Renda 2016
Renda 2016
Renda 2016
a r t i c l e i n f o a b s t r a c t
Article history: Acetic acid (HOAc) is a typical weak Brønsted acid that has been broadly used as a reactant in esterification reac-
Received 8 September 2015 tions and can be used as such in ionic liquid (IL) solutions. The nature of the HOAc molecule as a solute in the IL
Accepted 5 January 2016 1-ethyl-3-methylimidazolium acetate (EMIMOAc) solutions was investigated with thermodynamic, vibrational
Available online 15 February 2016
spectroscopic, conductivity, and viscosity measurements. Calorimetry and vapor pressure-based thermodynamic
measurements of the binary HOAc-EMIMOAc mixture were used to quantify the exothermic enthalpy of solution,
Keywords:
Acidic ionic liquids
ΔHsol, as ~3.2–4.9 kJ/mol. FTIR and Raman spectroscopy showed that the structures of the HOAc molecule and IL
Conductivity measurements molecular ions are unchanged in the solutions relative to the pure liquids, indicating that the HOAc is a negligibly-
Solution enthalpy dissociated weak acid in the IL solutions. Temperature-dependent conductivity measurements quantify the
Vapor pressure measurements effects of the HOAc on the IL solution conductivity and confirm that the HOAc remains a mostly neutral molecular
Vibrational spectroscopy solute that interacts with the EMIMOAc IL via ion-dipole and hydrogen bond interactions.
© 2015 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.molliq.2016.01.015
0167-7322/© 2015 Published by Elsevier B.V.
C.M. Renda et al. / Journal of Molecular Liquids 216 (2016) 710–715 711
also been used for chemical separations, e.g., extraction of HF from pe- the quantification of the AIL deviation from Raoult's Law using the
troleum products [31] or recovery of HOAc over water [32]. Optimizing following equations:
these applications will require a quantitative understanding of the
HOAc solvation and ionization mechanisms in ILs, including thermo-
dynamic solvation information and structural information about the P HOAc ¼ γ HOAc χ HOAc P HOAc : ð1Þ
HOAc ionization.
In this paper, we have characterized a series of EMIMOAc solutions
with increasing HOAc concentration using conductivity measurements In Eq. (1), PHOAC is the HOAc vapor pressure above the solution, χHAOc
to investigate the ion transport mechanism, vapor pressure and calo- is the molar fraction of HOAc in solution, and P°HAOc is the vapor pressure
rimetry measurements to study the HOAc-EMIMOAc solvation interac- over pure HOAc. γHAOc is the activity coefficient of HOAc in the AIL
tion, and vibrational spectroscopy to investigate the structure of the solution, which would be unity for an ideal solution (ΔHsol = 0).
solvent and solute. Considering that HOAc is an important reactant of Under the regular solution model [35] the value of ΔHsol is related to
the acid-catalyzed esterification reactions, a better understanding of the value of γHAOc through the β-parameter, which quantifies the net
the HOAc solvation and ionization in AILs will be helpful for the control HOAc-solvent interactions:
of acetylation reactions.
βχ 2solvent ¼ RT ln γ HAOc : ð2Þ
2. Experimental
ΔH sol ¼ βχ HOAc χ solvent : ð3Þ
2.1. Sample preparation
EMIMOAc (N 90%) and glacial acetic acid were purchased from In Eqs. (2) and (3), χsolvent is the molar fraction of the solvent, which
Sigma-Aldrich. EMIMOAc was de-colored by active carbon powder would be the EMIMOAc IL in this work. Hence, by measuring the vapor
and dried in a vacuum oven prior to use. Glacial acetic acid was pressure of HOAc over the AIL solutions, the ΔHsol can be calculated.
used as received. HOAc/EMIMOAc solutions were prepared by The HOAc vapor pressure was quantified using the “headspace” in-
carefully pipetting the two liquids with desired volume ratios. jection technique with a GC–MS system (Agilent 6890N GC, 5973 MS,
Molarities and molar fractions of HOAc were calculated based on and 7683 Injector). 0.5 mL of solutions (prepared with different HOAc:IL
the measured volume ratio and density values of pure HOAc and molar ratios) was sealed in 2 mL vials and shaken thoroughly for 10 min
EMIMOAc. The solutions were stored in capped containers for all to reach liquid–vapor equilibrium prior to the injection. 1 μL of the
experimental measurements. vapor in the head-space was injected (split, 10:1 ratio) into the column.
The oven temperature was ramped from 35 °C to 130 °C during the
2.2. Conductivity, viscosity, and thermodynamic measurements chromatography. Ions with m/z of 60 (HOAc molecular ion) were
extracted and chromatographic peak areas were used to calculate the
Conductivities of pure ILs and AIL solutions were measured by an HOAc vapor pressure.
AC Mode Traceable™ conductivity meter at a constant frequency of
3 kHz with a pair of parallel Pt plate electrodes and a temperature 2.3. Vibrational spectroscopy
probe. The cell constant is designed to be unity. The cell was calibrat-
ed by standard solutions with conductivity of 1 and 10 mS/cm, re- Fourier Transform Infrared (FTIR) spectra of all samples were mea-
spectively, prior to measurement. The conductivity was measured sured with a Varian FTS 7000 FTIR Spectrometer at 1 cm−1 resolution.
at room temperature, approximately 23 °C in our laboratory. For A thin layer of liquid samples was sandwiched between two KRS-5
temperature-dependent measurements, a water bath was used to salt plates. Samples were thoroughly purged by dry air before data ac-
control the temperature. The built-in temperature probe in the con- quisition to remove the absorbed moisture. Spectra demonstrated
ductivity meter allows the simultaneous reading of conductivity and were the average of 16 co-additions. Raman spectra were measured
temperature values. using a Horiba Jobin–Yvon LabRam Evolution Raman Microscope
The viscosities of pure ILs and AIL solutions were measured by a (Horiba, Edison, NJ) fitted with a liquid sample acquisition accessory.
Brookfield rotational viscometer with a built-in temperature probe. Samples were measured using either 532 nm or 633 nm excitation
All measurements were carried at ~ 23 °C. The instrument was cali- wavelengths. Fluorescence backgrounds were subtracted using the
brated by a mineral oil viscosity standard of 20 cP and 200 cP prior Labspec 6 software.
to use.
We measured the enthalpy of solution (ΔHsol) by two different
experiments. First, a standard “coffee-cup” calorimeter is used.
EMIMOAc and HOAc were mixed in a plastic container with volume
capacity of about 10 mL. The system was isolated with a rubber stop-
per and wrapped with an aluminum foil and polystyrene foam. The
temperature was monitored with time after HOAc/EMIMOAc mixing
by a thermocouple connected to a computer. We measured the
solution's specific heat capacity by calibrating the system with a
pre-heated copper slug. We also calibrated the systematic error
(loss of heat) using the dissolution of CaCl2 in H2O. With ~ 5–10 mL
of total liquid, it was estimated that there was a ~ 28% total heat
loss when the maximum temperature was reached within several
minutes [33]. The measured ΔHsol values were compensated for
this 28% heat loss. We also used this system to measure the heat ca-
pacity of the IL BMITFSI; the measured results were consistent with
the reported values in literature [34].
Second, ΔHsol can also be estimated by the HOAc vapor pressure Fig. 1. Temperature vs. time curve when HOAc and EMIMOAc are mixed at a 1:1 molar
values above the AIL solutions. Vapor pressure measurements enable ratio.
712 C.M. Renda et al. / Journal of Molecular Liquids 216 (2016) 710–715
which is not observed in the EMIMOAc spectrum, must correspond to and the mobility of these particles. The addition of HOAc molecules to
the carboxylic acid CO vibration. It is clearly broader in the AIL spectrum, the IL may affect the solution conductivity in several different ways.
which likely indicates hydrogen bond interactions between the −OH Once the solute–solvent cluster is formed, the HOAc solvated by IL mo-
group of the HOAc molecule and the IL molecular anion, OAc−. Howev- lecular ions may behave as a single charge carrier in the electric field.
er, the appearance of this band certainly indicates that the HOAc re- Because the size of the solute–solvent cluster would be larger than ei-
mains mostly undissociated in the AIL solution. In other words, the ther the cation or the anion alone, it would have a lower electrical mo-
dissociation of HOAc, if any, is not detectable by the Raman spectra. bility and hence diminished solution conductivity. On the other hand,
Hence, the Raman spectra show that the HOAc molecule and EMIMOAc the HOAc may also perturb the ion nanoclusters in ILs [43–44] and
ions are structurally unchanged in the AIL solution and HOAc remains a help release more “free” solvent ions as charge carriers, which would
weak acid, if it is acidic, since the HOAc ionization is not detected. enhance the solution conductivity [45]. Likely, the latter effect is domi-
Conductivity and viscosity measurements of ILs and AIL solutions nant in this system. Also, the increased solution conductivity with mo-
can provide insight into their collective transport properties [39]. Solu- lecular solutes in IL solutions has been correlated to the decrease of
tion conductivities are affected by the concentration of each component viscosity because of the “diluting” effect of the molecular solutes [46].
and their intermolecular interactions. Previous reports have indicated The measured viscosity values of pure EMIOAc and solutions are
that IL solution conductivities decrease with increasing concentrations shown in Fig. 4A; the results are consistent with the current discussion.
of ionic solutes such as Li+ and Na+ salts [33,40]. With strong acids To further characterize the HOAc effect on the EMIOAc solution
(strong electrolytes), increasing the acid concentration to create an conductivity at a molecular level, we measured the temperature-
AIL solution results in increased conductivity at lower concentrations dependence of the solution conductivities between 30 to 70 °C.
but decreased conductivity at higher concentrations due to reduced Arrhenius plots of the data are shown in Fig. 4B. At all temperatures,
acid dissociation (i.e., protonation) [15,18]. However, when a non- the conductivity increases with increasing HOAc molar fraction from
ionic molecular compound (nonelectrolyte or very weak acid) such as 0 to 0.5. The Arrhenius plots are not linear, and are therefore fitted
acetonitrile, methanol, or water is mixed with ILs, the solution conduc- with the Vogel–Fulcher–Tamman (VFT) equation:
tivity increases until the molecular solute becomes the majority compo-
nent [16,33,41–42]. A universal theory about the effects on conductivity ‐B
of ionic and molecular solutes has not been established due to the large σ ¼ σ 0e T‐T0
ð4Þ
diversity of the solute structures. This trend is valid empirically. Conduc-
tivity is tightly related to viscosity. In the cases mentioned above, the where σ0, B, and T0 are constants which have been described in the
viscosity changed to the opposite direction of conductivity, as described free-volume theory [47–48]. In this theory, σ0 is the infinite concen-
by Walden's rule. In general, the conductivity and viscosity of IL solu- tration conductivity (proportional of the number of charge carriers),
tions are both determined by the hindered drifting of ions driven by B has a temperature unit and is related to the activation energy of
an electric field or mechanical shear. In this regard, the Stoke–Einstein charge carrier's motion, and T0 is the “ideal” glass transition temper-
Equation is approximately valid. When ions associate together forming ature of the liquid, which is usually 30–50° below the measured
larger ion clusters (larger hydrodynamic radius) the viscosity increases glass transition temperature (Tg). Their values vary in different liq-
and the conductivity decreases. uids and solutions.
We present the conductivity and viscosity measurements for the The values of σ0, B, and T0 were obtained by fitting the experimental
HOAc/EMIMOAc AIL solutions in Fig. 4. For all conductivity and viscosity data with Eq. (4) and are summarized in Table 1. The number of charge
measurements, we estimate the errors as ~± 5% based on repeated carriers in the AIL solutions (σ0) increases with increasing HOAc con-
measurements of selected solutions. When HOAc is dissolved in centration. As previously mentioned this may be the result of HOAc
EMIMOAc, the conductivity increases significantly from ~2.5 mS/cm in molecules disturbing the ion pairs or ion clusters in the pure IL and
pure EMIMOAc to ~11 mS/cm in the 6.0 M HOAc (χHOAc = 0.60) AIL so- “freeing” the ions by dilution. Consequently, the apparent number of
lution (Fig. 4A). Based on the empirical conclusion mentioned above charge carriers increased. From Table 1, adding the HOAc to create the
that electrically neutral molecular solutes increase the IL conductivity, AIL solutions also reduces the glass transition temperature and increases
this conductivity result may support the conclusion from spectroscopic the apparent charge transport activation energy.
measurements that HOAc does not ionize in the AIL solution. HOAc may Overall, the changes in the VFT parameters help verify the con-
behave as a non-ionic molecular solute or at most as a very weak acid. In clusions regarding the nature of HOAc in the EMIMOAc solution and
classic electrolyte theory, the conductivity of a liquid solution depends provide insight into the microscopic solvation. The HOAc molecule re-
on the number density (concentration) of the charge carriers (ions) mains mostly non-ionized but may disrupt the clusters of IL molecular
Fig. 4. A) AIL solution conductivity (■) and viscosity (□) as a function of HOAc molarity at room temperature, 23 °C; (B) Arrhenius curves (conductivity vs. temperature). The errors in
conductivity and viscosity values are estimated at ~±5%.
714 C.M. Renda et al. / Journal of Molecular Liquids 216 (2016) 710–715
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This work is supported by NSF grants CHE-1362493 and MRI- [emim][acetate], [emim][trifluoroacetate], and [emim][acetate] + [emim]
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from ACS Project SEED program administered by the ACS South Jersey org/10.1021/je800701j.
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