Spectroscopic and MD Study of Dynamic and Structural Heterogeneities in Ionic Liquids

Abstract

The structure of ionic liquids (ILs) surrounding solute dyes and the effects of solvent structure on solute diffusion have been investigated using molecular dynamics (MD) and the experimental tools of confocal and fluorescence correlation spectroscopies. Although confocal microscopy and simulations show that the local environment around solutes in ILs is heterogeneous and that the structural heterogeneity is rather long-lived, the local polarity and the diffusion constant were found to be uncorrelated. Moreover, the complex diffusion observed experimentally is not due to the structural heterogeneity of the IL but rather due to the dynamic heterogeneity arising from the viscous glassy nature of the IL environment. MD simulations show that the degree of dynamic heterogeneity depends on the first nonvanishing electric multipole moment of the solute. The dynamics of a cationic solute are the least heterogeneous, whereas those of a solute without an electric multipole moment are the most heterogeneous. This indicates that the length scale over which the solute-solvent interactions occur, and thus the number of solvent degrees of freedom that couple to the solute, are the key factors governing the dynamic heterogeneity of the solute.

Neutron scattering studies on short- and long-range layer structures and related dynamics in imidazolium-based ionic liquids

“…[4][5][6] The structural investigations for CnmimX with n < 10 have been performed by using x-ray and neutron diffraction (ND) techniques [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] and molecular dynamics (MD) simulations. 12,17,[22][23][24][25][26][27][28][29][30] In these studies, a characteristic peak indicative of Q low , which is called "low-Q peak" in this paper, appeared at Q low ≈ 0.3 Å −1 in addition to the peaks at Q ion ≈ 1 Å −1 corresponding to the correlation among ions and at Q alkyl ≈ 1.5 Å −1 corresponding mainly to the correlation among alkylchains (Fig. 1).…”

Choline chloride and ethylene glycol based deep eutectic solvent (DES) versus hydroxyl functionalized room temperature ionic liquids (RTILs): assessing the differences in microscopic behaviour between the DES and RTILs

“…The correlation data were fitted to both single-component diffusion (b = 1) and anomalous diffusion (b o 1) using the equation 56 GðtÞ ¼ 1…”

“…The b values (which are well below unity) indicate that the diffusion dynamics of the solute molecules in these media are diverse in nature in terms of their translational diffusion time. 52,56 It has been well documented that for viscous heterogeneous media like DESs and RTILs, the mean square displacement (MSD) of solute molecules does not follow the normal Brownian diffusion (b = 1), instead it follows a sub-diffusive behaviour, r(t) 2 a t b (where b o 1). 56 As for conventional homogeneous solvents the diffusion of these solute molecules follows a single component diffusion model (b = 1), the deviation of the b value from unity indicates the presence of dynamic heterogeneity in all these media.…”

“…52,56 It has been well documented that for viscous heterogeneous media like DESs and RTILs, the mean square displacement (MSD) of solute molecules does not follow the normal Brownian diffusion (b = 1), instead it follows a sub-diffusive behaviour, r(t) 2 a t b (where b o 1). 56 As for conventional homogeneous solvents the diffusion of these solute molecules follows a single component diffusion model (b = 1), the deviation of the b value from unity indicates the presence of dynamic heterogeneity in all these media. 52,56 In the current study, as can be seen, the value of b deviates more for the hydroxyl ILs heterogeneity in the hydroxyl ILs as compared to the DES, which is also consistent with the result obtained from the time resolved anisotropy measurements.…”

Strongly diluted dimethyl-imidazolium chloride–alcohol solutions: solvents are structurally different but dynamic heterogeneities are similar

“…It also indicates that the mechanisms of diffusion, as well as the dynamic heterogeneity of the system changes, which correlates both with the experimental data obtained via neutron spin-echo and confocal microscopy methods, 80,81 and with computer simulations. 82,83…”