Dynamics of Methanol in Ionic Liquids: Validity of the Stokes-Einstein and Stokes-Einstein-Debye Relations

The effect of dilution on induced free charge density gradients in room temperature ionic liquids

We report on changes in the magnitude and length scale of the induced free charge density gradient, ρ_f, in three imidazolium room temperature ionic liquids (RTILs) with dilution by methanol and acetonitrile. Using depth- and time-resolved fluorescence measurements of cresyl violet rotational diffusion, we find that ρ_f persists in RTILs to varying degrees depending on RTIL and diluent identity, and in all cases the functional form of ρ_f is not a smooth monotonic diminution in either magnitude or persistence length with increasing diluent, but a stepwise collapse. This finding is consistent with changes in the bulk RTIL as a function of dilution seen using rotational diffusion measurements that show the rotating entity in bulk RTILs exhibits a larger effective hydrodynamic volume than would be expected based on bulk viscosity data for the diluted RTILs. This excess hydrodynamic volume can be understood in the context of aggregation of RTIL ion pairs in the diluted RTIL system. The size of the aggregates is seen to depend on RTIL identity and diluent, and in all cases aggregate size increases with increasing dilution. This finding is consistent with the ρ_f dependence on dilution data. The collapse of ρ_f is seen to correlate with the onset of RTIL ion pair dimer formation, a condition that may facilitate dissociated RTIL ion mobility in the binary system.

The molecular rotational motion of liquid ethanol studied by ultrafast time resolved infrared spectroscopy

In this report, ultrafast time-resolved infrared spectroscopy is used to study the rotational motion of the liquid ethanol molecule. The results showed that the methyl, methylene, and CO groups have close rotational relaxation times, 1-2 ps, and the rotational relaxation time of the hydroxyl group (-OH) is 8.1 ps. The fast motion of the methyl, methylene and CO groups, and the slow motion of the hydroxyl group suggested that the ethanol molecules experience anisotropic motion in the liquid phase. The slow motion of the hydroxyl group also shows that the hydrogen bonded network could be considered as an effective molecule. The experimental data provided in this report are helpful for theorists to build models to understand the molecular rotational motion of liquid ethanol. Furthermore, our experimental method, which can provide more data concerning the rotational motion of sub groups of liquid molecules, will be useful for understanding the complicated molecular motion in the liquid phase.

A "Universal" Spectroscopic Map for the OH Stretching Mode in Alcohols

Empirical maps are presented for the OH stretching vibrations in neat alcohols in which the relevant spectroscopic quantities are expressed in terms of the electric field exerted on the hydrogen atom by the surrounding liquid. It is found, by examination of the four lowest linear alcohols, methanol, ethanol, n-propanol, and n-butanol, that a single map can be used for alcohols with different alkyl groups. This "universal" map is in very good agreement with maps optimized for the individual alcohols but differs from those previously developed for water. This suggests that one map can be used for all alcohols, perhaps even those not examined in the present study. The universal map gives IR lineshapes in good agreement with measured spectra for isotopically dilute methanol and ethanol, while the two-dimensional IR photon echo spectra give results that differ from experiments. The role of non-Condon effects, reorientation dynamics, hydrogen bonding, and spectral diffusion is discussed.

Origin of heterogeneous dynamics in local molecular structures of ionic liquids

Room-temperature ionic liquids (ILs) are generally considered as structurally heterogeneous with inherent polar/apolar phase separation. However, even after a decade of research, local dynamics in the heterogeneous structures of ILs remain neglected. Such local dynamics may influence the ion transport of electrolytes, as well as the reaction rate of solvents. In this study, we performed molecular dynamics simulation to analyze the local dynamics for the structural heterogeneity of ILs. Calculations of the diffusion, reorientation, and association dynamics showed a distinct heterogeneous dynamics between the polar and apolar regions of ILs. Further studies demonstrated that such local dynamic differences originate from local structural heterogeneity. Different energy barriers determine a predominant fast reorientation dynamics in apolar regions and a locally vibrating behavior in polar regions. Additionally, we suggested a new jumping mechanism to clarify the dynamic heterogeneity of ions in the polar regions. The results will help determine the origin of the heterogeneous dynamics in IL local structures and provide a theoretical basis for tuning the dynamic properties of ILs used as electrolytes or reaction solvents.

Deuteron quadrupole coupling constants and reorientational correlation times in protic ionic liquids

We describe a method for the accurate determination of deuteron quadrupole coupling constants and reorientational correlation times in protic ionic liquids by means of NMR relaxations time experiments, DFT-calculations and molecular dynamics simulations.

Fractional Debye-Stokes-Einstein behaviour in an ultraviscous nanocolloid: glycerol and silver nanoparticles

One of the major features of glass forming ultraviscous liquids is the decoupling between translational and orientational dynamics. This paper presents studies of this phenomenon in glycerol, an accepted molecular glass former, concentrating on the impact of two exogenic factors: high pressures (P) up to the extreme 1.5 GPa and silver (Ag) nanoparticles (NPs). The analysis is focused on the fractional Debye-Stokes-Einstein (FDSE) relationship: σ(T,P)(τ(T,P))(S) = const, linking DC electric conductivity (σ) and primary (alpha, structural) relaxation time (τα). In glycerol and its nanocolloid (glycerol + Ag NPs) at atmospheric pressure only negligible decoupling (S ∼ 1) was detected. However, in the compressed nanocolloid, a well-defined transformation (at P = 1.2 GPa) from S ∼ 1 to the very strongly decoupled dynamics (S ∼ 0.5) occurred. For comparison, in pressurized 'pure' glycerol the stretched shift from S ∼ 1 to S ∼ 0.7 took place. This paper also presents the general discussion of FDSE behavior in ultraviscous liquids, including the new link between the FDSE exponent, fragility and the apparent activation enthalpy and volume.