MD Study of Solvation in the Mixture of a Room-Temperature Ionic Liquid and CO₂

Abstract

Solvation structure and dynamics of a saturated solution of carbon dioxide in the room-temperature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMI+PF6-) at 313 K and 0.15 kbar are investigated via molecular dynamics computer simulations by employing a diatomic probe solute. It is found that the mixture shows preferential solvation, which is mainly controlled by the solute-BMI+PF6- electrostatic interactions and thus dictates differing roles for CO2 as the solute charge distribution varies. The local structure and density of BMI+PF6- and CO2 in the vicinity of the solute become enhanced and reduced, respectively, as its dipole moment increases. As a result, equilibrium solvation dynamics of a nonpolar solute in the mixture have a strong CO2 character, whereas those of a dipolar solute are very similar to, albeit faster than, solvation dynamics in pure BMI+PF6-. Related nonequilibrium solvent response couched in dynamic Stokes shifts and accompanying solvation structure relaxation, in particular, CO2 structure reorganization, shows interesting dependence on the solute charge distribution. Ion transport in the mixture is much faster than in pure BMI+PF6-, indicating that the addition of cosolvent CO2 reduces the viscosity of the ionic liquid, significantly. The effective polarity of the mixture, measured as solvation-induced stabilization of a dipolar solute, is found to be comparable to that of neat BMI+PF6-, consonant with solvatochromic measurements.

Dynamic propensity as an indicator of heterogeneity in room-temperature ionic liquids

“…Viscous dynamics in RTILs have been studied theoretically through molecular dynamics simulations by many researchers. [9][10][11][12][13][14][15][16][17][18] Solvation and rotational dynamics of the solute are found to be described by the nonexponential relaxation in accordance with the experimental results. [19][20][21][22][23] Translational dynamics of the ions are characterized with the nonexponential structural relaxation, subdiffusive behavior, violation of the Stokes-Einstein relation, and the decoupling phenomena.…”

MD Study of Stokes Shifts in Ionic Liquids: Temperature Dependence

“…For the rigid diatomic solute, the solute atoms are separated by 3.5 Å, and they interact with solvent through both Lennard-Jones (LJ) and Coulomb potentials. The LJ parameters, σ = 4 Å and ϵ = 0.82890 kJ/mol, and mass, 100 amu, are identical for both atoms. ,, Two charge distributions are considered, a neutral pair (NP) with no dipole moment as the S 0 state, and an ion pair (IP) with a dipole moment of 16.8 D as the S 1 state. All other molecules are described with the fully flexible all atom description.…”

“…For EMI + , BMI + , and Tf 2 N – , the OPLS-based force fields of refs − were used. For PF 6 – , the LJ parameters and charge assignment of ref and the bonded parameters of ref were used as in ref .…”

“…One of the most studied solvent properties is polarity. For many chemical reactions involving charge shift and transfer, such as S N 1 and S N 2, the polarity of the solvent exerts a strong influence on reaction free energetics and thus the yield and rate of reaction. − Many different parameters, such as dielectric constant ε 0 , E T (30), or Kamlet–Taft parameters have been used to quantify solvent polarity. − While their typical dielectric constant (ε 0 = ∼10–16) is not as high as that of conventional polar solvents, − the effective polarity of many ILs, measured as their ability to stabilize polar solutes, is comparable to and even in excess of highly polar aprotic solvents (e.g., acetonitrile) due to strong Coulomb interactions of IL ions and polar solutes, according to experimental − and theoretical ,,− studies. Another interesting property of ILs is that the temperature dependence of ε 0 is considerably weaker than conventional polar solvents.…”

Carbon dioxide capture by aminoalkyl imidazolium-based ionic liquid: a computational investigation

“…13 The solubility of CO 2 in these ILs is typically around 0.02 mol CO 2 per mol IL at room temperature and atmospheric pressure. Molecular dynamics (MD) simulation has also been used to explore the important interactions associated with CO 2 solubility [28][29][30] and to reveal the effect of water on the interfacial transport dynamics of CO 2 in practical operation. 31 The MD simulation results show that anions dominate the interactions between anions and CO 2 , and that such physical interactions are weak Lewis acid-base or electrostatic interactions.…”