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We have all seen the following diagram (or similar) in our first chemistry class, depicting polar molecules aligned in an electric field. Is this just one of the half-truths of beginner chemistry or is this what really happens? I realize there will still be kinetic motion (spin, etc), but even statistically, is it possible for such a fluid/gas to have a net directional orientation throughout?

enter image description here

I have often seen the diagram depicted with water, but as the top answer on this question explains:

"...the ions will rush to the respective electrodes and form the so-called electric double layer. Its thickness, known as Debye length, is usually quite small. As for the rest of the solution, it will be effectively shielded from the electric field."

Perhaps this would be the same issue with hydrogen fluoride?
Is it possible the Debye length would be longer for a polar covalent molecule like Acetonitrile or Chloromethane?

If there are in fact polar molecules that exhibit this field-orientated behavior on a macro scale, what would be a way that it could be demonstrated?

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  • $\begingroup$ Your figure is only schematic. Yes they do try to align but thermal motion tends to have the opposite effect so the alignment is only partial; its a competition. For a given dipole, the stronger the field the better the alignment, the higher the temperature the worse the alignment. The time it takes to align is directly proportional to the solution viscosity and molecular volume. $\endgroup$
    – porphyrin
    Commented Oct 5, 2016 at 8:30
  • $\begingroup$ Related question on Physics: Can an electric field align water molecules? $\endgroup$
    – user7951
    Commented Oct 5, 2016 at 9:45
  • $\begingroup$ Liquid crystals exhibit enormous field orientation effects, e.g in liquid crystal displays and devices of all sorts. $\endgroup$
    – porphyrin
    Commented Oct 5, 2016 at 12:51
  • $\begingroup$ I should also add that the relative level of orientation is governed by statistical mechanics. You can probably approximate the relative ratio as $e^{-\beta \Delta E}$ where $E$ is the relative energy difference aligned versus anti aligned. Obviously, the stronger the field, the more aligned they dipoles will be on average. $\endgroup$
    – Zhe
    Commented Oct 6, 2016 at 0:42

2 Answers 2

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These are two entirely different and unrelated phenomena, to the point that you'd better unsee that other question and my answer to it, if you can. Ions are very much not like molecules. Ions are charged; molecules are not. Molecules don't form the double layer, nor do they even know what is Debye length. The electric field (though somewhat diminished, by a factor of dielectric constant) penetrates the whole volume of liquid, so all molecules feel it the same way; none is shielded.

Yes, polar molecules get oriented in the electric field, albeit to a very tiny degree. Think of it this way: the molecule still bounces and rotates around, but it spends a little bit longer in the field-aligned orientation than in any other. To orient all molecules in a truly stiff way, like soldiers in formation, the energy of alignment has to overtake the energy of thermal motion, which requires enormous electric fields and is ultimately impossible, because a breakdown would occur way sooner than you get there.

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  • $\begingroup$ Also, note that the correct function of LCDs depends on the alignment under an electric field. $\endgroup$
    – Zhe
    Commented Oct 5, 2016 at 21:54
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    $\begingroup$ True, but those are liquid crystals, and that's another story. They are already kinda ordered without any field, much like domains in a ferromagnetic. $\endgroup$
    – Ivan Neretin
    Commented Oct 5, 2016 at 23:44
  • $\begingroup$ Ok. Maybe water is just the oddball because it dissociates into ions; I'm reading that even pure water will self-ionize. Since HF is borderline ionic, would it break into ions and shield the rest of the volume, or would its bond be strong enough to hold it together? It has a high dielectric strength, but so does water supposedly. Is there a way to know or calculate if other polar molecules like Acetonitrile will break down instead of "statistically" orienting in an electric field? $\endgroup$
    – ericnutsch
    Commented Oct 6, 2016 at 4:58
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    $\begingroup$ Come to think of it, yes, water would contain ions and thus would form some double layer and have some Debye length. All that goes twice for $\ce{HF}$, to the point that it must have been a trouble to measure its dielectric constant at all. Still, it is there in the reference books. $\endgroup$
    – Ivan Neretin
    Commented Oct 6, 2016 at 5:46
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Yes, polar molecules can align with a strong field. If the field is strong enough to overpower the local effects and the molecules are mobile. There is a devise called an ELectret, which is roughly the electrostatic equivalent of a permanent magnet. An Electret can be formed using a polar polymer or wax that is solid at room temperature. The Polymer is installed as the dielectric of a capacitor. The material is melted and a large field potential is applied to the molten dielectric. The field is maintained while the mass solidifies. Once the material is solid, it will retain a permanent electrostatic charge. The piece can be make to act much like a piezo element and is the heart of a Electret Microphone. (A type of capacitance microphone that does not require a battery to charge the capacitor.)

https://sites.google.com/site/polymericmatrixmaterial/how-to-make-an-electret

I believe Carnauba Wax is one material that forms Electrets. Also, polyvinylidene difluoride (PVDF) has been used. I recall the report of an experimental detonator for a torpedo that used a coil of co-axial cable rapped around the nose-cone. The cable was made with PVDF as the dielectric, and was charged as an Electret. The explosive was triggered by the charge released when the nose-cone was struck and the dielectric was distorted, changing the volume of the charge that could be contained within.

I believe that Electrets are at the heart of "Swiffers" and other types of electrostatic cleaning cloths and air filters. The static fields around the fibers would attract and hold dust.

Homemade Electret microphones became more popular with the Ham radio crowd, during WWII when replacement microphones became difficult to obtain.

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  • $\begingroup$ Thanks Mike! I had forgotten about electrets. Strange no one sells them commercially for science experiments. Making one out of wax would definitely be a good demonstration project. $\endgroup$
    – ericnutsch
    Commented Oct 6, 2016 at 5:43

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