Development of asymmetric ammonium-based room temperature ionic liquids

Abstract

Ionic liquids (ILs) are salts with a melting point lower than 100 °C. Room temperature ionic liquids (RTILs) are a subset of these salts which are liquid at room temperature. Chapter one surveys the historical development of RTILs and their applications, concentrating in particular on quaternary ammonium-based systems. A series of asymmetric quaternary ammonium halide salts formulated as N11RR'X (where R = methyl, ethyl, propyl, butyl, hexyl, allyl, benzyl and cyclohexyl, R' = octyl, decyl, tetradecyl and octadecyl, X = I or Br) and N14R(14)I (where R = allyl, propyl, octyl and decyl have been prepared and characterised. Introduction of asymmetry into the quaternary ammonium salts is shown to have a profound influence on their melting points and has been investigated as a primary step for the development of RTILs. Attempts to introduce fluorinated alkyl chains into the cation based on Rf(CH2)nX compounds was successful when n = 3, but when n = 2, RfCH=CH2 and N11RHX resulted.Anion exchange using a range of fluorinated anions (including sulfonate, PF6ˉ, P(C2F5)3F3ˉ (FAP), heptafluoroniobates) resulted in 42 new RTILs. The TFSIˉ anion is found to be the most promising candidate, and it led to the development of 21 new stable, hydrophobic room temperature ionic liquids. Introduction of asymmetry into the anion, e.g. replacing PF6ˉ with FAPˉ also lowers the melting points of these quaternary ammonium salts.Investigation of the properties of these new ionic liquids showed that they are thermally stable (Td > 400 °C) with wide liquid ranges (over 420 °C), relatively low viscosity, high conductivity and wide electrochemical windows (> 4 V). Some physical properties were found to be related directly to the chain-lengths of the substituent groups in the cation. The X-ray crystal structures of N112(18)Br, N114(18)Br.H2O, N114HI, N11B(18)Br.H2O, N111(10)I and N111(14)TFSI are reported.

Date of Award	31 Dec 2012
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Alan Brisdon (Supervisor)