Effect of DBU (1,8-Diazobicyclo[5.4.0]undec-7-ene) Based Protic Ionic Liquid on the Volumetric and Ultrasonic Properties of Ascorbic Acid in Aqueous Solution Vickramjeet Singh, †,‡ Dharmendra Singh, † and Ramesh L. Gardas* ,† † Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India ‡ Department of Chemistry, Guru Nanak Dev University, Amritsar 143005, India * S Supporting Information ABSTRACT: In order to understand the strength and type of interactions involved in ternary solutions, the effect of solute or cosolute concentration and temperature is needed, as an understanding of these interactions is helpful in biochemical and biophysical chemistry. In this regard, we have studied the volumetric and ultrasonic properties of one of the most important vitamins, i.e., ascorbic acid, in water and in the presence of newly synthesized protic ionic liquid (1,8-diazabicyclo[5.4.0]undec-7- en-8-ium trifluoroacetate) at temperatures of 293.15-328.15 K and at atmospheric pressure. The experimentally measured density and speed of sound data were used to calculate apparent molar volume and isentropic compressibility, infinite dilution partial molar volume, and partial molar isentropic compressibility. Volume of transfer (Δ t V 2 ° and Δ t K° s,2 ), expansion coefficients, pair and triplet volumetric interaction coefficients were also evaluated and discussed in terms of various interactions occurring between ascorbic acid and PIL on the basis of the structural interaction model. 1. INTRODUCTION Ionic liquids (ILs) are molten salts having melting point generally below 100 °C. Owing to their unique physicochemical properties, 1-7 ILs are being used for analytical, biological, and industrial applications. 8-11 Physicochemical properties of ILs can be tuned by suitable combinations of anions or cations. As a result, ILs can be designed to become biocompatible. 12,13 Consequently, ILs are also being employed for the formation of aqueous biphasic systems (ABS). 14,15 ABS have been used for various applications such as for separation, purification, or extraction of biomolecules. 14-16 ABS employing ILs and polyhydroxy compounds (saccharides) demonstrated great potential use for extraction and purification. 17,18 The effect of temperature and IL on ABS has been reported, 19 as the chem- ical separation process using ABS requires accurate and reliable thermodynamic data. 19 ILs are also being used in the design and fabrication of modified electrodes and also to extract chemicals or com- pounds (synthetic colors) from food samples. 20,21 Recently, ILs were employed 20-23 for the extraction of Sudan and Para Red dyes from chilli powder, thus showing potential application of ILs in food industries. 20-23 An IL based carbon electrode 24 has been used for analysis of Sudan dyes, and similarly a novel carbon composite electrode of pyridinum based IL was used to determine the levels of ascorbic acid from food samples. 21 The increased utilization of ILs for various applications (chemical or separations processes) requires accurate determination of thermodynamic data. 25 One of the classes of ILs called switchable ionic liquids (SILs), i.e., solvents that can be changed to either ionic state or neutral state by introducing an external stimulus, has also been studied. 13,14 The switchable behavior of these solvents, i.e., from molecular liquid to highly polar ionic liquid, 26,27 also alters properties such as conductivity, polarity, viscosity, and solubilizing capacity. 28,29 As a consequence, these ILs can be employed for recycling or recovery processes. 26,30 SILs can be synthesized 13,26,30,31 by using molecular liquids, i.e., 1, 8-diazabicyclo[5.4.0]undec-7-ene (DBU) or 2-butyl-1,1,3,3- tetramethylguanidine (TMBG). Phan et al. 31 have reported switchable ILs based on DBU and TMBG, which have been used for the polymerization of styrene. The SILs have also been employed for a variety of purposes such as extraction, 32-34 dis- solution of biopolymers (lignin, hemicelluloses, etc.), 26,27,35,36 and also CO 2 capturing. 28,29 Recently, Lethesh et al. 37 have also reported synthesis and thermophysical properties of DBU based ILs and reported that there is a lack of thermophysical properties of DBU based ionic liquids. Thermodynamic properties of various vitamins have been reported in aqueous and mixed aqueous solutions. 38-42 Among these vitamins, ascorbic acid (vitamin C) is a favorite can- didate. 43-45 Ascorbic acid (AA), one of the most important vitamins for human health and nutrition is found in vegetables and fruits. It plays an important role in the suppression of superoxide radicals. 46-48 AA is a sugar acid known to have antioxidant properties, and it can treat/prevent scurvy and the common cold. 49 Degradation of AA is very important and is considered to be the major cause of quality and color changes during storage/processing of food materials. 50 The nature and strength of interactions between AA and various cosolutes (electrolytes, nonelectrolytes, surfactants, etc.) have been studied 38-42 by evaluating the thermodynamic properties of Received: December 19, 2014 Revised: February 4, 2015 Accepted: February 5, 2015 Published: February 5, 2015 Article pubs.acs.org/IECR © 2015 American Chemical Society 2237 DOI: 10.1021/ie504938v Ind. Eng. Chem. Res. 2015, 54, 2237-2245