Why are ionic liquid ions mainly associated in water? A Car–Parrinello study of 1-ethyl-3-methyl-imidazolium chloride water mixture

Abstract

In this study we present the results of a first principles molecular dynamics simulation of a single 1-ethyl-3-methyl-imidazolium chloride [C(2)C(1)im][Cl] ion pair dissolved in 60 water molecules. We observe a preference of the in plane chloride coordination with respect to the cation ring plane as compared to the energetic slightly more demanding on top coordination. Evaluation of the different radial distribution functions demonstrates that the structure of the hydration shell around the ion pair differs significantly from bulk water and that no true ion pair dissociation in terms of completely autonomous solvation shells takes place on the timescale of the simulation. In addition, dipole moment distributions of the solvent in distinct solvation shells around different functional parts of the [C(2)C(1)im][Cl] ion pair are calculated from maximally localized Wannier functions. The analysis of these distributions gives evidence for a depolarization of water molecules close to the hydrophobic parts of the cation as well as close to the anion. Examination of the angular distribution of different OH(H(2)O)-X angles in turn shows a linear coordination of chloride accompanied by a tangential orientation of water molecules around the hydrophobic groups, being a typical feature of hydrophobic hydration. Based on these orientational aspects, a structural model for the obvious preference of ion pair association is developed, which justifies the associating behavior of solvated [C(2)C(1)im][Cl] ions in terms of an energetically favorable interface between the solvation shells of the anion and the hydrophobic parts of the cation.

The properties of residual water molecules in ionic liquids: a comparison between direct and inverse Kirkwood–Buff approaches

“…Furthermore, it was reported that IL cations can exhibit a depolarization effect on water molecules. Thus, the dipole moment of water decreases with increasing IL concentration in several aprotic and protic ILs [122][123][124] , which can be counteracted in presence of strong hydrogen bond acceptor anions. Another interesting effect is also the formation of carbenes in presence of water 122 .…”

Association of ionic liquids in solution: a combined dielectric and conductivity study of [bmim][Cl] in water and in acetonitrile

“…Of course, the evaluation of L(c) also requires assumptions, see Section 3.1. It must also be remembered that the DRS experiments require rather high concentrations so that extrapolation of K A (I) with eqn (16) 15 and the excellent agreement between the rotational correlation time calculated for the CIP and the experimental value, SIP formation also seems unlikely in water.…”

“…As an alternative, the CIP dipole moments obtained from semiempirical MOPAC 50 calculations with solvent-effect corrections 77 were adopted. Since these calculations indicated the existence of various conformers of similar energy, their dipole moments were averaged, yielding the values listed in 15 These dipole moments were used to calculate ion-pair concentrations c IP (Fig. 8(a)) and K A values (Fig.…”

“…A Car-Parinello simulation revealed an unusual cooperativity in the hydration of the ion pair of 1-ethyl-3-methylimidazolium chloride ([emim][Cl]) as a major driving force for ion association. 15 Comparison of conductometrically-determined association constants for 1-butyl-3-methyl-imidazolium chloride ([bmim][Cl]) and tetrafluoroborate ([bmim] [BF 4 ]) in methanol and dimethylsulfoxide revealed a marked dependence on the H-bond acceptor strength of the anion. 16 Measurements of the molar conductivity, L, of dilute salt solutions are probably the most accurate route to ion-pair association constants, K o A , at least for symmetrical electrolytes.…”

“…16 Also, various theoretical and experimental investigations on [bmim][Cl] and related 1-alkyl-3-methylimidazolium chloride + (water or AN) systems have been published recently, which provide useful background information. 15,[25][26][27][28][29][30][31][32] The second aim of the present investigation is to determine reliable transport properties, especially the limiting ionic conductivities, l N + , of the [bmim] + cation as a function of temperature. Almost nothing is known about the temperature dependence of such quantities despite their importance as one of the two reference states (in addition to the pure RTIL) in mixture studies.…”