Ionic Liquids - American Chemical Society

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Chapter 22

Bulk and Interfacial Nanostructure in Protic Room Temperature Ionic Liquids Rob Atkin1,* and Gregory G. Warr2 1

School of Environmental and Life Sciences, The University of Newcastle, Callaghan, NSW 2308, Australia 2 School of Chemistry, The University of Sydney, NSW, 2006, Australia

Until recently, protic ionic liquids were thought to be structurally homogenous. However, small angle neutron scattering experiments using selectively deuteriated ethylammonium nitrate ([EtNH3][NO3]) and propylammonium nitrate ([PrNH3][NO3]) conclusively show that these ionic liquids are nanostructured in the bulk. Electrostatic and solvophobic interactions within the liquid lead to the formation of alternating polar and apolar layers. In this chapter, these results are described in detail, and the relationship between bulk structure and interfacial structure, determined using atomic force microscopy, is elucidated.

Introduction Ionic liquids are currently attracting a great deal of research interest as solvents for a wide range of reactions and processes (1,2). Much of this interest stems from the ability to predictably tune physical properties through the selection of appropriate cation and anion combinations. The cation structure, in particular, can be altered in a systematic fashion by varying the length or © 2009 American Chemical Society In Ionic Liquids: From Knowledge to Application; Plechkova, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

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318 number of aliphatic chains covalently bound to a permanently charged group such an imidazolium or quaternary ammonium. Ionic liquids may be broadly divided into two categories: protic and aprotic (3). To date aprotic ionic liquids have received far greater attention but as new applications for protic ionic liquids are uncovered, this imbalance is beginning to be addressed (3-7). Protic ionic liquids are formed when a proton is transferred from a Brønsted acid to a Brønsted base (4), the method used to create the first reported ionic liquid of any type, ethanolammonium nitrate in 1888 (8). A quarter of a century later, Walden reported the synthesis of ethylammonium nitrate (9), which has become by far the most widely studied protic ionic liquid. In the 1980s, Evans and co-workers demonstrated that many of the physical characteristics of [EtNH3][NO3] were similar to water, most importantly the capacity to form a three-dimensional hydrogen-bond network, thought to be essentially for inducing solvophobic [The term solvophobic was first used by Ray to describe interactions akin to the hydrophobic effect in water, but in a non-aqueous solvent (9).] interactions that drive amphiphilic self assembly (10,11). The formation of small micelles was reported in [EtNH3][NO3] a few years layer; the small size was attributed to relatively weak solvophobic interactions (12,13). The formation and properties of lipid bilayer phases in [EtNH3][NO3] were investigated around the same time, and many similarities with water-based structures were identified (14). In 1991, hexagonal and cubic liquid crystal phases were first reported in [EtNH3][NO3], again for cationic surfactants (15). In more recent studies, non-ionic surfactants in [EtNH3][NO3] have been shown to: (i) have liquid crystalline phase behaviour similar to aqueous systems (10); (ii) adsorb at the [EtNH3][NO3]-graphite interface in hemimicelle structures (16); (iii) have micellar structure similar to aqueous systems by neutron scattering (17); and (iv) form structured microemulsions when mixed with surfactant and oil (18). It has also been demonstrated that many other protic ionic liquids support surfactant self-assembly (3,19-22), recently reviewed in some detail (5,6). Despite these studies of surfactant aggregation in [EtNH3][NO3], it was never suspected that the same solvophobic forces that drive surfactant selfassembly would induce structure within the ionic liquid itself. Most likely, researchers believed that the cation alkyl group, typically between C2 and C4, was too short to respond to the solvophobic effect (surfactant molecules typically have alkyl groups C8 or longer). Nonetheless, small angle neutron scattering (SANS) experiments with selective deuteriation conclusively show that [EtNH3][NO3] and [PrNH3][NO3] possess nanoscale heterogeneity (23). Atomic force microscopy (AFM) experiments reveal similar structure at the interface between these ionic liquids and solid surfaces (24). In this chapter, the bulk and interfacial structures of [EtNH3][NO3] and [PrNH3][NO3] are reviewed in detail. We will also present data showing that disruption of the solvophobic interaction via the attachment of an alcohol moiety to the terminal carbon of the cation alkyl group decreases liquid nanostructure. Given the structural simplicity of these ionic liquids (cf. Table 1), these results are far reaching, and suggest many protic ionic liquids will exhibit such structural heterogeneity.

In Ionic Liquids: From Knowledge to Application; Plechkova, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

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Table 1. Molecular structure, molecular weight (MW), density (ρ), ion pair diameter (D) and repeat spacing (D*) of the ionic liquids investigated.

Ionic Liquid

Abbrev.

Ethylammonium Nitrate

[EtNH3][NO3]

Propylammonium Nitrate

[PrNH3][NO3]

Ethanolammonium [HO(CH2)2NH3][NO3] Nitrate Methylammonium Nitrate

[MeNH3][NO3]

Structure

MW D D* ρ (g) (g.cm-3) (nm) (nm) 108

1.21

0.53 0.97

122

1.16

0.56 1.16

124

1.26

0.54

94

1.26

0.50 1.04

-

D is determined from (ρ) assuming a cubic packing geometry according to the method of by Horn et. al. (26)

Preparation of Ionic Liquids [EtNH3][NO3] (m.p. 14 °C), [PrNH3][NO3] (m.p. 3.5 °C) and ethanolammonium nitrate, [HO(CH2)2NH3][NO3], used in AFM experiments were prepared by reacting equimolar amounts of the amine and conc. nitric acid to produce an aqueous solution (12,25). Excess water was removed by rotary evaporation followed by purging the concentrated ionic liquid solutions with nitrogen, then heating at 110–120oC for several hours under a nitrogen atmosphere. This leads to water contents undetectable by Karl Fischer titration (