Anion Dependent Dynamics and Water Solubility Explained by Hydrogen Bonding Interactions in Mixtures of Water and Aprotic Heterocyclic Anion Ionic Liquids

Abstract

Molecular dynamics simulations were used to compare water solubilities and the effects of water on the structure and dynamics of ionic liquids (ILs) composed of phosphonium cations paired with azolide and phenolate anions. The addition of water decreases ordering of the ions compared to the dry ILs with the exception of anion-anion ordering in the phenolate IL. The result is that the dynamics of the azolide ionic liquids increase significantly upon addition of water, whereas the phenolate IL dynamics show little change. The relative water solubilities were compared through calculation of Henry's law constants. Water is much more soluble in the phenolate IL due to strong hydrogen bonding interactions between water and the phenolate oxygen atom. Anions can therefore be selected to control IL-water hydrogen bonding for optimal performance in applications such as CO separation.

Solubility of Water in Aprotic Heterocyclic Anion (AHA) Ionic Liquids

“…The [P 66614 ] ILs with the 5-cyanoindolide and 2-cyanopyrrolide anions have the lowest water uptake values of the [P 66614 ]­[AHA] ILs, but they are an order of magnitude higher than those of the corresponding [Tf 2 N] − IL, as shown in Figure . Additionally, the hydrophobicity trend due to the anion observed for [P 66614 ]­[3-Triz] and [P 66614 ]­[2CNPyr] and shown in Figure agrees with the results obtained by Sheridan et al, where Henry’s law constants calculated from molecular simulations indicate that [P 2228 ]­[2CNPyr] is more hydrophobic than [P 2228 ]­[3-Triz]. These results highlight the well-known strong electron withdrawing capabilities of the cyano substituent which significantly increases the hydrophobicity of the ionic liquid. , The difference between the water solubility in [P 66614 ]­[5CnIn] and [P 66614 ]­[2CNPyr] is only 0.2 mol of H 2 O/mol of IL, but it is interesting that it is higher in the [P 66614 ]­[5CNIn], which is a larger anion.…”

“…There have been several experimental and theoretical studies regarding the interactions and the mutual solubilities between ILs and water. [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33]35 As expected, these studies show that both the anion and cation play an important role in the IL hydrophobicity. Furthermore, typical anions have stronger interactions with water than typical cations.…”

“…The most common anions studied are halides such as bromine and chloride, bis­(trifluoromethylsulfonyl)­imide, hexafluorophosphate, cyano, tosylate, sulfonate, and acetate based anions. − ,,,,− , In the case of imidazolium based ILs, small cations such as 1-butyl-3-methylimidazolium ([C 4 MIM]) when paired with [Cl] − , [Br] − , [CF 3 SO 3 ] − , or [BF 4 ] − are completely miscible with water. On the other hand, when [C 4 MIM] is combined with [C­(CN) 3 ] − , [CH 3 (C 2 H 4 O) 2 SO 4 ] − , [PF 6 ] − , or [N­(SO 2 CF 3 ) 2 ] − , there is a phase split at room temperature .…”

“…We are aware of only one theoretical study of the interaction of AHA ILs with water that has been published, which focuses on the triethyl­(octyl)­phosphonium cation and 2-cyanopyrrolide, phenolate, and 1,2,3-triazolide anions . This study indicates that the anion hydrophobicity increases in the following order: phenolate < 1,2,3-triazolide < 2-cyanopyrrolide.…”

“…19,22 The most common anions studied are halides such as bromine and chloride, bis(trifluoromethylsulfonyl)imide, hexafluorophosphate, cyano, tosylate, sulfonate, and acetate based anions. [20][21][22]24,27,28,[30][31][32][33]35 In the case of imidazolium based ILs, small cations such as…”