The dynamic process of radioactive iodine removal by ionic liquid 1-butyl-3-methyl-imidazolium acetate: discriminating and quantifying halogen bonds versus induced force

Abstract

With the increasing demand for nuclear energy and the Fukushima Daiichi nuclear disaster in 2011, the removal of radioactive and hazardous iodine has attracted more and more attention. Here, we investigate the dynamic process of radioactive iodine sorption in a representative acetate-based ionic liquid (AcIL), 1-butyl-3-methyl-imidazolium acetate [BMIM][Ac], via in situ UV-Vis spectroscopy in combination with a two-dimensional correlation technique. More importantly, the halogen bonds (including interior and exterior types) and induced force (only possessing an exterior form) resulting in iodine sorption in [BMIM][Ac] at specific time points are discriminated and quantified. The results show that the iodine sorption in [BMIM][Ac] can be divided into three zones. In the first 140 min, only halogen bonds occur (Zone 1). From 140 to 240 min, (exterior) halogen bonds and induced force occur simultaneously (Zone 2). After 240 min, only induced force occurs (Zone 3). Specifically, Zone 1 consists of two subzones, i.e., Zone 1a (before 90 min) and Zone 1b (90–140 min), corresponding to interior and exterior halogen bonds, respectively. Zone 2 is composed of three subzones, i.e., Zone 2a (140–180 min), Zone 2b (180–200 min), and Zone 2c (200–240 min), with (exterior) halogen bonds taking up the majority, approximately one half, and a small part of the total iodine sorption, respectively. The proportion of halogen bonds and induced force resulting in iodine sorption by [BMIM][Ac] can be approximately derived as 100% and 0% within 140 min, 96% and 4% within 240 min, and 91% and 9% within 570 min, respectively. Furthermore, the proportion of interior and exterior halogen bonds resulting in iodine sorption by [BMIM][Ac] could be approximately derived as 85% and 15% within 140 min, 80% and 20% within 240 min, and 80% and 20% within 570 min, respectively. These processes and quantifications can provide insight into the radioactive iodine removal by ILs in addition to the [BMIM][Ac] that we investigated here, and may motivate further experimental or theoretical studies on the application of halogen bonds for removal of iodine by designing new types of ILs.

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1. Introduction

The utilization of nuclear energy is increasing in order to meet world energy requirements, during which nuclear waste would be released. More importantly, a large quantity of nuclear waste will be produced if a nuclear accident takes place, such as the Fukushima Daiichi nuclear power plant disaster in 2011 and the Chernobyl nuclear accident in 1986.^1,2 Another source of nuclear waste is from the manufacture of nuclear weapons. The above three sources of nuclear waste will definitely cause harm to human health. One of the most common nuclear wastes is isotopes of iodine (e.g., ^125/129/131I), which are generated as byproducts after the fission of uranium and plutonium. Particularly, the half-life of ¹²⁹I is as long as 15.7 million years, and thus ¹²⁹I requires reliable entrapment and storage. ¹³¹I directly affects human metabolic processes and can trigger thyroid cancer, in spite of its half-life of 8 days.^3,4

Many types of adsorbents have been proposed for iodine capture. Silver-based zeolites, which are one of the inorganic solid adsorbents, have the capability to entrap iodine with favorable capacity and removal efficiency.⁵ Nevertheless, their relatively low porosity and adverse environmental impact limit their practical application, despite abundant nuclear waste. Metal–organic framework (MOF) materials have the ability to uptake iodine with enhanced capacity due to higher surface areas, and can trap iodine through pressure-induced amorphization.^6–9 Unfortunately, the water-instability of MOF may hinder much of their practical application in moist air. Other materials, such as molecular organic solids,¹⁰ metalloporphyrin-based conjugated microporous polymer,¹¹ activated carbon,¹² cyclodextrins,¹³ Ag₂O grafted titanate nanolamina,¹⁴ and silver aluminophosphate glasses¹⁵ have also been suggested for iodine capture. However, these materials still possess the drawback of low capacity for iodine removal. Therefore, the development of novel materials to reliably and efficiently store iodine is urgently needed.

We have previously proved that ionic liquids (ILs) can tightly hold iodine.¹⁶ ILs are salts that are liquid at room temperature, usually consisting of an organic cation and an organic/inorganic anion. The favorable features of ILs include low vapor pressure, liquidity at a wide range of temperatures, and high thermal stability. They have attracted much attention in many fields such as sour gas capture,^17,18 chemical synthesis and catalysis,^19,20 biomass utilization,^21,22 and analytical media.²³ Among all the types of ILs, acetate-based ionic liquids (AcILs) have emerged as a unique class of liquid salts with organic cations and acetate anions that show exceptional solvation capacity in both biomass utilization²² and carbon dioxide capture.^24,25 Also, AcILs possess the favorable features of low viscosity, low melting point, easy synthesis, low cost, and high biodegradation. Because AcILs also possess the common traits of ILs, they are highly tunable and relatively green. We have found that one of the common AcILs, 1-butyl-3-methyl-imidazolium acetate [BMIM][Ac], showed a much higher efficiency for iodine capture than the above referenced materials (e.g., MOF), mainly via halogen bonds.¹⁶

Halogen bonds, a counterpart of hydrogen bonds, have been extensively researched in the last two decades in many fields.^26–29 The general expression of halogens bonds is D⋯X–Y, where X is the electrophilic halogen atom as the halogen bond donor (e.g., F, Cl, Br, I); D is the electronegative group as the halogen bond acceptor (O, N, pi, F, Cl, Br, I); and Y is any other atom that is tethered to halogen atom X (e.g., C, F, Cl, Br, I).^30–35 Simply, a halogen bond is an intermolecular electrostatic force that occurs when a covalently-bonded halogen encounters an electronegative species.³⁶ Halogen bonds were initially recognized in the 19^th century.³⁷ Subsequently, the solutions and solid states were studied for a better understanding of the dihalogen/electron donor association.^38–43 The broad name of “halogen bonds” in halogenated electrophiles was only applied a decade ago.²⁶ Since then, halogen bonds have been widely studied in many areas, such as crystal engineering,²⁶ biological molecules,⁴⁴ supramolecular materials design,^45–47 organocatalysis or reactivity control,^48,49 self-assembly processes,⁵⁰ and rational drug design.^51–54 More interestingly, halogen bonds can also control the iodine sorption in ILs, as reported by our group.¹⁶

It was concluded in our previous report that [BMIM][Ac] absorbs iodine mainly via halogen bonds and little via induced force.¹⁶ However, how does the dynamic process of iodine sorption occur in [BMIM][Ac]? Could we determine the proportion of halogen bonds and induced force contributing to the iodine sorption? Furthermore, because the halogen bonds resulting in iodine sorption are induced to stay in the interior of the IL, are there any possibilities for iodine to be absorbed on the exterior of the IL, and what is the quantification? Understanding the microscopic sorption process and the halogen bond ratio will greatly assist with the design and synthesis of new ILs with higher efficiency and faster rate for radioactive iodine entrapment. We will discuss the above questions below.

Therefore, we selected a representative AcIL, [BMIM][Ac] (Scheme 1), to investigate the dynamic process of iodine (Scheme 1) sorption and its corresponding halogen bond quantification because [BMIM][Ac] possesses favorable properties as discussed above. Furthermore, the ILs with higher iodine sorption capacity than [BMIM][Ac] (e.g., [BMIM][Cl], [BMIM][Br], [BMIM][I], [BMIM][TFO]) contain halogen atoms,¹⁶ which still might adversely affect the environment. Some of these ILs (e.g., [BMIM][Cl], [BMIM][Br], and [BMIM][I]) exhibit a solid state at room temperature, which might hinder their use in spray technology applications despite their higher efficiency via stronger halogen bonds as compared to [BMIM][Ac].¹⁶ The remaining ILs (e.g., [BMIM][Tf₂N], [BMIM][PF₆], [BMIM][BF₄], [BMIM][ClO₄], and [BMIM][NO₃]) absorb iodine mainly via induced force, due to a less favorable halogen-bonding accepting ability of these anions as compared to the acetate anion in [BMIM][Ac].¹⁶ It thus will be more appropriate to select [BMIM][Ac] as a model IL to investigate the dynamic iodine sorption process and to quantify the proportion of halogen bonds and induced force. In this sense, although halogen bonds are the main contributor of iodine sorption, induced force might also play a role to some extent.

Scheme 1 Chemical structure and notation of [BMIM][Ac] and iodine I₂: 1-butyl-3-methyl-imidazolium [BMIM] cation (a), acetate [Ac] anion (b), and elemental iodine (c), Key: C (grey), N (blue), O (red), H (white), I (violet).

Here, we selected two-dimensional correlation (2D-COS) ultraviolet visible (UV-Vis) spectroscopy, i.e., 2D-COS UV, as a tool to detect this dynamic process of iodine sorption in [BMIM][Ac]. Generalized two-dimensional correlation spectroscopy (G-2D-COS) is the primary technique for comparing the intensity of two spectra along a perturbation variable (e.g., time, temperature, concentration, pH) and to decode the subtle information from those overlapped original spectra.^55,56 Later, auto-correlation moving-window two-dimensional correlation spectroscopy (auto-MW2D-COS) is proposed by replacing one of two spectral variables (wavenumber and wavelength) in the G-2D-COS perturbation variable with a perturbation variable.⁵⁷ Recently, a more advanced technique of perturbation–correlation moving-window two-dimensional correlation spectroscopy (PCMW2D-COS) has been developed for improved discrimination of critical transition, bandshift, and the complicated intensity variation along the perturbation variable.⁵⁸ Both G-2D-COS and PCMW2D-COS include the synchronous (s-G-2D-COS and s-PCMW2D-COS) and asynchronous (as-G-2D-COS and as-PCMW2D-COS) mode. The 2D-COS technique has already been successfully applied to investigate intermolecular ILs/solvent systems by Lendl,⁵⁹ Yu,^60,61 Li,⁶² Wu,^63–65 and Wang.⁶⁶ Recently, we also have used the 2D-COS technique to investigate the IL/water dynamic interaction.⁶⁷ UV-Vis spectroscopy is one of the most convenient and efficient tools to detect halogen bonds.^7,16,36 Thus, the combination of 2D-COS and UV-Vis (i.e., 2D-COS UV) can be considered a reliable method to analyze the dynamic process of iodine sorption in [BMIM][Ac].

2. Experimental section

2.1. Materials

[BMIM][Ac] (≥99%) was purchased from Lanzhou Greenchem ILs, LICP, CAS (Lanzhou, China). A [BMIM][Ac] sample was loaded in a vial, which was placed in a vacuum oven at 60 °C for 80 h for drying before use. For better drying efficiency, some P₂O₅ was loaded into another vial and then put in the vacuum oven. The water content in the [BMIM][Ac] was less than 790 ppm, as measured by Karl Fisher titration. Halogen ion was undetectable by AgNO₃ precipitation. The NMR spectra and IR spectra before and after drying indicated that the thermal decomposition of [BMIM][Ac] during the drying process was negligible.

2.2. UV-Vis spectra measurement

0.2429 g of IL sample was loaded into a cuvette with an optical length of 1 cm. Then, the sample in the cuvette was evenly distributed by ultrasonic oscillation for approximately 10 min. Subsequently, 1.7195 g of iodine cyclohexane solution (0.384 g L⁻¹) was added onto the [BMIM][Ac] phase at room temperature (approximately 20 °C) without stirring, followed by sealing of the cuvette with a self-contained cap in order to avoid evaporation of iodine and the uptake of water from the moisture in the air due to the high hygroscopicity of ILs.^68,69 Then, UV-Vis spectroscopy (Carry 5.0, Varian) was measured to characterize the supernatant (i.e., the upper phase containing the iodine cyclohexane solution) and determine the iodine concentration as a function of time from 0 min to 570 min with increments of 5 min, within the wavelength range from 280 nm to 700 nm. We used the same solvent, that is, pure cyclohexane, as a blank.

2.3. Difference in the UV-Vis spectra and peak area calculation

The differences in the UV-Vis spectra were derived from the original UV-Vis spectra as described in the above section. Specifically, we chose the starting spectra as the reference spectrum, subtraction of which in each case from the dynamic UV-Vis spectra yielded the corresponding difference UV-Vis spectra. The difference UV-Vis spectra could be used to demonstrate a more obvious change in intensity. The peak area of UV-Vis was calculated by Origin Professional 9.0 software in the form of absolute value.

2.4. Color change experiment

The color change was mainly referred to the supernatant iodine solution with cyclohexane as the solvent. A photograph was taken with a commercial camera every 1 h for 10 h. Note that the contents of the supernatant (approximately 0.2644 g) and subnatant (approximately 1.9358 g) were slightly different from that in the cuvette intended for UV-Vis measurement. Also, here we loaded the two-phase system in a glass vial (approximately 5 mL) sealed with a self-contained cap rather than a cuvette. The upper phase is the iodine cyclohexane solution, showing a violet color. The lower phase is pure [BMIM][Ac], showing a yellow color. The seemingly yellow color in the liquid surface in the glass bottle is not a phase but only a light reflection or refraction caused by the experimental conditions. A separate photograph was taken for each vial using the same white paper as the background, and then a composite photo of all the vials was created.

2.5. ATR-IR spectra measurement

An FTIR spectrometer (Prestige-21, Shimadzu, Japan, DTGS detector) was applied using the attenuated total reflection (ATR-IR, with a ZnSe ATR-8200H cell, 584 mm² surfaces, 45° incident angles) as the accessory. Before the experiment, [BMIM][Ac] was placed on the cell without covering the seal of the ATR cell or the lid of the FTIR spectrometer to allow direct exposure to the moisture in the air. Once the samples were loaded on the ATR cell, they were scanned every 5 min until 570 min later, when the change of intensity and positions of all peaks was negligible after 570 min. During this process, the samples on the ATR cell were kept static with no stirring or mixing. The parameters set for each ATR-IR sample were 40 scans, 4 cm⁻¹ resolutions, and 4200 cm⁻¹ wavenumber spans (ranging from 4600 cm⁻¹ to 400 cm⁻¹).

2.6. G-2d-COS, auto-MW2D-COS and PCMW2D-COS

All the two-dimensional correlation spectroscopy 2D-COS data (including s-G-2D-COS, as-G-2D-COS, auto-MW2D-COS, s-PCMW2D-COS, as-PCMW2D-COS) were obtained from the original data using the 2D Shige program (http://sci-tech.ksc.kwansei.ac.jp/%7Eozaki/2D-shige.htm). 2D Shige has been widely used in academic study, including that of intra- or inter-molecular interactions related to neat ILs and their mixtures,^58,63,64,66 and can be uploaded from the web site above as suggested by Shige Morita. We set the parameter of window size (2m + 1) as 11 for all the data processing of 2D-COS. Then, we plotted planar and stereoscopic figures with these data derived from 2D Shige.

3. Results and discussion

In order to provide a coherent overview of the results, all of the schemes and figures are summarized here prior to the discussion. Fig. 1 and 2 show the normal and difference UV-Vis spectroscopy of I₂ (i.e., in the state of cyclohexane solution in the upper phase) sorption in ionic liquid (IL, in the lower phase) [BMIM][Ac] as a function of time from 0 to 570 min, respectively. The corresponding peak area with time is described in Fig. 3 and 4, the latter of which presents a slice spectrum at 660 nm. Then, we obtained the corresponding s-G-2D-COS (Fig. 5) and s-G-2D-COS (Fig. 6) spectroscopy data via the 2D Shige software from the UV-Vis data. Furthermore, the auto-MW2D-COS (Fig. 7), s-PCMW2D-COS (Fig. 8), and as-PCMW2D-COS (Fig. 9 and 10) were derived for a better understanding of the dynamic process of iodine sorption in [BMIM][Ac], and particularly for improved discrimination and quantification of halogen bonds (XB) vs. induced force (IF), and interior halogen bonds (IXB) vs. exterior halogen bonds (EXB). Scheme 1 is the notation and structure of [BMIM][Ac] and iodine. Documentation of the solution color changes is given by the photographs in Scheme 2, while the proposed microscopically dynamic iodine sorption in [BMIM][Ac] is summarized in Scheme 3. The proportion of halogen bonds vs. induced force, and interior halogen bonds vs. exterior halogen bonds at representative time points (140, 240, and 570 min) is shown in Scheme 4.

Fig. 1 UV-Vis spectroscopy peak intensity of iodine cyclohexane solution in the upper phase for the iodine sorption in [BMIM][Ac] as a function of time from 0 min to 570 min with increments of 5 min: linear (a) and stereoscopic (b) presentation.

Fig. 2 Difference UV-Vis spectroscopy peak intensity of iodine cyclohexane solution in the upper phase for the iodine sorption in [BMIM][Ac] as a function of time from 0 min to 570 min with increments of 5 min: linear (a) and stereoscopic (b) presentation. The difference spectra are obtained by subtracting the reference spectrum at 0 min from the whole spectra.

Fig. 3 UV-Vis spectroscopy peak area of iodine cyclohexane solution in the upper phase for the iodine sorption in [BMIM][Ac] as a function of time from 0 min to 570 min with increments of 5 min: linear presentation.

Fig. 4 UV-Vis spectroscopy peak area of iodine cyclohexane solution in the upper phase for the iodine sorption in [BMIM][Ac] as a function of time from 0 min to 570 min with increments of 5 min: slice spectrum at 660 min (top) and stereoscopic presentation with profile (bottom).

Fig. 5 s-G-2D-COS UV-Vis spectroscopy peak intensity for the iodine sorption in [BMIM][Ac] as a function of time from 0 min to 570 min with increments of 5 min: planar (a) and stereoscopic (b) presentation.

Fig. 6 as-G-2D-COS UV-Vis spectroscopy peak intensity for the iodine sorption in [BMIM][Ac] as a function of time from 0 min to 570 min with increments of 5 min: planar (a) and stereoscopic (b) presentation.

Fig. 7 Auto-MW2D-COS UV-Vis spectroscopy peak intensity for the iodine sorption in [BMIM][Ac] as a function of time from 0 min to 570 min with increments of 5 min: stereoscopic presentation.

Fig. 8 s-PCMW2D-COS UV-Vis spectroscopy peak intensity for the dynamic process of iodine sorption in [BMIM][Ac] as a function of time from 0 min to 570 min with increments of 5 min: planar (a) and stereoscopic (b) presentation.

Fig. 9 as-PCMW2D-COS UV-Vis spectroscopy peak intensity for the iodine sorption in [BMIM][Ac] as a function of time from 0 min to 570 min with increments of 5 min: stereoscopic presentation.

Fig. 10 as-PCMW2D-COS UV-Vis spectroscopy peak intensity for the iodine sorption in [BMIM][Ac] as a function of time from 0 min to 570 min with increments of 5 min: slice spectrum at 522 nm (top) and planar presentation with profile (bottom). The thick blue and thin green dashed lines are the dividing point of zones and subzones, discriminated with the Arabic numerals (i.e., 1–3) and English letters (i.e., a–c), respectively. XB and IF are the abbreviations for halogen bonds and induced force, respectively.

Scheme 2 Color of iodine cyclohexane solution in [BMIM][Ac] as a function of time from 0 h to 10 h with increments of 1 h at room temperature of approximately 20 °C. The upper phase is the iodine cyclohexane solution. The lower phase is [BMIM][Ac], showing a yellow color. The seemingly yellow-colored meniscus at the liquid surface inside the glass bottle is not a phase but only the reflection or refraction of light that was captured when the photograph was taken. Each vial was photographed separately with the same white paper used as the background, and then a composite photo of all the vials was created.

Scheme 3 Proposed dynamic process of iodine sorption in [BMIM][Ac] as a function of time: beginning at 0 min (a), only interior halogen bonds before 90 min (b), only exterior halogen bonds between 90 min and 140 min (c), simultaneous exterior halogen bonds and induced force between 140 min and 240 min (d), and only induced force after 240 min (e), Exterior refers to the interface layer between the [BMIM][Ac] phase and iodine phase; interior represents the layer below the interface layer between the [BMIM][Ac] phase and iodine phase; subinterior denotes all the area below the interior until the bottom surface of the [BMIM][Ac] phase. The iodine phase (upper phase) is the iodine cyclohexane solution; the IL phase (lower phase) is the pure IL.

Scheme 4 Proportion of halogen bonds and induced force for the iodine sorption in [BMIM][Ac] as a function of time at 140 min (a), at 240 min (b), at 570 min (c), and proportion of interior (IXB) and exterior halogen bonds (EXB) within the overall halogen bonds at 140 min (d) and at 240 min (e) that correspond to the components of the halogen bonds in (a) and (b), respectively.

3.1. A direct color visualization for iodine sorption in [BMIM][Ac]

Scheme 2 shows the direct color change of iodine sorption in [BMIM][Ac]. The upper phase (supernatant) is the iodine solution with cyclohexane as the solvent (approximately 0.2644 g, 0.384 g L⁻¹), which is violet in color. The lower phase (subnatant) is the neat [BMIM][Ac] (approximately 1.9358 g), which is yellow in color. Both supernatant and subnatant are contained in a 5 mL glass bottle. The photographs in Scheme 2 show the color change resulting from the sorption of iodine in [BMIM][Ac] over time from 0 h to 10 h with increments of 1 h at room temperature. The color of the supernatant (the iodine phase) fades as time elapses; the color of the subnatant ([BMIM][Ac] phase) stays almost unchanged.

A close inspection of Scheme 2 indicates that a turning point occurs at 2 h (120 min), when a most drastic change in the color of the supernatant iodine cyclohexane solution takes place. The dramatic color change before 120 min also indicates a fast iodine sorption rate, which may be due to a strong interaction between iodine and [BMIM][Ac]. Similarly, there might exist another turning point at 4 h (240 min), after which the color change of the supernatant iodine cyclohexane solution is minor, which perhaps results from a weak interaction between iodine and [BMIM][Ac]; it also can imply a slow iodine sorption rate. Between 120 min and 240 min, there might be a mixture of strong and weak interaction between iodine and [BMIM][Ac] accompanying a moderate iodine sorption rate.

The two turning points (120 min and 240 min) divide the iodine sorption process into three zones. Zone 1 is before 120 min; Zone 2 is between 120 min and 240 min; and Zone 3 is after 240 min. Zone 1 is attributed to a very strong I₂/IL interaction, leading to a fast iodine sorption rate and color change. Only a moderate I₂/IL interaction occurs in Zone 2, resulting in a moderate iodine sorption rate and color change. However, there are also two important questions to ask.

The first question is: how is the strong and weak I₂/IL interaction as mentioned above specifically related to the zone-dividing speculation? Our previous study showed that the iodine would interact with ILs via two types of interactions: halogen bonds between the iodine (halogen atom, electrophilic species, electron acceptor, Lewis acid, XB donor) and the anion of ILs (electronegative species, electron donor, Lewis base, XB acceptor), and induced force ascribed to the high intrinsic electric field of ILs and the highly-induced electron shell of iodine.¹⁶ Generally, halogen bonds are stronger than the induced force due to the relatively closer distance between the two atoms involved in halogen bonds than induced forces. Our previous investigation also concluded that the iodine sorption in ILs mainly via halogen bonds (e.g., [BMIM][X] and [BMIM][TFO], X = Cl, Br, I) is more efficient than that mainly via induced force (e.g., [BMIM][BF₄], [BMIM][PF₄], [BMIM][Tf₂N], [BMIM][ClO₄]).¹⁶ Thus, we interpret the strong I₂/IL interaction in Zone 1 as the halogen bonds, and the weak I₂/IL interaction in Zone 3 as the induced force. I₂/IL interactions in Zone 3 include both halogen bonds and induced force.

The second question is: how is the turning time point for the zone-dividing pattern specifically related to the iodine sorption in [BMIM][Ac]? Deducing the turning points (120 min and 240 min) from the color change of the violet supernatant might be very difficult. Furthermore, the color change is recorded every 1 h, and trying to gauge the turning point by using subjective color change with such large time increments prompted us to seek other more accurate methods to discriminate these two turning points. It was anticipated that the turning points might be at approximately 120 min and 240 min, with differences that may not be very large. Next, we used in situ UV-Vis spectroscopy (including normal and difference peak intensity, and peak area as a function of time) in combination with two-dimensional correlation techniques (including G-2D-COS, auto-MW2D-COS, PCMW2D-COS) to discriminate the turning points accurately, and then quantified halogen bonds and induced force, and interior halogen bonds and exterior halogen bonds, for the I₂/IL system at specific time points.

3.2. Dynamic process of iodine sorption in [BMIM][Ac]

The s-PCMW2D-COS shown in Fig. 8 indicates that 140 min is a turning point. It can also be clearly observed in as-PCMW2D-COS of Fig. 9 and 10, which corroborate the existence of a turning point at 140 min. The color change of the supernatant iodine cyclohexane solution also suggests a similar turning point at 120 min. It also implies that PCMW2D-COS is more accurate than color visualization. The G-2D-COS discriminating ability (Fig. 5 and 6) is limited due to its averaging effect. The turning point might result from the iodine in the supernatant mainly interacting with [BMIM][Ac] via halogen bonds (including interior halogen bonds IXB and exterior halogen bonds EXB, which will be discussed later) before 140 min, while the induced force occurs after 140 min on the exterior of [BMIM][Ac].

Fig. 8 shows that another turning point in the s-PCMW2D-COS is at 240 min, after which the intensity change is negligible; before which, however, the change cannot be detected. Fig. 9 and 10 also support the existence of the second turning point at 240 min by as-PCMW2D-COS. The color change of the supernatant iodine cyclohexane solution discussed above also suggests a similar turning point at 240 min, which again provides evidence for the second turning point at 240 min by PCMW2D-COS. The G-2D-COS discriminating ability (Fig. 5 and 6) is limited due to its averaging effect. The change intensity after 240 min can be entirely ascribed to induced force on the [BMIM][Ac] exterior, while simultaneous induced force and halogen bonds would occur before 240 min on the [BMIM][Ac] exterior.

Thus, the dynamic process of iodine sorption in [BMIM][Ac] can be divided into three zones according to the above two discriminated dividing points (Fig. 2). The first 140 min belongs to Zone 1 as shown in Scheme 3a and b, where only halogen bonds occur (both interior halogen bonds in Scheme 3b and exterior halogen bonds in Scheme 3c). From 140 min to 240 min during Zone 2, halogen bonds (only including exterior halogen bonds) and induced force occur simultaneously on the [BMIM][Ac] exterior (Scheme 3d). In Zone 3 after 240 min, only exterior induced force takes place (Scheme 3e). Note that halogen bonds include interior and exterior, while induced force can only occur on the exterior.

Interestingly, there exists one minor turning point at 90 min in as-PCMW2D-COS of Fig. 10, which shows that Zone 1 could be divided into two subzones, i.e., Zone 1a (0–90 min in Scheme 3a) and Zone 1b (90–140 min in Scheme 3b). The change rate of Zone 1a is higher than that of Zone 1b, implying that Zone 1a involves a stronger form of halogen bonds (i.e., interior halogen bonds, Schemes 3a) while weaker halogen bonds (i.e., exterior halogen bonds, Schemes 3b) occur for Zone 1b. This is understandable because the first reacted iodine would penetrate into the [BMIM][Ac] interior via IXB before 90 min, which might in turn prevent the further entrance of iodine, and hence an exterior halogen bond occurs after 90 min.

Another interesting finding is the existence of two minor turning points at 180 min and 200 min, as shown in as-PCMW2D-COS of Fig. 10. It indicates that Zone 2 can be divided into three subzones, i.e., Zone 2a (140–180 min), Zone 2b (180–200 min), and Zone 2c (200–240 min). Also, s-PCMW2D-COS in Fig. 8 seems to show that 180 min and 200 min are two minor turning points despite less clarity than as-PCMW2D-COS in Fig. 10. Zones 2a, 2b and 2c correspond to most EXB, comparable and simultaneous EXB and exterior induced force, and most exterior induced force, respectively.

3.3. Microscopic explanation for the iodine dynamic sorption process in [BMIM][Ac]

The interaction contributing to iodine sorption in [BMIM][Ac] in Zone 1 is total halogen bonds, as shown in Scheme 3b and c. All halogen bonds in Zone 1 indicate that interaction between iodine and [BMIM][Ac] is the strongest, which prefers to occur first. It is possibly the iodine-anion halogen-bonding interaction because of a higher electrophilicity (a better XB donor) of the iodine atom and a higher electronegativity (a better XB acceptor) of the acetate anion of [BMIM][Ac]. After this zone (after 140 min), [BMIM][Ac] might also interact with other iodine molecules via a less strong interaction (i.e., induced force), and hence there is less change in intensity in Fig. 1, 2 and 7 after 140 min. The change in peak area also corroborates the same conjecture of the occurrence of a very strong I₂/IL interaction before 140 min. Specifically, this halogen-bonding interaction between I₂ and [BMIM][Ac] is not negligible until 200 min, as shown by the turning point in Fig. 10, which will be explained later. The strong halogen-bonding interaction might also induce a quick color change of the iodine cyclohexane solution in the corresponding time period, as shown in Scheme 2. Due to the higher strength of halogen bonds, a robust iodine sorption can be seen for many ILs to capture I₂.¹⁶ Our previous iodine entrapment experiments also suggested that the initial iodine sorption rate by ILs is faster than that of later time periods, particularly for those ILs with a greater halogen bond-donating ability (e.g., [BMIM][Br]). However, removing this part of iodine would be very difficult, suggesting improved efficiency for reliably storing radioactive iodine.¹⁶

Specifically, Zone 1 is divided into two subzones by a minor turning point at 90 min. The iodine is first drawn by [BMIM][Ac] into its interior via halogen bonds during the initial 90 min period. Our previous study also reported that the strong halogen bonds would draw iodine into the interior of ILs.¹⁶ The interesting thing is that they would form a more stable complex and a more viscous product. The stabilization of iodine by [BMIM][Ac] has been supported by a nitrogen sweeping experiment.¹⁶ The more viscous product could be visualized by adding some solid iodine powder directly into the [BMIM][Ac]. Both the stability and viscosity might prevent the iodine from penetrating into the interior of the [BMIM][Ac] and keep it on the exterior of [BMIM][Ac] after 90 min, despite the same type of halogen bonds though with slightly less strength. This may be evidenced by the dividing point at 90 min (as-PCMW2D-COS of Fig. 10). In order to further corroborate this idea, we also conducted an additional experiment by placing [BMIM][Ac] (0.5 mm height) on the bottom of the ATR-cell and then covering it with iodine cyclohexane solution (1 mm height). The results show that the IR spectra of the subnatant [BMIM][Ac] were nearly unchanged. It indicates that the iodine would not penetrate into the bottom of [BMIM][Ac], i.e., the subinterior of the IL phase as shown in Scheme 3. It also suggests that the [BMIM][Ac]/I₂ complex would hinder further penetration of iodine into the [BMIM][Ac] interior, which would in turn favor halogen-bonded iodine staying on the [BMIM][Ac] exterior to some extent. The proportion of exterior halogen bonds was found to be much less than that of the interior halogen bonds, which we will discuss in the quantification section.

Zone 3 is the process of iodine sorption via only induced force on the exterior of [BMIM][Ac], as shown in Scheme 3e. The turning point at 240 min can be clearly corroborated in the as-PCMW2D-COS (Fig. 10), and roughly determined in the s-PCMW2D-COS (Fig. 8). Interestingly, this turning point to divide halogen bonds and induced force could also be determined solely by eye visualization of the supernatant color change as shown in Scheme 2. In this process, iodine only interacts with the exterior of [BMIM][Ac] via induced force, which is weaker than the halogen bonds in Zone 1, and therefore the already halogen-bonded [BMIM][Ac] in Zone 1 is not affected by the iodine sorption in this zone. The possible reason might be due to the halogen-bonding acceptance limit at 240 min, after which the absorbed halogen-bonded iodine prohibits [BMIM][Ac] from absorbing more iodine via halogen bonds due to the full loading of iodine on the halogen-bonding accepting sites of [BMIM][Ac]. Other reasons could be the relatively high viscosity of neat [BMIM][Ac], and possibly a higher viscosity of the [BMIM][Ac]/I₂ complex. Also note that there is no mechanical mixing or stirring device in this entire process, which deters the iodine from percolating to the interior of the [BMIM][Ac] sample rather staying on the exterior of [BMIM][Ac]. The UV spectra of the peak intensities and area thus remain almost unchanged (Fig. 1–4 and 7); all the bands varying in this zone can thus be due to the iodine absorbed on the exterior of [BMIM][Ac] via induced force. Induced force is weaker than halogen bonds, and thus, iodine sorption in this zone is minor and slow, and can easily be removed to some extent. It is consistent with our previous report that iodine is adsorbed on the exterior of ILs with less XB-accepting ability (e.g., [BMIM][ClO₄]) and can thus be easily removed.¹⁶ A slow absorption rate could also be witnessed for the iodine sorption by many ILs in this zone.¹⁶

Zone 2 from 140 to 240 min is a transition zone with three subzones, as shown in Fig. 10 and Scheme 3. The main feature is the sorption of iodine via simultaneous halogen bonds and induced force. The as-PCMW2D-COS in Fig. 9 and 10, s-PCMW2D-COS in Fig. 8, and supernatant color change in Scheme 2 all support the existence of the second zone. In this zone, the strongest interaction (interior halogen bonds) is totally depleted; the stronger interaction (exterior halogen bonds) is almost exhausted; the weak interaction (i.e., exterior induced force) begins to emerge. Thus, the remaining exterior halogen bonds and exterior induced force combine to simultaneously play a role in determining iodine sorption in [BMIM][Ac]. Note that in this zone, all of the iodine is absorbed on the exterior of [BMIM][Ac] due to the occupation of the IL by iodine via interior halogen bonds as discussed above. Particularly, the existence of the three subzones in Zone 2 (Zone 2a, Zone 2b, and Zone 2c) is evidenced by the more accurate as-PCMW2D-COS in Fig. 10, which will be discussed below.

From 140 to 180 min (Zone 2a), the exterior halogen bonds dominate the iodine sorption (compared with Zone 1), and only a slight induced force takes place. In general, Zone 2a resembles Zone 1. In Zone 2a, there still exist some un-halogen-bonded acetate anions that the iodine needs; the interaction is also strong, although fewer anions participate in the halogen bonds with the supernatant iodine than that in Zone 1. Thus, in this subzone, alternation of intensity also exists but with a slower rate than that in Zone 1 (Fig. 1–4 and 7). Still, the halogen bonds in this subzone stay on the exterior of [BMIM][Ac]. From 180 to 200 min (Zone 2b), both exterior halogen bonds and induced force are contributing to the iodine sorption. In Zone 2b, fewer acetate anions are available to uptake supernatant iodine by halogen-bonding interactions. Thus, the electron shell of iodine is induced by [BMIM][Ac] with a high intrinsic electric field, which could be corroborated by the zone from 180 to 200 min in Fig. 10. From 200 to 240 min (Zone 2c), the (exterior) induced force takes up the majority of the iodine sorption while (exterior) halogen bonds are few. In general, Zone 2c resembles Zone 3. In Zone 2c, almost all of the anions of the IL that are able to halogen-bond with iodine have already interacted, and only a negligible amount of acetate anions remain that can absorb iodine via exterior halogen bonds. The most likely interaction for supernatant iodine to be absorbed on [BMIM][Ac] is with induced force.

3.4. Quantifying halogen bonds and induced force

Conclusions drawn above by s-PCMW2D-COS (Fig. 8), as-PCMW2D-COS (Fig. 9 and 10), and color change (Scheme 2) suggest that 240 min is a turning point, after which iodine is absorbed only on the exterior of [BMIM][Ac] via induced force; another turning point is discriminated at 140 min, before which iodine is absorbed only into [BMIM][Ac] via halogen bonds. Considering the two turning points, the following is a description of the dynamic process of iodine sorption by [BMIM][Ac]. During the first 140 min, only (interior and exterior) halogen bonds occur. From 140 to 240 min, (exterior) halogen bonds and (exterior) induced force occur simultaneously. After 240 min, only (exterior) induced force occurs. Specifically, Zone 1 consists of two subzones, i.e., Zone 1a (before 90 min) and Zone 1b (90–140 min), corresponding to interior and exterior halogen bonds, respectively. Zone 2 is composed of three subzones, i.e., Zone 2a (140–180 min), Zone 2b (180–200 min), and Zone 2c (200–240 min), with (exterior) halogen bonds taking up the majority, approximately one half, and a small part of the total iodine sorption, respectively.

Based on the clarification of the dynamic iodine sorption process, we quantify the halogen bonds vs. induced force of iodine sorption in [BMIM][Ac] by combining the sorption with its UV-Vis spectroscopy intensity. The intensity (Fig. 1a and b) derived from UV-Vis spectroscopy directly corresponds to the concentration of iodine cyclohexane solution according to the Beer–Lambert law, in which the molar absorptivity is estimated as 3.441 L g⁻¹ mol⁻¹.¹⁶ By using UV-Vis spectroscopy, we also measured the molar absorption coefficient for many kinds of ILs.⁷⁰ However, here we do not use the normal absorbance intensity developed from UV-Vis spectroscopy or the real concentration derived from the Beer–Lambert law. Rather, we select the absorbance from the difference UV-Vis spectroscopy (Fig. 2a and b), which could provide the change of concentration of iodine at the cyclohexane solution more directly and simply. All of the data are positive after ignoring the sign of UV-Vis difference spectroscopy. Therefore, the change of iodine sorption relative capacity at 0, 140 (the first turning point), 180, 200, 240 (the second turning point), and 570 min could be derived as 0 (A_{0 min}), 1.36 (A_{140 min}), 1.43 (A_{180 min}), 1.46 (A_{200 min}), 1.51 (A_{240 min}), and 1.59 (A_{570 min}), respectively.

First, we quantify the proportion of halogen bonds and induced force within 570 min. The components of iodine sorption in [BMIM][Ac] via halogen bonds include three categories as shown in eqn (1): (i) in Zone 1, the relative content of halogen bonds is approximately 1.36 by subtracting A_{140 min} from A_{0 min}; (ii) in Zone 2a, it is approximately 0.07 by subtracting A_{180 min} from A_{140 min} after assuming the total halogen bond contribution; (iii) in Zone 2b, it is approximately 0.015, half of the difference between A_{200 min} and A_{180 min}, based on the assumption that the halogen bonds and induced force are comparable in this subzone. After considering the three types of contribution, halogen bonds are calculated as 1.445. The overall contribution of induced force could be calculated with eqn (2). For a better understanding, we also divide it into three parts: (I) from Zone 3, the relative content of induced force is approximately 0.08, i.e., the difference between A_{570 min} and A_{240 min}; (II) from Zone 2c, it is approximately 0.05, by A_{240 min} minus A_{200 min}, on the basis of the premise of a totally induced force contribution despite almost all patterns; (III) from Zone 2b, it is approximately 0.015, which is the same situation as that discussed in (iii). The sum of the three species (0.08, 0.05, and 0.015) would equal 0.145 for the overall induced force within 570 min. Therefore, the proportion of halogen bonds and induced force resulting in iodine sorption by [BMIM][Ac] within 570 min could be derived approximately as 91% and 9% (Scheme 4c), according to eqn (3) and (4), respectively.

A_{XB-570 min} = (A_{140 min} − A_{0 min})_Zone1 + (A_{180 min} − A_{140 min})_Zone2a + (A_{200 min}/2 − A_{180 min}/2)_Zone2b (1)

A_{IF-570 min} = (A_{570 min} − A_{240 min})_Zone3 + (A_{240 min} − A_{200 min})_Zone2c + (A_{200 min}/2 − A_{180 min}/2)_Zone2b (2)

P_{XB-570 min} = A_{XB-570 min}/(A_{XB-570 min} + A_{IF-570 min}) (3)

P_{IF-570 min} = A_{IF-570 min}/(A_{XB-570 min} + A_{IF-570 min}) (4)

Then, the relative content of halogen bonds and induced force within 240 min would be quantified by referring to the methods previously discussed that were used within 570 min. Similar to 570 min, the constituents of halogen bonds within 240 min can also be divided into three compositions as shown in eqn (5). A closer inspection would reveal that the content of halogen bonds within 240 min is the same as that of 570 min. However, the amount of induced force resulting in iodine sorption in [BMIM][Ac] within 240 min is different from that within 570 min. Specifically, the content of induced force during Zone 3 (approximately 0.08 in I) is excluded within 240 min (eqn (6)), while it is included within 570 min (eqn (2)). If the contribution of induced force from Zone 2c (approximately 0.05 in II) and Zone 2b (approximately 0.015 in III) is added, then the overall induced force within 240 min, i.e., 0.065, would be obtained. After considering the relative content of halogen bonds (1.445) and induced force (0.065), the proportion can be estimated as 96% and 4%, respectively (as shown in Scheme 4b), by referring to similar equations (eqn (3) and (4) for the time region within 570 min) for halogen bond- and induced force-proportion quantification.

A_{XB-240 min} = (A_{140 min} − A_{0 min})_Zone1 + (A_{180 min} − A_{140 min})_Zone2a + (A_{200 min}/2 − A_{180 min}/2)_Zone2b (5)

A_{IF-240 min} = (A_{240 min} − A_{200 min})_Zone2c + (A_{200 min}/2 − A_{180 min}/2)_Zone2b (6)

Another time point at 140 min was also selected to analyze the proportion of halogen bonds and induced force because this was the first turning point, and it is a representative time point for halogen bond and induced force determination. In this case, the overall halogen bonds and induced force that resulted in iodine sorption in [BMIM][Ac] are more easily justified due to simpler constituents. Namely, only Zone 1, i.e., approximately 1.36 in (i), is included in the content of halogen bonds within 140 min as shown in eqn (7). Nevertheless, there is no induced force included in the same time region (eqn (8)). Thus, the proportion of halogen bonds and induced force could be determined quite simply, i.e., 100% and 0%, respectively, as shown in Scheme 4a. The most important feature of iodine sorption in [BMIM][Ac] is that halogen bonds dominate the entrapment process during a long-term exposure (e.g., 91% within 570 min, 96 within 240 min), particularly a short-term exposure (e.g., 100% within 140 min), while the contribution of induced force for iodine sorption in [BMIM][Ac] is minor (9% at most). These results are highly consistent with our previous work on selecting ILs ([BMIM][Ac]) to capture iodine mainly via halogen bonds, which demonstrated that the acetate anion is adept at accepting halogen bonds.¹⁶

A_{XB-140 min} = (A_{140 min} − A_{0 min})_Zone1 (7)

A_{IF-140 min} = 0 (8)

3.5. Quantifying interior halogen bonds (IXB) and exterior halogen bonds (EXB)

In the above quantification of halogen bonds and induced force, induced force contributes to an exterior sorption of iodine due to its weaker strength while halogen bonds leads to both an interior (IXB) and an exterior (EXB) iodine sorption. We cannot distinguish exterior- or interior-induced force, because induced force only includes the exterior pattern. However, in terms of halogen bonds, it contains interior halogen bonds and exterior halogen bonds. In terms of determining the proportion of halogen bonds and induced force, we do not differentiate interior or exterior halogen bonds because it is not necessary. The indiscrimination of interior or exterior halogen bonds makes the quantification process simpler and more convenient. Below, we discriminate between interior and exterior halogen bonds and then quantify them by choosing a representative time point (particularly at the two turning points and the experiment-ending time point). Before the quantification, we should determine the intensity of the first turning point at 90 min, discriminating the interior and exterior halogen bonds, A_{90 min} = 1.16, which is derived from the raw data directly.

We first quantified the proportion of interior and exterior halogen bonds within 240 min. The interior halogen bonds only occur before 90 min, and thus, the overall content was easily determined as approximately 1.16 in Zone 1a (eqn (9)). However, the components of exterior halogen bonds include three species according to eqn (10). The first species comes from Zone 1b, the difference between A_{140 min} and A_{90 min} (approximately 0.20); the second part is the same as that of (ii) as discussed above, which is included in Zone 2a, approximately 0.07; the third category is also equivalent to that of (iii) as discussed above in Zone 2b, approximately 0.015. The sum of the values for the three types (0.20, 0.07, and 0.015) is 0.285, which is the overall exterior halogen bonds needed for iodine sorption in [BMIM][Ac]. Therefore, the proportion of interior halogen bonds (80%, Scheme 4e) within 240 min could be derived after dividing 1.16 by the sum of 0.20 and 0.285, i.e., 1.445 (eqn (11)). Similarly, by utilizing eqn (12), the proportion of exterior halogen bonds (20%, Scheme 4e) within 240 min could also be obtained. Obviously, most (80%) of the iodine sorption takes place by way of interior halogen bonds within 240 min. Then, the subsequent exterior halogen bonds are inhibited (i.e., 20%) due to some physical (e.g., no stirring or mixing behavior, higher viscosity after sorption) or chemical reasons (e.g., weaker halogen-bonding accepting ability of the acetate anion) as suggested above.

A_{IXB-240 min} = (A_{90 min} − A_{0 min})_Zone1a (9)

A_{EXB-240 min} = (A_{140 min} − A_{90 min})_Zone1b + (A_{180 min} − A_{140 min})_Zone2a + (A_{200 min}/2 − A_{180 min}/2)_Zone2b (10)

P_{IXB-240 min} = A_{IXB-240 min}/(A_{IXB-240 min} + A_{EXB-240 min}) (11)

P_{EXB-240 min} = A_{EXB-240 min}/(A_{IXB-240 min} + A_{EXB-240 min}) (12)

When the time point is set as 140 min, the proportion of interior and exterior halogen bonds would vary to some extent. As to the content of interior halogen bonds, it is the same as that within 240 min, approximately 1.16 in Zone 1a (eqn (13)). In terms of exterior halogen bonds within 140 min, the content changes slightly when compared with that of 240 min (eqn (14)). Specifically, only the first component (approximately 0.20 in Zone 1b) of exterior halogen bonds discussed above within 240 min remains to 140 min, while the other two components (Zone 2a and Zone 2b) are not included. Thus, the overall content of exterior halogen bonds within 140 min is only approximately 0.20. In this way, the proportion of interior and exterior halogen bonds was obtained as 85% and 15%, respectively (Scheme 4d). In this case, the contribution of interior halogen bonds is also higher than that of exterior halogen bonds. If we choose the experiment-ending time point, i.e., 570 min, the proportion of interior and exterior halogen bonds could also be secured. In the case of within 570 min, it would be the same as that within 240 min, because no form of halogen bond is involved in iodine sorption after 240 min. Namely, the proportion of interior and exterior halogen bonds within 570 min is also 80% and 20%, respectively.

A_{IXB-140 min} = (A_{90 min} − A_{0 min})_Zone1a (13)

A_{EXB-140 min} = (A_{140 min} − A_{90 min})_Zone1b (14)

In all three cases (140 min, 240 min, and 570 min), the function of interior halogen bonds is higher than that of exterior halogen bonds, resulting in iodine sorption in [BMIM][Ac]. It suggests that [BMIM][Ac] binds iodine mainly via halogen bonds into its interior rather than on the exterior. Our previous study also anticipated that it would take less time to reach maximum iodine sorption with stirring.¹⁶ Another interesting finding is that the proportion increase (4–0% = 4% from Scheme 2a and b) of induced force from 140 min to 240 min is comparable to that (20–15% = 5% from Scheme 2d and e) of exterior halogen bonds during the same time zone (Zone 2). It is consistent with our assumption of simultaneous and comparable (exterior) halogen bonds and induced force in Zone 2 from 140 to 240 min, as suggested above. Before 140 min (Zone 1), the interior halogen bonds are totally exhausted, and the exterior halogen bonds are almost depleted. In Zone 2, induced force and the remaining exterior halogen bonds would have a synergistic effect on iodine sorption by [BMIM][Ac].

4. Conclusion

Here, for the first time, we investigated the dynamic process of radioactive iodine sorption in a representative AcIL, [BMIM][Ac]. Furthermore, the halogen bonds (including interior and exterior types) and induced force (only owning an exterior form) resulting in iodine sorption in [BMIM][Ac] at some specific time point were discriminated and quantified.

The results showed that the dynamic iodine sorption in [BMIM][Ac] could be divided into three zones. In Zone 1, only (interior and exterior) halogen bonds occur during the first 140 min. From 140 to 240 min, (exterior) halogen bonds and induced force occur simultaneously (Zone 2). After 240 min, only induced force occurs (Zone 3). Specifically, Zone 1 consists of two subzones, Zone 1a (before 90 min) and Zone 1b (90–140 min), corresponding to interior and exterior halogen bonds, respectively. Zone 2 is composed of three subzones, Zone 2a (140–180 min), Zone 2b (180–200 min), and Zone 2c (200–240 min), with (exterior) halogen bonds taking up the majority, approximately one half, and a small part of the total iodine sorption, respectively. The proportion of halogen bonds and induced force resulting in iodine sorption by [BMIM][Ac] could be derived as approximately 100% and 0% within 140 min, 96% and 4% within 240 min, and 91% and 9% within 570 min, respectively. Furthermore, the proportion of interior and exterior halogen bonds resulting in iodine sorption by [BMIM][Ac] could be derived approximately as 85% and 15% within 140 min, 80% and 20% within 240 min, and 80% and 20% within 570 min, respectively.

These processes and quantifications provide insight into how iodine is removed by [BMIM][Ac] in the form of sorption, and may motivate further experimental or theoretical studies on the application of halogen bonds for removal of iodine by designing new types of ILs, particularly for those task-specific ILs tethered with amines, hydroxyl, or unsaturated bonds that are assumed to be a good halogen-bonding acceptor for halogen-bonding donor iodine. Note that although halogen bonds and induced force have been discriminated in this study, more work is still needed to clarify the process. Other types of ILs might also show a similar pattern, but that is beyond the scope of this article. The external factors affecting the dynamic process and the quantification of halogen bonds and induced force are also our interest. For the first time, an investigation has been conducted to understand the dynamic process of radioactive iodine sorption in [BMIM][Ac], and approximately quantify the halogen bonds vs. induced force, and interior vs. exterior halogen bonds.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21473252). We also thank Morita (Division of Energy Science, EcoTopia Science Institute, Nagoya University, Furo-cho, Chigusa-ku, Nagoya 464-8603, Japan) and Ozaki (Department of Chemistry, School of Science and Technology, Kwansei-Gakuin University, Sanda 669-1337, Japan) for assisting us via e-mail to obtain PCMW2D-COS with the software 2D Shige.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra06069e

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