Glass Transition Behavior of the Quaternary Ammonium-Type Ionic Liquid : N , N-Diethyl-N-methyl-N-( 2-methoxyethyl ) ammonium Bromide-H 2 OMixtures

By a simple differential thermal analysis (DTA) system, the concentration dependence of the glass transition temperatures (Tgs) for the quaternary ammonium-type ionic liquid, N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bromide [DEME][Br] and H2O mixtures, after quick precooling was measured as a function of water concentration x (mol% H2O). We compared the results with the previous results of [DEME][I]-H2O and [DEME][BF4]-H2O mixtures in which a double-glass transition behavior was observed. Remarkably, the [DEME][Br]-H2O mixtures basically show one-Tg behavior and the Tg decreases monotonically with increasing H2O content up to around x = 91.5. But it suddenly jumps to higher Tg value at a specific x =∼ 92. At this very limited point, two Tgs (Tg1,Tg2) which we might consider as a transition state from the structure belonging to the Tg1 group to another one due to the Tg2 group were observed. These results clearly reflect the difference in the anionic effects among Br−, I−, and BF4 −. The end of the glass-formation region of [DEME][Br]-H2O mixtures is around x = 98.9 and moves to more water-rich region as compared to those of [DEME][BF4]-H2O (x = 96.0) and [DEME][I]-H2O (x = 95.0) mixtures.


Introduction
Room temperature ionic liquids (RTILs) are molten salts with melting temperature below (<373 K) and well known to have many attractive properties as solvents, for example, almost zero vapor pressure, wide electrochemical window, high recyclability, nonflammability, and so forth [1,2], so that RTILs stimulate the interests of a wide range of applications [3,4].Many of the attractive features require a thorough knowledge of their thermophysical properties.Interactions in RTILs are characterized by the presence of Coulomb interactions among the constituent ions.Hydrogen-bonding, van der Waals, and π-π interactions also take place in RTILs and affect their nature and the liquid structures.Thus, the liquid structures of RTILs are determined by a balance be-tween long-range electrostatic forces and local geometric factors.As a result, many RTILs can be easily supercooled and form the glassy state [5][6][7].Changes in the cation and anion combinations allow the physical and chemical properties of ionic liquids to be effectively tuned, for example, for manipulating the solvent properties of RTILs in order to achieve different purposes [7][8][9].
Although thermodynamic properties including T g have been so far investigated mainly on the imidazolium-based series of pure RTILs [10][11][12], we point out that investigations of glass transition behavior of the RTILs-H 2 O mixtures are still scarce.In previous work, we reported the glass transition behavior of the quaternary ammoniumtype ionic liquid, N,N-diethyl-N-methyl-N-( − and I − on the water structure.It is interesting to quote that the ionic radius of BF 4 − anion (229 pm [15,16]) is slightly larger than that of I − (216 pm [17]) anion.We suspect that the subtle difference in the ionic radii between BF 4 − and I − anions together with the anionic nature plays an important role in determining the regions where doubleglass transitions occur.
Then, what happens to the phase behavior of other quaternary ammonium-type ionic liquid, [DEME][Br]-H 2 O mixtures, in which the anionic radius of Br − (195 pm [18]) is smaller than that of I − anion?Does the double-glass transition phenomenon occur or not?The aim of the present paper is to show the glass transition behavior of  [DEME][Br] was synthesized following procedures reported in the literature [19].The synthetic route of this material is shown in Scheme 1.The schematic chemical structures of [DEME][Br] are shown in Figure 1.

DTA Measurements.
To detect the glass transition temperature, T g , a simple differential thermal analysis (DTA) system designed for quick cooling experiments was used.As a reference material for the measurements, we used benzene (Wako Pure Chemical Co.).A sample cell (about 35 mm long and 2 mm i.d.glass tube with one side sealed) was filled with a mixture, and then a thermocouple junction was placed 25∼30 mm from the mouth of the sample cell.As a precooling procedure, a vitrification was done by putting the whole sample solution directly into liquid nitrogen where the quenching rate was estimated to be about 500 K/min.After taking out the sample from the liquid nitrogen, the DTA traces were recorded.The detailed procedure is basically the same as the previously reported ones [13,14].T g values were found to be reproducible in this study to within 0.5 K, and the accuracy of temperature reading is estimated to be 1 K from a determination of melting temperatures of several guaranteed grade reagents (absolute ethanol, acetone and chloroform).

Raman Spectral Measurements.
Raman spectra were typically measured by a JASCO NRS-1000DT Raman spectrophotometer equipped with a single monochromator and a CCD detector at room temperature (298 K).The 533 nm from green laser (Showa Optronics Co., Ltd.) with a power of 100 mW was used as an excitation source.

Glass Transition Behavior of [DEME][Br]-H 2 O Mixed Solutions. Schematic DTA thermograms of [DEME]
[Br]-H 2 O mixed solutions at three typical concentrations (x = 65.0,92.0, 95.0) are shown in Figure 2. At x = 65.0, the T g was observed at 182 K.After the glass transition, we could not see the cold crystallization [20] with a large exothermal peak, which was observed in the case of [DEME][I]-H 2 O mixed solutions [14].Just at x = 92.0,we captured the intriguing result: the DTA trace gives a first glass transition followed by an exothermic crystallization-like peak at around  [14] are also shown.The T g value monotonically decreases with increasing x up to ∼ 91.5 mol%.But it suddenly jumps to higher T g value at x ≥ 91.5.At this very limited point (x = 91.5 ∼ 92.5), two T g s ( : T g1 , : T g2 ) were observed.As to an explanation for the mechanism of a double-glass transition

Second run T g2
Requenching to liquid nitrogen temperature phenomenon, the following was proposed [21][22][23].When we lower a temperature of the sample, the homogeneous solution splits into two phases due to its thermodynamic instability where a metastable liquid-liquid immiscibility occurs.Based on the idea, we consider that the [DEME][Br]-H 2 O mixed solution may separate into two phases from the original solution at x = 91.5 ∼ 92.5.It is important to quote that the T g of a glass-forming liquid has a correlation with its viscosity [24].Thus, the two-T g s behavior implies that the viscosities (and thus the structures) of two glassy phases are very different from each other.Looking into the results more closely, we may consider this as a transition state from the structure belonging to the T g1 group (x < 91.5) to another one due to the T g2 group (x > 92.5).At x ≥ 92.5 some inclusions of H 2 O ice crystals in the quenched samples were visually confirmed.However, the solutions keep showing the clear but small T g value up to around 98.9 mol% meaning that the system still holds the glassy state.We find that the edge of a glass-forming composition range extends to more water-rich region as compared to those of the   region, though the T g1 value decreases with increasing x.This behavior is similar to that in the aqueous solutions of "normal salt" such as symmetrical tetraalkyl ammonium halide (R 4 NX; R = Ethyl (Et), n-Propyl (n-Pr), and X = Cl − , Br − ) [21][22][23].

Comparison with the Glass Transition Behaviors of [DEME][I]-H 2 O and [DEME][BF
On the other hand, the two-T g s region moves to a very water-poor side in the case of [DEME][BF 4 ]-H 2 O solution and the concentration range of the double glass transitions is much wider (13.5 mol%).These clearly reflect the difference in the anionic effects on the water structure.Previously, we explained that the differences in the results between the [DEME][BF 4 ] mixtures and the [DEME][I] mixtures come from the variation in the solvation abilities and also the relative positions of the respective anions (BF 4 − and I − ) and [DEME] + cation in the RTILs [14].The interactions between water molecules and the anions are significantly different in the two RTILs.Considering the smaller ionic radius of the I − anion rather than BF 4 − anion, the anion-water interaction in the [DEME][I] solution should be stronger than that in the [DEME][BF 4 ] solution.This was partly evidenced by the existence of nearly free hydrogen-bonded Raman band (NFHB) of water molecules in the [DEME][BF 4 ]-H 2 O system, which will be described in the next section.NFHB is assigned to the water molecules which exist as single molecules (not self-associated state) without forming the hydrogen-bonding network among themselves as in pure liquid

Raman Spectra.
To look further into states of the water structure in the mixtures, we measured Raman spectra.A vibrational spectroscopy is highly sensitive to the intermolecular interactions including hydrogen bonding among water molecules.Firstly, Figure 5 On the contrary, peaks in the range from 3200 to 3800 cm −1 belong to the OH stretching vibrational mode of water molecules.Figure 5(b) shows the enlarged spectra in this region.In viewing the results, the difference in the Raman spectra are very distinct.As pointed out in a previous paper [25], the spectrum of [DEME][BF 4 ]-H 2 O typically displays a small peak at around 3565 cm −1 with a shoulder of 3620 cm −1 .This peak is assigned to the nearly free hydrogenbonded band (NFHB) of water molecules [26], as mentioned in the previous section.Here water molecules of the NFHB are probably very weakly interacting via H-bonding with the BF 4 − anions.There is no water molecules due from NFHB in the case where the anion is iodide.Instead, an intenisty of the Raman peak centered at ∼3440 cm −1 is appreciable.This band is mainly due to the OH stretching vibrations of water molecules weakly hydrogen bonded to halide ions [27], suggesting that the interactions between water molecules and the anions are significantly different in the two RTILs,  ) mixtures.Similar studies on other different RTILs with, for example, imidazolium-based cation-H 2 O systems, will deepen our understanding of the property of mixed solutions.Actually, we reported the glass transition temperatures of 1-butyl-3methylimidazolium tetrafluoroborate-H 2 O mixed solutions as a function of H 2 O concentration [28].Interestingly, in contrary to the results of quaternary ammonium-type ionic liquids, the multiple glass transition behavior was not observed and the system shows only one T g throughout the whole concentration range.

Figure 3 :
Figure 3: The requenching procedure for detecting the T g2 at x = 92.0 is shown.

Figure 4 :
Figure 4: T g variations as a function of x are shown.For comparison, T g data of [DEME][I]-H 2 O and [DEME][BF 4 ]-H 2 O mixed solutions are also shown.: T g1 , : T g2 , : T g .(only one glass transition was observed except for the region x = 91.5 ∼ 92.5).
H 2 O [14].Unfortunately nor the detailed liquid structure of [DEME][BF 4 ] or [DEME][I] has apparently not been repeted as yet.It is interesting to point out that the double-glass transition behavior is systematically changing from [DEME][BF 4 ]-H 2 O (two-T g s behavior in waterpoor region, x = 16.5∼30.0),[DEME][I]-H 2 O (two-T g s behavior in water-rich region, x = 77.5 ∼ 85.0), and finally to [DEME][Br]-H 2 O (two T g s at very limited concentrations, x = 91.5∼92.5) with decreasing the anionic radii.Clearly the solvation abilities of the respective anions provide significant effect on the structures of the RTILs-H 2 O mixtures which was reflected in the glass transition behavior.
(a) shows a comparison of the Raman spectra in the CH and OH stretching regions among [DEME][BF 4 ]-H 2 O, [DEME][I]-H 2 O, and [DEME][Br]-H 2 O at x = 65 in the liquid state.The signals arising from the CH stretching modes of the [DEME] cation appear ranging from 2800 to 3100 cm −1 , though unfortunately the precise assignments of the respective peaks are not available at present.The overall CH frequency of the solutions shifts to a lower frequency side on going from [DEME][BF 4 ]-H 2 O to [DEME][Br]-H 2 O with slight changes in the spectral shapes.This indicates that the environment around the alkyl chains of [DEME] cation is perturbed with the change of anion.
[DEME][BF 4 ] and [DEME][I] solutions; the anion-water interaction in the [DEME][I] solution is much stronger than that in the [DEME][BF 4 ] solution.In viewing the results, the results of [DEME][Br]-H 2 O show basically similar behavior to that of [DEME][I]-H 2 O.The important result obtained from the comparison

Figure 5 :
Figure 5: Comparison of the Raman spectra among [DEME][BF 4 ]-H 2 O, [DEME][I]-H 2 O, and [DEME][Br]-H 2 O at x =∼ 65 in the liquiud state.Peak intensities are normalized by the strongest peak of CH stretching vibrational mode of [DEME] + cation.(a) The CH and OH stretching vibrational modes of the solutions and (b) the OH stretching vibrational region.
DEME][I]-H 2 O [14] mixtures, after quick precooling.Remarkably, double-glass transitions were observed in both mixtures.But concentrations of the two-T g s region are completely different from each other; the doubleglass transitions for the [DEME][BF 4 ]-H 2 O mixtures were observed in the RTIL-rich region of x = 16.5 ∼ 30.0 (mol% H 2 O), whereas the region moves to a water-rich side of x = 77.5 ∼ 85.0 (mol% H 2 O) for the [DEME][I]-H 2 O mixtures.These clearly reflect the difference in the anionic effect between BF 4 2-methoxyethyl)ammonium tetrafluoroborate [DEME][BF 4 ]-H 2 O [13] and iodide [ [DEME][Br]-H 2 O mixtures and to compare the results with those of [DEME][BF 4 ]-H 2 O [13] and [DEME][I]-H 2 O [14] mixtures.Here we have measured the T g variation of [DEME][Br]-H 2 O mixtures as a function of H 2 O concentration x (mol% H 2 O).A limit of the glass-formation region is also determined.