Some ionic liquids suffer from more or less some drawbacks such as toxicity, preparation and high cost in the process for use.

Some points are given below.

1. Size of cationb and anion must be different largely as they can't fit in unit cell.

2. One of the ion must be very large as the z/r become low.

3. Ions must not be in regular symmetrical spherical shape.

Dear Marwa Abd El Kader Zaater many thanks for sharing this important technical question with the RG community. Limitations in the formation of ionic liquids have been discussed in the following potentially useful review article:

IONIC LIQUIDS: PREPARATIONS AND LIMITATIONS

http://journal.ui.ac.id/science/article/viewFile/677/645

This paper is freely accessible as public full text on the internet (please see the attached pdf file). For a more recent article (published in 2020) please also have a look at the following interesting review article:

Key Applications and Potential Limitations of Ionic Liquid Membranes in the Gas Separation Process of CO2, CH4, N2, H2 or Mixtures of These Gases from Various Gas Streams

This paper has also been published Open Acess (see attached).

I hope this helps. Good luck with your work and please stay safe and healthy! With best wishes, Frank Edelmann

Dear Nayim Sepay , thanks for your answer. Can you provide a reference or more for such critical factors for ionic liquid formation.

Also is there a computational method that can predict those factors?

Dear Prof. Frank T. Edelmann , thank you very much for your help and those useful articles.

I'm wondering if I want to predict computationally whether a specific pair of ions will form ionic liquid or not, what will be the factors I should be looking for?

Dear Marwa Abd El Kader Zaater many thanks for your kind response and additional question. I just came across the following potentially useful article which might help you in your analysis:

Prediction of Ionic Liquids Properties through Molecular Dynamics Simulations

Article Prediction of Ionic Liquids Properties through Molecular Dyn...

Unfortunately this paper has not yet been posted as public full text on RG. However, the pricipal investigator of this study has an RG profile. Thus you can easily contact him directly via RG at https://www.researchgate.net/profile/Jose-Gomes-20 and request the full text. As he is an expert in the field, it is certainly worth a try to send him a message and ask for more advice about the computational prediction of ionic liquid formation.

There is also an entire book about this topic available. Perhaps you can access it through your institition:

Theoretical and Computational Approaches to Predicting Ionic Liquid Properties

https://www.elsevier.com/books/theoretical-and-computational-approaches-to-predicting-ionic-liquid-properties/joseph/978-0-12-820280-7

Good luck and best wishes, Frank Edelmann

Thanks Prof. Frank T. Edelmann that was really helpful!

Dear Marwa Abd El Kader Zaater, in addition to the previous fruitfull contributions, I advise you to read about 'Deep Eutectic Solvants DES'. They are by far easy and cheap to prepare, and with competitive performance with ionic liquids. My Regards

Dear Marwa

Hope you are doing well

First of all ionic liquids is NOT LIQUIDS!. It is is molten salts: a type of salts that can be at the molten state in the low temperature ( or 25 oC to below100 oC). The term "liquids" is a sophistry or a fallacy term. However, although these materials are behaving like a liquids, but (Again) are NOT liquids.

Moreover, beside the size difference between the Bulk (huge) cation and the small anions, the key point of these materials is the asymmetry of the cations.

There are there generations of these ILs,

1st: the unstable one or chloro-XXX

2ed: the stable imidazolium ILs

3rd bis- IL

Remember, again ILs are NOT liquids, are molten salta!

wishing you all best

Dear Ahmed MS Dawelbeit ,

1. The division of ionic liquids into generations look as follows:

1st generation - ILs with designed physical properties (like solvents)

2nd generation - ILs with designed physicochemical properties (like catalysts)

3rd generation - ILs with designed physicochemical and biological properties (like herbicidal ILs or pharmaceutical ILs)

Source: Article Ionic Liquids: Properties, Application, and Synthesis

2. Molten salts ARE LIQUIDS, because after you apply enough heat to them, they melt, the crystalline structure undergo phase transition into a liquid. You can check that for particular ILs by DSC method (Differential scanning calorimetry).

You can find thermogram made by DSC which proves that ILS are liquids at appropiate temperature in many papers, for example: Article Toward revealing the role of the cation in the phytotoxicity...

Salts mean compounds which consists of ions, liquid is a state of matter. Definition of ILs sounds as follow: "Compounds which consist of ions, which are liquids at 100 degrees Celsius." So, even in the definition it is stated, that they must be liquids at appropiate temperature.