Monolayer to Bilayer Structural Transition in Confined Pyrrolidinium-Based Ionic Liquids

• DOI: 10.1021/jz301965d
• Publication Date: 2013-01-03T18:42:28Z
• Authors: Alexander M. Smith; Kevin R. J. Lovelock; Nitya Nand Gosvami; Peter Licence; Andrew Dolan; Tom Welton; Susan Perkin
• Keywords: 01 natural sciences; 0104 chemical sciences

Ionic liquids can be intricately nanostructured in the bulk and at interfaces resulting from a delicate interplay between interionic and surface forces. Here we report the structuring of a series of dialkylpyrrolidinium-based ionic liquids induced by confinement. The ionic liquids containing cations with shorter alkyl chain substituents form alternating cation-anion monolayer structures on confinement to a thin film, whereas a cation with a longer alkyl chain substituent leads to bilayer formation. The crossover from monolayer to bilayer structure occurs between chain lengths of n = 8 and 10 for these pyrrolidinium-based ionic liquids. The bilayer structure for n = 10 involves full interdigitation of the alkyl chains; this is in contrast with previous observations for imidazolium-based ionic liquids. The results are pertinent to these liquids' application as electrolytes, where the electrolyte is confined inside the pores of a nanoporous electrode, for example, in devices such as supercapacitors or batteries.