Ionic liquids (ILs) are a class of organic salts composed of ions that are exist in the liquid state at low temperatures (below 100 °c) [1–8]. Generally, ILs consists of organic cation (for example, imidazolium, pyridinium, phosphonium and etc.,) and organic/inorganic anion (for example, acetate, nitrate, tetrafuoroborate, alkyl-sulfate and etc.,) [9–19]. Because of their unique properties, they have attracted research interests in both academia and industries as they applied in organic/inorganic synthesis, catalysis, electrochemical devices and so on [20–26]. Different kinds of ILs are developed and studied for various applications such as room-temperature ionic liquids (RTILs) [2, 16, 27], poly-ionic liquids (PILs) [28, 29], amino-acid ionic iquids (AAILs) [30–32] and task-specific ionic liquids (TSILs) [33, 34]. Similarly, new class of ionic liquid called surface active ionic liquid (SAILs) have possess diverse applications in organic synthesis, biotechnology, nanotechnology, surfactant flooding and so on [35–39]. It is considered that the SAILs have both the properties of ILs and surfactant. Based on the structural property, SAILs can be divided into three category, (i) cationic SAILs (like imidazolium, pyridinium with long alkyl chain), (ii) anion SAILs (like carboxylate anion, sulphate anion and sulfonate anion) and (iii) catanionic SAILs (cations and anions showing surface active properties) [38, 39]. The nature of SAILs can be tuned by the choice of cation and anion, and the properties of SAILs are studied extensively in colloidal chemistry, supramolecular chemistry, eletrochemisty, pharmaceutical industry and etc., [40–43]
The presence of hydrogen bond (H-bond) between cation and anion of ILs plays a vital role in the behaviour of ILs [44–48]. In general, the H-bond is denoted as X–H···Y interaction in which X (usually, C, N and O) and Y (usually, N and O) are strong electronegative atoms. It was found that the cations are the active H-bond donors and anions are the active H-bond acceptors. Based on the strength of the H-bond, the ILs are divided into two types, (i) protic ILs and (ii) aprotic ILs. In protic ILs, H-bond is formed by proton transfer from a Brønsted acid to a Brønsted base whereas in aprotic ILs, the cationic C–H unit be the major H-bond donor unit . Since, ILs have large numbers of cations and anions, the H-bonding in ILs are highly system dependent. Unlike, the H-bonding in traditional neutral system, the H-bonding present in the ILs have different interesting behaviours.
Nowadays, the water aggregation or micellization properties of SAILs are actively studied by both experimental and theoretical studies [38, 49, 50]. Experimentally, surface tension, conductivity, steady state fluorescence spectra, viscosity, and dynamic light scattering measurements are used to determine the water aggregation properties of SAILs. Theoretically, density functional theory (DFT) is used to study the water aggregation properties of SAILs. Mostly, the studies are performed on water aggregation properties of imidazolium cations with sulfonate anions of SAILs [39, 51]. It promoted us to study the water aggregation properties of SAILS. Here, we have studied the water aggregation properties of the methyl-imidazolium cation with six fatty-acid anion of SAILs with different water molecules (1–3) using DFT. This study will be useful to understand the interactions of cation-water, anion-water, cation-anion and ionic liquid-water towards the water aggregation properties of SAILs.