Heterogeneous Chlorination of Squalene and Oleic Acid

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Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Heterogeneous Chlorination of Squalene and Oleic Acid Heather Schwartz-Narbonne,† Chen Wang,† Shouming Zhou,† Jonathan P.D. Abbatt,*,† and Jennifer Faust*,†,‡ †

Department of Chemistry, University of Toronto, Toronto, ON Canada M5S 3H6 Department of Chemistry, College of Wooster, Wooster, Ohio 44691, United States

‡

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S Supporting Information *

ABSTRACT: Washing with chlorine bleach leads to high mixing ratios of gas-phase HOCl. Using two methods that are sensitive to surface film compositionattenuated total reflection fourier transform infrared (ATR-FTIR) spectroscopy and direct analysis in real time mass spectrometry (DART-MS)we present the first study of the chlorination chemistry that occurs when gaseous HOCl reacts with thin films of squalene and oleic acid. At mixing ratios of 600 ppbv, HOCl forms chlorohydrins by adding across carbon−carbon double bonds without breaking the carbon backbone. The initial uptake of one HOCl molecule occurs on the time scale of a few minutes at these mixing ratios. For oleic acid, ester formation proceeds immediately thereafter, leading to dimeric and trimeric chlorinated products. For squalene, subsequent HOCl uptake occurs until all six of its carbon−carbon double bonds become chlorinated within 1−2 h. These results indicate that chlorination of skin oil, which contains substantial carbon unsaturation, is likely to occur rapidly under common cleaning conditions, potentially leading to the irritation associated with chlorinated bleach. This chemistry will likely also proceed with cooking oils, in the human respiratory system which has unsaturated surfactants as important components of lung fluid, and with organic components of the sea surface microlayer.

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INTRODUCTION Household chlorine bleach is a pH 12 aqueous solution commonly used to disinfect surfaces. The active ingredient that gives bleach its cleaning power is hypochlorite (OCl−). NaOCl dissociates in water to form OCl−, which can form HOCl (pKa (20 °C) = 7.5) or Cl2 depending on the pH of the solution and the presence of Cl−.1 These processes are illustrated in Reactions 1 and 2 below. HOCl(aq) F OCl−(aq) + H+(aq)

(1)

HOCl(aq) + Cl−(aq) + H3O+(aq) F Cl 2(aq) + 2H 2O(l)

(2)

chloroform, chloramines (from the mixing of bleach and ammonia), and large quantities of chlorine gas (from the mixing of bleach and acid).5−7 Most recently, a study carried out by Wong et al. (2017) determined that the major gaseous byproducts released from floor washing, following standard cleaning procedures, are Cl2, HOCl, nitryl chloride (ClNO2), and chloramines (NHCl2, NCl3).8 All of these species were observed in the headspace of the cleaning fluid, except HOCl, which was only observed to a significant extent after floor washing. In the floor washing experiment, gas-phase mixing ratios of hundreds of ppbv of HOCl and tens of ppb of Cl2 were observed; in a less well ventilated and more confined room, the mixing ratios would have been considerably higher. Gas-phase HOCl was observed to decay from the room atmosphere at a rate 1.4 times faster than the air exchange rate, indicating that some gas phase or heterogeneous (i.e., gassurface) process was taking place. Light levels had no impact on the observed decay rate. These results raise the question, what is the fate of these gases, in particular HOCl, in the indoor atmosphere? Both HOCl and Cl2 participate in electrophilic addition reactions with double bonds in bulk-phase reactions. Dichlorine adds

Cl2 is a volatile species, whereas HOCl will only volatilize when present in its undissociated form. HOCl is known to be a biologically active molecule, and its strong oxidizing power makes it a potent antimicrobial agent. In the body, HOCl is created by the myeloperoxidase enzyme from H2O2 and Cl− as part of the inflammatory process, and it can react with proteins, nucleic acids, carbohydrates, and lipids.2 It can cause oxidative stress and kill bacteria and pathogens,3,4 but it is also associated with halogenative stress, which is in turn linked with cardiovascular, neurodegenerative, and infectious diseases.3 Few studies have examined the gaseous products and chemical consequences of indoor bleach use. To date, these have primarily focused on unintended byproducts of the bleaching process such as release of carbon tetrachloride, © XXXX American Chemical Society

Received: July 31, 2018 Revised: September 27, 2018 Accepted: October 10, 2018

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DOI: 10.1021/acs.est.8b04248 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology across the bond to form a dichloride.9 Similarly, HOCl adds across the double bonds of unsaturated molecules to produce cholorohydrins, which, if the appropriate functional groups are present, can further dimerize to form esters.2,4 See Figure 1 for reaction diagrams.

in real time. Direct analysis in real time mass spectrometry (DART-MS) was used to more precisely determine the molecular products of the heterogeneous oxidation reactions as they progressed, and to provide an initial estimate of their kinetics.

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EXPERIMENTAL SECTION Chemicals. Sodium hypochlorite (11.5% active chlorine by mass), dichloromethane (≥99% purity), hexanes (≥98% purity), isopropyl alcohol (≥99.5% purity), oleic acid (≥99% purity), squalene (≥98% purity), and squalene 2,3-epoxide (≥92% purity) were purchased from Sigma-Aldrich. Sodium phosphate monobasic (≥98% purity) was purchased from Caledon Laboratories. Sodium chloride (≥98% purity) was purchased from ACP Chemicals. Acetone (≥99% purity), hydrochloric acid (≥97% purity), and acetonitrile (≥99% purity) were purchased from Thermo Fisher Scientific. All aqueous solutions were prepared using 18.2 MΩ·cm deionized water. All the chemicals and reagents were used as received. Gas-Phase Source. For the heterogeneous oxidation experiments, chlorinated gases were delivered by bubbling nitrogen gas through NaOCl solutions prepared at a known pH. The gas-phase concentrations of HOCl and Cl2 in the nitrogen flow were characterized by measuring their absorption at 242 and 330 nm, respectively, for over 2 h. Specifically, a 348 mM solution of NaH2PO4 was prepared, and NaOCl was added to achieve a concentration of 367 mM and a pH of 6.3, which is below the pKa of 7.5 for HOCl. Fifty standard cubic centimeters·minute−1 (sccm) of N2 were bubbled through a frit in the solution, and the gases then passed through Teflon tubing to a 139 mm-long gas cell inside a PerkinElmer Lambda 1050 UV/vis Spectrophotometer, with N2 gas used as a blank. The gas-phase absorbance at 240 nm, where HOCl absorbs strongly, was continuously monitored. Concentrations of HOCl in the gas phase were calculated using the absorption cross section of 2.03 × 10−19cm2 molecule−1 at 242 nm.17 Because the HOCl source was prepared by bubbling gas through acidified aqueous solutions of hypochlorite, the presence of Cl2 is unavoidable based on the authors’ experience both in this work and in an earlier study.18 As a result, the use of Henry’s law to determine the HOCl partial pressure over these solutions is not possible given that some of the hypochlorite in solution is converted to Cl2, requiring the use of spectroscopic determination of the gas-phase concentration instead. Although Cl2 is present in the flow, there is little overlap between the absorption spectra of Cl2 and HOCl at 242 nm, and thus the absorbance at this wavelength arose solely from HOCl.19 Similarly, gas-phase Cl2 was monitored at 330 nm, where its cross section is 2.55 × 10−19cm2 molecule−1 and that of HOCl is very low.17 Beer’s law was used to calculate the gasphase concentrations of the HOCl and Cl2 analytes in equilibrium with this solution to be (2.5 ± 0.3) × 1015 and (6.4 ± 0.5) × 1015 molecules cm−3, respectively. For the infrared and mass spectrometric experiments described below, the gas-phase reagent flows consisted of 3 sccm of N2 bubbled through the above NaOCl solution, diluted by 600 sccm of N2. This resulted in the organic substrates being exposed to HOCl and Cl2 concentrations of (1.2 ± 0.2) × 1013 and (3.2 ± 0.2) × 1013 molecules cm−3, respectively. In addition to the solution characterized above, additional solutions were prepared with sufficient acidity to produce a

Figure 1. (A) Reaction mechanism depicting the addition of HOCl to an alkene, forming a chlorohydrin.9 (B) Reaction mechanism depicting oleic acid chlorohydrin dimerization, forming an ester.4

It is poorly characterized whether the heterogeneous reactions of gaseous bleach byproducts with unsaturated organic molecules proceed at an environmentally significant rate.10 The only study in the literature of such heterogeneous oxidation processes is that of Popolan-Vaida et al. (2015), who examined the reaction between chlorine gas and singlecomponent aerosol particles composed of a variety of unsaturated molecules, including squalene and oleic acid. They found that a moderately fast reaction occurs, with reactive uptake coefficients on the order of 10−5 to 10−4, and they also observed evidence for addition of Cl2 to molecules such as squalene that have multiple carbon−carbon bonds. To our knowledge, there are no previous studies of HOCl-organic heterogeneous interactions. In this study, squalene and oleic acid were chosen as representative unsaturated reactant molecules. Squalene is a highly unsaturated, unconjugated triterpene (Supporting Information (SI) Figure S1), which is a major component of human skin oil.11−13 It has been shown to be highly reactive with ozone, and it is one of the most important ozone scavengers in indoor air environments.12,14 Oleic acid is an example of monounsaturated fatty acids (SI Figure S1), which are commonly found in plant and animal membranes, in skin oil, and in cooking oils.11,13,15 In the aqueous phase, oleic acid has been shown to react with HOCl, forming chlorohydrins and dimeric and trimeric ethers and esters.4 Long-chain fatty acids are also found in the marine environment, in both primary sea spray particles and in the sea surface microlayer.16 The primary goal of this project is to determine if gaseous bleach byproducts, especially HOCl given its high abundance, react heterogeneously with thin films of liquid unsaturated hydrocarbons. Emphasis is given to the products that are formed in these reactions, with preliminary kinetics studies performed. In particular, attenuated total reflection fourier transform infrared spectroscopy (ATR-FTIR) was used to probe molecular structural changes in squalene and oleic acid B


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Figure 2. ATR-FTIR spectra of squalene (A) and oleic acid (B) pre (green) and post (red) oxidation. The difference changes in the spectra are plotted in blue. The times for the reactions were 180 and 60 min, in the cases of squalene and oleic acid, respectively. The difference spectrum has been vertically shifted for clarity. The concentrations of HOCl and Cl2 in the flow were (1.2 ± 0.2) × 1013 and (3.2 ± 0.2) × 1013 molecules cm−3, respectively.

concentration of Cl2 of up to 2 × 1016 molecules cm−3(see eq 2). However, HOCl concentrations were roughly the same as above, that is, 1.4 × 1013 molecules cm−3. These solutions were used to evaluate whether the reactivity with organic films was due to Cl2 or HOCl, as described below. Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR). The methodology for the ATR-FTIR experiments was adapted from Roberts et al. (2016).20 A Thermo Scientific iS50 spectrometer equipped with a Pike Technologies VeeMax II variable angle ATR accessory (set at 45°) and a 2.0 cm diameter germanium crystal (cleaned prior to use with isopropyl alcohol) was used for real-time reaction monitoring. For each measurement, the instrument collected and averaged 8 scans, with a resolution of 4.0 cm−1. A DTGS detector with KBr window was used. The experimental setup is depicted in SI Figure S2. N2 was flowed through Teflon tubing to a custom-designed stainless steel cell in which the carrier gas passed over the ATR crystal. N2 flow was used to generate a background spectrum. A solution of 13.3 mM squalene dissolved in hexanes was prepared the day of each experiment and stored in the dark when not in use throughout the day. Thirty μL of this solution

(i.e., 164 μg of squalene) were deposited onto the crystal via a micropipette over an area of roughly 3.14 cm2, and the hexanes were given 2 min to evaporate, as confirmed by the infrared spectrum. The evanescent wave at 45° for a Ge crystal with a sample of refractive index 1.5 has a depth of penetration between 0.39 and 2.0 μm for a wavenumber range between 600 and 3000 cm−1.21 Using the mass and density, and assuming a homogeneous layer was formed, we calculate that the squalene layer produced was 0.64 μm thick, meaning that nearly the entire depth of the sample was probed. This calculation is just a rough approximation given that the film is unlikely to be entirely uniform. Measurements of the squalene sample under N2 flow were recorded. The diluted HOCl/Cl2 flow described above was then flowed through the cell, with data collection commencing when the chlorinated flow first passed over the crystal, and measurements continued until there were no changes observed between subsequent spectra. The time between each measurement was 69−71 s. Oleic acid reaction monitoring was carried out using the same parameters, except oleic acid was dissolved in acetone to C


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906 cm−1, and 780 cm−1, which correspond to the O−H stretch, H−O−H bend in water, C−OH stretch, C−C−O bend, and C−Cl stretch, respectively.23,24 All ATR-FTIR assignments are listed in SI Tables S1 and S2. These data are fully consistent with the addition of OH and Cl groups to double bonds in the squalene molecule via the chlorohydrin reaction (Scheme A in Figure 1). As well, the film becomes more hygroscopic with sorbed water. The C−Cl peak is relatively broad, which could be due to the variety of chemical environments that arise for the Cl atoms in different positions on the molecule. Given that both HOCl and Cl2 pass through the reaction cell, we evaluated whether HOCl is the primary reactant by performing an experiment wherein the reactant flow that contains the very high ratio of Cl2/HOCl (roughly 3 orders of magnitude) was passed through a vessel containing a slurry of 30 g NaCl acidified with 10 mL HCl. It has been shown previously that this mixture quantitatively converts HOCl to Cl2 by promoting Reaction 2, thus removing HOCl from the gas stream.8,25 In this mode of operation, the ATR-FTIR spectrum does not change compared to the regular experiments (see SI Figure S3), confirming that HOCl is indeed the primary reagent in this system. If Cl2 is reacting, it does so much slower than does HOCl. In a separate control experiment, we also tested whether epoxide functional groups are forming in the reaction by measuring the ATR-FTIR spectrum of squalene 2,3-epoxide. Although chlorohydrins can eject HCl to form epoxide functional groups, SI Figure S4 shows that there is no spectral evidence that epoxides are forming. The rate of change of major spectral features (see SI Figure S5 for the peak at 910 cm−1) are linear at early times consistent with zero-order kinetics. This is because the supply of HOCl to the squalene substrate controls the kinetics, that is, all the gasphase reagents are being consumed and are not in excess to the amount of squalene on the surface. The changes in the spectrum cease when the squalene sample is fully consumed. As shown in SI Figure S5, the plateau region is reached at 130 min of HOCl exposure, by which point 1.8 × 10−6 moles of HOCl have flowed through the cell. By comparison, there are 4.0 × 10−7 moles of squalene on the ATR crystal, corresponding to 2.4 × 10−6 moles of double bonds given that there are 6 double bonds per squalene molecule. This value closely matches the number of moles of HOCl flowing over the first 130 min, confirming complete conversion of all double bonds in squalene to chlorohydrins. Note that uptake kinetics, in the form of a reactive uptake coefficient, cannot be extracted from these data because the HOCl concentration is not constant in the reaction cell. DART-MS spectra are shown for squalene oxidation in Figure 3A, illustrated at short (10 min) and long (90 min) reaction times. The reactant spectrum at t = 0 min has been subtracted from the spectra in Figure 3. Positive mode spectra are shown in Figure 3A whereas negative mode spectra, which are consistent with those in the positive mode, are shown in SI Figure S7. The complex nature of the mass spectra arises from the large number of reaction products associated with the six carbon−carbon double bonds in squalene and also because of the two isotopes of chlorine, because DART-MS ionization can proceed via multiple reagent ions, and because ion fragmentation can occur. Despite this complexity, mass spectral assignments from some of the key peaks in the spectra are presented in SI Tables

a concentration of 13.3 mM, the film thickness and mass were 0.40 μm and 113 μg, and the time between scans was 44−47 s. All results were found to be reproducible over at least three separate experiments. Direct Analysis in Real Time Mass Spectrometry (DART-MS). The methodology for the DART-MS experiments was adapted from Zhou et al. (2016).14 A 2.43 mM solution of squalene or 3.54 mM solution of oleic acid in 1:1 dichloromethane to acetonitrile was prepared. Ten glass melting point capillary tubes with their ends sealed were held in a homemade Teflon capillary mount. The capillaries were heat treated to remove contaminants, and a micro syringe (10 μL, SGE Analytical Science) was used to deposit 1 μL of solution (1 μg of squalene or oleic acid) onto each tube. The solvent was allowed to evaporate, producing thin layers of analyte on each capillary. Assuming a homogeneous film area of roughly 4 mm2, the thicknesses of the squalene and oleic acid layers were 3.6 × 1016 molecules cm−2 or 5.3 × 1016 molecules cm−2, respectively, that is, on the order of 0.25 μm. (It should be noted that a previous study performed by Zhou et al. (2015) showed that Benzo[e]pyrene films formed via this method were not uniform, but this assumption was made in the interest of calculating the preliminary reactive uptake coefficient.22) The same diluted chlorinated flow of nitrogen was then passed through a cylindrical glass flow tube roughly 1 L in volume with an inlet and outlet flange mounted on both ends. The capillaries were placed inside the cell and exposed to the reagents for a range of experimental reaction times: 0, 5, 10, 30, 45, 60, 90, and 120 min. The experimental setup is depicted in SI Figure S2. As in the ATR-FTIR experiments, the concentrations of HOCl and Cl2 in the flow tube were (1.2 ± 0.2) × 1013 and (3.2 ± 0.2) × 1013 molecules cm−3 of HOCl and Cl2, respectively. The oxidized samples from the flow tube were stored in a desiccator, where a flow of nitrogen was added to prevent contamination from the room air before analysis with DARTMS. The method has been described previously in further detail.13 In brief, the capillary holder was placed on a rail motorized at a speed of 0.3 mm s−1 between the ion source and MS inlet. A flow of helium (∼3.0 standard liters per minute (slm)) was used in the DART source that was held at ∼500 °C (giving rise to a temperature of roughly 280 °C at the capillary tube).13 A Vapur Interface (IonSense Inc., Saugus, MA) with a ceramic inlet was mounted between the linear rail and the MS inlet. A diaphragm pump was used to adjust a total flow of ∼3.8 slm through the ceramic tube. DART-MS was operated under both positive and negative modes. The MS instrument was a JMS-T100LC time-of-flight mass spectrometer (JEOL USA Inc., Peabody, MA) with a mass resolution of a few thousand at m/z 600. Only mass-to-charge ratios with nominal unit mass resolution were used in this work given that the DART-MS signal intensity was generally not sufficient to permit more accurate mass analysis.

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RESULTS AND DISCUSSION i. Squalene Reaction. From the ATR-FTIR experiments, the main spectral changes observed for squalene during the course of the experiment are illustrated in Figure 2A. In particular, there were decreases in the peaks at 2967 cm−1, 2913 cm−1, and 2854 cm−1, which correspond to C−H stretches.12 Furthermore, the peak at 1668 cm−1, which corresponds to the CC stretch, disappeared.12 At the same time, peaks appeared at 3081 cm−1, 1645 cm−1, 1240 cm−1, D


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S3 and S4. As supported by the literature,26 we see clear evidence in the positive ion mode for ionization via addition of both + H+ and + NH4+. In the negative ion mode, we see evidence for deprotonation (−H+) and for adducts (+Cl−). Also, there is the potential for loss of H2O or HCl after ionization of a chlorohydrin. In SI Tables S3 and S4 (and in SI Tables S5 and S6 for oleic acid), we have indicated the massto-charge ratio associated with the most or second-most intense peak that arises from the species indicated, noting that there are many additional peaks associated with each ionized molecule given the many chlorine isotope combinations possible. There is clear evidence from Figure 3A and SI Table S3 that HOCl is added to the squalene backbone forming chlorohydrins, in support of the ATR-FTIR results. Figure 3A shows that there is a progression from lower molecular weight products present at 10 min of HOCl exposure, transitioning to much higher molecular weight products at 90 min exposure. To illustrate the temporal progression of HOCl addition to the squalene backbone, Figure 4 shows the intensities of mass spectral peaks up to 120 min exposure time, where the six individual panels in the figure represent mass spectral assignments of peaks with 1−6 additions of HOCl. For example, the first panel plots the intensities for all peak areas for mass-to-charge ratios identified as arising from one HOCl addition, the second panel plots peaks arising from two HOCl additions, and so on. There is a clear transformation from early times (approximately 10 min exposure time) when one HOCl

Figure 3. Positive-mode DART-MS spectra of squalene (A) and oleic acid (B) both at 10 min and at 90 min of reaction time. For clarity, just a few of the key peaks are labeled with their mass-to-charge ratio (m/z). The concentrations of HOCl and Cl2 in the flow were (1.2 ± 0.2) × 1013 and (3.2 ± 0.2) × 1013 molecules cm−3, respectively.

Figure 4. Intensities as a function of time of positive mode DART-MS mass spectral peaks, where panels A−F correspond to peaks assigned to product molecules with 1−6 HOCl molecules that have been added to squalene (see text for explanation and SI Table S3 for assignments). The labels correspond to the specific mass-to-charge ratio for which the data are plotted. E


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reaction (Figure 1). This chlorinated ester can then add a second HOCl molecule to the double bond in the second oleate chain, yielding a molecule detected at m/z 632. Further esterification can occur involving the addition of a third oleic acid molecule, forming molecules detected at m/z 918, 932, 933, and 987 at long reaction times; see SI Table S5 for peak assignments. We believe that considerable fragmentation occurs in the DART-MS spectrum with these highly functionalized, high molecular weight products, producing the additional peaks (for example, at m/z 688) seen in the 90 min spectrum; for that reason we do not make assignments for these peaks. Similar evidence for the formation of esters is present in the negative mode DART-MS spectra (see SI Figures S8 and SI Table S6). The time profile for the conversion of dimeric products (which peak at about 10 min of reaction time) into trimeric products (which peak at about 50 min) is shown in SI Figure S10. The rate constant for the decay of oleic acid as observed at m/z 281 in the negative mode (2.8 ± 0.5) × 10−3 s−1 (see SI Figure S11) corresponds to a reactive uptake coefficient of 1.1 × 10−3. This is likely a lower limit to the true reactive uptake coefficient given gas-phase diffusion restrictions. This value is similar to the reactive uptake coefficient for ozone with oleic acid, which has been measured by many research groups to be in the range from 0.6 × 10−3 to 1.6 × 10−3.29 By contrast, Cl2 reacts a hundred times more slowly with a reactive uptake coefficient value of 2.3 × 10−5.10

addition dominates, to later times (100−120 min) when the peaks arising from five and six HOCl additions dominate. Interestingly, the peaks in the plots for the first three additions of HOCl, at 15, 30, and 45 min respectively, are all equally spaced. This pattern implies that addition of one HOCl molecule to squalene does not significantly enhance or diminish the rate of the addition of a subsequent molecule. To gain another indication of the reaction kinetics for this system, we note that the first order rate constant for squalene decay, detected in the positive mode DART-MS spectra as the protonated and ammoniated ion, is (8.9 ± 0.8) × 10−4 s−1 (see SI Figure S9). Using a standard approach (for example, see Fu et al. (2013))12, this rate constant corresponds to a reactive uptake coefficient of 2.5 × 10−4, which represents the fraction of collisions of HOCl with the squalene film that lead to reaction. Given that this calculation requires knowledge of the film surface area, which is quite uncertain in this work with roughly a factor of 2−3 uncertainty, we consider this a preliminary measurement of the reactive uptake coefficient. As well, gas-phase diffusion restrictions are challenging to estimate with this reaction geometry. A more appropriate method for the measurement of reactive uptake coefficients is the use of an aerosol flow tube. To put this preliminary value into context, the reactive uptake coefficient for ozone on squalene has been reported to be between 1 × 10−5 and 5 × 10−4.12,14,27,28 Similarly, using an aerosol flow tube, Popolan-Vaida et al. (2015) report that the reactive uptake coefficient of Cl2 to squalene is 6.6 × 10−5.10 We conclude that HOCl reacts at least as efficiently with squalene as does ozone, and faster than Cl2. ii. Oleic Acid Reaction. In the case of oleic acid, there is also clear evidence for HOCl addition to the carbon−carbon double bond via chlorohydrin formation. However, although there is only one carbon−carbon double bond in oleic acid, the system has additional chemical complexity given the potential for ester formation to occur via reaction between the OH group in the chlorohydrin and the carboxylic acid functionality in oleic acid (see Scheme B of Figure 1). In the oleic acid ATR-FTIR spectrum (see Figure 2B and SI Table S2), there were intensity decreases in the peaks at 2981 cm−1, 2921 cm−1, and 2853 cm−1 related to C−H stretches. Furthermore, the peak at 3010 cm−1, which corresponds to an allylic C−H stretch, disappeared.20 Concurrently, there was the pronounced disappearance and appearance of peaks at 1710 and 1738 cm−1, which correspond to the carboxylic acid CO stretch and an ester CO stretch, respectively.20,24 This result agrees with the observations made by Schröter in 2016 that HOCl reacts in the bulk phase with oleic acid to form esters.4 Given that only one CCl bond is forming in oleic acid, the weak CCl feature that appears in the spectrum of squalene chlorination products is not observable in the oleic acid experiments. As with squalene, we note that the reaction ceases after roughly 25 min of HOCl exposure, corresponding to the flow of 3.4 × 10−7 moles of HOCl. This matches very closely the number of moles of oleic acid on the ATR crystal (4.0 × 10−7 moles). DART-MS data support the rapid formation of esters after initial formation of a chlorohydrin. In particular, Figure 3B shows that large peaks arise at early times at m/z 599 and 616, which correspond to the mass of two oleic acid molecules plus the mass of HOCl minus the mass of water (see SI Table S5). Thus, one chlorohydrin reacts with another oleic acid molecule, ejecting water in the ester-forming condensation

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ENVIRONMENTAL IMPLICATIONS Given that chlorine bleach solutions are moderately alkaline, there is little gaseous HOCl in the headspace of bleach solutions. However, washing with bleach releases large quantities of both HOCl and Cl2 to indoor spaces; indeed, a substantial fraction of the active chlorine in bleach is ultimately released from washing clean floors as HOCl vapor.8 The additional proton may either arise from acidic substances on the floor or from equilibration of the washing solutions with atmospheric CO2. The HOCl photolytic lifetime indoors is much longer than the air-exchange rate, but nevertheless gasphase HOCl was observed by Wong et al. (2017) to be removed faster than the air exchange rate.8 This observation implies that HOCl is subject to other loss processes in the room. The HOCl mixing ratios reported by Wong et al. (2017) of roughly 100 ppbv were measured in a tall room with extremely fast air exchange (13/hour). The values used in this work (600 ppbv) are more representative of values that will prevail in less well ventilated spaces. The results reported here indicate that an indoor uptake mechanism of HOCl is reaction with unsaturated molecules present on indoor surfaces. Examples of such surface constituents include (i) skin oils and flakes, which contain squalene, unsaturated fatty acids, and unsaturated triglycerides, and (ii) cooking oils, which contain unsaturated triglycerides such as triolein. In particular, the key experimental result is that HOCl reacts rapidly with unsaturated molecules such as squalene and oleic acid. The initial uptake of one HOCl molecule occurs on the time scale of minutes. In the case of squalene, which has six carbon−carbon double bonds, the sixth HOCl molecule can be incorporated into the squalene backbone within one to 2 h at a 600 ppbv mixing ratio of HOCl. The reactive uptake coefficient consistent with this uptake, approximately 2.5 × 10−4, is sufficiently large that the movement of HOCl from the gas phase to indoor surfaces F


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Environmental Science & Technology coated by these oils, as potentially in a kitchen, will be fast. Of course, HOCl may also react with other components of surface films. For example, a selective mechanism is reaction with thiols, as present in some proteinaceous materials.30 The potential human health consequences of HOCl exposure include impacts on both the skin and respiratory systems. As opposed to oxidation by ozone, which breaks carbon−carbon bonds, leading to the formation of volatile oxygenated end products, oxidation by HOCl does not lead to carbon−carbon bond breakage. Rather, the products formed are relatively nonvolatile, with molecular weights much higher than that of the precursor. In the case of squalene, this transformation occurs via the formation of highly chlorinated products, whereas in the case of oleic acid the end products are lightly chlorinated oleate esters. We note that the reactive uptake coefficients of HOCl and O3 with squalene, measured by the same technique, are within a factor of 2 of each other: 2.5 × 10−4 and 4.2 × 10−4, respectively.14 Thus, during the time that HOCl mixing ratios are high during bleach washing (i.e., 100s of ppb or higher), HOCl will dominate the oxidation rate of squalene in skin oil, given that ozone mixing ratios are usually only a fraction of outside values.31 Studies of bleach exposure have pointed out the hazards that arise when bleach is mixed with ammoniated cleaners, leading to the formation of harmful chloramines, for example.7 Other work has evaluated exposure responsessuch as via skin irritation teststhat primarily use application of alkaline NaOCl solutions.32 However, the OCl− ion is not as reactive as HOCl toward molecules with carbon−carbon double bonds,1 which are widely present not only in skin oil but also in the phospholipids that line the lung. This distinction suggests that it is important to better understand the relationship between HOCl exposure and irritation of both human skin and respiratory systems. Lastly, we point out that HOCl is also an important molecule in the marine boundary layer, acting as a reservoir compound for free radical chlorine. To our knowledge, it has not been evaluated whether HOCl may react with unsaturated components of marine aerosol, or with the biologically produced molecules that make up a large part of the sea surface microlayer.16

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(Figure S6); DART-MS spectrum in the negative mode during chlorination of squalene and oleic acid (Figure S7 and S8); decay of squalene signal as a function of time at m/z 411 and 428 in positive ion mode (Figure S9); intensities as a function of time of positive mode DARTMS mass spectral peaks for the chlorination of oleic acid (Figure S10); decay of oleic acid signal as a function of time at m/z 281 in negative ion mode (Figure S11) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(J.A.) E-mail: [email protected]. *(J.F.) E-mail: [email protected]. ORCID

Heather Schwartz-Narbonne: 0000-0003-4776-0974 Chen Wang: 0000-0001-9565-8777 Jonathan P.D. Abbatt: 0000-0002-3372-334X Jennifer Faust: 0000-0002-2574-7579 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Alfred P. Sloan Foundation under its Chemistry of Indoor Environments program, and by NSERC.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.8b04248. Peak assignments for ATR-FTIR observations for chlorination of squalene and oleic acid (Tables S1 and S2); peak assignments for DART-MS positive and negative ion observations for chlorination of squalene and oleic acid (Tables S3−S6); structures of squalene and oleic acid (Figure S1); schematic of experimental setup for ATR-FTIR and DART-MS measurements of the oxidation of squalene and oleic acid films (Figure S2); ATR-FTIR data for squalene, where the chlorine source is passed through an acidified NaCl scrubber that removes gas-phase HOCl (Figure S3); ATR-FTIR data where the spectra for squalene and squalene 2,3-epoxide are compared to the difference spectrum arising via chlorination of squalene (Figure S4); peak height at 910 cm−1 during chlorination of squalene (Figure S5); peak height at 1738 cm−1 during chlorination of oleic acid G


Article


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Heterogeneous Chlorination of Squalene and Oleic Acid

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