Conductivity, Viscosity, Spectroscopic Properties of Organic Sulfonic Acid solutions in Ionic Liquids

Anh T Tran,Phuoc H Lam,Timothy D Vaden,Brittany L Stinger,Jay Tomlin,Dustin J Walczyk,Omar Cruz,Lei Yu,Alexandra D Miller

doi:10.3390/chemengineering3040081

Abstract

Sulfonic acids in ionic liquids (ILs) are used as catalysts, electrolytes, and solutions for metal extraction. The sulfonic acid ionization states and the solution acid/base properties are critical for these applications. Methane sulfonic acid (MSA) and camphor sulfonic acid (CSA) are dissolved in several IL solutions with and without bis(trifluoromethanesulfonyl)imine (HTFSI). The solutions demonstrated higher conductivities and lower viscosities. Through calorimetry and temperature-dependent conductivity analysis, we found that adding MSA to the IL solution may change both the ion migration activation energy and the number of “free” charge carriers. However, no significant acid ionization or proton transfer was observed in the IL solutions. Raman and IR spectroscopy with computational simulations suggest that the HTFSI forms dimers in the solutions with an N-H-N “bridged” structure, while MSA does not perturb this hydrogen ion solvation structure in the IL solutions. CSA has a lower solubility in the ILs and reduced the IL solution conductivity. However, in IL solutions containing 0.4 M or higher concentration of HTFSI, CSA addition increased the conductivity at low CSA concentrations and reduced it at high concentrations, which may indicate a synergistic effect.

Highlights

Investigations and reports about ionic liquids (ILs) have been burgeoning in the past two decades due to their unique chemical and physical properties as organic liquid substances
We investigated the properties of a series of acid solutions in ILs such as 3-butyl-1-methylimidazolium tetrafluoroborate (BMIBF4) [3], 3-butyl-1-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMITFSI) [4,5], N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PyrrTFSI) [6], and 3-ethyl-1-methylimidazolium acetate (EMIOAc) [7]
We investigate the properties of organic sulfonic acid (R-SO3H)/IL solution properties with a combination of various methods, including conductivity, viscosity, calorimetry, spectroscopy, and computational studies

Summary

Introduction

Investigations and reports about ionic liquids (ILs) have been burgeoning in the past two decades due to their unique chemical and physical properties as organic liquid substances. ILs’ low volatilities and high thermal stabilities allow IL-based PEM works at temperatures as high as ~300 ◦C without observable IL evaporation and loss of electrochemical functionality For this purpose, many cross-linked polymers with sulfonic acid functional groups have been synthesized and swollen with ILs. Extra sulfonic acids may be added to further enhance the proton conductivity. The AILs exhibit large contact area with no vapor or gas produced These applications, in general, are based on the strong acidic property of sulfonic acids and non-volatile liquid properties of the ILs. the sulfonic acid dissociation, microenvironment of active protons, and the channels and mechanisms of proton transportation are very essential knowledge to know in order to understand their performance in applications and to design and develop optimistic systems for the applications. The collective properties of the interactions are reflected by the solution enthalpy (∆Hsol) that is measured by solution calorimetry

Materials and Methods

MSA in PyrrTFSI

Conclusions