Chemistry is a continually evolving field, offering new solutions to age-old problems. The discovery and application of ionic liquids is one such innovation. Ionic liquids are a special class of solvents, also called molten salts, revered for their unique characteristics and versatile properties. This blog is a comprehensive review on The Fascinating World of Ionic Liquids & Their Usage in Liquid-Liquid Chromatography:

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Ammara Waheed

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Introduction

Unlike typical solvents comprising molecules, ‘Ionic liquids are entirely composed of ions and essentially present in the molten state. An equal number of positive and negative charges results in an overall neutral salt’’.

Ionic liquids melt at lower temperatures and possess tunable properties such as low volatility, high thermal stability, and high solvating power as compared to traditional salts. Hence establishing their authority as mobile phase solvents and stationary phase modifiers in liquid-liquid chromatography.

So, in this article, we will walk you through the fascinating world of ionic liquids; their structure, properties, synthesis, and applications, particularly focusing on the usage of these miracle solvents in liquid-liquid extraction via chromatography.

Shall we start? Come along!

Understanding Ionic Liquids- Their Chemistry, Structure, & Properties

Ionic liquid (IL) is a complex term that principally stands for defining all such inorganic and organic salts which are entirely composed of ions. These ions can be organic cations such as imidazolium or pyridinium and inorganic anions such as chloride, sulfate, hexafluorophosphate, or a combination of both.

In contrast to the traditional inorganic salts, which melt at significantly high temperatures owing to their symmetric, compactly packed crystalline structures, ionic liquids melt at lower temperatures. This property of ILs is attributed to the presence of dissymmetrical heterocyclic rings based on nitrogen and/or phosphorus such as imidazolium or pyridinium rings and/or the presence of a quaternary ammonium cation with different alkyl chains attached which make their structures comparatively less stable than the counter salts.

Ionic liquids with melting temperatures below 298 K are thus termed room-temperature ionic liquids (RTILs).

Other attractive attributes assigned to this unique class of compounds include their:

• High density (~ around 1-2.4 g/mL)
• Negligible vapour pressure
• High thermal stability
• High conductivity
• Non-flammability
• Tunable viscosity  and surface tension

The cations and anions present in the IL structure can manipulate its polarity, which in turn governs its hydrophobicity/hydrophilicity, miscibility in water, and other specific electrochemical properties.

The possibility of different cationic-anionic combinations and the easy adjustability of their structures have hailed the ionic liquids as designer solvents of modern analytical chemistry.

The melting point range associated with ionic liquids is also adjustable and is highly dependent on the charge distribution in the ions. An IL with its ionic charges relatively delocalized has a lower melting point. Similarly, ILs involving asymmetrical cations/anions possess lower melting points as compared to their contemporaries.

Synthesis of Ionic Liquids

Multiple methods can be used for the synthesis of ionic liquids, depending on the desired cations and anions. Some of the most common ones are:

1. Ion Exchange: This method is based on exchanging the cation or anion of a pre-existing IL with the desired ion, resulting in a new ionic liquid.
2. Neutralization: An acid-base reaction of a strong acid with a strong base yields an ionic liquid i.e., a salt.
3. Metathesis: Metathesis reactions involve the exchange of ions between two precursor compounds.
4. Coordination Chemistry: Utilizing coordination chemistry, ionic liquids can be synthesized by coordinating cations with specific anions to form stable complexes.

The synthetic flexibility and recycling ability offered by ionic liquids opens a new windowpane for a diversity of IL-related applications, which elevates their status above the single-component molecular solvents.

Three Generations of Ionic Liquids

Based on the cations and the anions present, ionic liquids can be distinguished into three major categories namely:

1. 1^st Generation ionic liquids, which are composed of alkylimidazolium and/or alkylpyridiunium cations with a metal halide such as haloaluminate (chloroaluminate) as the anion. These are oxygen and moisture-sensitive due to the hygroscopic nature of AlCl₃ and are thus described toxic and non-biodegradable because of which they find very limited applications.
2. 2^nd Generation ionic liquids replaced water and oxygen-sensitive anions with simple halides (Cl^-1, Br^-1,and I^-1) and/or anions such as nitrate (NO₃^-1), hexafluorophosphate (PF₆^-1) and tetrafluoroborate (BF₄^-1) while maintaining the same cations.  The 2^nd generation offered lower melting points and greater miscibility.
3. 3^rd Generation ionic liquids also called task-specific/designer ionic liquids incorporate natural i.e. readily available, biodegradable, low toxicity, and stable anions such as amino acids, amino-alcohols, carbohydrates/sugars, organic acids, and other plant-derived compounds such as menthol. Chiral ILs derived from natural amino acids and amino acid esters possess appreciable characteristics such as reduced cost and toxicity, greater biodegradability, and biocompatibility.

The Scope of Bio-Based Ionic Liquids in Green Chemistry

The 3^rd generation ionic liquids derived from renewable and natural sources, formally known as biogenic or bio-based ionic liquids follows the Green Chemistry protocols.

Bio-based ionic liquids promote environmental protection by offering the following advantages:

• Green Synthesis and Sustainability
• Reduced Energy Consumption 
• Reduced Environmental Impact
• Waste Minimization
• Resource Efficiency
• Biodegradability
• Safer Handling
• Versatile applications

Owing to their low toxicity, biodegradability, and low volatility, bio-based ILs are a viable option for replacing harmful organic solvents in chemical reactions. Moreover, due to their high thermal stability and ability to conduct ions, bio-based ILs can potentially be used in energy storage devices (supercapacitors and lithium-ion batteries).

Bio-based ILs used as solvents in food delivery systems and for the extraction and recovery of bioactive compounds mitigate the risk of contamination. The key point here is that all the above benefits are offered based on the production of ILs using natural substances, resulting in a reduced reliance on fossil fuels and non-renewable energy sources. 

Ionic Liquids & Liquid-Liquid Chromatography

Ionic liquids pose a serious competition to environmentally toxic organic solvents such as methanol and acetonitrile commonly used as mobile phase components in liquid-liquid chromatography. The dual-character property of ionic liquids allows interaction with a wide range of compounds. 3^rd generation tailor-made ionic liquids offers synthetic flexibility with a wider choice of cation-anion combinations. Ionic Liquids & Their Usage in Liquid-Liquid Chromatography          

The coating ability of ionic liquids related to their viscosity and surface tension can very well adsorb them on the surface of a polar adsorbent such as silica while performing column chromatography. The spherical, porous morphology of silica particles facilitates this adsorption by providing a large surface area and a greater sorption capacity. The active sites present on the surface of silica that facilitate IL binding include silanol and siloxane groups. However, physical adsorption always comes with the risk of column bleeding and sample contamination. In contrast, chemical modification of the stationary phase with the ionic liquid is a better approach. Both reverse-phase and normal-phase columns can be developed depending on the structure of the ionic liquid used.

Stationary phase modification with an ionic liquid results in multi-modal retention properties for a variety of solutes. The cationic ends of the ionic liquids interact with the anionic silanol groups of the silica stationary phase. The organofunctional group of the silane-coupling agent reacts with silica support through covalent bonding while the hydrocarbon chains bind with the cationic end of the ionic liquid, leaving the anionic end exposed.

The polarity of the groups exposed to the sample components serves as an indicator for the intermolecular interactions governing the retention of solutes. The sample components may bind with these exposed functional groups employing different interaction mechanisms mainly hydrogen bonding, hydrophobic interactions, and 𝜋-𝜋 interactions.

One limitation associated with ionic liquids immobilization on a solid support is that this act may mask some of the properties of neat ionic liquids.

Challenges & Future Directions

While the potential of ionic liquids as the solvents of the future is undeniable, however, there are still some challenges and considerations that need to be addressed i.e. 

1. Cost: Ionic liquid synthesis is an expensive process, limiting its widespread use. Research efforts are being carried out to develop cost-effective IL synthesis methods.
2. Performance Optimization: Further research is needed to optimize the properties and performance of ionic liquids in liquid-liquid extraction.
3. Environmental Evaluation: Even though it is confirmed via multiple sources that ionic liquids (especially 3^rd generation ILs) are less toxic than traditional solvents, there is still a need to thoroughly assess their safety and potential impacts on the environment.
4. Knowledge Gaps: Despite significant progress, there is still room for investigating the behavior of new cation-anion combinations in complex chemical environments.
5. Recycling: The recycling and recovery of ionic liquids is essential to ensure sustainability, and is the very factor keeping researchers on their toes.

Also Read: Women in Chemistry: Pioneers, Innovators, and Unsung Heroes

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