The field of ionic liquids (ILs) has recently gained huge worldwide scientific and industrial recognition due to the many beneficial properties of these materials. The ability to tailor the physical, chemical and biological properties of ILs has been the major driving force behind the surge of interest in this rapidly growing field of chemistry. In this mini-review, the history of ILs, their description, properties and applications are discussed.

A Historical Précis
Although ILs are often represented as a new class of solvents, the concept of low melting ILs is not new but goes back to the 19th century. It is generally acknowledged that the birth of room temperature ionic liquids (RTILs) took place in 1914, when Walden reported ethyl ammonium nitrate a salt that is liquid at room temperature [1]. However, this discovery did not trigger any significant interest in ILs at the time, as this compound was highly reactive. The first discoveries of striking ILs were the aluminum chloride-based salts that drew attention for their use in electroplating in the 1940s [2]. The second discovery of prominent ILs were the alkylimidazolium salts introduced in the early 1980s [2]. It was the introduction of 1-ethyl-3-methylimidazolium-based chloroaluminate in 1982 which led to the quest for RTILs. This IL turned out to be one of the most widely studied IL of its era. Unfortunately, the chloroaluminate-based ILs were reactive in water. The third major discovery was the advent of less corrosive, air and water stable ILs in 1992 [2, 3], which propelled new developments in this field and led to the current growth in academic and industrial research. Indeed the number of publications on the exciting fundamental developments of ILs has grown exponentially in the last decade as shown in fig.ure 1. This has been closely followed by a significant increase in the number of published patents on technological applications of ILs. This surge in the number of publications is indicative of the growing interest in IL technology by the scientific community. The historical pathway toward the development of ILs is summarized in figure 2.

General Description and Properties of ILs
There are many synonyms used for ILs in literature including; RTIL, molten organic salt, ambient temperature ionic liquid, fused organic salt, low-melting salt, non-aqueous ionic liquid (NAIL), designer salt and neoteric solvent [4].

ILs can be subdivided into two groups: I) protic ILs, that are formed by combining a Brønsted acid and Brønsted base and II) aprotic ILs, which consist entirely of cations that are not protonated and anions. The basis for this division is that protic ionic liquids have high volatility by virtue of a proton transfer from the acidic cation back to the basic anion [6]. This review focuses on aprotic ionic liquids.

Currently, aprotic ILs are ‘officially' referred to as salts composed solely or almost solely of cations and anions with melting points and/or glass-transition temperatures below 100 °C [4]. Salts that are liquid at room temperature are known as RTILs, whereas, salts with higher melting points are referred to as molten salts. Nearly all RTILs consist of large nitrogen or phosphorus-containing organic cations with associated inorganic or organic anions. The most popular cations and anions employed in ILs are shown in figure 3. One important advantage arising from the chemical structure of RTILs is that the anion or cation can be altered to provide tailor-designed properties that suit a particular application. Given the wide range of available combinations of cations and anions, it is estimated that 1018 different potential ILs can be designed. Consequently, ILs have been described as designer solvents [7]. Typically, the physical-chemical properties of RTILs are determined by a combined fit of both the cation's and anion's size, geometry and charge distribution. For example, while the structure of RTILs is similar to simple inorganic salts such as table salt (sodium chloride), most RTILs are liquids with low melting points (> 100 oC), while table salt melts at 801 oC. The low melting point of RTILs is usually attributed to the less efficient packing of their ions compared to the tightly packed ions of inorganic salts [4]. The combination of large asymmetric cations and small or in some cases large anions lowers the lattice energy of the crystalline structure and results in ILs with remarkably low melting points [8]. The melting point of RTILs decreases with increase in the anions asymmetry and size of ions. Conversely, the melting point increases as the symmetry of the cation increases [4]. ILs are also attractive because of their other beneficial and unique properties such as low volatility, non flammability, good thermal stability, good solvating properties, tunable viscosity, high heat capacity and conductivity, wide electrochemical window and a density that is generally greater than water. The thermal stability and solubility of ILs in water is dependent on the anion while, their viscosity, density and surface tension is dependent on the shape and length of the alkyl chain in the cation as well as on the interactions and symmetry of the ions. Generally, the anion contributes to the overall characteristics of the ILs [9]. A detailed coverage of the physical-chemical properties can be found in Ref. 9.