Nappy Science Gang are doing a joint project at the moment with Cardiff Catalysis Institute, partly funded by a grant from the Royal Society for Chemistry. We are trying to work out more about the mineral exoskeleton we found on some of the nappies we got tested last year. In this chat we talked to Jennifer and Jonathan Bartley from the Cardiff Catalysis Institute about hydroxyl apatite and how we can work together.
To start with Jennifer and Jonathan were welcomed to the group chat and thanked for their time. NSG then outlined the findings of the group to date. We got several nappies tested, which all had similar ‘symptoms’ – persistent smell of ammonia or a ‘barnyard’ smell when freshly weeed in. The nappies were tested to see if they had anything soluble in them, but came up clean. They were tested for bacteria, and all came up not very clean. They were tested for insoluble contents using an ‘ash test’, and 2 out of 5 had a lot of hydroxyl apatite (about 20% hydroxyl apatite by weight). There are more details and analysis in the blogs at https://nappysciencegang.wordpress.com/2016/03/15/strip-washing-results-part-2/. The lab people who did our tests said that an ash value of 1-2% would be normal, 3-4% would be high, 20% was unheard of – I’d be interested to know how likely this is. However, it was thought that this amount made sense for the amount that a nappy would be washed in its lifetime though.
Question: To start with, what is hydroxyl apatite?
Jonathan: Calcium hydroxy apatite is a naturally occurring mineral that is the main constituent of some bones like vertebrae and teeth. It is made up of calcium ions that are found in hard water; phosphate groups that are often added to washing powders; and hydroxide groups that are from water with the chemical formula Ca5(PO4)3OH.
NSG Member: Love the formula. Is it entirely plausible that the calcium and phosphate could be coming from different sources than those you have identified, for instance the phosphate in urine.
Question: Do we know if these nappies also have a crunchy feel to them? Does the hydroxy apatite make them feel crunchy? I have found my daughter’s nappies became smelly after getting wet – not even necessarily with wee. They were also crunchy. I also live in a very hard water area.
Jonathan: Urea breaks down into ammonia over time so the smell is probably common to nappies with or without apatite in them. As the hydroxy apatite formation is like growing bone around the nappy fibres it won’t smell, but could trap other things in them.
NSG Member: I think if we polled people in the group they might say they’d expect to smell ammonia after say an overnight use, but smelling it as soon as the nappy is wet is pretty unusual.
NSG Member: We obviously commonly found ammonia in nappies, but there is a pattern here of *some* nappies starting to really smell of it, when most don’t.
Question: We understand that phosphates were banned (only allowed in a tiny quantity at least) in domestic detergents in 2011. Do you think laundry can still be exposed to enough phosphate to accumulate high levels of apatite? Do you think the calcium or the phosphate would be limiting in a wash? Only because we have wondered whether hard water makes it worse. Is it actually possible with very soft water?
Jonathan: The point about phosphates in washing powder is a good one and it is no longer used in modern detergents, so you are right it could be coming from the urine as it is found in quite a few foods including milk – although not sure if that applies to breast milk.
NSG Member: That’s an interesting thought. I wonder if we could test two samples from the same nappy – one from the wee zone and one as far from it as possible?
Question: We know that phosphates are added to water in some areas, and that phosphate is sometimes found in wee (depending on diet and medical conditions) – could these be the source of the phosphate? Or is detergent still the most likely source? How could we work that out?
Jonathan: From my point of view I would like to know what the possible phosphate sources are, i.e. what is added to the water and by whom. It may depend on the area you live in. Soft water areas won’t have as much calcium, and I am not aware of where and how much phosphate is added to water (that was mentioned earlier in the discussion), but that would be a factor too.
Jennifer: Then we can simulate hard water/soft water/pH etc etc and see what is driving the formation. Even down to nappy fibres.
Question: Does hydroxyl apatite build up exponentially? Does having some there already mean more can be accumulated next wash?
Jennifer: Could the hydroxy apatite be trapping bacteria? Were there any signs – that would catalyst the urea decomposition? We can check if the hydroxyapatite can do the same.
Question: If we were able to identify a plausible source could you make any suggestions as to what sorts of experiments we could conduct to try to replicate the apatite build up?
Jennifer: Yes definitely.
NSG Member: Can you remember the nappy that had the huge hydroxy apatite build-up? I don’t remember it feeling particularly crunchy…
NSG Member: We still have some of those nappies so we can maybe have a blind feel test. NSG Member: Wasn’t the nappy with the huge deposit described as ‘pebbly’; I wonder if crunchy becomes pebbly when the crystals get so big they start bridging the gap between fibres?
NSG Member: I would think so, but don’t know. Maybe gives the bacteria something to hang onto? There are some Scanning Electron Microscope pictures in the post I linked to (aboveD.
NSG Member: I wonder how much the pebbly feel is due to the apatite and how much is just how it has been washed and dried?
Jennifer: Crystal (mineral) growth is much easier to facilitate when there is a crystal “seed” already present.
Question: Does the seed need to be a crystal of the same type, or could it for example, be that the fibres of the nappy act as a ‘seed’?
Jennifer: The seed doesn’t have to be the same compound; a rough surface will do it. In organic chemistry when purifying a compound by recrystallization it’s very common to add a small amount of the pure compound to make it all crash out of the solution. Even scratching the glassware can to this.
NSG Member: It suddenly seems plausible how that 20% ash could come about if the conditions are repeated again and again.
NSG Member: the one we have the Scanning Electron Microscope picture of was one of the 20% apatite ones, and the fibres are coated in it, but definitely not joining the fibres together, if you see what I mean.
Jonathan: I think if you have small crystals they could move with the fibres they have grown on, but when they become too big they won’t be able to do so, some could crack instead, although this could also be due to different crystals growing towards each other and not matching up so leaving cracks between them.
NSG Member: Maybe it could be that when the coating is light the crystals snap and as it strengthens they don’t? I’ve no idea if that would make sense.
NSG Member: Looking at the pictures – I think that as the coating gets thicker, it has more tendency to ‘crack’.
NSG Member: We definitely have to do a ‘feel’ test. I wonder if different bits of the nappy, for example, not the wee zone would be different or not?
Question: Does the source of phosphate make any difference to the crystal? Actually, what COULD you tell by physically examining the growth, structure, etc. of the crystal?
Jonathan: It would be impossible to identify the phosphorous source from studying the apatite itself, but identifying the source and the form it is in you could then carry out experiments to see if it apatite can be made from that source.
Question: Does it make sense that you’d have more phosphate/apatite making stuff in general when you’re growing fast, since your body is growing bones and teeth? Like, a well-nourished baby from whatever type of milk would likely have an excess of calcium.
Jonathan: I’m not a biologist, but I would think that when you are growing your body would use a lot of the calcium and phosphate from your diet to grow bones. You would pass less of it through your body, but you would never store it all so it would always come out in your wee, like if you take in more vitamins and minerals than you need most of it will pass straight through you.
Question: Can you explain how the bacteria could catalyse the urea decomposition and why that is relevant? We need to identify source so we can attempt to replicate the phenomenon.
Jennifer : It’s well known that bacteria are a catalyst for urea decomposition, which would turn the urine into ammonia (causing nappy rash) I don’t know for sure (will check the literature) but I’m not sure the hydroxy apatite will be capable of doing this.
Jonathan: Hydroxy apatite is quite a stable compound that is relatively easy to make, so trying to make it from different sources at different temperatures is something we would look at trying to do.
Question: What’s going on at a molecular level when urine is turned into ammonia with the rough surface?
Jonathan: When crystals grow the dissolved compounds need to nucleate which can happen on the fibres and then there is crystal growth where more material is precipitated on top. The size and shape of the crystals is a balance between these two processes.
Question: What does nucleate mean, please?
Jonathan: Nucleate is when a compound goes from being dissolved to forming the first bit of solid, that then grows into a bigger crystal. Jenny is posting a link to a video to help visualise it.
Question: Could you maybe give us an overview of the sorts of tests or experiments that your laboratory can do? NSG members can we come up with a list of questions we want to start with – like what are the first things we want to find out?
Jennifer : We can run a laboratory scale experiment, then take the nappy section out to mimic drying conditions no problem. We have reactors, of a fixed volume, that we can control the temperature and stirring speed in. So we can add urea, a section of nappy, hard/soft water, detergent and see if we can mimic the formation of the hydroxyl apatite. We can also take fresh and used real nappies, and characterise them using a microscope, or spectroscopy to see what elements/mineral s are present. What else do you guys want to know? If you can provide maybe a few key questions we can plan out the experiments. We may struggle with people power, we don’t have a full time student to work on this so identifying key questions is crucial.
NSG Member: So we want to grow some crystals. Maybe replicate the test nappy conditions. Find out as much history about it as we can.
Jennifer: here’s a pretty awesome video of crystal growth: https://www.youtube.com/watch?v=K297toCvHtY
Question: NSG Member: We were told (by Ian Strudwick) that hydroxyl apatite forms on ‘cellulosic fibres’ (no idea why that specifically) that have been washed and dried many times. Because the calcium and phosphate ions are in the water the nappy comes out of the washing machine with a lot.
Jonathan: A crystal is a solid when it has formed, but before it nucleates the components will be in solution.
Jennifer: I think we need to determine that we can replicate the previous findings on real nappies. Then can we simulate the growth in the laboratory.
Question: Jennifer I’m wondering if the actually structure of cellulose fibres is in some way friendly to crystal growth. Are there any variables which are crucial to the growth (nappy fibre, powder composition etc) particular things like charge, the geometry of the fibre (viscose smooth and natural fibres jaggy but both cellulosic) that make it easier?
Jennifer : We can test the hypothesis of fibre structure affecting the mineral growth for sure. I think in the first instance, without even using a nappy, we can form hydroxyl apatite in solution from washing powder, wee and tap water (and its additives). Once we can form it, we can look at what affects the nucleation to the fibres.
Jonathan: If you think about having some sea water and you left the water to evaporate you would form salt crystals.
Questions: Is charge involved in any way in the crystal formation? I’m asking because it’s involved in the feel/softness of the cloth after the wash and nappies often end up pretty rough because of avoiding softener and tumble drying.
Question: Has hydroxy apatite been detected on all nappy fabrics? (Cotton, bamboo, hemp, microfibre, etc) or just one?
NSG Member: We only found it on viscose in our tests. But 4 out of 5 nappies we got tested were viscose.
NSG Member: We found it on 50% of smelly viscose nappies. We need to get some more but no-one on Nappy Science Gang will have any now!
NSG Member: I think that under dosing with detergent is key to allowing this reaction to pivot so I would be interested in a) the source is it urine, tap water or detergent. I hypothesise that it isn’t detergent. How can we test this?
Jennifer: We haven’t covered it on this chat yet, but looking at the best way to remove it is very pertinent!
Jonathan: We should be able to answer the detergent dosing question fairly easily once we know how it is being made we can then do some experiments after washing with different amounts. I know proctor and gamble that make a lot of the big brands don’t use phosphates any more so your hypothesis is pretty likely.
Thank you so much everyone for coming!