11 research outputs found
Influence of the Organized Structure of 1‑Alkyl-3-Methylimidazolium-Based Ionic Liquids on the Rotational Diffusion of an Ionic Solute
To
understand the influence of organized structure of the ionic
liquids on the rotational diffusion of a hydrogen bond donating ionic
solute, reorientation times (Ď„<sub>r</sub>) of rhodamine 110
(R110) have been measured in 1-alkyl-3-methylimidazolium ([Rmim<sup>+</sup>]) based ionic liquids with anions tetrafluoroborate ([BF<sub>4</sub><sup>–</sup>]) and hexafluorophosphate ([PF<sub>6</sub><sup>–</sup>]).
The viscosity (η) was varied by changing the temperature (<i>T</i>) and also the alkyl chain length on the imidazolium cation
(ethyl, butyl, hexyl, and octyl). It has been noticed that τ<sub>r</sub> versus η/<i>T</i> plots contain two slopes
corresponding to lower and higher values of η/<i>T</i> for ionic liquids with [BF<sub>4</sub><sup>–</sup>] as well
as [PF<sub>6</sub><sup>–</sup>] anions. For lower values of
η/<i>T</i> (<0.2 and <0.3 mPa s K<sup>–1</sup>, respectively, for [Rmim<sup>+</sup>]Â[BF<sub>4</sub><sup>–</sup>] and [Rmim<sup>+</sup>]Â[PF<sub>6</sub><sup>–</sup>]), rotational
diffusion of R110 follows Stokes–Einstein–Debye hydrodynamic
theory with stick boundary condition due to specific interactions
between the solute and the anions of the ionic liquids. In contrast,
at higher η/<i>T</i>, the rotational diffusion of
the solute is faster than the stick predictions and this trend could
not be explained by the quasihydrodynamic theories of Gierer–Wirtz
and Dote–Kivelson–Schwartz as well. Diminishing hydrogen
bonding interactions between the solute and the anions, which transpire
as a consequence of the organized structure of the ionic liquids,
are responsible for the observed behavior
Influence of the Organized Structure of 1‑Alkyl-3-methylimidazolium Tetrafluoroborates on the Rotational Diffusion of Structurally Similar Nondipolar Solutes
To understand how the organized structure
of the ionic liquids
influences the location and mobility of nondipolar solutes, rotational
diffusion of 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrroloÂ[3,4-<i>c</i>]Âpyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrroloÂ[3,4-<i>c</i>]Âpyrrole (DPP) has been examined in 1-alkyl-3-methylimidazolium
(alkyl = ethyl, butyl, hexyl, and octyl) tetrafluoroborates. Both
the solutes are structurally similarî—¸the sole difference being
the two NCH<sub>3</sub> groups of DMDPP are replaced by two NH groups
in DPP. The rotational diffusion of DPP is found to be significantly
slower than DMDPP due to specific interactions between the NH groups
of the solute and the anion of the ionic liquid. It has been observed
that for a given viscosity and temperature, the rotational diffusion
of DMDPP becomes progressively faster with an increase in the length
of the alkyl chain on the imidazolium cation. DMDPP resides in the
nonpolar domains of these ionic liquids whose sizes increase with
an increase in the length of the alkyl chain, and as a result it experiences
microviscosity that is lower than the bulk viscosity. However, an
increase in the length of the alkyl chain has no apparent effect on
the rotational diffusion of DPP because specific interactions with
tetrafluoroborate necessitate the solute to be located in the vicinity
of the anion. The results of this work exemplify that despite having
similar size and shape, the rotational diffusion of DMDPP and DPP
is quite contrasting as their sites of solubilization and the nature
of interactions with the surroundings are vastly different owing to
subtle variations in their chemical structures
Rotational Diffusion of Nonpolar and Ionic Solutes in 1‑Alkyl-3-Methylimidazolium Bis(trifluoromethylsulfonyl)imides: Is Solute Rotation Always Influenced by the Length of the Alkyl Chain on the Imidazolium Cation?
In an attempt to find out whether the length of the alkyl
chain
on the imidazolium cation has a bearing on solute rotation, temperature-dependent
fluorescence anisotropies of three structurally similar solutes have
been measured in a series of 1-alkyl-3-methylimidazolium (alkyl =
methyl, ethyl, propyl, butyl, and hexyl) bisÂ(trifluoromethylsulfonyl)Âimides.
Solute–solvent coupling constants obtained from the experimentally
measured reorientation times with the aid of Stokes–Einstein–Debye
hydrodynamic theory indicate that there is no influence of the length
of the alkyl chain on the rotation of nonpolar, anionic, and cationic
solutes 9-phenylanthracene (9-PA), fluorescein (FL), and rhodamine
110 (R110), respectively. It has also been noticed that the rotational
diffusion of 9-PA is closer to the predictions of slip hydrodynamics,
whereas the rotation of negatively charged FL and positively charged
R110 is almost identical and follows stick hydrodynamics in these
ionic liquids. Despite having similar shape and size, ionic solutes
rotate slower by a factor of 3–4 compared to the nonpolar solute.
Interplay of specific and electrostatic interactions between FL and
the imidazolium cation of the ionic liquids, and between R110 and
the bisÂ(trifluoromethylsulfonyl)Âimide anion, appear to be responsible
for the observed behavior. These results are an indication that the
length of the alkyl chain on the imidazolium cation does not alter
their physical properties in a manner that has an effect on solute
rotation
Fluorescence Anisotropy of a Nonpolar Solute in 1‑Alkyl-3-Methylimidazolium-Based Ionic Liquids: Does the Organized Structure of the Ionic Liquid Influence Solute Rotation?
Temperature-dependent
fluorescence anisotropies of a nonpolar solute 9-phenylanthracene
(9-PA) have been measured in 1-alkyl-3-methylimidazolium-based ionic
liquids with anions such as bisÂ(trifluoromethylsulfonyl)Âimide ([Tf<sub>2</sub>N<sup>–</sup>]), trisÂ(pentafluoroethyl)trifluorophosphate
([FAP<sup>–</sup>]), tetrafluoroborate ([BF<sub>4</sub><sup>–</sup>]), and hexafluorophosphate
([PF<sub>6</sub><sup>–</sup>]) to find out if the organized structure of the ionic liquid has
a bearing on solute rotation. Analysis of the experimental data using
the Stokes–Einstein–Debye hydrodynamic theory indicates
that there is no significant variation in the solute–solvent
coupling constants (<i>C</i><sub>obs</sub>) with an increase
in the length of the alkyl chain on the imidazolium cation for the
ionic liquids with [Tf<sub>2</sub>N<sup>–</sup>] and [FAP<sup>–</sup>] anions. However, in the case of ionic liquids with
[BF<sub>4</sub><sup>–</sup>] and [PF<sub>6</sub><sup>–</sup>] anions, the rotation of 9-PA for a given viscosity at constant
temperature becomes progressively faster and <i>C</i><sub>obs</sub> decreases by a factor of 2.4 from ethyl to octyl derivatives.
Quasihydrodynamic theories of Gierer–Wirtz and Dote–Kivelson–Schwartz
could not account for the significant decrease in the <i>C</i><sub>obs</sub> values. The observed behavior has been rationalized
in terms of the organized structure of the ionic liquids having [BF<sub>4</sub><sup>–</sup>] and
[PF<sub>6</sub><sup>–</sup>] anions, which results as a consequence of the high charge-to-size
ratio of these anions compared to [Tf<sub>2</sub>N<sup>–</sup>] and [FAP<sup>–</sup>]
Effect of Alkyl Chain Length on the Rotational Diffusion of Nonpolar and Ionic Solutes in 1‑Alkyl-3-Methylimidazolium-bis(trifluoromethylsulfonyl)imides
Rotational diffusion of a nonpolar
solute 9-phenylanthracene (9-PA)
and a cationic solute rhodamine 110 (R110) has been examined in a
series of 1-alkyl-3-methylimidazolium (alkyl = octyl, decyl, dodecyl,
tetradecyl, hexadecyl, and octadecyl) bisÂ(trifluoromethylsulfonyl)Âimides
to understand the influence of alkyl chain length on solute rotation.
In this study, reorientation times (Ď„<sub>r</sub>) have been
measured as a function of viscosity (η) by varying the temperature
(<i>T</i>) of the solvents. These results have been analyzed
using the Stokes–Einstein–Debye (SED) hydrodynamic theory
along with the ones obtained for the same solutes in 1-alkyl-3-methylimidazolium
(alkyl = methyl, ethyl, propyl, butyl, and hexyl) bisÂ(trifluoromethylsulfonyl)Âimides
(Gangamallaiah and Dutt, <i>J. Phys. Chem. B</i> <b>2012,</b> <i>116,</i> 12819–12825). It has been noticed that
the data for 9-PA and R110 follows the relation τ<sub>r</sub> = <i>A</i>(η/<i>T</i>)<sup><i>n</i></sup> with <i>A</i> being the ratio of hydrodynamic volume
of the solute to the Boltzmann constant and <i>n</i> = 1
as envisaged by the SED theory. However, upon increasing the alkyl
chain length from methyl to octadecyl significant deviations from
the SED theory have been observed especially from the octyl derivative
onward. From methyl to octadecyl derivatives, the value of <i>A</i> decreases by a factor of 3 for both the solutes and <i>n</i> by a factor of 1.4 and 1.6 for 9-PA and R110, respectively.
These observations have been rationalized by taking into consideration
the organized structure of the ionic liquids, whose influence appears
to be pronounced when the number of carbon atoms in the alkyl chain
attached to the imidazolium cation exceeds eight