Room temperature ionic liquids (RTILs) are the environment-friendly alternatives for organic volatile solvents in a host of synthetic, catalytic, and electrochemical applications. These are also being used for various R&D works in nuclear fuel cycle research such as the recovery and purification of nuclides of interest from spent nuclear fuel matrices. In this work, density, refractive index ( ) at sodium line, and thermodynamic water activity of the aqueous solution of 1-hexyl-3-methyl imidazolium bromide were determined in dilute aqueous solutions at 298.15？K. These results were used to calculate the apparent molal volumes of each solute over various concentration ranges. The measurements were performed as per ASTM procedures. Vapour pressure was derived using water activity values. Information on excess properties and structural interaction was also reported. 1. Introduction Room temperature ionic liquids are considered as young chemicals having variety a of applications in all types of areas in chemical industry due to their unique properties. They are referred to as designer solvents as we can design them for different reactions by changing the cation and anion. They can act as both polar and nonpolar solvents which minimizes the use of lot of chemicals making our environment neat and clean and thus referred to as “green solvent” [1]. Researchers have found that ionic liquids are more than a solvent and have several applications in electrochemistry, biological media, catalysis, organic synthesis, and so on [2–7]. Low vapour pressure, thermal stability, high thermal conductivity, large electrochemical window, and nonflammability are some of their properties which make them an innovative solvent. As a solvent, ionic liquids possesses several advantages over conventional organic solvents making it environmentally compatible [8]. The potential advantage of ionic liquids depends upon their thermophysical properties like density, refractive index, activity, vapour pressure, conductivities, viscosities, and so forth. Various research groups have studied the thermophysical properties of various ionic liquids to extract important information about them [9–16]. RTILs are extensively used for the extraction of metal ions in solvent extraction [17–19]. In nuclear industry these are used as green solvent for the extraction of various fission products in liquid-liquid extraction [20–22]. Currently, variants of 1,3 dialkyl imidazolium salt are being tried in processing of materials. In this paper one of such 1,3-dialkyl imidazolium salts that is, 1-hexyl-3-methyl
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