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Chem1 General Chemistry Virtual Textbook → States&Solids → Ionic Liquids & Plasmas

Ionic liquids and plasmas

Some special states of matter

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- Ionic liquids
- Ionic gases or Plasmas

All beginning courses in Chemistry describe ions as they exist in aqueous solutions or in the form of solids such as sodium chloride. But what about ionic liquids? In recent years, these have become sufficiently important that it is worth knowing something about them. And to round this off, have you heard of ionic gases? You have certainly seen them, and ought to at least know what they are!

1 Ionic liquids

If sodium chloride is heated to 801° C it melts to form a liquid, which, being electrically-conductive, must consist of mobile electrically-charged particles which we can assume to be ions.

As a matter of interest, molten salt will boil at 1465°C, but the vapor contains no ions. Instead, spectroscopic observations indicate the presence of weakly-bound molecules of NaCl and Na2Cl2. Molten salt has been proposed as a medium for the short-term storage of solar energy.

Although all molten salts consist of ions, the term ionic liquid now more commonly refers more specifically to a new class of substances that have mostly been developed since the early 1990s, and which have melting points near or below 100° C. Those that remain liquid at room temperature (room-temperature ionic liquids or RTILs) are of special interest for two general reasons:

- Unlike normal liquids, they are non-volatile; the attractions between oppositely-charged ions prevents any one ion from escaping. This is of great importance in "green chemistry" in which one tries to minimize the escape of undesirable vapors into the air.
- Their special properties often allow processes to occur that cannot take place in ordinary solvents. Some examples:

- Deposition of metal on the surface of an ionic liquid can produce a liquid mirror that has been proposed for use on a moon-based telescope (New Scientist)
- An ionic liquid aids the formation of metal alloy nanoparticles under high-vacuum conditions that would cause ordinary liquids to boil away (ChemTech).

What prevents these salts from freezing to solids at the very high temperatures characteristic of ordinary salts? The secret is to employ cations and anions that have such different shapes and characters that it is energetically very difficult for them to fit together in a compact (and energetically efficient) way.

Thus a common formulation is to use a bulky organic cation whose electric charge is spread out over a portion of the molecule, making it difficult for it to "dock" with the compact inorganic cation in a stable packing arrangement.

Ionic liquids, and particularly RTILs, have opened many new and exciting approaches to chemistry process development. A variety of other structures and more about RTILs generally can be found here, and also on this informative Wikipedia page.

Typical of some of the surprising properties of ionic liquids is their ability to dissolve wood — something no other kind of solvent can do. [image]

##### Electroneutrality rules!

As mentioned above, ionic liquids do not vaporize; unlike ordinary liquids, they have virtually zero vapor pressure. This is an example of the electroneutrality principle, which says that no phase of matter can have more than a negligible net electric charge. The work needed to separate a chemically-significant quantity of oppositely-charged ions from each other would far exceed that which is thermally available at even the highest attainable temperatures.

2 Ionic gases: plasmas

As we have seen above, heating an ionic solid or liquid to the point of boiling or decomposition does not yield an ionic vapor. But it is possible for atoms or molecules already in the gaseous state to become ionized by various means:

- collision with high-kinetic energy particles or with electrons from an external source
- very high temperatures (in which ordinary intermolecular collisions are energetic enough to knock electrons off of atoms)
- absorption of intense ultraviolet radiation or other ionizing radiation
- intense electric fields

Once gaseous ions have formed, they tend to react with other species present or to recombine into neutral particles, so in general they must be continually re-created in order to persist.

A gas consisting mainly of ions or of ions and electrons is known as a plasma. Everyone has seen plasmas (or their effects) without even knowing it: electric sparks, lightning, neon signs, fluorescent lamps, plasma video displays, and if you live in the right places, auroral displays. The plasma-like conditions in the earth's upper atmosphere (ionosphere) are responsible for around-the-world transmission of mid-frequency radio signals; see here for more on this.

"Plasma globes" became a popular novelty item in the 1980's. These consist of a glass vessel containing a low-pressure mixture of inert gases such as neon and argon. When excited by a high voltage alternating current, they produce luminous streamers having interesting shapes and colors. Especially fascinating is the way in which these streamers direct themselves to wherever one touches the outside of the globe. (Touching provides a capacitively-coupled return path from the single excitation electrode.) [image]

The study of plasmas tends to fall more into the area of physics than of chemistry. For more information on this topic, see

- this Wikipedia article
- an on-line Plasma Physics text from U. Texas
- this list of plasma tutorial sites