10

I recently saw "hydrophobic water" in my school science fair. I have no idea on the procedure of how to make it, so, can I make it? I did google it and read some articles (zero helpful) and see some images (which seem to match with the one I saw). So, could you tell me how to make hydrophobic water if I even can? Thanks in advance.

Articles that didn't help:

How to make stuff super hydrophobic

How to make stuff hydrophobic or hydrophilic

Karsten
  • 40,170
  • 8
  • 67
  • 183
dotmashrc
  • 373
  • 4
  • 14

3 Answers3

26

You can't make hydrophobic water molecules. You can, however, make hydrophobic droplets containing mostly water. They are not made of pure water, but are coated with a substance that remains on the surface of the water droplet and changes the properties of the surface.

Here is an example of making such a hydrophobic drop: https://www.youtube.com/watch?v=H0spGzO2FSo.

And here is a demonstration of the properties (this drop also contains hydrophilic dye which remains mixed with the water in the bulk of the droplet: https://www.youtube.com/watch?v=MkLbVLGcn-A.

Here is another demonstration with a drop of pure water (top left) encountering a coated drop (bottom right, pushed with a finger):

enter image description here

Source: https://www.youtube.com/watch?v=Z4bVP7hEcKI

Karsten
  • 40,170
  • 8
  • 67
  • 183
  • 3
    Well done.... :-) – Poutnik Jul 05 '21 at 14:57
  • 3
    I think writing "hydrophobic water droplets" is misleading. The droplets are not hydrophobic, they repel each other, but not because they contain water. All the same, I guess it might not be worth arguing about (but still rankles!). – Buck Thorn Jul 06 '21 at 10:23
  • @BuckThorn I changed it to hydrophobic droplets containing mostly water. – Karsten Jul 06 '21 at 11:51
  • I'm not sure that edit addresses the point I was trying to make. My point is that whatever is inside the droplets is of secondary importance, as it is what is stuck on the surface that is responsible for repulsion. So "hydro" in hydrophobic seems a misnomer in this context. Still, that's presumably how people refer to the phenomenon when they see it, so pressing the subject seems pointless. – Buck Thorn Jul 06 '21 at 11:58
  • 4
    Here's an analogy: if I throw a water balloon at you, you will not regard yourself as hydrophobic or hydrophilic based on whether the balloon ruptures on impact. – Buck Thorn Jul 06 '21 at 11:59
  • @BuckThorn The pure droplet of water is hydrophilic. If you touch it with your finger, it will wet it. The other drop is hydrophobic. If you touch it with your finger, it rolls away. In your analogy, the balloon is hydrophobic, not me. To see whether I am hydrophilic or hydrophobic or Telfon, you have to try water and oil droplets. – Karsten Jul 06 '21 at 13:34
  • 1
    Ok, yes, you are right of course. The coated droplet repels an uncoated water droplet. Does that make water hydrophobic? I don't know anymore. – Buck Thorn Jul 06 '21 at 15:54
  • 3
    @Buckthorn Is hydrophobicity property of a substance or a phase ? If the latter then the phase originally based on water can be made hydrophobic without making water hydrophobic and the question would be rather Can we make water phase hydrophobic? – Poutnik Jul 07 '21 at 08:41
10

Water cannot be made hydrophobic. Water drops can be set to act like if water were hydrophobic, but it would not be pure water anymore.

The drop surface can be made hydrophobic by hydrophobic liquid, film or dust. See the links in the Karsten's answer.

A dirty trick could be electrostatic charging of separated water drops, e.g. in the Kelvin_water_dropper, eventually modified for drop repulsion demonstration.

Poutnik
  • 41,273
  • 3
  • 49
  • 106
  • I think it is perhaps the first explanation. I don't think it would be the second as their grades depend on it. I don't completely understand the last explanation but it might be the last one as well. – dotmashrc Jul 05 '21 at 13:42
  • 1
    I vote for the first one as well, I have added the rest for alternatives. – Poutnik Jul 05 '21 at 14:00
1

tl;dr– Hydrophobic-water would be water that doesn't significantly cohere to itself. So, water's hydrophobic when it wouldn't form a condensed-phase (e.g., if it boils and preferentially exists as a gas at equilibrium). Can't have hydrophobic-water in an equilibrium-preferred condensed-phase, as a condensed-phase means that the material's sticking to itself and thus isn't phobic of itself.

Two species are phobic within some medium under some conditions when they don't here together (cohere if they're the same species; adhere otherwise).

To be clear: phobic species don't really repel so much as they're not very attracted to each other.

Species can become self-phobic. Of course, it's important to remember that members of the species aren't repulsing others of their own kind so much as just aren't very attracted to them. And then since they're not really attracted, they do the natural thing and just tend to disperse, filling the volume containing them. In other words, they vaporize.

Water can be vaporized, e.g. through heating.

Note: Can't have hydrophobic-water as a condensed-phase.

A condensed-phase (like liquid or solid) is defined, in part, by the material sticking to itself.

You can have hydrophobic-water physically inside of a condensed-phase, e.g. you can have molecular-water as a minority-component of a larger material-phase in a scenario in which the molecular-water wouldn't be so cohesive, but you can't have hydrophobic-water as a condensed-phase, because it wouldn't cohere (and thus wouldn't be condensed).

Nat
  • 371
  • 1
  • 4
  • 15
  • aren't the water molecules still attracted to each other even in gaseous state and their intermolecular forces are just too weak? – dotmashrc Jul 07 '21 at 10:57
  • @ShirsakMajumder: Yup! If you check out this data page for the Van der Waals equation-of-state, the values for $a$ (which corresponds to intermolecular attraction) are positive. – Nat Jul 07 '21 at 12:58
  • then doesn't that make them hydrophilic? – dotmashrc Jul 07 '21 at 14:07
  • @ShirsakMajumder: Nope! Hydrophobic stuff attracts water too, just relatively weakly. When you see water bead up on a hydrophobic surface, it's not that the surface is repelling the water, but rather water's self-attracting itself away (often in a quasi-spherical shape, like a droplet) and the surface's own attraction is too mild to counteract it. – Nat Jul 07 '21 at 20:57