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Anomalous phase behavior of excess iodide in room-temperature ionic liquid: 1-methyl-3-propylimidazolium iodide
Chemical Physics 502 (2018) 72–76
Contents lists available at ScienceDirect
Chemical Physics journal homepage: www.elsevier.com/locate/chemphys
Anomalous phase behavior of excess iodide in room-temperature ionic liquid: 1-methyl-3-propylimidazolium iodide Hiroshi Abe ⇑, Hiroaki Kishimura, Masami Aono Department of Materials Science and Engineering, National Defense Academy, Yokosuka 239-8686, Japan
a r t i c l e
i n f o
Article history: Received 21 September 2017 In final form 15 January 2018 Available online 3 February 2018 Keywords: Polyiodides Room-temperature ionic liquids 7 mol% anomaly Excess iodides Desorption process
a b s t r a c t Phase diagrams of room-temperature ionic liquid (RTIL), polyiodides, were obtained by simultaneous Xray diffraction and differential scanning calorimetry measurements. The original RTIL is 1-methyl-3propylimidazolium iodide, [C3mim][I]. By adding iodine to [C3mim][I], polyiodides were formed in the mixtures, which are expressed as [C3mim][Im]. At both low temperature and high pressure, I 3 is found to be a crystal forming factor (Abe et al., 2017). Upon cooling, an amorphous phase appeared at around m = 3.66. The mixture [C3mim][I3.66], as a non-stoichiometric system containing excess iodide, was redefined as [C3mim][I3] – 7.1 mol% I2, assuming that I3 is an anion. The desorption process of polyiodides in the mixture was measured under vacuum. A relatively long desorption time was observed due to ionic interactions. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Polyiodides, Im, have geometrical varieties both in liquid and solid states [1]. Branched polyiodides were simulated by density functional theory (DFT) [2,3]. Not only the molecular shapes but also the intra-bonding distances differ depending on the sites of the polyiodides [3]. In industrial applications, the polyiodides are assembled in dye-sensitized solar cells (DSSCs), which can achieve high conversion efficiency. Recently, room-temperature ionic liquids (RTILs) were utilized in DSSCs as electrochemically stable electrolytes [4–10]. Hydrophilic 1-alkyl-3-methylimidazolium iodides, [Cnmim][I], were selected as solvent-free electrolytes for DSSCs, where n is the alkyl chain length [8,11]. By first-principle calculations, the transport of polyiodides was examined in a C2mim+ cation-mediated environment [12]. Possible mass transfers of iodide ions in the RTILs were considered to be the indirect drive-one (Grotthuss exchange mechanism) [11,12]. In fact, high molar conductivities in the [C3mim][I]/[C3mim][I3] mixtures were observed [13], and the conducting process could be explained by the Grotthuss exchange mechanism. Recently, in 127I-NMR (nuclear magnetic resonance) experiments, peak splitting of [Cnmim][Im] occurred depending on m [14]. For large m, doublet and triplet peak splitting implied that the local environments of iodides were modified by the Grotthuss exchange mechanism. At low temperature (LT) and ambient pressure, a phase diagram of the ⇑ Corresponding author. E-mail address: [email protected] (H. Abe). https://doi.org/10.1016/j.chemphys.2018.01.013 0301-0104/Ó 2018 Elsevier B.V. All rights reserved.
[C3mim][Im] system was obtained by differential scanning calorimetry (DSC) [15,16]. Pure [C3mim][I] amorphized upon cooling, and cold crystallization was suppressed by heating. At 2.7 m, crystal (C) and liquid crystal (LC) phases subsequently occurred upon heating. Furthermore, at room temperature, high pressure (HP) phases of the [C3mim][Im] system were investigated, compared with LT ones [17]. Both at LT and HP, I is found to promote amorphization, while I 3 contributes to crystallization. Thus, I3 is regarded as an anion that promotes stable crystallization. Moreover, complicated LT and HP phase changes were observed in non-stoichiometric [C3mim][I3.66]. Excess iodide caused additional fluctuations in the mixtures. Desorption was measured in a mixed system to estimate molecular interactions [18,19]. Assuming that RTILs having nearly zero vapor pressure could not evaporate even under vacuum, the molecular interactions between the RTILs and additives were estimated. Since the evaporation time in the pure systems was proportional to the boiling point [19], the observed time under vacuum could reflect the molecular interactions. Moreover, in the mixed systems, the propanol isomer effect was clearly obtained by the desorption experiments [19]. The geometrical factor and hydrophobicity [20,21] of the propanol isomers in the RTILs were distinguished on the desorption time. Therefore, the desorption experiment under vacuum is one of methods to evaluate the molecular interactions in the RTILs. In this study, at ambient pressure, we obtained LT phase diagrams of [C3mim][Im] (or [C3mim][I3]-y mol% I2) via simultaneous X-ray diffraction and DSC measurements. At around y = 7 mol% I2,
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an amorphous (A) phase appeared discretely upon cooling. The desorption of [C3mim][Im] was also examined under vacuum. Below m = 1.5, the desorption rate decreased. 2. Materials and methods The RTILs used in this study were hydrophilic [C3mim][I], and N, N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate, [DEME][BF4], (Kanto Chemical Co.). Iodine (99.8%), CH3OH (99.8%), distilled H2O (99.8%) (Kanto Chemical Co.), and distilled D2O (99.9%) (Cambridge Isotope Laboratories, Inc.) were used as additives. In [C3mim][Im], mixtures with iodine were dark brown despite pure [C3mim][I] being transparent (Fig. 1). The mixtures were prepared inside a glove box through which helium gas was passed, and the relative humidity was suppressed below 10% using silica gel. To determine the LT phases, simultaneous X-ray diffraction and DSC measurements were carried out at ambient pressure using SmartLab and a 1D detector (Rigaku Co., Japan) [22–25]. The wavelength of the incident X-ray was k = 1.542 Å (Cu Ka radiation). The temperature range was 100 to 30 °C. The cooling and heating rates were fixed at 5 °C/min. The desorption process was examined by monitoring the weight loss under vacuum [19]. The temperature was fixed at room temperature. The weight of the mixtures was monitored with an electric balance (TW223N, Shimadzu Co.) inside a vacuum-type dry box. The mass of the mixtures was converted to mol%, that is, [DEME][BF4] – x mol% H2O. In [C3mim][Im], the mass is converted to the m value. We achieved 400 Pa from ambient pressure (0.1 MPa) using a rotary pump. The vacuum was monitored with a Pirani gauge (VG-10, Tokyo Electronics Co.) and a digital multimeter (2000, Keithley Instruments Co.).
Fig. 2. Phase diagrams of [C3mim][I3] – y mol% I2 (see text) upon (a) cooling and (b) heating. L, scL, C, and A denote liquid, supercooled liquid, crystal, and amorphous phases, respectively. C’ phase reveals coexistence of C + (L or A).
3. Results and discussion 3.1. Phase behavior of [C3mim][I3] – y mol% I2 We used mol%, i.e., [C3mim][I] – x mol% I2, to express the addi tive concentration. To distinguish between I anions, we 3 and I introduced the new notation, [C3mim][I3] – y mol% I2. The relation between x and y is y = x–50, since [C3mim][I3] is equivalent to [C3mim][I]-50 mol% I2. Fig. 2(a) and (b) reveal phase diagrams of [C3mim][I3] – y mol% I2 upon cooling and heating, respectively. The iodine concentration, y, is recalculated by assuming I 3 anions. The relation between m and x is expressed as,
y¼
m1 100 50 mþ1
ð1Þ
In the LT phase diagrams, an amorphous (A) phase appeared below m = 2.9 as reported previously [15,16]. Due to the influence of the crystallization factor, I 3 , the liquid (L) to crystal (C) phase transition occurred above m = 3.0. The C phase existed at 0 y 2.5 mol%. Above 2.5 mol%, C phase and non-crystal phase (L or A) coexisted.
This is because broad halo pattern of non-crystal phase was observed even below crystallization temperature. Here, by X-ray diffraction, the liquid or amorphous state is not distinguishable. Then, coexistence of C phase and non-crystal phase is denoted by C’ phase (Fig. 2(a)). As the iodine concentration increases, the A phase exists at around y = 7 mol% upon cooling (Fig. 2(a)). According to Equation (1), non-stoichiometric [C3mim][I3.66] is equivalent to [C3mim][I3] – 7.1 mol% I2. The amorphization region is quite narrow at 7 mol%. Interestingly, the C’ phase appeared above 8 mol%. It should be noticed that the A phase occurs at around 7 mol% between two C’ phases, and the concentration region is quite narrow. Focusing on the heating process, a more complex phase diagram was obtained by simultaneous measurements (Fig. 2(b)). Below m = 2.6, a simple glass transition without cold crystallization was detected in the same manner with pure [C3mim][I] [15]. In contrast, at 2.6 m, cold crystallization occurred from the supercooled liquid (scL) upon heating (Fig. 2(b)). More importantly, multiple melting was observed above m = 3. The multiple melting implies that multiple steps of cold crystallization occurred at 0 < y < 6 mol
Fig. 1. Samples of [C3mim][Im] system. The samples are dark brown except for pure [C3mim][I].
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% I2. However, multiple melting was suppressed at around 7 mol%, corresponding to the A phase upon cooling. The simple melting indicates that a single step of cold crystallization occurred in the narrow region of the A phase. Above 8 mol%, multiple steps of melting appeared accompanied by the C’ phase upon cooling. Upon both cooling and heating, the appearance of the discrete A phase at 6 < y < 8 mol% I2 was similar to that of [DEME][BF4]-water system [22,23]; the common features are amorphization upon cooling and cold crystallization upon heating in [DEME][BF4] – 7 mol% H2O. The 7 mol% anomalies were interpreted by fluctuations of the water additive between the DEME+ cation and BF 4 anion (Fig. 3) [26]. Another similarity of H2O and I2 additives is that mass transfer of proton [27,28] and iodide [11] is explained by the Grotthuss exchange mechanism. Furthermore, the anomalies in [DEME] [BF4] – 7 mol% H2O disappeared with D substitutions at a fixed water concentration [29]. It is proved that weak hydrogen bonding of H contributes to A phase formation upon cooling. A possible explanation for the discrete anomalies in the [C3mim][I3] – 7 mol % I2 mixture is that I2 additives could fluctuate extensively between cations and anions (Fig. 3) [17]. Here we interpret that the HP anomaly at 7 mol% I2 [17] is derived from additional fluctuation of the excess iodide.
Table 1 Normalized evaporation time, se, of pure H2O, D2O, and CH3OH.
se (h/mol)
H2O
D2O
CH3OH
13.6
15.0
7.2
3.2. Desorption of [DEME][BF4]-based mixtures Desorption in RTIL-based mixtures involves molecular interactions on the surface. Under vacuum, the evaporation of pure H2O, D2O, and CH3OH as additives in the mixtures was examined [19]. Since iodine is in the solid state at room temperature, it was impossible quantitatively to estimate the evaporation time of iodine. To evaluate the evaporation time, te, in a pure system, we introduce the normalized evaporation time, se [19], which is given by,
se ¼ te =ðma =Ma Þ;
ð1Þ
where Ma is the additive molecular weight and ma is the initial mass of a pure additive. The se values of pure H2O, D2O, and CH3OH are listed in Table 1. Proportionally to boiling points, the se of pure D2O is longer than that of H2O. To compare with the desorption of polyiodide in [C3mim][Im], we carried out vacuum drying using [DEME][BF4]-based mixtures as non-iodide systems. Here, we assume that the RTILs could not evaporate from the mixtures even under vacuum. Therefore, the mass of the IL, mIL, is constant during vacuum drying. Fig. 4 reveals the normalized desorption time, sd, of [DEME][BF4]-based mixtures. By using the observed desorption time, td, sd is also defined as,
sd ¼ td =ðmIL =MIL Þ
ð2Þ
Desorption using the normalized sd is shown in Fig. 4. The time dependence of the additive concentrations qualitatively describes
Fig. 4. Desorption under vacuum of [DEME][BF4]-based mixtures; sd is the normalized desorption time, which is divided by the number of moles of IL. Blue, red, and green curves indicate the time dependences of the concentrations of CH3OH, H2O, and D2O additives, respectively.
the molecular interactions. The shortest sd of the [DEME][BF4]-CH3OH system indicates a weak interaction between [DEME][BF4] and the CH3OH additive on the liquid surface. On the other hand, a relatively long sd was observed in water-based mixtures. The hydrogen bonding of water is the main contributor to the long desorption time. Another reason for this is that water molecules are captured in two sites as shown in Fig. 3: the oxygen of the DEME+ cation and the fluorine of the BF 4 anion. More importantly, the D2O desorption time in the mixture is longer than that of H2O. This result is consistent with the evaporation time of pure H2O and pure D2O (Table 1) [19]. Generally, in bulk liquids, it is well known that the hydrogen bonding strength of D is stronger than that of H. 3.3. Desorption of [C3mim][Im] The desorption of [C3mim][Im] is expressed by m as shown in Fig. 5. For the normalized desorption time, sd, the I2 additive is less desorbed compared with [DEME][BE4]-based systems. Here, we emphasize that, in both systems, there are two additive adsorption sites as shown in Fig. 3. The longer sd of [C3mim][Im] indicates that
Fig. 3. Molecular interactions of [DEME][BF4] – H2O and [C3mim][I3] – I2 systems.
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Yoshimura of National Defense Academy for the helpful discussions. This work is supported by a grant of Shinki Yosan (NDA). References
Fig. 5. Desorption of [C3mim][I]-polyiodide system. A relatively long desorption time, sd, was observed.
Im exists in the mixture and that the coulombic interaction between C3mim+ and I m is consistent with the longer sd. The coulombic interaction prevents dissociation of polyiodides near the liquid surface. Consequently, the different sd values of the two systems are derived from different types of molecular interactions: the hydrogen bonding of the water additive is dominant or ionic interaction of polyiodides is formed by iodine additives. The desorption rate changed below m = 1.5 (x = 20 mol% I2). At the early stage of desorption, it is predicted that large polyiodides easily dissociate to neutral I2 near the liquid surface. One reasons for this is that the coulombic interaction is inversely proportional to the size of the polyiodides. In larger polyiodides, the surface charge density decreases, satisfying the negative (1) charge. Also, the non-equivalent bonding distances of the polyiodides cause asymmetric distributions of their surface charge density. Loosely bonded parts of polyiodides near C3mim+ cations could easily dissociate to form neutral I2. At the later stage of desorption, the I 3 anion as a crystallization factor might prevent dissociation and desorption near the liquid surface. Below m = 3, it is considered that only I and I 3 anions exist in the mixtures. Since both anions are strongly coupled with the C3mim+ cation, dissociation of I 3 and desorption of I2 are suppressed.
4. Conclusions Phase anomalies of non-stoichiometric [C3mim][I3.66] (or [C3mim][I3] – 7.1 mol% I2) were clarified in phase diagrams both upon cooling and heating. The stoichiometric [C3mim][I3] (y = 0 mol% I2) crystallized upon cooling, while crystallization of the 7 mol% mixture was suppressed upon cooling. It is indicated that the I 3 anion is regarded as a crystallization factor. Compared with the 7 mol% anomalies of the [DEME][BF4] – H2O system, the specific fluctuations of excess iodine between the C3mim+ cation and I 3 anion disturb the crystal nucleation of [C3mim][I3]. The 7 mol% anomaly of [C3mim][Im] at LT is related to non-stoichiometric anomalies under HP. The long desorption time of [C3mim][Im] indicates that Im anions exist in the mixture.
Acknowledgments We thank Dr. S. Tsuzuki of National Institute of Advanced Industrial Science and Technology, Dr. T. Takekiyo and Prof. Y.
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Contents lists available at ScienceDirect
Chemical Physics journal homepage: www.elsevier.com/locate/chemphys
Anomalous phase behavior of excess iodide in room-temperature ionic liquid: 1-methyl-3-propylimidazolium iodide Hiroshi Abe ⇑, Hiroaki Kishimura, Masami Aono Department of Materials Science and Engineering, National Defense Academy, Yokosuka 239-8686, Japan
a r t i c l e
i n f o
Article history: Received 21 September 2017 In final form 15 January 2018 Available online 3 February 2018 Keywords: Polyiodides Room-temperature ionic liquids 7 mol% anomaly Excess iodides Desorption process
a b s t r a c t Phase diagrams of room-temperature ionic liquid (RTIL), polyiodides, were obtained by simultaneous Xray diffraction and differential scanning calorimetry measurements. The original RTIL is 1-methyl-3propylimidazolium iodide, [C3mim][I]. By adding iodine to [C3mim][I], polyiodides were formed in the mixtures, which are expressed as [C3mim][Im]. At both low temperature and high pressure, I 3 is found to be a crystal forming factor (Abe et al., 2017). Upon cooling, an amorphous phase appeared at around m = 3.66. The mixture [C3mim][I3.66], as a non-stoichiometric system containing excess iodide, was redefined as [C3mim][I3] – 7.1 mol% I2, assuming that I3 is an anion. The desorption process of polyiodides in the mixture was measured under vacuum. A relatively long desorption time was observed due to ionic interactions. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Polyiodides, Im, have geometrical varieties both in liquid and solid states [1]. Branched polyiodides were simulated by density functional theory (DFT) [2,3]. Not only the molecular shapes but also the intra-bonding distances differ depending on the sites of the polyiodides [3]. In industrial applications, the polyiodides are assembled in dye-sensitized solar cells (DSSCs), which can achieve high conversion efficiency. Recently, room-temperature ionic liquids (RTILs) were utilized in DSSCs as electrochemically stable electrolytes [4–10]. Hydrophilic 1-alkyl-3-methylimidazolium iodides, [Cnmim][I], were selected as solvent-free electrolytes for DSSCs, where n is the alkyl chain length [8,11]. By first-principle calculations, the transport of polyiodides was examined in a C2mim+ cation-mediated environment [12]. Possible mass transfers of iodide ions in the RTILs were considered to be the indirect drive-one (Grotthuss exchange mechanism) [11,12]. In fact, high molar conductivities in the [C3mim][I]/[C3mim][I3] mixtures were observed [13], and the conducting process could be explained by the Grotthuss exchange mechanism. Recently, in 127I-NMR (nuclear magnetic resonance) experiments, peak splitting of [Cnmim][Im] occurred depending on m [14]. For large m, doublet and triplet peak splitting implied that the local environments of iodides were modified by the Grotthuss exchange mechanism. At low temperature (LT) and ambient pressure, a phase diagram of the ⇑ Corresponding author. E-mail address: [email protected] (H. Abe). https://doi.org/10.1016/j.chemphys.2018.01.013 0301-0104/Ó 2018 Elsevier B.V. All rights reserved.
[C3mim][Im] system was obtained by differential scanning calorimetry (DSC) [15,16]. Pure [C3mim][I] amorphized upon cooling, and cold crystallization was suppressed by heating. At 2.7 m, crystal (C) and liquid crystal (LC) phases subsequently occurred upon heating. Furthermore, at room temperature, high pressure (HP) phases of the [C3mim][Im] system were investigated, compared with LT ones [17]. Both at LT and HP, I is found to promote amorphization, while I 3 contributes to crystallization. Thus, I3 is regarded as an anion that promotes stable crystallization. Moreover, complicated LT and HP phase changes were observed in non-stoichiometric [C3mim][I3.66]. Excess iodide caused additional fluctuations in the mixtures. Desorption was measured in a mixed system to estimate molecular interactions [18,19]. Assuming that RTILs having nearly zero vapor pressure could not evaporate even under vacuum, the molecular interactions between the RTILs and additives were estimated. Since the evaporation time in the pure systems was proportional to the boiling point [19], the observed time under vacuum could reflect the molecular interactions. Moreover, in the mixed systems, the propanol isomer effect was clearly obtained by the desorption experiments [19]. The geometrical factor and hydrophobicity [20,21] of the propanol isomers in the RTILs were distinguished on the desorption time. Therefore, the desorption experiment under vacuum is one of methods to evaluate the molecular interactions in the RTILs. In this study, at ambient pressure, we obtained LT phase diagrams of [C3mim][Im] (or [C3mim][I3]-y mol% I2) via simultaneous X-ray diffraction and DSC measurements. At around y = 7 mol% I2,
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an amorphous (A) phase appeared discretely upon cooling. The desorption of [C3mim][Im] was also examined under vacuum. Below m = 1.5, the desorption rate decreased. 2. Materials and methods The RTILs used in this study were hydrophilic [C3mim][I], and N, N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate, [DEME][BF4], (Kanto Chemical Co.). Iodine (99.8%), CH3OH (99.8%), distilled H2O (99.8%) (Kanto Chemical Co.), and distilled D2O (99.9%) (Cambridge Isotope Laboratories, Inc.) were used as additives. In [C3mim][Im], mixtures with iodine were dark brown despite pure [C3mim][I] being transparent (Fig. 1). The mixtures were prepared inside a glove box through which helium gas was passed, and the relative humidity was suppressed below 10% using silica gel. To determine the LT phases, simultaneous X-ray diffraction and DSC measurements were carried out at ambient pressure using SmartLab and a 1D detector (Rigaku Co., Japan) [22–25]. The wavelength of the incident X-ray was k = 1.542 Å (Cu Ka radiation). The temperature range was 100 to 30 °C. The cooling and heating rates were fixed at 5 °C/min. The desorption process was examined by monitoring the weight loss under vacuum [19]. The temperature was fixed at room temperature. The weight of the mixtures was monitored with an electric balance (TW223N, Shimadzu Co.) inside a vacuum-type dry box. The mass of the mixtures was converted to mol%, that is, [DEME][BF4] – x mol% H2O. In [C3mim][Im], the mass is converted to the m value. We achieved 400 Pa from ambient pressure (0.1 MPa) using a rotary pump. The vacuum was monitored with a Pirani gauge (VG-10, Tokyo Electronics Co.) and a digital multimeter (2000, Keithley Instruments Co.).
Fig. 2. Phase diagrams of [C3mim][I3] – y mol% I2 (see text) upon (a) cooling and (b) heating. L, scL, C, and A denote liquid, supercooled liquid, crystal, and amorphous phases, respectively. C’ phase reveals coexistence of C + (L or A).
3. Results and discussion 3.1. Phase behavior of [C3mim][I3] – y mol% I2 We used mol%, i.e., [C3mim][I] – x mol% I2, to express the addi tive concentration. To distinguish between I anions, we 3 and I introduced the new notation, [C3mim][I3] – y mol% I2. The relation between x and y is y = x–50, since [C3mim][I3] is equivalent to [C3mim][I]-50 mol% I2. Fig. 2(a) and (b) reveal phase diagrams of [C3mim][I3] – y mol% I2 upon cooling and heating, respectively. The iodine concentration, y, is recalculated by assuming I 3 anions. The relation between m and x is expressed as,
y¼
m1 100 50 mþ1
ð1Þ
In the LT phase diagrams, an amorphous (A) phase appeared below m = 2.9 as reported previously [15,16]. Due to the influence of the crystallization factor, I 3 , the liquid (L) to crystal (C) phase transition occurred above m = 3.0. The C phase existed at 0 y 2.5 mol%. Above 2.5 mol%, C phase and non-crystal phase (L or A) coexisted.
This is because broad halo pattern of non-crystal phase was observed even below crystallization temperature. Here, by X-ray diffraction, the liquid or amorphous state is not distinguishable. Then, coexistence of C phase and non-crystal phase is denoted by C’ phase (Fig. 2(a)). As the iodine concentration increases, the A phase exists at around y = 7 mol% upon cooling (Fig. 2(a)). According to Equation (1), non-stoichiometric [C3mim][I3.66] is equivalent to [C3mim][I3] – 7.1 mol% I2. The amorphization region is quite narrow at 7 mol%. Interestingly, the C’ phase appeared above 8 mol%. It should be noticed that the A phase occurs at around 7 mol% between two C’ phases, and the concentration region is quite narrow. Focusing on the heating process, a more complex phase diagram was obtained by simultaneous measurements (Fig. 2(b)). Below m = 2.6, a simple glass transition without cold crystallization was detected in the same manner with pure [C3mim][I] [15]. In contrast, at 2.6 m, cold crystallization occurred from the supercooled liquid (scL) upon heating (Fig. 2(b)). More importantly, multiple melting was observed above m = 3. The multiple melting implies that multiple steps of cold crystallization occurred at 0 < y < 6 mol
Fig. 1. Samples of [C3mim][Im] system. The samples are dark brown except for pure [C3mim][I].
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% I2. However, multiple melting was suppressed at around 7 mol%, corresponding to the A phase upon cooling. The simple melting indicates that a single step of cold crystallization occurred in the narrow region of the A phase. Above 8 mol%, multiple steps of melting appeared accompanied by the C’ phase upon cooling. Upon both cooling and heating, the appearance of the discrete A phase at 6 < y < 8 mol% I2 was similar to that of [DEME][BF4]-water system [22,23]; the common features are amorphization upon cooling and cold crystallization upon heating in [DEME][BF4] – 7 mol% H2O. The 7 mol% anomalies were interpreted by fluctuations of the water additive between the DEME+ cation and BF 4 anion (Fig. 3) [26]. Another similarity of H2O and I2 additives is that mass transfer of proton [27,28] and iodide [11] is explained by the Grotthuss exchange mechanism. Furthermore, the anomalies in [DEME] [BF4] – 7 mol% H2O disappeared with D substitutions at a fixed water concentration [29]. It is proved that weak hydrogen bonding of H contributes to A phase formation upon cooling. A possible explanation for the discrete anomalies in the [C3mim][I3] – 7 mol % I2 mixture is that I2 additives could fluctuate extensively between cations and anions (Fig. 3) [17]. Here we interpret that the HP anomaly at 7 mol% I2 [17] is derived from additional fluctuation of the excess iodide.
Table 1 Normalized evaporation time, se, of pure H2O, D2O, and CH3OH.
se (h/mol)
H2O
D2O
CH3OH
13.6
15.0
7.2
3.2. Desorption of [DEME][BF4]-based mixtures Desorption in RTIL-based mixtures involves molecular interactions on the surface. Under vacuum, the evaporation of pure H2O, D2O, and CH3OH as additives in the mixtures was examined [19]. Since iodine is in the solid state at room temperature, it was impossible quantitatively to estimate the evaporation time of iodine. To evaluate the evaporation time, te, in a pure system, we introduce the normalized evaporation time, se [19], which is given by,
se ¼ te =ðma =Ma Þ;
ð1Þ
where Ma is the additive molecular weight and ma is the initial mass of a pure additive. The se values of pure H2O, D2O, and CH3OH are listed in Table 1. Proportionally to boiling points, the se of pure D2O is longer than that of H2O. To compare with the desorption of polyiodide in [C3mim][Im], we carried out vacuum drying using [DEME][BF4]-based mixtures as non-iodide systems. Here, we assume that the RTILs could not evaporate from the mixtures even under vacuum. Therefore, the mass of the IL, mIL, is constant during vacuum drying. Fig. 4 reveals the normalized desorption time, sd, of [DEME][BF4]-based mixtures. By using the observed desorption time, td, sd is also defined as,
sd ¼ td =ðmIL =MIL Þ
ð2Þ
Desorption using the normalized sd is shown in Fig. 4. The time dependence of the additive concentrations qualitatively describes
Fig. 4. Desorption under vacuum of [DEME][BF4]-based mixtures; sd is the normalized desorption time, which is divided by the number of moles of IL. Blue, red, and green curves indicate the time dependences of the concentrations of CH3OH, H2O, and D2O additives, respectively.
the molecular interactions. The shortest sd of the [DEME][BF4]-CH3OH system indicates a weak interaction between [DEME][BF4] and the CH3OH additive on the liquid surface. On the other hand, a relatively long sd was observed in water-based mixtures. The hydrogen bonding of water is the main contributor to the long desorption time. Another reason for this is that water molecules are captured in two sites as shown in Fig. 3: the oxygen of the DEME+ cation and the fluorine of the BF 4 anion. More importantly, the D2O desorption time in the mixture is longer than that of H2O. This result is consistent with the evaporation time of pure H2O and pure D2O (Table 1) [19]. Generally, in bulk liquids, it is well known that the hydrogen bonding strength of D is stronger than that of H. 3.3. Desorption of [C3mim][Im] The desorption of [C3mim][Im] is expressed by m as shown in Fig. 5. For the normalized desorption time, sd, the I2 additive is less desorbed compared with [DEME][BE4]-based systems. Here, we emphasize that, in both systems, there are two additive adsorption sites as shown in Fig. 3. The longer sd of [C3mim][Im] indicates that
Fig. 3. Molecular interactions of [DEME][BF4] – H2O and [C3mim][I3] – I2 systems.
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Yoshimura of National Defense Academy for the helpful discussions. This work is supported by a grant of Shinki Yosan (NDA). References
Fig. 5. Desorption of [C3mim][I]-polyiodide system. A relatively long desorption time, sd, was observed.
Im exists in the mixture and that the coulombic interaction between C3mim+ and I m is consistent with the longer sd. The coulombic interaction prevents dissociation of polyiodides near the liquid surface. Consequently, the different sd values of the two systems are derived from different types of molecular interactions: the hydrogen bonding of the water additive is dominant or ionic interaction of polyiodides is formed by iodine additives. The desorption rate changed below m = 1.5 (x = 20 mol% I2). At the early stage of desorption, it is predicted that large polyiodides easily dissociate to neutral I2 near the liquid surface. One reasons for this is that the coulombic interaction is inversely proportional to the size of the polyiodides. In larger polyiodides, the surface charge density decreases, satisfying the negative (1) charge. Also, the non-equivalent bonding distances of the polyiodides cause asymmetric distributions of their surface charge density. Loosely bonded parts of polyiodides near C3mim+ cations could easily dissociate to form neutral I2. At the later stage of desorption, the I 3 anion as a crystallization factor might prevent dissociation and desorption near the liquid surface. Below m = 3, it is considered that only I and I 3 anions exist in the mixtures. Since both anions are strongly coupled with the C3mim+ cation, dissociation of I 3 and desorption of I2 are suppressed.
4. Conclusions Phase anomalies of non-stoichiometric [C3mim][I3.66] (or [C3mim][I3] – 7.1 mol% I2) were clarified in phase diagrams both upon cooling and heating. The stoichiometric [C3mim][I3] (y = 0 mol% I2) crystallized upon cooling, while crystallization of the 7 mol% mixture was suppressed upon cooling. It is indicated that the I 3 anion is regarded as a crystallization factor. Compared with the 7 mol% anomalies of the [DEME][BF4] – H2O system, the specific fluctuations of excess iodine between the C3mim+ cation and I 3 anion disturb the crystal nucleation of [C3mim][I3]. The 7 mol% anomaly of [C3mim][Im] at LT is related to non-stoichiometric anomalies under HP. The long desorption time of [C3mim][Im] indicates that Im anions exist in the mixture.
Acknowledgments We thank Dr. S. Tsuzuki of National Institute of Advanced Industrial Science and Technology, Dr. T. Takekiyo and Prof. Y.
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