Temperature Effect on the Molecular Interactions between Ammonium Ionic Liquids and N,N-Dimethylformamide Pankaj Attri, † Pannuru Venkatesu,* ,† and Anil Kumar ‡ Department of Chemistry, UniVersity of Delhi, Delhi 110 007, India, and Physical Chemistry DiVision, National Chemical Laboratory, Pune 411 008, India ReceiVed: August 24, 2010; ReVised Manuscript ReceiVed: September 22, 2010 In view of the wide scope of molecular interactions between the highly polar compound of N,N- dimethylformamide (DMF) and ammonium ionic liquids (ILs), we have measured thermophysical properties such as densities (F) and ultrasonic sound velocities (u) over the whole composition range at temperatures ranging from 25 to 50 °C under atmospheric pressure. To gain some insight into the several aggregations of molecular interactions present in these mixed solvents, we predicted the excess molar volume (V E ) and the deviations in isentropic compressibilities (ΔK s ) as a function of the concentration of IL. These results are fitted to the Redlich-Kister polynomials. The materials investigated in the present study included the hydroxide series of ammonium ILs of tetramethylammonium hydroxide [(CH 3 ) 4 N][OH] (TMAH), tetraethylammonium hydroxide [(C 2 H 5 ) 4 N][OH] (TEAH), and tetrapropylammonium hydroxide [(C 2 H 7 ) 4 N][OH] (TPAH). The intermolecular interactions and structural effects were analyzed on the basis of the measured and the derived properties. A qualitative analysis of the results is discussed in terms of the ion-dipole and ion-pair interactions, and hydrogen bonding between ILs and DMF molecules and their structural factors. Introduction During the past decade, interest in the utility of environmen- tally friendly ionic liquids (ILs) and the broad variety of applications have increased immensely. ILs are a relatively new class of compounds that are liquid at or relatively near room temperature. These compounds appear to be the replacements for volatile organic compounds (VOCs). Due to the ionic nature of these materials, ILs have essentially insignificant vapor pressure and they therefore can be envisioned as being useful in a variety of applications. 1,2 The development of neoteric solvents, i.e., ILs, for chemical synthesis holds great promise for green chemistry applications. 3 ILs were initially synthesized in the early 20th century and at present there are over 200 types of ILs prepared. Some of these have been successfully applied in organic synthesis and other aspects. 4–6 One of the major objectives of the chemical industry today is to search for safer alternatives of VOCs that will minimize air pollution, climatic changes, and human health-related problems. ILs exhibit certain desirable physical properties, wide electrochemical window, wide thermal window, nonflammability, wide range of densities and viscosities, high potential for recycling, and high solvating capacity for organic compounds. Their perceived status as “designer”, alternative “green” solvents has contributed largely to this interest, i.e., the existence of fluids with no measurable volatility, and are able to selectively dissolve different types of solute merely by exchanging one of the ions that form the IL, or even more subtly, by altering one of the organic residues within a given ion. Recently, our research group has demon- strated the application and limitation of ILs as solvent in chemical processes. 7–10 The research on ILs still is in its preliminary phase; the reasons are (i) pure ILs and mixtures containing ILs differ noticeably from so-called “normal liquids”, and their behavior is not fully understood, and (ii) new compounds with unknown properties are being synthesized. The ILs possess many unique properties as compared to ordinary fluids. On the other hand, their structures are varied with designable functions. Although the number of research articles on ILs is increasing exponen- tially, there is still a lack of data on their thermophysical description and molecular modeling properties. The aim is to achieve exactly the desired chemical and physical properties by a judicious combination of an anion and a cation. The study of the properties and structure of complex liquid mixtures is necessary from both the theoretical and experimental points of view. Mixed solvents are almost ubiquitous in the industry in different fields ranging from petrochemistry to pharmaceutical industries. Thermodynamic properties of mixed solvents have been particularly informative in elucidating the solute-solute and solute-solvent interactions that exist in these solutions. Experimental data of thermodynamic and thermo- physical properties of liquids and liquid mixtures are fascinating and of high fundamental, practical importance for the industry. Although a qualitative connection between the macroscopic and microscopic features is feasible, quantitative conclusions are of interest to both academic and industry communities. In spite of the importance of properties of ILs in different solvent media, only a small amount of physicochemical data is available in the literature, which mainly characterizes ILs. 7,11–15 In this context, our aim is to explore closely two key thermophysical properties such as density (F) and ultrasonic sound velocity (u) for the mixed solvents of ILs and polar solvent. Till date, there is no systematic documentation for the ammonium hydroxide ILs with polar solvents. For these reasons, three ammonium ILs were synthesized in our laboratory by the simplest methods, which increase its utility. Interactions of ILs with organic molecular liquids have been rather scarcely investigated up to * To whom correspondence should be addressed. E-mail: venkatesup@ hotmail.com; pvenkatesu@chemistry.du.ac.in. Tel: +91-11-27666646-142. Fax: +91-11-2766 6605. † University of Delhi. ‡ National Chemical Laboratory. J. Phys. Chem. B 2010, 114, 13415–13425 13415 10.1021/jp108003x  2010 American Chemical Society Published on Web 10/06/2010