A new insight into the nanostructure of alkylammonium alkanoates based ionic liquids in water

Abstract

In this paper, small angle X-ray scattering has been used to study a series of ionic liquids, alkylammonium alkanoates ([N0 0 0 n][CmCO2]), with varying alkyl chain lengths in the cation and the anion. We investigate the behaviour and the structure of such ionic liquids in their neat state, and in binary mixtures with water. To the best of our knowledge, this is the first structural study dealing with the behaviour of propylammonium propanoate [N0 0 0 3][C2CO2], butylammonium propanoate [N0 0 0 4][C2CO2], propylammonium butanoate [N0 0 0 3][C3CO2] and butylammonium butanoate [N0 0 0 4][C3CO2] when mixed in water. Generally, in the ionic liquids containing alkyl chains on both the cation and the anion, the correlation distance and the resulting scattering peak, which signal intermediate range order, are affected equally by both of the chains. The interesting result obtained regarding these systems is that the shift of the pre- and principal peaks with the addition of water depends on the overall molar concentration of the chains and is generally cumulative. Although, for a given sum of cation and anion chain lengths, the shift is reliant on the cation-anion combination in the neat state, it is not the case in their mixtures with water. In some recent papers, it has been reported that with addition of water, the pre-peak position remains constant, but our results show a shift in both pre- and principal Q peaks.

Induced Protic Behaviour in Aprotonic Ionic Liquids by Anion Basicity for Efficient Carbon Dioxide Capture

“…This can be envisaged as multiple water molecules aggregating around a single carboxylate headgroup, up to 0.8, promoted by repulsion of the hydrophobic alkyl chains of the cation and anion with water. [18] Thus, resulting in delocalisation of the negative charge of the anions causing the system to expand resulting in a "swelling effect", inducing water separated ion-pairs. [19] This is corroborated by the 1 H-1 H NOESY spectra, with water correlating to all 1 H environments of the cation, and becomes more evident with increasing water concentration (Figures S1.19-S1.36).…”

“…[19] The repulsion between water and the alkyl chains dominates, causing the polar head group to expand, which has been regarded as a "swelling effect". [18] The analysis of effect of the addition of water to the octanoate IL sample is more complicated, with the diffraction of the pre-peak initially moving to lower values until 0.41 (less compact) before jumping to significantly higher values at 0.67 (greater compactness) and returning downfield (Table S4 .3, Figure 3). This dramatic change suggests a re-orientation of the ions moving closer to one another, probably through greater interdigitation of the alkyl chains of the cation and anion promoted by van der Waals forces.…”

“…An increase in water content sees a gradual decrease in average anion‐water interaction, up to 0.8, which is reflected in the upfield shifts before plateauing. This can be envisaged as multiple water molecules aggregating around a single carboxylate headgroup, up to 0.8, promoted by repulsion of the hydrophobic alkyl chains of the cation and anion with water . Thus, resulting in delocalisation of the negative charge of the anions causing the system to expand resulting in a “ swelling effect ”, inducing water separated ion‐pairs .…”

“…The lowering of the q values with increasing water content of the pre‐peak for the ethanoate and propanoate ILs suggests ion‐pair separation, as the polar domains enlarge . The repulsion between water and the alkyl chains dominates, causing the polar head group to expand, which has been regarded as a “ swelling effect ” . The analysis of effect of the addition of water to the octanoate IL sample is more complicated, with the diffraction of the pre‐peak initially moving to lower q values until 0.41 (less compact) before jumping to significantly higher values at 0.67 (greater compactness) and returning downfield (Table S4.3, Figure ).…”

Ionic Liquids As Tunable Toxicity Storage Media for Sustainable Chemical Waste Management

“…Dynamic microscale processes occur in ionic liquid-water systems during extraction . Water tends to form various structures, depending on its concentration in the ionic liquid, and these structures possess high levels of micro- and nanoscale organization in ionic liquid/water mixtures. − High efficiency of ionic liquid-water mixtures for processing plant biomass was demonstrated . Moreover, production costs of ionic liquids are decreasing rapidly, and the possibility of their recycling also can significantly alleviate the expenses. , …”