Antimicrobial Ionic Liquids

Abstract

First described almost a century ago (Walden, 1914), ionic liquids are a novel class of low temperature (typically <100°C) molten salts, comprised of discrete anions and cations (Seddon, 1997; Scammells et al., 2005; Stark & Seddon, 2007;). Many are liquid at room temperature. The majority of room temperature molten ionic liquids are salts with large nitrogen or phosphorous-bearing cations with alkyl chain substituents and anions such as halides, fluorophosphates, fluoroborates and so on. Over one million simple ionic liquids are theoretically possible, with mixtures of two or more ionic liquids making the possibilities for new reaction media almost limitless. Ionic liquids research has experienced a massive upsurge of interest in the past decade, primarily driven by their application in ‘Green’ chemistry, for example, as replacements for conventional organic solvents and volatile organic compounds (VOCs) in the chemical industry. Furthermore, ionic liquids have been utilized in multitude of diverse applications from synthetic chemistry (separation/extraction/catalysis) to novel biological applications. The most commonly used and extensively described cations and anions employed in ionic liquids are detailed in Figure 1 (adapted from Seddon et al, 2000). The ability to ‘tune’ the physical, chemical and biological property sets of ionic liquids, by independent modification of the properties of the constituent anions and cations, has been the major driving force behind the huge interest in this rapidly expanding field of chemistry. ‘Tuneability’ of ionic liquids introduces an unparalleled flexibility in the design of reagents for a particular functional niche, these ‘designer solvents’ (Earle et al., 2006) are capable of providing a range of new reaction media potentially having greater diversity of character and application than that of the traditional solvents they are designed to replace (Scammells et al., 2005; Earle et al., 2006; Stark & Seddon, 2007). A summary of the physicochemical properties of common ionic liquids is given in Table 1. Whilst the majority of industry in this field has, to date, been directed towards ‘green’ applications, biological issues such as stability, biodegradability, recyclability and toxicity (Scammells et al., 2005) have received relatively little attention. However, these issues have attracted increased scrutiny recently, and the biological properties of ionic liquids, which in themselves are ‘tuneable’ have become one of the most debated topics in the ionic liquids arena. Ionic liquids generally have properties such as near-zero vapour pressure (Earle et al., 2006) and thermal stability (Kosmulski et al., 2004). However, by altering the cation and anion, ionic liquids can be specifically created for a purpose or to possess particular properties suited to a given functional niche, and can therefore be described as tunable or designer