A modular, ionic liquid (IL)-based strategy allows compartmentalized molecular level design of a wide range of new materials with tunable biological, as well as the well known physical and chemical, properties of ILs, which thus deserve consideration as ‘tunable’ active pharmaceutical ingredients (APIs) with novel performance enhancement and delivery options. IL strategies can take advantage of the dual nature (discrete ions) of ILs to realize enhancements which may include controlled solubility (e.g., both hydrophilic and hydrophobic ILs are possible), bioavailability or bioactivity, stability, elimination of polymorphism, new delivery options (e.g., slow release or the IL-API as ‘solvent’), or even customized pharmaceutical cocktails. Here we exemplify this approach with, among others, lidocaine docusate (LD), a hydrophobic room temperature IL which, when compared to lidocaine hydrochloride, exhibits modified solubility, increased thermal stability, and a significant enhancement in the efficacy of topical analgesia in two different models of mouse antinociception. Studies of the suppression of nerve growth factor mediated neuronal differentiation in rat pheochromocytoma (PC12) cells suggests potential differences between LD and lidocaine hydrochloride at the cellular level indicating an entirely different mechanism of action. Taken together these results suggest that the unique physiochemical properties of ILs in general, may confer a novel effect for the bioactivity of an API due to (at least) slow-release properties in addition to novel delivery mechanisms.

As highlighted by the recent ChemComm web themed issue on ionic liquids, this field continues to develop beyond the concept of interesting new solvents for application in the greening of the chemical industry. Here some current research trends in the field will be discussed which show that ionic liquids research is still aimed squarely at solving major societal issues by taking advantage of new fundamental understanding of the nature of these salts in their low temperature liquid state. This article discusses current research trends in applications of ionic liquids to energy, materials, and medicines to provide some insight into the directions, motivations, challenges, and successes being achieved with ionic liquids today.

The non-flammability of ionic liquids (ILs) is often highlighted as a safety advantage of ILs over volatile organic compounds (VOCs), but the fact that many ILs are not flammable themselves does not mean that they are safe to use near fire and/or heat sources; a large group of ILs (including commercially available ILs) are combustible due to the nature of their positive heats of formation, oxygen content, and decomposition products.

The dual nature of ionic liquids has been exploited to synthesize materials that contain two independent biological functions by combining anti-bacterial quaternary ammonium compounds with artificial sweetener anions. The synthesis and physical properties of eight new ionic liquids, didecyldimethylammonium saccharinate ([DDA][Sac]), didecyldimethylammonium acesulfamate ([DDA][Ace]), benzalkonium saccharinate ([BA][Sac]), benzalkonium acesulfamate ([BA][Ace]), hexadecylpyridinium saccharinate ([HEX][Sac]), hexadecylpyridinium acesulfamate ([HEX][Ace]), 3-hydroxy-1-octyloxymethylpyridinium saccharinate ([1-(OctOMe)-3-OH-Py][Sac]), and 3-hydroxy-1-octyloxymethylpyridinium acesulfamate ([1-(OctOMe)-3-OH-Py][Ace]), are reported, as well as the single crystal structures for [HEX][Ace] and [1-(OctOMe)-3-OH-Py][Sac]. Determination of anti-microbial activities is described for six of the ILs. While some exhibited decreased anti-microbial activity others showed a dramatic increase. For two of the ionic liquids, [DDA][Sac] and [DDA][ACE], oral toxicity, skin irritation, and deterrent activity was also established. Unfortunately, both ILs received a Category 4 (harmful) rating for oral toxicity and skin irritation. However, deterrent activity experiments point to use as an insect deterrent, as both ILs scored either “very good” or “good” against several types of insects.

Due to the high interest in the applications of ionic liquids, new, cheaper, multifunctional ionic liquids which are easy to prepare are highly desired. Here, we present a new group of air- and moisture-stable, hydrophobic ammonium-based ionic liquids and their properties, including the single-crystal X-ray structure of benzethonium nitrate. These salts have utility as anti-bacterial, anti-fungal agents. Additionally, the potential application of these ionic liquids for wood preservation was tested with positive results. The toxicity of benzalkonium and didecyldimethylammonium nitrates were studied and are presented herein.