Abstract
The bulk nanostructure of 15 mol% propylammonium chloride (PACl) dissolved in propylammonium nitrate (PAN) and 15 mol% ethanolammonium chloride (EtACl) in ethanolammonium nitrate (EtAN) has been determined using neutron diffraction with empirical potential structure refinement fits. For both the PAN:PACl and EtAN:EtACl mixtures, data for three different scattering contrasts were simultaneously fit, and the structures determined and compared to that of the pure ionic liquids. Strong electrostatic interactions between chloride and cation charged groups, as well as the alcohol moiety of EtAN, lead to marked changes in local ion packing that alter the liquid structure. In PAN, the addition of chloride modifies but does not significantly disrupt the bicontinuous amphiphilic nanostructure of the IL. Tight packing of ammonium groups around chloride favours a gauche conformer for the cation which shrinks the apolar domains and brings the terminal methyls nearer the polar domains. The weakly-clustered nanostructure of EtAN, a consequence of the terminal hydroxyl, is overwhelmed by strong chloride-cation interactions. Ethanolammonium binds tightly to chloride in a monodentate fashion via either its alcohol or ammonium charge centre, or through both in a bidentate arrangement by adopting a gauche or eclipsed conformer.
Highlights
Ionic liquids (ILs) are salts with melting points less than 100 1C
A series of isotopically substituted 15 mol% propylammonium chloride (PACl) in propylammonium nitrate (PAN) and 15 mol% ethanolammonium chloride (EtACl) in ethanolammonium nitrate (EtAN) mixtures were prepared for neutron diffraction experiments; H-PACl:H-PAN, d3-PACl:d3-PAN and d7-PACl:d7-PAN and H-EtACl:H-EtAN, d4-EtACl:d4-EtAN and dalk-EtACl:dalk-EtAN
The final water content of the ILs was o0.1% by Karl–Fischer titration. 1H-NMR experiments reveal that, on average, 2.5 out of 3 amino hydrogen atoms are replaced with deuterium. d7-PAN and dalk-EtAN were synthesised by performing the acid–base reaction using N-propyl-d7-amine (CDN isotopes) and ethanol-1,1,2,2-d4-amine (CDN isotopes) respectively. d10-PAN was prepared from d7-PAN by washing d7-PAN with 3 molar equivalents of D2O and followed by drying by rotary evaporation for several hours at 25 1C, repeating this step twice before final drying by rotary evaporation for several hours at 25 1C and heating to B110 1C under N2 purge
Summary
ILs are composed entirely of ions and often boast an array of desirable physiochemical properties, including high electrochemical stability,[1] high thermal stability,[2] low vapour pressure[1,3,4] and the ability to dissolve both organic and inorganic substances.[5] A vast number of ion structures can be employed to form ILs, which enables the properties of ILs to be tuned. While it is relatively simple to determine what materials dissolve in an IL, it is much more difficult to experimentally determine how it is solvated, and how the solute in turn affects IL nanostructure.[34,36,39,40,44,46]
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