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$Department of Physics, Indian Institute of Technology Bombay, Mumbai -400076, India ... The paint water interface is gaining the attention of the nuc...
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Heterodyne-Detected Sum Frequency Generation Study of Adsorption of I− at Model Paint−Water Interface and Its Relevance to Post-Nuclear Accident Scenario P. Mathi,*,† B. N. Jagatap,§ and Jahur A. Mondal†,‡ †

Radiation & Photochemistry Division, Bhabha Atomic Research Centre, ‡Homi Bhabha National Institute, Trombay, Mumbai-400085, India § Department of Physics, Indian Institute of Technology Bombay, Mumbai-400076, India ABSTRACT: Organic iodides constitute a significant fraction of radioactive iodine released into the environment in the event of a nuclear power plant accident. The painted surfaces inside the reactor containment play a key role in the formation of organic iodides. In this study, heterodyne detected vibrational sum frequency generation (HD-VSFG) spectroscopy has been used to gain insight into the origin of organic iodides from paint surfaces. Model polymeric compounds dimethylhexadecyl amine (DHDA) and Nylon6, which resemble the constituents of containment paints, are selected for this study. Our investigations on the DHDA−water interface and Nylon6−water interface reveal the existence of positive surface field at acidic conditions (bulk pH: 2, 6) due to protonation at amine functional group and adsorption of H+ at amide groups; and a negative surface field at pH 11 due to adsorption of OH− ions at both amine and amide functional groups. In the presence of CsI in the aqueous phase, this surface field is altered by the counterion effect of I− (at pH 6 and pH 2) and Cs+ ions (at pH 11) at the DHDA−water and Nylon6−water interfaces. These studies highlight that at acidic bulk pH (pH < 7), both DHDA and Nylon6 participate in adsorption of I− at the interface, and compared to Nylon6, DHDA is more effective in adsorbing I−. On the other hand, at bulk pH = 11, I− is repelled from both the Nylon6−water interface and the DHDA−water interface, suggesting the lower probability of organic iodide formation at alkaline condition. for example) are difficult to retain.15 These VOI-r compounds, which escape into the environment, have high radiotoxicity, as iodine is biologically active and in mammals it accumulates readily in the thyroid. Exposure to high levels of radioactive iodine may increase the risk of radiogenic thyroid cancer in later life.16 Given the hazardous consequences of VOI-r compounds, it is therefore necessary to elucidate the molecular level details of the origin of such species at the paint−water interface. I− ion present in the aqueous phase may also play a significant role in the formation of organic iodides and is the basis of this investigation. Adsorption of inorganic halides (e.g., NaX; X− = F−, Cl−, Br−, and I−) at the aqueous interface (e.g., the air−water interface) has been studied by molecular dynamics simulations, which suggested a surface excess of halide ions, the surface propensity being more for highly polarizable anions like I−.17,18 These results are in good agreement with interface-selective experimental results, such as vibrational sum frequency generation (VSFG)19,20 and its phase sensitive variant,

1. INTRODUCTION Interfaces represent the initial reaction site in many natural processes such as transport of ions across membranes,1,2 ion sorption at mineral water interface,3 formation, growth, and aging of aerosols,4−7 heterogeneous catalysis,8,9 and corrosion,10 to name a few. The corrosion of painted material surfaces is a serious concern to paint manufacturers and applicators and metallic structure maintainers. The paint−water interface is gaining the attention of the nuclear power plant community as well. It is now believed that painted structural surfaces within containment buildings of nuclear power plants11,12 are the new source of volatile organoiodineradioactive (VOI-r) compounds, which constituted a significant amount of radioactive iodine (∼160 PBq of 131I) emitted into the atmosphere during the Fukushima Dai-ichi nuclear power plant accident.13,14 However, under manageable accident conditions, the high temperature, high pressure gases are released from the reactor containment to the outside via a filtered containment venting system in order to preserve the structural integrity of the containment. During this process, molecular iodine is easily trapped by these post-accident filtration systems that employ filter materials such as charcoal, scrubber, and gas masks, while organic iodides (RI, R = CH3, © XXXX American Chemical Society

Received: February 10, 2017 Revised: March 21, 2017 Published: March 23, 2017 A

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The Journal of Physical Chemistry C heterodyne detected VSFG (HD-VSFG) studies21−23 of NaXcontaining aqueous interfaces. A significant counterion effect of halide ions has been observed at positively charged interfaces, and the counterion effect increases as F− < Cl− < Br− < I−. Thus, these investigations provide a general description of the interfacial behavior of iodide anion at the air−water as well as charged surfactant−water interfaces, but do not shed light on the prevalence of iodide anion at the paint−water interface, which is different (depending on the bulk pH) from the air− water or charged surfactant−water interfaces. Therefore, for a molecular level understanding of the origin of organic iodides in the aftermath of a nuclear accident, it is essential to investigate the adsorption characteristics of iodine species at the paint−water interfaces pertinent to the containment chamber of nuclear power plants. To this end, cesium iodide (CsI) has been chosen as the model compound to represent the predominant iodine species for this study. The rationale is that in the event of a postulated nuclear accident, iodine is released into the containment atmosphere, in both gaseous and particulate forms whose composition and distribution evolves over time. The prominent species are the CsI aerosol particles, which are likely to settle in the sump and instantaneously dissociate into Cs+ and I−.24 Further, under such accident conditions, the pH of sump water, calculated to be alkaline (∼10−11), can also become acidic due to dissolution of nitric acid, formed from nitrate/nitrite generated during the radiolysis of humid air inside the containment, as well as absorption of HCl formed from Cl− ions generated during radiolytic decomposition of cable insulation and other polyvinyl chlorides.25,26 Regarding the paints used in containment buildings, they are mostly of epoxy type based on diglycidyl ether of bisphenol A (DGEBA) and cured with hardeners such as amides, amines, or alcohols.27 Since the paint is a complex mixture of a variety of organic compounds, it represents a complicated system to study using the interface-selective and phase-sensitive HD-VSFG technique (vide infra). Therefore, we have simplified the system by choosing model polymeric compounds dimethylhexadecyl amine (DHDA) and Nylon6 (a polyamide consisting of repeating units containing six carbon atoms and one amide group) that mimic the structural characteristics of paints used in French nuclear power plant containments28 (see Figure 1). The aqueous interface formed by DHDA and Nylon6 monolayers on water (at different bulk pH) was measured using HD-VSFG spectroscopy. The power of this technique (as compared to conventional VSFG) lies in its ability to provide

information on orientation of the molecules at the interface. In conventional VSFG, the SFG intensity is proportional to the absolute square of the second-order nonlinear susceptibility (|χ2|2); hence, any spectral information pertaining to amplitude and phase can only be extracted by fitting, the parameters for which are not unique.29 On the other hand, the HD-VSFG technique provides the phase information through the imaginary part of the complex nonlinear susceptibility Imχ(2).30 Unlike the intensity spectrum (|χ2|2 spectrum) in the conventional VSFG measurement, the Imχ(2) spectrum is directly comparable to the linear absorption spectrum (Imχ1 spectrum) of the molecules and hence provides unambiguous information on an interface. Specifically, the Imχ(2) signal is linearly proportional to the surface density of probe molecules (Ns), i.e., Imχ(2) ∞ Ns, and hence, linearly additive with surface density, similar to that of the absorbance in a bulk phase (absorbance ∞ concentration). The linear concentration dependence makes the quantitative analysis of Imχ(2) signal more straightforward than that of the |χ(2)|2 signal (i.e., |χ(2)|2 ∞ |Ns|2) obtained in conventional VSFG measurement. We have exploited this feature of HD-VSFG spectroscopy to evaluate the relative electric fields at model paint−water interfaces with varying bulk pHs and electrolytes. Moreover, the sign of the Imχ(2) spectrum reveals the absolute orientation of the molecules at the interface and hence the charged characteristics of the interfacial region.31,32 The Imχ(2) spectra (OH stretch of the interfacial water and CH stretch of the alkyl chains of model paint molecules) of model paint−water interfaces in the presence of CsI in the aqueous phase of varying pH reveal that both DHDA and Nylon6 participate in adsorption of iodide at the interface even at weakly acidic pH (pH = 6) and compared to Nylon6, DHDA is more efficient in I− adsorption. On the other hand, at alkaline conditions (pH = 11), I− is adsorbed neither at the Nylon6−water interface nor at the DHDA−water interface, suggesting the lower chance of organic iodide formation at alkaline condition.

2. EXPERIMENTAL SECTION 2.1. Materials. N,N-Dimethyl hexadecylamine (DHDA; ≥98.0%) was purchased from TCI. Nylon6 pellets and CsI (≥99.0%) were purchased from Sigma-Aldrich, and n-hexane (HPLC grade) was from SD FineChem, India. All these chemicals were used as received. Stock solutions of DHDA and Nylon6 were prepared in n-hexane. While the concentration of DHDA stock solution was typically 1 mg/mL, Nylon6 was sparingly soluble in n-hexane. Milli-Q water (18.2 MΩ cm resistivity) was used for all the measurements. CsI salt solution was prepared in Milli-Q water. The pH of water was varied from 2 to 11 by adding diluted HCl/NaOH, without additional buffer. This precaution has been taken so as to preclude the effect of additional cations and anions (constituents of buffer). The pH (bulk) of the experimental solution was measured before and after the HD-VSFG experiment, and was found to vary within a narrow range (pH = ±0.3) for all the measurements. For carrying out HD-VSFG measurement, self-assembled monolayers of DHDA and Nylon6 on water were prepared by depositing ∼10 μL of the stock solution on water, kept in a Petri dish (diameter: 6 cm). The surface pressure was monitored with a commercial surface tension meter (Kibron, Finland) and was maintained within the range 25 ± 5 mN/m. Upon addition of CsI to the aqueous subphase, the surface pressure increased to ∼48 ± 5 mN/m. HD-VSFG measure-

Figure 1. (a) Chemical structure of paint used in French nuclear reactor containments. Model systems used in this study: (b) Nylon6, (c) dimethylhexadecyl amine (DHDA). B

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molecular orientation with respect to surface normal. Furthermore, the net Imχ(2) signal in PPP combination has contributions from all nonvanishing χ(2) (four effective χ(2)) at the interface, whereas that in SSP polarization measures only one effective χ(2) (i.e., χ(2) yyz ). This makes the orientational analysis of the Imχ(2) signal in SSP configuration more straightforward than that in PPP.

ments on the DHDA−water and Nylon6−water interfaces were carried out after ∼10 min of addition of the respective stock solutions on water, which ensures evaporation of n-hexane from the water surface. 2.2. HD-VSFG Measurements. The Imχ(2) spectra were recorded in an indigenously developed heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectrometer. The experimental setup has been described in detail in our previous publications.33−35 Briefly, a narrow visible pulse (ω1) (λcenter: 800 nm, fwhm: 1 nm, 20 μJ/pulse) and a broad infrared pulse (ω2) (λcenter: 3200 nm, fwhm: 350 nm, 6 μJ/pulse) were temporally and spatially overlapped on the sample surface (DHDA, Nylon6 monolayer on water) to generate the sum frequency from the sample (SF1). Both beams were focused prior to impinging on the sample surface. The visible and infrared beams reflected from the sample surface were further refocused onto a GaAs(110) surface (local oscillator, LO) to generate another sum frequency (SF2). The SF1 was delayed with respect to the SF2 by passing through a 1-mm-thick glass plate. Both SF1 and SF2 pulses were collimated and focused into the slit of a polychromator and detected as an interference fringe (raw sample spectrum) by a thermoelectrically cooled (−70 °C) CCD. A similar interference fringe (raw reference spectrum), obtained from a Z-cut quartz crystal, is used for intensity and phase calibration of the sample SFG signal. The quartz does not have any resonance; hence the quartz spectrum can be used as an “external reference”. Following the procedure of Yamaguchi et al.,36 the Imχ(2) and Reχ(2) spectra of the sample (i.e., the model paint−water interface) is obtained by dividing the sample interferogram by the Z-quartz reference interferogram. It needs to be mentioned here that the procedure of using “external reference” may contain “phase-error” in the Imχ(2) spectra due to optical path difference between the sample and quartz surfaces. Extra care was taken to minimize the height of the Z-cut quartz fixed in such a way that the image of the sum frequency signal from the reference quartz is on the same pixels of the CCD as that of the sample. Further, the change in the height of the DHDA and Nylon6 monolayers due to evaporation of water during acquisition was compensated by raising the sample (placed on a computer-controlled translational stage) height by 0.4 μm in every 5 s (without feedback; laboratory temperature: 23.5 °C and humidity: ∼45%). This exercise ensures that the sum frequency image (from the sample) falls on the same pixels of the CCD as that of the reference quartz. With this improvisation, the Imχ(2) spectra could be recorded with a phase accuracy of ±20°, which might still be critical for the correct spectral interpretation, especially for interfaces with weak Imχ(2) signal. For such cases, the experimentally Imχ(2) spectra were further phase-corrected (mathematically) by internal referencing,35 such that the Imχ(2) signal is close to zero in the region beyond the vibrational resonance (e.g., 2700−2900 cm−1 regions for the air−H2O interface). The phase-corrected Imχ(2) spectrum thus provides accurate absorption characteristics of interfacial molecules and the sign of Imχ(2) spectra reveals the net orientation of interfacial molecules (and in turn, the direction of the interfacial electric field). The SSP polarization (S-sum frequency, S-visible, and P-infrared) combination was used in all the experiments. This is because, among the different possible polarization combinations (SSP, PPP, PSS, and SPS), it is only the SSP and PPP combinations which are sensitive to dipole derivative normal to the surface, providing information about the

3. RESULTS 3.1. Effect of CsI on DHDA−Water Interface under Different Bulk pH Conditions. 3.1.1. pH 6. Figure 2a (red

Figure 2. Imχ(2) spectrum of the DHDA−H2O interface in the region 2750−3650 cm−1 as a function of [I−] concentration in the aqueous subphase at different bulk pH (6, 2, and 11). [I−] concentration: red, 0 mM; green, 50 mM; blue, 100 mM; cyan, 200 mM; purple, 500 mM; black curve represents the Imχ(2) spectrum for the air−water interface.

line) shows the Imχ(2) spectrum of the DHDA−H2O interface in the region 2750−3650 cm−1 at pH 6. The spectral features (sharp negative peaks) at 2875 cm−1and 2935 cm−1 are assigned to the symmetric stretch of the terminal CH3 group of the alkyl chain, and its Fermi resonance with its bending overtone, along with the CH3 asymmetric stretch mode (∼2960 cm−1) at the shoulder, respectively.37 The broad band in 3000− 3600 cm−1 with the maximum at 3400 cm−1 is attributed to a continuum of OH stretch of interfacial water molecules.30,32 The negative sign of the 3000−3600 cm−1 band is assignable to a net H-down orientation of interfacial water (i.e., the water dipoles are pointed toward the bulk water). It is important to note that such orientational assignment is based on the hyperpolarizability analysis of the isolated water molecule,38 though the OH stretch vibrations of liquid water are intra- and intermolecularly coupled.39,40 The magnitude of this signal is C

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The Journal of Physical Chemistry C relatively larger than the corresponding signal for neat air− water interface (cf., Figure 2a, black curve). Upon addition of 50 mM [CsI], the amplitude of Imχ(2) signal for OH band (3000−3600 cm−1) decreases (Figure 2a, green line). Further increase in the salt concentration in solution (up to 100 mM) is marked by a steady decrease in the strength of OH signal (Figure 2b). At still higher concentrations (200−500 mM), there is a marginal change in the OH signal strength. There is, however, no perceptible change in the CH3 peak region following addition of salt. 3.1.2. pH 2. The behavior of the DHDA−water interface at moderately acidic subphase (pH 2) is illustrated in Figure 2b. The amplitude of this signal is significantly larger compared to the corresponding signal at pH 6, suggesting an increase in the number of “H-down”oriented water molecules at the interface and/or an increased orientational order of the water molecules. In either case, the enhanced amplitude of the OH stretch band is reflective of an increase in the positive surface field strength as the pH is lowered. On addition of CsI (concentration ranging from 50 mM to 500 mM), the amplitude of the Imχ(2) signal (3000−3600 cm−1 region) decreases, following the same trend as that of the pH 6 case. 3.1.3. pH 11. The DHDA−water interface at bulk pH 11 presents an interesting case. The sign of the Imχ(2) signal (Figure 2c) corresponding to OH stretch (3000−3600 cm−1region) is reversed (now positive) indicating reorientation of interfacial water molecules in H-up configuration. This reversal in the alignment of interfacial water dipoles confirms the presence of a negative electric field at the surface. On introducing 50 mM CsI, it is observed that the amplitude of the Imχ(2) signal corresponding to OH stretch region decreases. This effect is amplified on further addition of salt (100−500 mM). 3.2. Effect of CsI on Nylon6−Water Interface under Different Bulk pH Conditions. 3.2.1. pH 6. Figure 3a (red line) shows the Imχ(2) spectrum for the Nylon6−H2O interface in the region 2750−3650 cm−1 at pH 6. The CH stretch region (2800−3000 cm−1) of Nylon6 monolayer is qualitatively similar to that of DHDA monolayer (cf., section 3.1.1). The OH stretch band (3000−3600 cm−1region), as discussed in section 3.1.1, has major contributions from interfacial water molecules. The Imχ(2) spectrum for the OH stretch region exhibits a negative sign corresponding to net H-down orientation of interfacial water molecules suggesting that the interface is positively charged at pH 6. Addition of CsI weakens the positive surface field as can be seen from the reduced OH band intensity (Figure 3a, green line). At low concentration of CsI (up to 100 mM), the changes in the interfacial water alignment are similar to that observed at the DHDA−water interface at pH 6. The amplitude of the Imχ(2) signal of the OH band decreases (as discussed earlier), while the sign of signal remains negative. The net electric field at the surface and amplitude of OH signal thus decreases with addition of CsI. It is interesting to note the changes in the interfacial water alignment effected by increasing the concentration of CsI. As the ionic strength is increased (200, 500 mM), there is a reversal in the orientation of interfacial water molecules, which are now aligned H-up with respect to the surface field. 3.2.2. pH 2. Under moderately acidic conditions (pH 2), the amplitude of Imχ(2) signal (3000−3600 cm−1) is significantly larger than the pH 6 case. The negative sign suggests the presence of a positive surface field. Addition of salt weakens this

Figure 3. Imχ(2) spectrum of Nylon6−H2O interface in the region 2750−3650 cm−1 as a function of [I−] concentration in the aqueous subphase at different bulk pH (6, 2, and 11). [I−] concentration: red, 0 mM; green, 50 mM; blue, 100 mM; cyan, 200 mM; purple, 500 mM; black curve represents the Imχ(2) spectrum for air−water interface.

positive surface field and causes a reduction in the OH band intensity; this trend is similar to the behavior of DHDA−water interface at pH 2. 3.2.3. pH 11. At moderately basic condition (pH 11), the Nylon6−water interface behaves qualitatively similarly to the DHDA−water interface (at pH 11). The positive sign of the Imχ(2) signal of the OH band indicates a negative surface field. Upon addition of I−, the magnitude of the Imχ(2) signal of the OH band decreases.

4. DISCUSSION 4.1. Effect of pH. 4.1.1. DHDA−Water Interface at Different pH. DHDA is a model compound resembling the tertiary amines which are important constituents of hardeners in paints used in the nuclear reactor containment. Amines are weak bases; the pKa of the conjugate acid, i.e., protonated amine, can be estimated as pKa = (10.5 ± 0.2) − n# × 0.2, where n# is the number of methyl groups bound to the basic nitrogen atom.41 For DHDA, pKa ≈ 10.05; therefore, in the aqueous bulk, DHDA molecules are expected to be fully protonated even at a mildly acidic condition, e.g., pH 6. Nevertheless, the interfacial pH and pKa values are different from the corresponding bulk values,42,43 and hence DHDA molecules may exist in an equilibrium of protonated− deprotonated forms at the interface. Accordingly, partially protonated DHDA molecules on water surface (pH = 6) creates a positive electric field at the interface so that the interfacial water molecules gets oriented as “H-down”. As the D

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Figure 4. Scheme illustrating I− adsorption and associated preferential orientation of interfacial water at (a) DHDA and (b) Nylon6 containing model paint−water interface at different pH (bulk).

be seen from the Imχ(2) spectrum of the Nylon6−water interface at pH 6, 2, and 11 (see Figure 3). The Imχ(2) spectrum for the OH stretch region (3000−3600 cm−1) exhibits a negative sign suggesting that the interface is positively charged at pH 6 and pH 2. Given the fact that the amide moieities in Nylon6 will not be protonated at mildly and moderately acidic conditions (see earlier discussion), the only plausible explanation for the origin of the positive surface field can be accumulation of H+ ions at the Nylon6−water interface. The electron-rich O and N centers in the polyamide may be possible sites at which H+ ions are adsorbed. At moderately basic condition (pH 11), the Nylon6−water interface behaves similarly to the DHDA−water interface (at pH 11). The Imχ(2) signal of the OH band has a positive sign, which indicates the presence of a negative surface field that can arise from OH− adsorbed at the polyamide monolayer (cf., section 4.1.1). It is interesting to note that these signal intensities are similar to that at the protonated DHDA−water interface (pH 2) where the head groups are positively charged. This means the densities of H+ and OH− ions are quite high at the Nylon6− water interface at the acidic and basic pHs. We speculate that the presence of H-bond donating (amide N−H) and accepting (amide carbonyl) sites of Nylon6 stabilizes the adsorbed H+ and OH− ions at the low and high pH values, respectively.46 The DHDA−water and Nylon6−water interfaces represent the surface of the model paint−water system, which becomes positively charged at pH 2 and pH 6 due to protonation at the amine functional group and adsorption of H+(or H3O+) at amide groups, while it is negatively charged at pH 11 due to adsorption of OH− ions at both amine and amide functional groups. In the following section, we examine the changes in the pH affected interface when the ionic strength of the subphase is altered by addition of CsI. 4.2. Effect of Addition of CsI to the pH Affected Interface. 4.2.1. DHDA−Water/CsI. The decrease in the amplitude of Imχ(2) signal (3000−3600 cm−1) from the DHDA−water interface, i.e., a reduction in the thickness of the H-down oriented interfacial water layer upon addition of CsI (cf., Figure 2a,b, green line) to the acidic (pH = 2, 6) aqueous subphase is related to the surface propensity of I− ion47 as well as the interfacial electric field created by the protonated DHDA molecules (at acidic pH). For large

pH of the aqueous subphase is lowered (i.e., moderately acidic pH 2), most of the amine headgroups at the DHDA−water interface get protonated. The positive field at the surface is further strengthened (compared to pH 6) and more OH dipoles of interfacial water are oriented in H-down configuration. This increase in thickness of H-down oriented interfacial water layer is reflected as an increase in OH band intensity (compared to pH 6). Under moderately basic conditions in the subphase (pH 11), the amine headgroups in the DHDA−water interface are expected to remain uncharged (since pH > pKa ) and the Imχ(2) spectra corresponding to OH band should, in principle, be similar to that of the air−water interface (black line, Figure 2c). Surprisingly, the DHDA−water interface spectra at bulk pH 11 present an altogether different scenario. The sign of the Imχ(2) signal (Figure 2c) corresponding to OH stretch (3000− 3600 cm−1) is reversed (now positive), implying that the OH dipoles of interfacial water molecules are oriented away from the bulk (i.e., H-up); this suggests the existence of a negative electric field at the surface. The negative surface field arises due to the accumulation of anions at the interface; in this case, it is the OH− ions from NaOH which are added to maintain the pH at 11. This observation is in agreement with our recent HDVSFG study of the primary amine monolayer−water interface, which also shows adsorption of OH− ions at basic bulk pH.44 4.1.2. Nylon6−Water Interface at Different pH. Another important constituent of hardeners in many epoxy paints is polyamide. Amides may be considered amine derivatives where one nitrogen is substituted by a carbonyl moiety. This structural modification facilitates a “conjugated system” in which the nonbonding electrons (NBEs) of nitrogen are delocalized into the adjacent carbonyl (CO) group. The NBEs of amides are not as readily shared with a proton as compared to the NBEs of an amine and therefore amides are considered to be “nonbasic”. The pKa value of an O-protonated amide is close to zero, whereas an N-protonated amide has a pKa value of about −7.45 With regard to their acidity, amides are considered to be “nonacidic” as well. The pH range in our HD-VSFG experiments varies between moderately acidic (pH 2) to moderately basic (pH 11). The Nylon6−water interface is therefore expected to remain uncharged under our experimental conditions. This presumption is proven wrong as can E

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The Journal of Physical Chemistry C polarizable ions such as I− (radius: 2.05 Å, polarizability: 7.5 Å3), the energy cost for solvating an ion in water by breaking the H-bond network (cavitation energy) outweighs the gain in electrostatic self-energy arising from redistribution of charges on the surface as the ion moves from the bulk to the interface, thereby favoring I− to accumulate at the interface.48 At the DHDA−water interface, these surface-active I− ions bind to the protonated amine headgroups of DHDA, and as a result, the positive surface field is severely attenuated. This weakened field reorients fewer interfacial water molecules, and hence, a decrease in OH stretch band signal has been observed (Figure 2a,b, green line). As the concentration of CsI in the aqueous subphase is raised (i.e., ionic strength increases), the number of surface-active I− ions at the DHDA−water interface increases causing the positive surface charge at the amine head groups to be screened within a small distance. (i.e., shorter Debye screening length). Consequently, fewer interfacial water molecules are aligned with this field, hence the decrease in the strength of the OH signal. Similar experiments under moderately basic condition (pH 11) showed a decrease of the positive Imχ(2) signal for the OH band with increasing ionic strength ([CsI]) in the aqueous subphase. The positive signal is due to creation of negative electric field via the adsorption of OH− at the interface. Therefore, the counterion effect observed (due to addition of CsI) is due to the Cs+ cation (not the I− anion). This effect is amplified as the concentration of salt is increased (100−500 mM). Figure 4a illustrates the I − adsorption behavior and the associated preferential orientation of interfacial water at the DHDA−water interface at different bulk pH. 4.2.2. Nylon6−Water/CsI. Under moderately acidic conditions, the Nylon6−water interface has a positive surface field arising from adsorption of H+ ions. The decrease in the amplitude of the Imχ(2) signal (3000−3600 cm−1) from the Nylon6−water interface following addition of CsI to the aqueous subphase is attributed to the attenuation of positive surface field by the more surface-active I− ions (more than H+).49 Under mildly acidic conditions (pH 6), the Nylon6− water system, which has a weak positive surface field, is found to display a change in the sign of the surface field (positive to negative) with increasing concentration of CsI in the aqueous subphase (Figure 3a). The orientation of interfacial water molecules is reversed from H-down (at low [CsI] ∼100 mM) to H-up (high concentration [CsI]: 200−500 mM). This behavior of the Nylon6−water interface is related to the “forces” responsible for the surface field. At pH 6, because of the lower concentration of H+ ions (compared to that at pH 2), the surface field thus generated (due to adsorbed H+) is weaker compared to that at pH 2. This is why the addition of CsI in the aqueous phase ([CsI] >200 mM) leads to reversal of water orientation at pH 6 but not at pH 2. For the Nylon6−water system at pH 11, the decrease in the magnitude of the Imχ(2) signal of the OH band with increasing ionic strength is attributed to the countercation effect of Cs+ cation (Figure 4b), similar to what has been discussed for the DHDA−water/CsI system at pH 11 (cf., section 4.2.1). 4.3. Consolidated Picture at the Model Paint−Water Interface. The surface field is altered by the counterion effect of I− (at pH 6 and pH 2) and Cs+ ions (at pH 11) at the DHDA−water and Nylon6−water interface. The strength of this surface field can be qualitatively estimated by comparing the integrated OH stretch band amplitude, since the water molecules are aligned by the interfacial electric field.50,51 It

needs to be mentioned here that the orientation profile of water molecules at the air−water interface is unaffected by changes in the pH from 2 to 11 (see Imχ(2) spectra in Figure 2a−c) and salt concentration (0.2−1 m).52 Therefore, by subtracting the Imχ(2) signal (3000−3600 cm−1) of air−water interface, (signal)water, from the corresponding signal (signal)samp of the sample (i.e., either DHDA−water or Nylon6−water interface) in the presence of salt (CsI), we can get the response of interfacial water molecules to the changes in surface field (RE) at the DHDA−water and Nylon6−water interface, for varying salt concentration at different pH (see eq 1 and Figure 5): 3600

RE =

∫3000

(signal)samp − (signal)water

(1)

Figure 5. Response of the electric field at DHDA−water and Nylon6− water interfaces to changes in the concentration of I− ions in the aqueous subphase at different pH (bulk).

A large negative value of RE implies a strong response (alignment) of interfacial water molecules in H-down configuration with respect to a strong positive surface field. There are three striking features that emerge from the plot in Figure 5. First, under moderately acidic (pH 2) and weakly acidic (pH 6) conditions, the negative RE decreases steadily with increasing salt concentration up to 100 mM and then approaches saturation at higher concentration (500 mM). The decrease of RE (due to decreased orientational order of interfacial water) is a consequence of screening of positive surface field (arising from protonated amines or H+ adsorbed on amides) by I−. Second, the RE values for pH 2 are more negative than the corresponding values for pH 6. This is presumably due to the higher surface density of protonated DHDA at pH 2 than that at pH 6. Third, the amplitudes of the RE values for Nylon6−water interfaces (at pH 2 and pH 6, respectively) are smaller compared to the corresponding DHDA−water interfaces, which means the interfacial electric F

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The Journal of Physical Chemistry C field at the Nylon6−water interface is weaker than that of the DHDA−water interface at the same acidic bulk pH. In other words, the DHDA-containing paint−water interface could be more effective in adsorbing I− than the Nylon6-containing paint−water interface. For moderately basic condition pH 11, the RE whose values are positive for both DHDA− and Nylon6−water interface decreases with increasing salt concentration (up to 100 mM), and thereafter remains almost constant. A large magnitude of RE with a positive sign implies that the corresponding paint−water interfaces are under the influence of negative electric fields, though the functional groups (amine for DHDA and amide for Nylon6) in the paints are uncharged. The negative surface field, as discussed in the previous section, is due to adsorption of OH− ions at the DHDA−water and Nylon6−water interfaces. The decrease in RE (i.e., increase in misalignment of interfacial water molecules) is due to attenuation of negative surface field by counterion effect of Cs+ cation. With increasing addition of CsI, the positive signal does not decrease substantially after the initial decrease of ∼40%. This indicates that apart from the preferential orientation of interfacial water by the adsorbed OH− ions, the uncharged paint headgroups may also be involved in the preferential H-up orientation of water. This may be caused by the interaction of interfacial water with the hydrogen bond accepting sites of the headgroups of Nylon6 and DHDA.46 The above analyses based on HD-VSFG studies on DHDA− water and Nylon6−water interfaces show that at moderate (pH 2) and weakly acidic (pH 6) conditions, I− is adsorbed at the interface; while at moderately basic conditions (pH 11), it prefers to remain in the bulk. Since the DHDA−water and Nylon6−water interfaces are analogous to amine and amide functional groups in paint surfaces exposed to water (inside the containment), the HD-VSFG results can be used to explain the adsorption behavior of I− on paint surfaces as reported in the OECD Behavior of Iodine Project (BIP) tests.26,27 The BIP tests on iodine behavior (where the initial iodine species was an aqueous mixture of I2 and I−) in an Amercoat66 (containment paint) painted vessel had revealed deposition of I− on the paint surfaces at pH 5. This observation is verified from our HDVSFG results, which, in addition to confirming I− adsorption at the interface at moderate (pH 2) and weakly acidic (pH 6) conditions, also provide molecular level insight on the possible adsorption sites as well as their relative effectiveness. The HDVSFG experiments show that at acidic bulk pH (pH < 7), DHDA-containing paint−water interface is more effective to the adsorption of I− from the aqueous phase than that of the Nylon6-containing paint−water interface. The observed surface activity of I− may be the missing link in the formation of organic iodides from paint surfaces immersed in water (inside the containment). Though the exact mechanism is still under debate, the widely accepted mechanism28 involves reaction of HI with paint eventually breaking bonds in the polymer matrix. HI, due to its strong acidic nature and polar character, reacts with the amine group in the paint matrix as follows:

amine part of the polymer are the most relevant. This is corroborated by our HD-VSFG findings which show that iodide can be adsorbed to a greater extent at the amine (DHDA) containing paint−water interface. Furthermore, these findings have bearing on current practices vis-a-vis iodine mitigation. In the event of a nuclear power plant accident, engineered safety systems such as containment sprays are activated to reduce the pressure inside the containment, and to wash out aerosols and volatile iodine. Additives in the containment spray water such as sodium hydroxide (NaOH) and sodium thiosulfate (Na2S2O3) increase the water pH and serve to inhibit the revolatilization of iodine.24 The alkaline conditions in the containment spray, will also inhibit the adsorption of I− ions at the paint surface (in contact with water), as seen in our HD-VSFG studies, and eventually lower the chances of formation of organic iodides.

5. CONCLUSION Our HD-VSFG studies on model paint−water interfaces represented by DHDA−water interface and Nylon6−water interface have provided molecular-level insight into the origin of volatile organoiodine compounds from painted structural surfaces within containment buildings of nuclear power plants. The DHDA−water and Nylon6−water interfaces become positively charged at pH 2 and pH 6 due to protonation at the amine functional group and adsorption of H+(or H3O+) at amide groups, while it is negatively charged at pH 11 due to adsorption of OH− ions at both amine and amide functional groups. In the presence of CsI, this surface field is altered by the counterion effect of I− (at pH 6 and pH 2) and Cs+ ions (at pH 11) at the DHDA−water and Nylon6−water interface. The HD-VSFG studies reveal that at moderate (pH 2) and weakly acidic (pH 6) conditions, I− is adsorbed at the interface. Furthermore, at acidic bulk pH (pH < 7), DHDA-containing paint−water interface is more effective to the adsorption of I− from the aqueous phase than the Nylon6-containing paint− water interface. At moderately basic conditions (pH 11), I− prefers to remain in the bulk. Alkaline conditions thus inhibit the adsorption of I− ions at both the DHDA and Nylon6 containing paint−water interface, eventually lowering the chances of formation of organic iodides.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

P. Mathi: 0000-0003-0860-686X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the support from Dr. Vinu V. Namboodiri, Dr. A. K. Singh, and Dr. D. K. Palit, RPCD, BARC.

Here, P(P′) denotes a polymer that can contain various functional groups such as a carbonyl, amine, and so forth. DFT calculations reported28 on related model compounds, amino alcohol (NH2CH2CH(OH)CH2OH), and amide (CH3CH2(CONH)CH2CH3) indicate that reactions with the

ABBREVIATIONS HD-VSFG, heterodyne detected vibrational sum frequency generation; DHDA, dimethylhexadecyl amine; OECD, Organisation for Economic Co-operation and Development

P−NH−CH 2−P′ + HI → P−NH 2 + P′−CH 2I

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