p. 2200420. doi: 10.1002/macp.202200420.
• Zhuang, B., Ramanauskaite, G., Koa, Z. Y. and Wang, Z. G. (2021) ‘Like dissolves like: A first-principles theory for predicting liquid miscibility and mixture dielectric constant’, Science Advances, 7(7), p. eabe7275. doi: 10.1126/sciadv.abe7275.