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I'm working on a science fiction short story and was wondering if there were any liquids that would be safe to bathe in while traveling on a space craft with artificial (spin-based) gravity. In such a context hydrogen and oxygen are valuable resources not to be wasted. While bath water could be recycled, it is not available for other uses while someone is sitting in it, so I was wondering if there are other liquids at Standard Temperature Pressure that would be safe to bathe in (potentially ingesting small amounts).

I thought of using carbon bucky-ball fullerenes in a fluid but the viscosity looks to be much too high (essentially tar-like). I know there are waterless chemicals that can be used for cleaning (e.g. DryBath), the purpose of this is more to have a hot-tub to relax in than to disinfect and clean. If the addition of some hydrogen and oxygen (either as H2O or in other compounds) can improve the viscosity or is necessary to form the liquid, that is acceptable as long as less than 10% of the liquid is comprised of hydrogen and oxygen. The purpose is to save water and oxygen for air while still being able to soak in a hot-tub.

Unfortunately most oils contain a substantial amount of hydrogen and some oxygen, so they don't qualify unless the amount of carbon offers substantial savings in volume. I'd prefer something without any hydrogen or oxygen if such a chemical exists.

This is my first post here and I'm unfamiliar with the tags, so feel free to edit or add any you feel are appropriate.

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    $\begingroup$ This is a very interesting question! I can see oxygen being a potentially rationed element, but hydrogen is the most common element in the universe. Oxygen is the third most common element in the universe. With the right technology, it should be easy to mine hydrogen from gas giants. We could mine water from comets and other Kuiper belt objects $\endgroup$
    – Ben Norris
    Mar 19, 2015 at 14:40
  • $\begingroup$ I know hydrogen is abundant (and even exists in space in low quantities), but for the purposes of the story, hydrogen is also a fuel source and so water (and thus hydrogen) is still rationed for habitats in addition to oxygen for both water and air. The space craft is not in a position to harvest resources from gas giants, asteroids, or planets. $\endgroup$
    – hatch22
    Mar 19, 2015 at 14:47
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    $\begingroup$ If you want to relax your muscles similarly to a hot tub, without insisting on getting clean, you don't need a liquid. Granulated material of size and composition which doesn't allow it to cling to skin is a good replacement, as well as simply donning a suit with an active heating system. I have back problems and lying on a warmed wax pad (closed up, no skin contact with the wax) is just as good as taking a hot bath. $\endgroup$
    – rumtscho
    Mar 19, 2015 at 16:40
  • $\begingroup$ This is a good suggestion, though I am still most interested in a liquid rather than a solid or gel, since I specifically asked for a liquid that could be a water substitute. $\endgroup$
    – hatch22
    Mar 19, 2015 at 19:58
  • $\begingroup$ Assuming abundant electricity, water is a safe and compact way to transport large amounts of hydrogen and oxygen. You just need electricity to split them apart again. So water may not need to be as constrained a resource as you imagine. $\endgroup$
    – RBerteig
    Mar 20, 2015 at 0:49

2 Answers 2

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Perfluorocarbons would be a possibility, $\ce{(-CF2-)_{n}}$, they contain only carbon and fluorine and are relatively low cost and widely available in large quantities. They are inert (e.g. non-reactive) and depending upon chain length and structure, you could pick one that can be warmed up nicely to a bath temperature. They are also used industrially in degreasing operations, so they might have some cleaning ability as well.

Some perflurocarbons are actually being investigated as human blood substitutes - they can transport and release oxygen to tissues. Perfluorodecalin is a prime example. As shown below, it is basically a naphthalene ring in which all of the hydrogens and double bonds have been replaced with fluorine.

PFDecalin image

Ingesting small amounts would not be a problem. In fact, these same materials have been used in liquid breathing experiments.

There are a number of other scientific aspects of these perfluorocarbons that could play into science fiction. For example, the small size of the molecules (compared to say, haemoglobin) allows it to travel to smaller places that haemoglobin cannot access due to its larger size. Perhaps the perfluorcarbon could also carry small drugs to relax, rejuvenate, repair body parts, or enhance sensory experiences during the bath. Since these perfluorocarbons act as oxygen transport agents they could also be worked into the story as a bath medium that also scavenges "loose" oxygen.

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    $\begingroup$ Wow, perfluorocarbons have even been used in liquid breathing research. This is a really interesting possibility (+1). I'll wait a while to see if others have any good ideas while I do some more reading, but this is excellent. $\endgroup$
    – hatch22
    Mar 19, 2015 at 15:00
  • $\begingroup$ Thanks everyone for the excellent ideas and comments. I've decided to accept this answer, as I feel it comes the closest to what I asked for in the question out of all the suggestions made in comments and Yomen's answer. Most suggestions other than this one involved gels or solids. This is the only answer that supplied an actual liquid, and @ron also suggested ways to incorporate other properties of perfluorocarbons into a scifi setting. $\endgroup$
    – hatch22
    Mar 20, 2015 at 13:11
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I think we should propose something that uses some water. We cannot forget that our body is compatible with water as a solvent. The chemistry in our body is based on the chemistry of aqueous solutions.

On the other hand, we have to take into account the large quantities of solvents, other than water to be transported to the spacecraft for astronauts baths. The water used for my proposed solution is a secondary product of the fuel cells actually used to generate electricity in the spacecraft. The fuel for the fuel cells are oxygen and hydrogen. And as @Ben Norris added hydrogen is the most common element in the universe and oxygen is the third most common element in the universe. So, with the right technology, we should have them on the space craft.
I would like to suggest to use "Bentonite based superabsorbent polymer composite". It's a kind of hydrogels that absorbs water and loaded up to 50% wt/wt with a clay, bentonite (aluminium phyllosilicate). This composite is taken to space as powder. When astronauts want to take a bath, they can add some (water+fragrance) to the powder that swells and forms a gel.

The value of bentonite as stabilizing and rheological agents is due to the colloidal structure in water. Each smectite particle is composed of thousands of submicroscopic platelets stacked in sandwich fashion with a layer of water between each. A single platelet is one nanometer thick and up to several hundred nanometers across. The faces of these platelets carry a negative charge, while edges have a slightly positive charge.

Bentonite is largely used in cosmetics, it is bio-compatible and has excellent rheological properties to get perfect emulsions and optimize flow properties. On the other hand, the composite I'm suggesting has been formulated and tailored to absorb moderate quantities of water and fragrance that has a soothing properties to relax astronauts.

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  • $\begingroup$ The question I have about this approach is: How much water would be necessary for such a gel to be used as a bath. Remember that the primary purpose in this case is to relax and soak, not get clean (though that is a potential side benefit). For astronauts in real life what you are suggesting makes excellent sense, but I'm dealing with a futuristic fictional context where people living on a water constrained habitat would like to soak. I'm not opposed to gels, as that would be similar to a mud-bath in some respects, but it shouldn't be more than 10% water. $\endgroup$
    – hatch22
    Mar 19, 2015 at 16:32
  • $\begingroup$ Bentonite is largely used in cosmetics, it is bio-compatible and has excellent rheological properties to get perfect emulsions and optimize flow properties. On the other hand, the composite I'm suggesting has been formulated and tailored to absorb small quantities of water 10% wt/wt + fragrance that has a soothing properties to relax astronauts. $\endgroup$
    – Yomen Atassi
    Mar 19, 2015 at 17:03
  • $\begingroup$ Please, take into account the quantities of solvents, other than water to be transported to the spacecraft for astronauts baths. $\endgroup$
    – Yomen Atassi
    Mar 19, 2015 at 18:35
  • $\begingroup$ Point taken that we don't want to carry material that is only useful for baths, although the medical benefits of perfluorocarbons (as a blood substitute and breathable fluid) might justify their inclusion for space travel. Nevertheless, a Bentonite hydrogel as you've outlined is a valid choice that meets the requirements I outlined (+1). $\endgroup$
    – hatch22
    Mar 19, 2015 at 19:35

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