Sunday 10 September 2017

Rubbing alcohols plural - what are the differences and what's the alternative?

This post came from a conversation I had with Wendy Vecchi this morning on Twitter, where the usual "is surgical spirit just the UK name for rubbing alcohol?" thing came up. I thought I'd explain why this is wrong and what the key differences are and what you can use instead.

SAFETY: all alcohols mentioned herein are flammable and you should exercise care when using them. Some can cause lightheadedness when inhaled so I recommend you work with the windows open!

People often use rubbing alcohol in techniques like adding it to watercolour or oil-based inks to produce new effects, but in the UK, it isn't really sold widely. Many people do assume "rubbing alcohol" to just be the US name for "surgical spirit" - and in terms of their medical uses, they are analogous - but they don't have similar compositions and whilst surgical spirit may work to some extent in your work, it may also have undesired effects. Herein I've written out their different properties and explained why they do/don't do what they do - some of it is quite technical but I've tried to use "normal" English and laymanise everything! If you're not interested in the whys and wherefores - please scroll down and just read the "bottom line" bit and ignore the details!

Rubbing alcohol is a US product and was originally used to rub onto muscles and had various oils added. It's not mostly used in medicine as an antiseptic or to dissolve certain things like when removing adhesives from the skin. It comes in two key varieties in the USA - one is based on ethanol (aka ethyl alcohol or grape alcohol - the same type of alcohol found in all alcoholic drinks) at either 70% (v/v) or 90% (v/v) concentration i.e. 70mL ethanol and 30mL water or 90mL ethanol and 10mL water. The second type is based on iso-propanol (aka iso-propyl alcohol, 2-propanol, propan-2-ol - this type of alcohol is NOT found in any beverages) - this also comes at 70% 70% (v/v) or 90% (v/v). ALL types of rubbing alcohol are denatured by the addition of denatonium compounds (e.g. denatonium acetate aka BITTERANT-A, or denatonium benzoate aka BITTERANT-B or Bitrex, or denatonium saccharide aka BITTERANT-S), which are painfully bitter and make it near impossible to consume, along with the other common bittering denaturant sucrose octacetate.

Surgical spirit is a UK product and is much more like the original 1920s version of rubbing alcohol and it is still used for thickening the skin of the feet (hikers), hands (rowers) or back (bed-bound patients). It has variable compositions but is most commonly in 2017 based on ethanol, but it has various oils added to quite high concentrations - 5-10% of the total volume will be a mix of castor oil, oil of wintergreen (aka methyl salicylate) and solvents like diethyl phthalate that help to keep the oils in solution. The ethanol used is in the form of industrial methylated spirit (IMS or white meths or industrial alcohol), which is about 90-95% (v/v) ethanol with the remaining 5-10% comprised of methanol (aka wood alcohol, methyl alcohol, hence "methylated" spirit), which is a deadly poison and 1) makes it harder to drink and 2) is very hard to separate from the ethanol by freezing or distilling so prevents people trying to purify it. Industrial methylated spirit should not be confused with methylated spirit (aka meths, pink meths) which contains a dye (purple in the UK, green-yellow in some countries), pyridine (foul smelling, to make it even harder to drink - this is the stuff that gives meths its characteristic smell) and compounds like Bitrex etc.

So how do they compare side-by-side? Let's compare - I've ignored the 70% rubbing alcohols as they're not much good for arts and crafts other than for cleaning up inks and paints!

Comparison of rubbing alcohols (USA) and surgical spirit (UK). All values are % v/v. KEY: EtOH = ethanol; iso-PrOH = iso-propanol; MeOH = methanol; oils = any oils/water-insoluble solvents. Bittering agents are omitted since they are present at ≤1% (w/v).
Product EtOH Oils Water MeOH iso-PrOH
Rubbing alcohol (90% ethanol) 90 0 10 0 0
Rubbing alcohol (90% iso-propanol) 0 0 10 0 90
Surgical spirit 85.5 5 0 9.5 0

So where does that leave us? First of all, surgical spirit is 1/20th (5% v/v) oils - that means you will get residues left on your work, and it will smell of oil of wintergreen for some time, which is not unpleasant to be honest! Castor oil is not a "drying oil" like the oils used in oil paintings, it won't harden over time and will remain greasy, but it may smell rancid over time. Not what you want! Should we be worried about methanol toxicity? In the above table, I used 90:10 ethanol:methanol in the IMS component of the surgical spirit, but if it was a 95:5 one used, the methanol would be down to 4.75% (v/v) - it may even be less than that - there are 99:1 mixes out there but I don't know which is used in the bottle of surgical spirit that I keep in my bathroom medicine cabinet! So no, in short, given that the amount is little AND surgical spirit is designed to be rubbed into the skin, I would not be concerned about it overly. 

How do ethanol and iso-propanol based rubbing alcohols compare? Can they be used in the same techniques?
Here's a comparison of the chemical and physical properties of the two alcohols which will help us understand a bit more about what they can and can't do - don't panic about unfamiliar words or symbols - I'm going to explain them and you DON'T need to remember or refer to this table ever again - I've only put it here so you can more easily see "this one has a bigger value than that one" or "this one is 10 times higher than that one" and get a feel for how different they are.
Comparison of properties of ethanol and iso-propanol at 20 °C unless otherwise stated. I have also added water, as a reference point
Property Ethanol iso-Propanol Water
No. C atoms 2 3 0
No. H-bond donors 1 1 2
Melting point (°C) -114 -89 0
Boiling point (°C) +78 + 83 + 100
Density (g/ML) 0.789 0.786 0.998
Viscosity (mPa.s) 1.20 2.34 1.00
Water solubilityMiscible Miscible Miscible
Vapour pressure (kPa)5.95 4.40 2.30
Acidity, pKa (in water)15.9 16.5 14.0
Partition coefficient0.66 1.12 0.05
Surface tension (mN/m)22.4 21.4 72.9
Solubility of table salt (g/L)0.03 0.65 359

There are only a few key properties that matter to us as artists and crafters and I'll explain them - firstly let's think about a typical use of alcohols in crafting. You've painted some watercolour and whilst it's still wet, you drip onto it a form of rubbing alcohol so that you can make interesting 'bubble' shapes in the paint. When the alcohol lands, it pushes away the water before dissolving in it, then evaporating, which gives you that characteristic pattern. Let's work backwards - evaporation - drying the paper after use. Whilst water has the highest boiling point (the point liquid turns into gas) of the three, ethanol evaporates faster at room temperature owing to the vapour pressure being the highest. The higher the vapour pressure, the more a liquid wants to be a gas, so the faster it will evaporate - you can see that iso-propanol will dry slower off of the page than ethanol but still faster than water. 

When those droplets of alcohol land on your page, the viscosity and surface tension dictate how they will behave versus the water on the page. Both are more viscous (less runny) than water but iso-propanol is considerably less runny than ethanol - so when dropped onto a wet page, it will move more slowly across it. The surface tension can tell us how well a drop will behave. If you think about dropping water onto your non-stick craft sheet, you will get nice domed droplets as water has a very high surface tension, but if you do the same with alcohols, they will run around and form a flat layer as their surface tension is so very much lower - the two alcohols are similar in this regard. When it comes to adding alcohols to water-based media, density (how heavy the liquid is based on how much 1mL weighs) is important. Water has a density of close to 1g per 1mL but alcohols are lower - so they will want to float on the water - think about what happens if you add alcohol inks to paper wetted with water, for example.

Solubility is important too - look at how much table salt dissolves in each solvent. Water is excellent for dissolving what we call ionic compounds - thinks with charges that split in water and disperse really well - but what water can't do is dissolve greasy molecules. Think back to the surgical spirit with castor oil in it - you can dissolve oils (and thus oil-based inks like Ranger Archival) in alcohols really easily but they don't dissolve in water - this is the basis of many cool techniques. If you want to try these, iso-propanol is generally better as those greasy molecules like water-insoluble dyes generally prefer dissolving in it than in ethanol so you'll be able to get better results. The partition coefficient is a related measure - this is a measure of if you give a chemical a choice of dissolving in water or in a greasy alcohol (octan-1-ol), will it pick the water (lower numbers) or the greasy solvent (higher numbers) - you can see from the table that iso-propanol likes all things greasy and keep this in mind - oil pastels, oil paints, gel pastels, crayons - anything waxy or greasy can get swollen, dissolved or lifted by iso-propanol but far less so by ethanol and not at all by water - this CAN be a huge benefit but it can also be a nightmare when a few drops ruin your work.

The bottom line - which is best for what?
Use ethanol when you want fast-drying and you want your alcohol to travel pretty far on wet paper leaving you bigger 'bubbles'.
Use iso-propanol when drying time doesn't matter so much and when you want denser, smaller 'bubbles' in watercolour. This alcohol is better for dissolving oil-based dye inks like Ranger Archival - whilst ethanol will work, you may need to use more of it to get the dye to dissolve fully.

But I live in the UK - what can I use instead of surgical spirit?
You can use Ranger's Alcohol Blending Solution (which contains a mix of ethanol and iso-propanol with some other solvents to thicken it up a little bit so that it evaporates a bit more slowly) - but this is quite costly at around £5 per 100mL (£50 per litre!), so if you need a lot, you can buy various cheap 70-99% (v/v) iso-propanol solutions on UK Amazon - but you won't see them in shops very often. If you buy the 99.0% version or the 99.9% version, you need to dilute them for use as follows:

To make 90% from 99.0 or 99.9%, just mix 9 parts with 1 part water - store away from children in a clearly labelled jar and store away from heat or naked flames. If you want 70% too so that you can use it for cleaning brushes, it's just 7 parts of the 99.0% with 3 parts water.

Hope that this helps - you can use the store below to pick up any items you may need! 

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