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 article info 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 abstract Phase diagrams of room-temperature ionic liquid (RTIL), polyiodides, were obtained by simultaneous X- ray diffraction and differential scanning calorimetry measurements. The original RTIL is 1-methyl-3- propylimidazolium iodide, [C 3 mim][I]. By adding iodine to [C 3 mim][I], polyiodides were formed in the mixtures, which are expressed as [C 3 mim][I m ]. 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 [C 3 mim][I 3.66 ], as a non-stoichiometric system containing excess iodide, was rede- fined as [C 3 mim][I 3 ] – 7.1 mol% I 2 , assuming that I 3  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, I m  , 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 liq- uids (RTILs) were utilized in DSSCs as electrochemically stable electrolytes [4–10]. Hydrophilic 1-alkyl-3-methylimidazolium iodides, [C n mim][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 C 2 mim + cation-mediated environment [12]. Possible mass trans- fers 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 [C 3 mim][I]/[C 3 mim][I 3 ] mixtures were observed [13], and the conducting process could be explained by the Grotthuss exchange mechanism. Recently, in 127 I-NMR (nuclear magnetic resonance) experiments, peak splitting of [C n - mim][I m ] 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 tem- perature (LT) and ambient pressure, a phase diagram of the [C 3 mim][I m ] system was obtained by differential scanning calorimetry (DSC) [15,16]. Pure [C 3 mim][I] amorphized upon cool- ing, 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 [C 3 mim][I m ] system were investigated, com- pared with LT ones [17]. Both at LT and HP, I  is found to promote amorphization, while I 3  contributes to crystallization. Thus, I 3  is regarded as an anion that promotes stable crystallization. More- over, complicated LT and HP phase changes were observed in non-stoichiometric [C 3 mim][I 3.66 ]. Excess iodide caused additional fluctuations in the mixtures. Desorption was measured in a mixed system to estimate molec- ular 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 esti- mated. Since the evaporation time in the pure systems was propor- tional 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 dia- grams of [C 3 mim][I m ] (or [C 3 mim][I 3 ]-y mol% I 2 ) via simultaneous X-ray diffraction and DSC measurements. At around y = 7 mol% I 2 , https://doi.org/10.1016/j.chemphys.2018.01.013 0301-0104/Ó 2018 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: ab@nda.ac.jp (H. Abe). Chemical Physics 502 (2018) 72–76 Contents lists available at ScienceDirect Chemical Physics journal homepage: www.elsevier.com/locate/chemphys