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. 2024 Aug 2;128(32):6739–6744. doi: 10.1021/acs.jpca.4c03010

Spontaneous Production of I2 at the Surface of Aqueous Iodide Solutions

Junwei Song †, Christian George †,*, D James Donaldson ‡,§,*
PMCID: PMC11332398  PMID: 39092462

Abstract

graphic file with name jp4c03010_0006.jpg

Several groups have recently reported spontaneous production of atmospherically reactive species, including molecular iodine (I2) at the air–water interface of droplets. In this study, glancing angle laser-induced fluorescence spectroscopy was used to track the luminol fluorescence at the surface of sodium iodide (NaI) and sodium chloride (NaCl) solutions. Although luminol fluorescence is hardly quenched by halide anions, even up to fairly high concentrations, it is effectively quenched by I2. We observe luminol fluorescence quenching at the surface of NaI solutions but not at the surface of NaCl solutions, pointing to the formation of I2 at the surface of NaI solutions. This provides further support for the proposal that the strong electric field or the reduction solvation present at the air–water interface can initiate spontaneous iodide activation and other chemistry there. The spontaneous production of I2 at the surface of aqueous iodide solutions presents a previously unconsidered source of iodine in the atmosphere.

1. Introduction

Several recent studies have reported spontaneous formation of hydroxyl radical (OH•) and hydrogen peroxide (H2O2) at the air–water interface of microdroplets.1−8 Those studies hypothesized that the strong electric field (∼109 V m–1) present at the air–water interface promotes the charge separation of hydroxide anions (OH–) to the hydroxyl radical (OH•), which then recombine to form H2O2. A study by Xing et al.,9 reported the formation of triiodide (I3–) in iodide-containing microdroplets, and they proposed that the oxidation of iodide (I–) to produce atomic iodine (I•) and subsequently I3– was induced by the spontaneously generated OH• at the air–water interface. The formation of I3– at aqueous-air interfaces was also reported by a very recent publication.10 Similarly, we have observed I3– formation in aqueous droplets but also the evolution of gas phase I2, both with aqueous iodide concentration dependences suggestive of a surface-mediated process.11 We proposed that the lower oxidation potential of iodide compared to hydroxide will allow the conversion of I– to I• at the air–water interface, likely as a charge separation process induced by the strong electric field.10,11 Specifically, the strong electric field at the air–water interface of iodide-containing microdroplets could dissociate I– into I• and a free electron (E1). Once produced, two I• can combine to produce I2 in the aqueous phase (E2), followed by the formation of I3– (E3).

1. E1
1. E2
1. E3

In Guo et al.,11 we explore this mechanism at some length. The presence of O2(g), or other electron scavengers, was seen to enhance I3– production over that observed in a pure nitrogen atmosphere, presumably by reducing the influence of the reverse reaction (E1). The formation of O2– by electron capture is expected to give rise to HO2, via reaction with H+ or water. This species may also play a role in the oxidation of iodide at the air–water interface.

In general, the photodissociation of organo-iodine compounds released by the marine biota is regarded as the major source of atomic and molecular iodine (I• and I2) in the coastal and marine atmospheres.12−14 These reactive iodine species are one of the most important species responsible for the depletion of tropospheric ozone.15 Furthermore, iodine chemistry can contribute to atmospheric new particle formation, especially in the marine boundary layer, and therefore cloud properties and radiative forcing.16,17 Despite the importance of reactive iodine species in the atmosphere, their sources or formation mechanisms are not fully understood. Therefore, the experimental observations discussed above9−11 could provide an additional source of reactive iodine to the atmosphere. In those previous laboratory studies, the formation of I3– within microdroplets or I2 emission into the gas phase from iodide-containing droplets was measured using absorption spectroscopy and mass spectrometry techniques, respectively.10,11 However, the formation of aqueous I2 at the air–water interface (E2), which is expected to be the controlling step of I3– production or gaseous I2 emissions, has not been observed experimentally. The present article aims to expand the earlier measurements by monitoring aqueous I2 production at the air–water interface without any added oxidizing agents and catalysts or UV illumination.

Our detection scheme relies on the interaction of molecular iodine with luminol. Luminol (C8H7N3O2, 3-aminophthalhydrazide) is used as a probe for OH• (or H2O2, in the presence of a Fenton reagent) via a blue chemiluminescence observed when it reacts with strong oxidants.18−21 Luminol chemiluminescence has been widely used to detect reactive species such as H2O2 and I2, but the chemiluminescent reaction requires strong basic conditions (pH ∼ 11–13) for maximum chemiluminescence quantum yield.22−26 However, luminol is fluorescent at lower pH, with an excitation peak at ∼350 nm and an emission peak at ∼425 nm.27 Therefore, one can potentially also track the changes in luminol fluorescence intensity to monitor its reactive or quenching interactions with other species.28−30 Here we use glancing angle laser-induced fluorescence spectroscopy (GALIF) to provide experimental evidence for spontaneous I2 formation at the surface of aqueous iodide solutions through tracking the loss of fluorescence intensity as a probe. GALIF experiments have been used extensively by us in the past decades to study chemistry at the air–water interface.31−36

2. Experimental Details

A luminol stock solution was prepared by weighing 173 mg of luminol (TCI America, >97.0% purity) into a 1 L Erlenmeyer flask, then adding 500.0 mL of 18 MΩ water. Had the sample fully dissolved, this would result in a concentration of 2 mM luminol. However, there remained obvious solid samples in the flask after preparing the solution and stirring overnight. The actual concentration of luminol stock solution with 100 times dilution was determined by measuring the absorbance spectrum using a PerkinElmer LAMBDA 365 double-beam UV–vis spectrometer. Luminol showed clear absorption peaks at ∼285 and ∼350 nm, respectively (Figure S1), consistent with previous studies.20,27 The concentration was calculated assuming a molar absorptivity coefficient of ∼7.6 mM–1 cm–1 at 350 nm.25 The actual concentration of luminol stock solutions was estimated to be ∼750 μM.

Fluorescence experiments were carried out using a setup based on those we have used in the past,31−35 and described fully in McLay et al.36 The unfocused output of a wavelength-tunable pulsed Nd:YAG laser-pumped optical parametric oscillator (OPO) system operating at a pulse frequency of 20 Hz was directed toward either a circular sample dish, 6 cm in diameter and about 1 cm deep, or a 1 cm path-length cuvette. Typical pulse energies at the laser head were between ∼0.5 and ∼1.2 mJ per 3 ns pulse. Irises were used to select the central region of the circular beam. All experiments were carried out in an open container under ambient air. This ensured that oxygen was present to act as an electron scavenger, enhancing the production of I2, as discussed in Guo et al.11

For glancing angle laser-induced fluorescence (GALIF) experiments, an adjustable mirror directed this beam to the sample dish such that it impinged the surface of the liquid at approximately 87° to the surface normal. This arrangement gives an estimated penetration depth of the evanescent wave of a few tens of nanometers. A 7 mm diameter liquid light guide was positioned around 5 mm above where the laser interacted with the sample. Light collected by the guide was sent to a 1/4 m focal length monochromator and detected by a photomultiplier tube, whose output was visualized with a digital oscilloscope that averaged over 16 laser pulses. The oscilloscope output was sampled and collected by a computer for subsequent analysis. For experiments probing the bulk solution, the sample dish was replaced with a 1 cm path-length quartz cuvette, and the light guide was positioned at the side of the cuvette, perpendicular to the laser beam direction. Signal collection and analysis were the same as for the glancing angle experiments.

To measure the surface adsorption isotherm of luminol using GALIF, the sample dish was filled to its top with 32.0 mL of luminol solution (∼3.75 μM). Dilutions were done by carefully removing the desired amount of solution with an autopipette and replacing it with 18 MΩ water of the same volume.34,36 The concentrations of each solution were back-calculated using the initial concentration of the first solution and the amount of solution removed each time. Different pipet tips were used for removing solution and adding water to prevent any carryover of solution. In total, about 15 dilutions were carried out, with duplicate measurements being taken for each concentration. A fluorescence measurement of pure 18 MΩ water was obtained at the end. The same successive dilution process was used for establishing the fluorescence intensity and concentration in the bulk.

A preliminary fluorescence spectrum of bulk luminol solution excited at 337 nm was measured to confirm that the maximum is found near 425 nm. A complete spectrum was not measured, either in the bulk or at the surface. Previous work31,33,35 has shown that the bulk aqueous and aqueous surface fluorescence spectra are essentially identical for clean water surfaces. The luminol fluorescence intensity at 420 nm was monitored following excitation at 350 nm in the bulk and at the water surface as a function of the concentration of added salt. For the bulk measurements, the cuvette was filled with the salt solution, and then 100 μL of luminol stock solution (∼750 μM) was added and allowed to mix. This resulted in a luminol concentration of ∼7.5 μM after the mixing in the cuvette. Three measurements of intensity were made on the sample, and then the cuvette was washed. This experiment was done in triplicate for solutions of NaCl and NaI from 1.0 to 10.0 mM. All NaCl (ACS reagent, ≥99.0% purity) and NaI (ACS reagent, ≥99.5% purity) solutions were prepared fresh prior to use. In the GALIF measurements, the sample dish was filled with the solution of choice and allowed to sit for 15–20 min in dark room conditions; then 100 μL of luminol stock solution (∼750 μM) was added to the surface, and the fluorescence measurements began immediately. Fluorescence intensity was measured every minute for 5 min, and then the solution was discarded, the dish was cleaned, and a new measurement began. The same range of NaCl and NaI concentrations was examined on the surface as in the bulk.

After the GALIF experiments, another stock solution was prepared by weighing 6 mg of luminol (Sigma-Aldrich, 97% purity) into a 1 L flask and then adding 1.0 L of 18 MΩ water. The flask was stirred overnight, and luminol was fully dissolved, resulting in a concentration of 32 μM. A commercial fluorescence spectrometer (RF 6000, Shimadzu) was used to measure the fluorescence spectra of solutions containing 3.2 μM of luminol mixed with concentrations of I2 (Alfa Aesar, 99.5% purity) between 0 and 150 μM in the bulk. For these measurements, the excitation and emission wavelengths used were 350 and 425 nm, respectively.

3. Results and Discussion

Our I2 detection scheme relies upon efficient quenching of luminol fluorescence by molecular iodine but not by iodide. To demonstrate that luminol fluorescence is effectively quenched by molecular iodine, we measured the luminol fluorescence spectrum in aqueous bulk solutions in the presence of I2. Figure 1a shows representative fluorescence emission spectra measured by a commercial fluorescence spectrometer for bulk solutions having a fixed concentration of luminol (3.2 μM) mixed with varying concentrations of I2. The spectrum is in line with previous studies that show the maximum fluorescence intensity of luminol peaks at 425 nm.28,29 Luminol fluorescence exhibits a clear intensity decrease as the I2 concentration increases, consistent with fluorescence quenching of luminol by I2 in the bulk. To confirm this, we performed triplicate fluorescence measurements of bulk solutions having a fixed concentration of luminol (3.2 μM) mixed with varying concentrations of I2 from 0 to 150 μM. Figure 1b shows a Stern–Volmer analysis of the fluorescence intensity at 425 nm as a function of the I2 concentrations. A good linear fit is seen (R2 = 0.93), confirming that the decrease of luminol fluorescence is due to quenching by I2. The slope of the linear fit yields a Stern–Volmer constant of (3.1 ± 0.1) × 104 M–1, which combined with a reported fluorescence lifetime of 10.0 ns,29,30 predicts a quenching rate constant of 3.1 × 1012 M–1 s–1. This value is almost 2 orders of magnitude greater than the maximum diffusion-limited quenching rate constant (∼2 × 1010 M–1 s–1), suggestive of complex formation between I2 and luminol in the ground state, as previously proposed.28,29 We will use this quenching effect as an indicator for the production of aqueous I2.

Figure 1.

Figure 1

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(a) Fluorescence emission spectra of solutions of 3.2 μM luminol mixed with varying I2 concentrations in the bulk measured using a commercial fluorescence spectrometer (RF 6000, Shimadzu). (b) Stern–Volmer analysis of the results of three separate experiments (red, blue, and yellow dots) including the data shown in panel (a). The slope of the line yields a Stern–Volmer constant of (3.1 ± 0.1) × 104 M–1.

We next demonstrate that we can use this quenching effect to probe for I2 at the air–water interface. In Figure 2, we display luminol fluorescence intensity measured as a function of luminol concentration in the bulk and on the aqueous surface using GALIF. The bulk results show a good linear correlation of fluorescence intensity with luminol concentration (R2 = 0.98). The water surface measurements are not linear but show an approach to leveling off the fluorescence intensity as the luminol concentration increases. The dependence of fluorescence intensity at the water surface on luminol concentration is well fit (R2 = 0.96) using a Langmuir formulation for describing the absorption isotherm,31−36 indicating that luminol partitions to the air–water interface. This gives confidence that changes in luminol fluorescence at the air–water interface can be well tracked using GALIF.

Figure 2.

Figure 2

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Fluorescence intensity of varying concentrations of luminol in the bulk (red triangles) and on the water surface (blue circles) using GALIF. A linear fit was made for the bulk measurements and a hyperbolic fit describing a Langmuir adsorption isotherm is shown for the surface measurements.

We next investigated whether any reactions in the bulk phase occur between luminol and either NaCl or NaI at concentrations of 1–50 mM. Figure 3 shows the fluorescence intensities of fixed concentrations of luminol (7.5 μM) mixed with different concentrations of salt solutions in the bulk. At salt concentrations <2 mM, no significant changes of luminol fluorescence intensity are observed, indicating that no reaction is occurring for luminol with low concentrations of NaI or NaCl in the bulk. At higher concentrations, a modest decrease is seen with increasing concentrations for both salts. These observations are consistent with modest fluorescence quenching of excited luminol by both salts at higher salt concentrations.

Figure 3.

Figure 3

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Fluorescence intensity of a fixed concentration of luminol as a function of varying concentrations of NaI and NaCl in the bulk. The error bars represent the standard deviation of three separate measurements.

The results presented above show that luminol is present and measurable via GAILF at the interface and that its fluorescence is efficiently quenched by molecular iodine but not by iodide. To detect the formation of I2 at the interface, a small volume of luminol (100 μL) was added to the surface of NaI solutions which had been allowed to sit for at least 20 min exposed to the atmosphere in the dark. The final concentration of luminol in solution is estimated to be 7.5 μM, which is the same concentration used for the bulk measurements discussed above. GALIF measurements were commenced immediately following the luminol addition to the solution surface. As shown by some representative results in Figure 4, a rapid increase in fluorescence intensity is observed when luminol is added to the surface of water or to that of a 1 mM NaI solution. After 1 min measurement time, the fluorescence intensity at the water surface decreases slightly, which is probably related to the diffusion of luminol into the bulk solutions. Note that the fluorescence intensity at the water surface remains at relatively high values even at the end of the 5 min measurement period. By contrast, a large decrease of fluorescence intensity at the surface of the 2 mM NaI solution is observed after 1 min measurement time, dropping to low values over the measurement time. At the surface of NaI solutions with higher concentrations (5–10 mM), we observe significantly lower fluorescence at the beginning of the measurement period. This observation stands in dramatic contrast to what is seen in the bulk, where there is no loss of fluorescence from iodide-containing solutions over 20 min after preparation. Similar experiments using NaCl rather than NaI as the solute did not exhibit any such consistent decrease in luminol fluorescence at higher salt concentrations (Figure S2). The results displayed in Figure 1 show efficient fluorescence quenching by molecular iodine; the results in Figure 3 indicate no such effect due to iodide, at least over the concentration range explored here. We therefore propose that the large decrease of fluorescence at the surface of NaI solutions, but not in the bulk, or at the surface of NaCl solution, is associated with the formation of I2 at the surface of NaI solutions, as implied by previous studies.9−11

Figure 4.

Figure 4

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Representative traces showing the time evolution of the luminol fluorescence intensity at the sample surface following the addition of different concentrations of NaI solution.

Figure 5 shows the percentage change in surface fluorescence intensity 5 min after addition of luminol relative to what is observed at the pure water surface ([Isalt – IH2O]/IH2O) as a function of salt concentrations (1–10 mM) for NaCl and NaI solutions. As implied by the results shown in Figure 4, a significant loss of luminol fluorescence is obvious at the surface of NaI solutions, while no such fluorescence loss is observed at the surface of NaCl solutions. The loss of surface luminol fluorescence exhibits a nonlinear dependence on the concentrations of NaI solutions, showing a saturation-type dependence on the initial iodide concentration. This behavior is well fitted (R2 = 0.92) using a Langmuir isotherm, yielding a result essentially the same as that seen for gas phase I2(g) production as a function of bulk NaI concentrations reported in our previous study.11 Since gas phase I2 is thought to be produced in or at the surface of iodide solutions,11 the striking similarity of the results displayed in Figure 5 to the dependence of gas phase I2 production on iodide solution concentration, shown in Figure 3 of Guo et al.,11 implies that gas phase I2 appears due to volatilization of I2 at the aqueous surface. Of course, this conclusion does not negate the report by Seki et al.10 of I3– also being present (perhaps formed) at the interface.

Figure 5.

Figure 5

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Percentage change of luminol fluorescence intensity 5 min after its addition to the surface of salt solutions with noted concentrations, relative to the intensity measured at the pure water surface ([Isalt – IH2O]/IH2O). The solid blue and red lines show the average values of three separate experiments for NaI solutions (shown as blue circles) and two separate experiments for NaCl solutions (illustrated by the red circles) respectively. The error bars represent the standard deviation of the percentage change of luminol fluorescence intensity over the measurement time of 180–300 s as shown in Figure 4.

4. Conclusions

In conclusion, using GALIF spectroscopy, we have presented strong evidence confirming that molecular I2 is present and perhaps generated spontaneously at the surface of aqueous NaI solutions. This observation provides additional evidence to support the observation of gas phase I2 and solution phase I3– from the reaction of I– and I2 at the air–water interface of microdroplets reported in recent studies.10,11 Volatilization of interfacial I2(aq) can act as an important but previously unconsidered source of iodine in atmospheric environments such as the sea spray aerosol and cloud microdroplets.

Acknowledgments

This research was supported by the NSERC and the European Research Council (ERC) under the Horizon 2020 Research and Innovation Program/ERC Grant Agreement 101052601-Spontaneous interfacial oxidant formation as a key driver for aerosol oxidation (SOFA).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpca.4c03010.

  • UV–vis absorption spectra of 100 times diluted stock luminol solutions (Figure S1); time evolution of fluorescence intensity of luminol when it was added to the surface of varying concentrations of NaCl solutions (Figure S2) (PDF)

The authors declare no competing financial interest.

Supplementary Material

jp4c03010_si_001.pdf (162.6KB, pdf)

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Associated Data

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Supplementary Materials

jp4c03010_si_001.pdf (162.6KB, pdf)

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