An ionic liquid (IL) is a salt in the liquid state. In some contexts, the term has been restricted to salts whose melting point is below some arbitrary temperature, such as 100 °C (212 °F). While ordinary liquids such as water and gasoline are predominantly made of electrically neutral molecules, ionic liquids are largely made of ions and short-lived ion pairs. These substances are variously called liquid electrolytes, ionic melts, ionic fluids, fused salts, liquid salts, or ionic glasses.

Ionic liquids have many applications, such as powerful solvents and electrically conducting fluids (electrolytes). Salts that are liquid at near-ambient temperature are important for electric battery applications, and have been used as sealants due to their very low vapor pressure.

Any salt that melts without decomposing or vaporizing usually yields an ionic liquid. Sodium chloride (NaCl), for example, melts at 801 °C (1,474 °F) into a liquid that consists largely of sodium cations (Na+) and chloride anions (Cl−). Conversely, when an ionic liquid is cooled, it often forms an ionic solid—which may be either crystalline or glassy.

The ionic bond is usually stronger than the Van der Waals forces between the molecules of ordinary liquids. For that reason, common salts tend to melt at higher temperatures than other solid molecules. Some salts are liquid at or below room temperature. Examples include compounds based on the 1-Ethyl-3-methylimidazolium (EMIM) cation and include: EMIM:Cl, EMIM dicyanamide,

(C2H5)(CH3)C3H3N+2·N(CN)−2, that melts at −21 °C (−6 °F); and 1-butyl-3,5-dimethylpyridinium bromide which becomes a glass below −24 °C (−11 °F)

Low-temperature ionic liquids can be compared to ionic solutions, liquids that contain both ions and neutral molecules, and in particular to the so-called deep eutectic solvents, mixtures of ionic and non-ionic solid substances which have much lower melting points than the pure compounds. Certain mixtures of nitrate salts can have melting points below 100 °C.

The above answer is full of truth, but I am still trying to find a cheap ionic liquid which contains NH4 cations and is liquid at 20 oC.

please try to prepare DES using ammonium salt, than you can get a kind of ionic liq.

Some sort of deep eutectic solvent should be possible, but I do not think you will be able to get a single pure RTIL based on NH4+ it is just too small, too symmetrical and the charge is too concentrated.

I was hoping for a deep eutectic, my reasoning is that choline chloride forms plenty of deep eutectics. As chloride is a small and perfectly round anion I then reasoned that with the right anion a eutectic mixture could be made of a NH4 salt and something else.

Mark, there is a huge difference between choline and ammonium, in symmetry, bond strengths, ability to form h-bonds and so on. Consider even nh4 bf4 and nh4 bta are solids

Yes I know, but if chloride is a perfect sphere and yet things like [Mg(H2O)6].Choline.3Cl are liquids then I reason that a substance NH4.C.2A should exist where C is a cation and A is a very unsymmetric anion.

Consider for a moment the classic BMIM ionic liquids, they tend to use very symmetric anions such as PF6 with a very unsymmetric organic cation. I reason that this idea can be reversed.

Hi Mark, the catch with the symmetric anions is that the charge is stronly delocalized, and that they are as whole non or low coordinating, , and ammonium is none of those.

As Boyan already mentioned, you can not compare the charges of anion and cation. Addtionally, the NH4+ cation involves comparably strong hydrogen bridge bonds to any anion (even WCAs). You can see this when you compare the melting points of [NX4]+[OTf]- with X= H, Me, Bu, etc.). Your only option is an eutectic mixture of [NH4][X] with a low melting ionic liquid constituded off the the same weakly coordinating anion.

How about betaine forming deep eutectic solvents

You can purchase e.g. choline citrate, but it forms eutectic (with other compounds) with higher viscosity than choline chloride. You can also prepare choline acetate by methathesis (from choline chloride) or by reaction of choline hydroxide with acid. Unfortunatelly I don't know what is the value of melting point and viscosity of such kind of salt

Meybe these links will be helpful

http://pubs.rsc.org/en/content/articlehtml/2013/ra/c3ra46149a  (see Table 1, number 19)

http://onlinelibrary.wiley.com/doi/10.1002/anie.201207548/full 

http://pubs.acs.org/doi/pdf/10.1021/jp981159p

Choline OAc is solid at RT, even the BTA is solid, when dry and clean enough.

As a choline citrate. But  it can be mixed with e.g. urea, polyalcohols or caboxylic acids to form liquid DES at room temperature.

Choline-based salts (not DES) seems to be liquid according this paper: http://pubs.rsc.org/en/content/articlepdf/2007/gc/b706571j

Thanks everyone, but the problem is that while it is very clear that a deep eutetic solvent can be made from choline chloride (an ammonium ion) and many things like lactic acid, glycolic acid, ethylene glycol and urea. It is not so clear if it is possible to make an ionic liquid or deep eutetic from an ammonium salt, when I say ammonium salt I mean NH4 cations with no alkyl / aryl groups on the nitrogen