Modern nappies are designed to be able to absorb and lock in a considerable amount of liquid, meaning that — where nappy rash is not an issue — they do not need to be changed as often as they did in the past. However, odour remains an unpleasant problem that nappy manufacturers have failed to quite overcome. Accordingly, inorganic and surface chemist Dr Isabelle Simonsson of the University of Gothenburg set out to explore what material properties are important for trapping the odour molecules released by urine.
Dr Simonsson said: “The odour molecule is called p-cresol and is an organic, volatile hydrocarbon. It’s what causes the strong odour associated with pig farming and horse stables.
“p-Cresol is also found in human urine and is hydrophobic, which means it avoids water.
“That’s one of the reasons why it is released from urine into the surrounding air, in other words, that the odour spreads.”
Manufacturers of hygiene products, Dr Simonsson explained, have long known that odour molecules can be adsorbed by electrically charged surfaces.
(Adsorption is the process by which molecules of a gas or liquid are held on a surface — not to be confused with absorption, where a liquid is soaked up through small openings.)
In fact, she said, there is a patent covering this very principle, but repeated tests with various materials had, until now, failed to provide a viable practical application.
In her study, Dr Simonsson realised the potential of “activated carbon”, an inexpensive and environmentally-friendly material already used in kitchen fans to eliminate odours from food.
Made largely out of graphene, it is “activated” because it is processed to have small pores that give it a larger surface area, a quality that turned out to be great for trapping smells.
Testing the ability of various carbon materials to attract p-Cresol, Dr Simonsson found that carbon with the least charge was the most effective at attracting the odour molecules — with activated carbon being the best of them all.
The chemist explained: “[My] findings show a direct ‘ion-specific effect’ on the material’s properties and adsorption capability in synthetic urine.
“Activated carbon has a large surface area, which is good at adsorbing odour molecules.”
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While largely at the fundamental research stage, the findings could have applications in various industrial settings — from mining and water or sewage treatment to the development not only of new hygiene products but also pharmaceuticals and construction materials.
Dr Simonsson concluded: “These results are promising, but there are obstacles to developing an odourless diaper.”
She mused: “Like colour. Can you sell a diaper that’s black?”
The full findings of the study were published on the Gothenburg University Publications Electronic Archive.