Viscoelastic, Surface, and Volumetric Properties of Ionic Liquids [BMIM][OcSO 4 ], [BMIM][PF 6 ], and [EMIM][MeSO 3 ] Manish Pratap Singh, † Satish Kumar Mandal, ‡ Yogendra Lal Verma, † Abhishek Kumar Gupta, † Rajendra Kumar Singh,* ,† and Suresh Chandra † † Department of Physics, Banaras Hindu University, Varanasi 221005, India ‡ Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur 721302, West Bengal, India * S Supporting Information ABSTRACT: Thermophysical properties viz. surface tension, viscosity, density, and ultrasonic velocity of three ionic liquids 1-butyl-3-methyl imadazolium octyl sulfate [BMIM][OcSO 4 ], 1-butyl-3-methyl imadazolium hexafluorophosphate [BMIM]- [PF 6 ], and 1-ethyl-3-methyl imadazolium methanesulfonate [EMIM][MeSO 3 ] have been measured in a wide temperature range. Experimental data so obtained have been used to calculate isentropic compressibility, isothermal expansion coefficient, surface entropy, surface enthalpy, and critical temperature (temperature where the distinction between liquid and gas phase vanishes and the surface tension tends to zero). Structure-property correlation for different ILs is also discussed. ■ INTRODUCTION A novel class of ionic materials, termed ionic liquids (ILs), has recently attracted much attention. These are essentially low melting point salts consisting of self-dissociated cations and anions. The combination of large and asymmetric cations like imidazolium or ammonium, phosphonium, pyridinium, etc. with anions like hexafluorophosphate [PF 6 ] or [BF 4 ], [MeSO 3 ], [OcSO 4 ], etc. makes them liquid down to unusually low temperatures. 1 ILs have some interesting properties such as extremely low vapor pressure, high thermal and chemical stability, recyclability, wide range of solubility, wide electro- chemical window, and liquidus range. 1-8 Due to these exotic properties, ILs have found applications as lubricants, 9 solvents for chemical synthesis, 1 catalysts, 4 as electrolytes for electro- chemical devices like batteries 10-12 fuel cells, 13 solar cells, 14 super capacitor, 15 sensors, 16 actuators, 17 electrode deposition, 18 templates for obtaining porous silica matrices, etc. 19-21 Thermophysical properties (viscosity, density, surface tension, elastic properties, etc.) of ILs are important from its technological and industrial applications point of view. For example, viscosity plays an important role from engineering point of view as it largely affects stirring, flow and lubricating properties; transport properties like duffusion; mixing and pumping processes. Another important thermophysical prop- erty is surface tension which affects some important steps related to production process like catalysis, adsorption and extraction. Ultrasonic velocity is an important thermophysical parameter which can give precise information about the processes occurring in the time scale 10 -3 to 10 -10 s and is very sensitive to molecular organization and interaction. Therefore, it can be used as important tool for characterizing materials. Using ultrasonic velocity together with density, viscosity etc. we can evaluate many interesting properties viz. thermal expansion coefficient, isentropic compressibility, attenuation, and relaxation time. Recently, we have reported 22 the correlations between many thermophysical quantities viz. ultrasonic velocity, density, viscosity, and surface tension for three imidazolium based ILs. The correlations developed by us can be used to evaluate the less studied property of ILs viz. speed of sound. In the present work, we report experimental data on surface tension, fluidity (=1/viscosity), density, and ultrasonic velocity as a function of temperature for three ILs and examine the effect of cation, anion, interactions, and the effect of temperature (since one of the most attractive features of these materials is the wide liquidus range they offer). From these experimental data we have evaluated some important volumetric, surface, and mechanical properties viz. thermal expansion coefficient, molecular volume, surface entropy, surface enthalpy, and isentropic compressibility. The ILs used for the present study are 1-butyl-3-methyl Imidazolium octyl sulfate [BMIM][OcSO 4 ], 1-butyl-3-methyl Imidazolium hexa- fluorophosphate [BMIM][PF 6 ], and 1-ethyl-3-methyl Imidazo- lium methanesulfonate [BMIM][MeSO 3 ]. All these ILs are commercially available, out of which [BMIM][PF 6 ] is the most studied. However, [BMIM][PF 6 ] has been chosen in order to Received: July 23, 2013 Accepted: June 25, 2014 Published: July 8, 2014 Article pubs.acs.org/jced © 2014 American Chemical Society 2349 dx.doi.org/10.1021/je5000617 | J. Chem. Eng. Data 2014, 59, 2349-2359