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Physics/chamistry of liquid chalk

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 hw 09 May 2020

Recently we've seen several discussions of the internet on the use of liquid chalk and the possible effects it has on the Corona virus; will the alcohol stay on your hands long enough to break down the virus, is the alcohol concentration high enough etc.

The question I have, is on the addition of water to the mixture. Isn't the idea that chalk (magnesium-carbonate) is very hygroscopic, and it will drive water off your hands by a reaction of the pure magnesium-carbonate to one of its hydrated forms (di, tri or pentahydrate).

In my thought you would want to get a suspension of pure magnesium-carbonate in alcohol. Any addition of water to the mix will cause formation of a hydrate, and thereby lower the ability of the chalk to take up liquid from your hands. Of course, after initial evaporation of the alcohol, your hands will be bone dry regardless of the chalk in the mixture. But climbing with nice white coated hands, will the white layer do anything for you if its hydrated chalk, other than lower the friction.

So I've bought all the ingredients to make liquid chalk myself (isopropanol and chalk). Could probably make a litre of the stuff for the price of two small bottles liquid chalk in the shops.

Now I've done the experiment of mixing chalk with 99.7% isopropanol, and a second mixture using 70% isopropanol and water.

  • adding water immediately thickens the mixture
  • the thicker mixture smears better, but the mixture with pure alcohol feels nicer on the skin
  • after the alcohol evaporated I sprinkled a few drops of water on my white coated hands, and I have the idea that the pure alcohol mixtures leaves a layer that absorbs more water

I then tried to do some proper science, by mixing equal amounts (10 grams of chalk) and leaving the mixture to dry on a very precise laboratory scale. The scale should be accurate enough to measure the differences between pure and hydrated magnesium-carbonate. And then the battery died, and I had no spares

Any chemists around that can contribute?

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 mark s 09 May 2020
In reply to hw:

I do gpc and gc testing at work, dont think that makes me a chemist though. So id say still to bog standard chalk 

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 hw 09 May 2020
In reply to mark s:

Same here, just chalk.

I don't know how it is in the UK, but in other European countries some gyms are reopening, and (for now) have made the use of liquid chalk mandatory. If I'm mixing up my own, I just want to make the best mixture.

I also experimented a bit more:

  • the 70% isopropanol mixture, after drying, becomes very slimy with a few drops of water.
  • the pure isopropanol mixture after evaporation leaves a very dry layer on your hands. I can blow it off, but it stics perfectly with some rubbing
  • I'll be going for the pure mixture then, just would like to understand why most companies add water to their liquid chalk. 
  • OK it will kill my skin, but all long sessions in the gym kill my skin anyway.
Post edited at 11:23
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 Andy Chubb 09 May 2020
In reply to hw:

https://www.gymclimber.com/how-to-make-liquid-chalk-at-home/

The suggestion here is isopropyl alcohol, no water

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 mikespooner 09 May 2020
In reply to hw:

(Recovering) Chemist here. This is what I think is going on here, caveat being I don't know this exact system and happy to be corrected by people that know specifically.

Anhydrous magnesium carbonate is very hygroscopic. The solid form will absob water, hence it's use to dry our hands. For this to happen, you must break magnesium-to-carbonate bonds in the solid to incorporate water molecules in the structure. Important to note here, you must also break a number of water-water bonds in the bulk liquid for this to happen. In this case, we know this is a favourable process as the solid will absorb water, and if you keep on adding water the solid will dissolve completely.

It's important to note that in your 70 % IPA mixture, the magnesium carbonate isn't fully dissolved, you've got particles of solid that are suspended. So the majority of those solid particles are still anhydrous.

You've said yourself that the 70 % IPA water is much thicker. You'll get an almost gel-like structure in the liquid with much more order. On the surfaces of your solid particles there's going to be large networks of hydrogen bonds between the surface and the surrounding water and IPA molecules.

If your solid were to absorb water from this mixture, you'd have to break down all of these strongly-bonded networks. I think it's likely that in this case you don't get the energy payback as you would in pure water, therefore it's not energetically favourable for the solid to absorb water molecules into the structure. So the majority of your solid particles remain anyhydrous (notwithstanding that you'll get some water adding at the surface).

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 mikespooner 09 May 2020
In reply to hw:

As to why manufacturers use water in their liquid chalk, they're probably aiming for that thick easy to spread texture. I think (memory's a bit hazy here) it would also be less flammable! A thick gel is safer for the end user to handle than pure liquid alcohol.

The second reason is as you say, rubbing your hands regularly with pure IPA is not going to do them any good, and I'd be surprised if it would pass many cosmetic safety tests (which I presume would be the regs these would fall under, not sure). I'm personally quite reticent about using commercial liquid chalk, but I've been conditioned by working in a chemistry lab for 5 years to be pretty cautious about this stuff. I wouldn't normally rub my hands regularly with 70 % IPA, so I don't like doing that on top of rubbing them regularly with another drying agent!

Post edited at 13:47
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 marsbar 09 May 2020
In reply to hw:

I remember reading something about how 100% alcohol doesn't kill bacteria and viruses as well as an alcohol and water mix of around 70%.

I can't remember the details.  If I can find it again I will post a link. 

https://www.researchgate.net/post/Why_is_70_ethanol_used_for_wiping_microbiological_working_areas

Post edited at 15:56
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 mikespooner 09 May 2020
In reply to marsbar:

This is true. Alcohols work as disinfectants by disrupting the cell membranes of organisms. In 100% alcohol, the biomolecules in a bacterial cell wall/membrane are going to pack more tightly together, as they don't like to interact with non-polar alcohols. There's a sweet spot where you add enough water that allows the alcohol to penetrate more easily, so it works more efficiently to kill off the cell.

Edit: I've now seen the link, pretty much covers it!

Post edited at 16:17
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 nufkin 09 May 2020
In reply to mikespooner:

>  In 100% alcohol, the biomolecules in a bacterial cell wall/membrane are going to pack more tightly together, as they don't like to interact with non-polar alcohols

Would a similar effect be witnessed with viruses? My biology is hazy, but I am aware that they don't necessarily respond in the same way as bacterial cells

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 mikespooner 09 May 2020
In reply to nufkin:

With 'enveloped' viruses that have a lipid/protein shell, yeah you'd get the same effect. Others without a shell are much harder to kill. Luckily coronaviruses are in this easy to kill category. There was a good article about how this works here: https://www.google.com/amp/s/amp.theguardian.com/commentisfree/2020/mar/12/science-soap-kills-coronavirus-alcohol-based-disinfectants

Equally there's a massive amount of variation in bacterial cell membranes meaning some of them are far harder to kill this way than others

Post edited at 16:32
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 marsbar 09 May 2020
In reply to mikespooner:

You explained it better.  

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 Monk 09 May 2020
In reply to hw:

It's been a while since I was a full-on chemist, but alcohol and water form an azeotropic mixture where they interact rather than just sit in solution together. I suspect this would have an effect on forming hydrates, but I've not given it much thought. Are you mixing the solution before adding chalk?

I suspect there's a lot more going on in how chalk works than simply forming a hydrate, much in the same way dry soil forms mud - it's a physical macroscale process not molecular change.

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 hw 09 May 2020
In reply to mikespooner:

That’s why I started the experiment with the laboratory scale. If the scale is accurate enough, I should be able to measure how much hydrates were formed after the IPA and water evaporated. Unfortunately, I don’t have access anymore to the more accurate equipment at work. 

Trying to summarise all your very helpfully comments, optimise for maximum climbing performance, leave out the water, optimise for virus/bacteria destruction, mix to 75% IPA (WHO recommendation).

Post edited at 19:38
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 mikespooner 09 May 2020
In reply to hw:

Good luck with it, I'd be interested to find out how much water is absorbed if you ever get to finish the experiment. My hunch is that it won't affect the absorption properties significantly (although it might mix it one way or other to cover your hands in a way that you prefer!)

The one factor missing for anyone looking to develop a liquid chalk that's anti-coronavirus certified is contact time. The European testing standard for consumer hygienic handrubs normally requires a contact time (that is, keeping the hands wet) for 30s. I don't know if there's any legit data that says a quick splash and let it evaporate is actually effective.

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 Toerag 11 May 2020
In reply to hw:

> just would like to understand why most companies add water to their liquid chalk. 

Probably because water is a lot cheaper than IPA in terms of raw material costs?

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 hw 19 May 2020
In reply to hw:

I've mixed my own 80% IPA 20% water and then added chalk. Haven't had time to look into the science part, but so far the climbing experience was much more positive than expected.

I always frowned upon the use of liquid chalk (just a way for the companies to sell us more expensive chalk) and climbing every other move I dipped my hands in my chalk bag (habit, addiction??).

Now with the liquid chalk my hands were so dry for the length of an route that I didn't feel the urge to re-chalk my hands at all.

Of course, the fact that here we are only allowed to climb on outside walls, on short 17m topropes, and the weather is dry, helps a lot as well.

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 AlanLittle 19 May 2020
In reply to hw:

> I don't know how it is in the UK, but in other European countries some gyms are reopening, and (for now) have made the use of liquid chalk mandatory.

Here in Bavaria some are, some are only recommending it but not making it mandatory. The ones that aren't making it mandatory are also providing *real* hand sanitiser, which strikes me as a better idea than requiring the use of something that may or may not have a useful effect.

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In reply to hw:

I have made my own too. 

I used a mix of sanitiser and chalk, no added water. I don't think added water is needed as my mix is slower to evaporate than sanitiser alone and from memory slower than commercial liquid chalk.

The chalk is surprising good. 

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 Reach>Talent 19 May 2020
In reply to Toerag:

I would assume adding water increases the viscosity, this has a couple of desirable effects:

- Suspensions settle out much more slowly with an increase in viscosity, a comparatively small change can make the difference between it crashing out almost instantly and needing a centrifuge to get it apart.

- Consumers like thicker products and believe that they are more effective.

There are other ways to increase viscosity but most cost more or have undesirable side effects (silica is great for this but screws up your skin and your lungs).

Mike (Very lapsed formulation chemist)

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