Aromaticity index (AI) and double-bond equivalent (DBE) were studied to seek a simpler approach in identifying suitable ionic liquids for the desulphurization process. Manifestation of interaction mechanism by COSMO-RS and Raman spectroscopy is discussed. The findings demonstrate that AI and DBE calculations can be used to analyze and arrogate potential combination of cation and anion for fuel desulphurization purposes. 1. Introduction Due to the limited efficiency of hydrodesulphurization towards removing aromatic organosulphur compounds, a number of recent researches had focused on extracting them using ionic liquids [1, 2]. Notable characteristics of ionic liquids (ILs) are their nonmeasurable vapour pressure, thermal stability with wide liquid range, and solvating properties for diverse materials. Because of these notable characteristics, ILs have drawn great attention in which their properties are believed to leave minimal impact on the environment as well as human health [3–9]. The prospect of ILs in becoming an industrial solvent for the desulphurization process may be illustrated from the solute-solvent properties’ point of view [7, 9–13]. These may include the polarity of the solute, the dipole moment, molecular size, and molar volume. As for the aromatic organosulfur functioning as a solute, its miscibility in ILs can mostly be described through its -electron density. The solute with a strong -electron density would be highly miscible in ILs and vice versa. This leads to our observation that -electron density could be a vital point in screening a potential IL for desulphurization [1, 11–14]. However, to determine the -electron density is not simple whereby it needs to be calculated using the quantum chemical calculations [13, 14]. Thus, the quantification of the -electron density for ILs using simple calculation needs immediate justification. Therefore, here, by suggesting a new complimentary method that estimates the aromaticity index and double-bond equivalent which are closely related to -electron density, the possibility of using it to preliminarily select an appropriate IL for fuel desulphurization purposes shall be explored. 2. Experimental and Theory From Conductor-like Screening Model for Realistic Solvents (COSMO-RS) database, 407 combinations of organic ILs (11 of cations and 37 of anions) were investigated, while metal-based ILs were excluded. First, three important parameters that determine the extractive ability of the ILs which are predicted using COSMO-RS were basically calculated from the estimated activity coefficient ( ) at
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