Photoemission of Iodide from Aqueous Aerosol Particle Surfaces - The

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A: Environmental, Combustion, and Atmospheric Chemistry; Aerosol Processes, Geochemistry, and Astrochemistry

Photoemission of Iodide from Aqueous Aerosol Particle Surfaces Ephraim Woods, Casey A. Konys, and Sean R. Rossi J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b12323 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 10, 2019

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The Journal of Physical Chemistry

Photoemission of Iodide from Aqueous Aerosol Particle Surfaces Ephraim Woods III,* Casey A. Konys, and Sean R. Rossi Department of Chemistry, Colgate University, 13 Oak Drive, Hamilton, NY 13346 [email protected]

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract

The photoemission of iodide from aqueous aerosol particle surfaces measures the surface concentration of iodide in predominantly supersaturated NaCl aerosol particles. Using the Langmuir model to describe the adsorption to the surface of aqueous iodide anions, the standard Gibbs free energy of adsorption is -15 kJ/mol in these systems. The presence of charged surfactants on the particle surfaces changes the adsorption behavior of iodide. The addition of sodium docecylsulfate (SDS) reduces the coverage of iodide, consistent with a competitive adsorption scenario. For surfaces coated with C12-, C14-, or C16-trimethylammonium chloride, the addition of iodide results in the formation of iodide-surfactant ion pairs at the surface with enhanced photoemission. The adsorption free energy for iodide in these systems is -21 kJ/mol. The results demonstrate the surface enhancement of iodide in supersaturated, atmospherically relevant conditions and demonstrate important differences between single-salt solutions and mixtures in the limit of high concentration.


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1. Introduction Halogens play several key roles in the heterogeneous chemistry of the troposphere, especially in the marine environment. Iodide, in particular, reacts with O3 at the air-water interface, producing both I2 and IO and contributing to catalytic ozone destruction.1-3 Iodine chemistry also contributes to new particle formation.4-7 For aqueous particles, the surface concentration of both bromide and iodide is enhanced relative to the bulk concentration,8-10 and the effect amplifies the importance of this chemistry. Experiments that can selectively probe the ions in the surface environment are powerful tools for studying the structure of these interfaces. Second harmonic generation (SHG) spectroscopy measures the surface concentration of iodide using the intensity of the light generated from frequency doubling at a solution-air interface.9, 14 Photoelectron spectroscopy (PES) also provides a means of measuring near-surface or surface-adsorbed species. A number of PES approaches exist. Extreme ultraviolet (EUV) and X-ray PES measurements from liquid beams in vacuum establish vertical binding energies and study the effect of counterion and salt concentrations.15-16 Laser-based UV PES experiments also make use of liquid beams in vacuum, demonstrating surface sensitivity and establishing the time-scales of solvated electron dynamics that arise from UV photodetachment processes.12, 17 Ambient-pressure photoelectron spectroscopy has also been applied to the air-solution interface, both with liquid jet18 and static sample sources.8, 19 These experiments demonstrate the enhancement of polarizable anions relative to cations in the surface layer. For NaI and KI solutions in the millimolar concentration range, the SHG experiments reveal a surface enhancement consistent with the Jones-Ray effect20-21 and the Gibbs free energy of & adsorption of iodide to the surface, Δ𝐺#$% (𝐼 ) ), of approximately -25 kJ/mol in both cases. At



& higher concentrations Δ𝐺#$% (𝐼 ) ) is smaller, but the SHG results were unable to distinguish

between 0 kJ/mol and the -3.3 kJ/mol result implied from liquid beam studies of NaI solutions.9, 15

Molecular dynamics (MD) simulations find a similar result.22-23 For the surface-active salt,

tetrabutylammonium iodide (TBAI), fitting the SHG data with a Langmuir adsorption model yields -21 kJ/mol.9 Analyzing the liquid beam photoemission data24 in the same way gives the slightly lower value of -17 kJ/mol.9 Petersen and Saykally attributed the discrepancy between the two experimental methods to the likely greater probe depth of the liquid beam experiment.9 There are no prior experiments that examine the surface segregation of iodide as a minor constituent of aqueous aerosol particles with high ionic strength. The static dielectric constant of aqueous solutions decreases markedly with increasing ionic strength,25 influencing the hydration free energy of solutes. MD simulations22 show that bromide almost completely displaces chloride at the surface of a NaCl/NaBr mixture with high concentration. This result, which does not follow naturally from the extrapolation of the properties of single-salt solutions, highlights the importance of measurements at high concentration. Aqueous marine aerosols represent a case where high ionic strength is common. Tropospheric aerosol particles may include supersaturated aqueous salt phases at relative humidity (RH) values between the efflorescence and deliquescence relative humidity. Marine aerosol, in particular, contains iodide26 as well as surfactant species.27 Krisch et al.19 showed that the model neutral surfactant, 1-butanol, altered the depth profile of ions near the interface. Studying the photoemission of iodide from aqueous aerosols with added surfactants provides a method of further probing the adsorption of iodide to the air-water interface under atmospherically relevant conditions. The general scheme of the experiment presented here is similar to the liquid beam experiments15 in that photoemission yield can serve as a proxy for surface or near-surface iodide


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concentration, but there are important differences. In particular, this aerosol experiment measures only the total photoemission yield rather than the photoelectron spectrum. Though it is more limited in the level of spectroscopic detail, the benefit of the experimental approach presented here is a greater flexibility in controlling the composition of aerosol particles. It affords control of the aerosol flow RH over a broad range, and, thus, control over the activity of water in the aerosol particles. Because the crystallization of supersaturated aerosol particles is negligibly slow at values of RH above the efflorescent point, this experiment can access metastable conditions that are inaccessible to other methods. Further, particles with complex composition have a greater length of time to equilibrate their morphology, including the partitioning of surfaceactive species to the gas-liquid interface, than in liquid beam experiments. As a result, these experiments provide information that is complementary to other liquid surface probes. Another difference, in principle, between this experiment and others that selectively probe surfaces is the probe depth of the experiment. For non-linear spectroscopies, such as SFG, the probe depth is a function of the thickness of the non-centrosymmetric region near the interface. The surface sensitivity of photoemission experiments, on the other hand, depends on the inelastic mean free path (IMFP) of electrons, or, more generally, the effective attenuation length (EAL), which is the distance over which the number of kinetic electrons decreases by a factor of 1/e owing to both elastic and inelastic effects. There are no experimental measurements of the IMFP in liquid water, but models28-29 predict a function that decreases with increasing kinetic energy between 10 and 90 eV, has a minimum of 1 to 2 nm near 100 eV, and increases with increasing kinetic energy to more than 20 nm at 1000 eV. Laser-based photoemission experiments sample kinetic energies below 10 eV, where the agreement among the models is poor. One recent model29 for low-energy electrons, which improves the description of disordered materials with



substantial band gaps such as water, predicts an IMFP of approximately 3 nm at the energy of our experiment. The EAL, in turn, should be 10-30% lower than the IMFP, depending on the energy.30 Suzuki et al.31 and Thurmer et al.32 measured the EAL for liquid water at energies near 10 eV, finding values between 1 and 3 nm. Despite these uncertainties, UV laser-based liquid jet photoionization experiments indicate a clear surface enhancement in the experimental signals.17 The results presented here imply a pronounced surface sensitivity, as well. Accordingly, we treat the photoemission yields as an indicator of surface concentration, but interpret the results as a limiting case.

2. Experimental Methods Figure 1 shows the experimental apparatus, which is similar to that in previous works, especially with regard to the generation and control of the aerosol flows.33-34 We generate atmospheric pressure flows of aqueous aerosol particles using a constant output atomizer (TSI model 3076). The atomizer solution is a mixture of NaCl, KI, and, in some cases, surfactants, and has a total ionic strength of roughly 0.05 mol/L. The droplets from the atomizer then pass through a diffusion drier where the RH drops below 20%, causing the particles to effloresce. A Po-210 static elimination device brings this flow into charge equilibrium before it enters a differential mobility analyzer (DMA), which size selects the dried particles. For these experiments, we select particles with a geometric mean mobility diameter near 210 nm for low RH flows.


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Figure 1. Diagram of the experimental apparatus. The arrows indicate the direction of flow.

The size-selected particles then pass through a flask containing water where the RH increases to 80-90% RH, which is sufficient to deliquesce the particles. A separate 2.0 standard liter per minute RH-controlled dilution flow joins with the deliquesced output from the DMA. The RH of the merged flow is the ultimate RH of the measurement. We maintain the RH at 60-61% for these experiments, ensuring the particles remain aqueous and supersaturated in NaCl. A scanning mobility particle sizer (TSI) monitors the final size distribution of the particles. The RH-controlled flow of particles passes through the ionization cell, where 230-nm laser light produces photoemission from iodide near the surface of the droplets. A Nd:YAG-pumped OPO operating at 10 Hz produces 460 nm light, and frequency doubling in a BBO crystal produces the 230-nm photons. To limit the overall charge on the particles, we typically operate the OPO such that the power is only 15 µJ/pulse, measured after the light passes through the cell.



The beam is columnated with a diameter of roughly 5 mm. Photoelectrons that enter the gas phase accelerate in a small electric field (10 V/cm) toward a precipitator electrode (likely in the form of 𝑂,) ), while the relatively large aerosol particles continue unaffected by the field. The particles, which now carry additional positive charges, flow through an aerosol electrometer, which measures the current in the flow. This current is proportional to the number of ionized iodide anions. Because the DMA produces a flow of singly-charged particles, the current with the laser blocked is proportional to the number of particles in the flow. The ratio of the photoemission signal to the signal with the laser blocked is the average number of charges per particle. Subtracting one from the result to account for the unit charge on particles prior to the laser ionization gives the photoemission yield. The likely mechanism of photoionization for these experiments is a one-color, two-photon process involving a solvated electron intermediate. Lubcke and coworkers12, 17 established this process as the dominant mechanism for laser-based methods at wavelengths resonant with the charge-transfer-to-solvent (CTTS) state. Figure 2 illustrates the scheme, with binding energies taken from photoelectron spectroscopy.11 The 230-nm light excites the aqueous iodide CTTS state associated with the 2P3/2 state of atomic iodine.35 Following the rapid relaxation to a solvated electron, a second photon within the same laser pulse ejects the electron. Time resolved photoelectron spectroscopy experiments12 show that, although a two-photon photodetachment from the iodide is energetically accessible, the process makes negligible contribution to the PES. The experiments of Lubcke12, 17 made use of laser pulses shorter than 100 fs, and this experiment employs nanosecond laser pulses. The much lower peak fluence of the nanosecond laser pulses is unlikely to drive direct photodetachment more efficiently than the ultrafast laser pulses.


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Figure 2. Photoemission from aqueous iodide through the CTTS, with binding energies for iodide11 and solvated electrons12-13 from photoelectron spectroscopy (PES). One 230nm photon, shown with blue arrow, excites the CTTS associate with the 2P3/2 state of atomic iodine and another ejects the solvated electron. Direct photodetachment from iodide, shown with the grey arrow, is energetically accessible, but not observed in PES experiments.12

Although the ionization process requires two photons, the laser power dependence of the signal in this work is very close to linear for the purely inorganic particles, suggesting that the CTTS excitation is saturated. The power dependence is weaker than linear for surfactant-containing particles, and the difference between these two cases is the subject of ongoing experiments. We note that the solvated electron transient signal in time-resolved experiments also indicates different dynamics in the presence of cationic surfactants.17 The supporting information further describes the laser dependence of the signal.

3. Results and Discussion Figure 3 shows fractional surface coverage of iodide, represented by the normalized photoemission yield, from three different aqueous aerosol samples composed primarily of NaCl



Figure 3. The fractional surface coverage as a function of iodide concentration for three predominantly aqueous NaCl aerosol mixtures: uncoated (¡), coated with DTAC (£), and coated with SDS (¯). The solid lines are fits to a Langmuir adsorption isotherm, as described in the text, and the numerical labels are the maximum, unscaled photoemission yield.

with variable amounts of KI. One sample contains only NaCl and KI, and the other two include either the anionic surfactant, sodium dodecyl sulfate (SDS), or the cationic surfactant, dodecyltetramethylammonium chloride (C12TAC). In both of the latter cases, the concentration of surfactant is sufficient to create a saturated monolayer on the surface for the majority of the particles. Some particles on the small end of the size distribution have a large enough surface area to volume ratio such that the surface coverage is less than one monolayer. The RH of the experiment is 60% in all cases, and the particles have a lognormal size distribution with a geometric mean of 340 nm and 1.09 geometric standard deviation. The supporting information provides more detail about the particle size distribution and expected surfactant coverage. We estimate the concentration of iodide by controlling the iodide to chloride ratio in the particle and using the E-AIM model36-38 to calculate the chloride concentration, which is approximately 7.5 mol/L under these conditions. The E-AIM calculation ignores the relatively small amount of iodide and surfactant. We normalize the photoemission signal so that the value corresponds to


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the fractional surface coverage implied by fits to the Langmuir isotherm. The highest point in each dataset bears a label that represents the unscaled photoemission yield, or the average number of positive charges per particle created by photoionization. The Langmuir adsorption isotherm, described well for the present context in Ref. 9, models these data effectively. The fractional surface coverage of the ith solute, 𝜃. , depends on the bulk solute mole fraction, 𝑥. according to the expression,

𝜃. =

23 9 ) 456,8 𝑥. 𝑒 :;

𝑥 +

23 9 ) 456,8 ∑. 𝑥. 𝑒 :;

(1)

& where 𝛥𝐺#$% is the corresponding standard Gibbs free energy change associated with adsorption

to the surface from the bulk solution. The summation is over all possible solutes. In the experiments described here, there are several solutes that may compete for surface sites. We use a number of simplifications to reduce the complexity of the system. For the uncoated system, we & as 0 for Cl- and we ignore the contributions of cations (Na+ and K+), approximate the 𝛥𝐺#

%$which are not surface active. As a result, the denominator in Equation 1 reduces to 𝑥 + 𝑥BC D + 𝑥ED 𝑒

D FG9 456(H ) IJ

)

, which is similar to the expression in Ref. 9. We assume that 𝑥 and 𝑥BCD

are constant over this range of iodide concentrations, and we use the E-AIM calculated mole fractions associated with the iodide-free particles. For the C12TAC-containing particles, we assume that iodide and the surfactant cations occupy the same surface sites, making ion pairs. Iodide, and other polarizable counterions, tend to associate more strongly with cationic surfactant species at the interface than does chloride.39 In this case, there is no competitive adsorption with the surfactant, and we exclude it from the summation in Equation 1. We suggest that the formation of these densely packed ion pairs is, at least in part, responsible for the larger



magnitude of the signal for C12TAC-covered particles indicated in Figure 3. For high concentrations of iodide, dense double layers produce large signal enhancements in the SHG experiment, as well.9 & The Langmuir fitted Δ𝐺#$% (𝐼 ) ) values for surfactant-free and C12TAC-covered surfaces are PQ

PQ

−15 ± 1 RSC and −21 ± 2 RSC, respectively. These quantities are lower limits on the magnitude of the free energy change. As discussed above, this experiment likely has a longer probe depth than non-linear spectroscopies, and contributions to our signal from bulk iodide is possible. These contributions would mask the surface excess, causing the fitted free energy changes to be closer to zero. This effect may explain the differences in the free energy of adsorption for TBAI measured with X-ray photoionization and SFG experiments.9 The result for the surfactant-free surface of −15 ± 1

PQ RSC

is smaller in magnitude than that for

dilute iodide in water20 and larger than that for concentrated iodide in solutions of single salts.9 We attribute the smaller free energy change, compared to low ionic strength solutions, to the substantial reduction in the relative dielectric constant of the supersaturated aqueous phase of the aerosol droplet compared to a dilute aqueous solution. That there is a still a substantial driving force to the surface under these conditions is consistent with the partitioning of polarizable anions to the surface of mixed salts when exposed to moderate RH.40 Molecular dynamics simulations22 show an analogous result for mixtures of bromide and chloride at high concentration, wherein bromide displaces nearly all of the chloride at the interface. For both iodide and bromide, it is clear that conclusions drawn from results involving single salt solutions cannot be extended to mixtures, especially in the limit of large concentrations. The result for the C12TAC-coated surface is very similar to tetrabutylammonium iodide (TBAI),9 suggesting that the attraction of the iodide to the surface active cation controls the


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energetics in both cases. The C12TAC Langmuir fit excluded the two highest concentration points, where the photoelectron yield is slightly lower than the maximum value. This decrease may be attributable to micelle formation, which may be promoted by the addition of iodide,41 or a morphological change associated with precipitation of hexadecyltrimethylammonium iodide (C12TAI) in the aerosol phase. A latter paragraph further describes this possibility. The uncertainty is larger than the surfactant-free measurement because of the limited range of & concentration used in the fit, but the Δ𝐺#$% (𝐼 ) ) value is clearly more negative than the

surfactant-free case. We consider three approaches for the SDS-coated particles. Using a model where the dodecylfulfate anion (DS-) and iodide compete for surface sites, we use the literature value, PQ

& ( 𝛥𝐺#$% 𝐷𝑆 ) ) = −31 RSC ,42 and approximate the 𝑥XYX as a constant. We calculate the 𝑥 at

RH=60% using the E-AIM model using a model surfactant molecule in the place of SDS. Fitting PQ

& ( )) to the Langmuir equation yields 𝛥𝐺#$% 𝐼 = −30 RSC , a value that is difficult value to justify

on the basis of the above and prior results. In particular, it is difficult to reconcile a more favorable adsorption for the negatively charged surface than for the neutral one. One alternative approach is to assume that the surface is not saturated with DS-, leaving open sites for iodide PQ

& ( )) adsorption. Assuming that the 𝛥𝐺#$% 𝐼 = −15 RSC , as it does for the surfactant-free case, we

can fit the data using the product, 𝑥XYX 𝑒 parameter. The result is 𝑥XYX 𝑒

DFG9 456 ([\[) IJ

DFG9 456([\[) IJ

in the denominator of Equation 1, as a

= 7.35, which implies a coverage fraction for DS- of

approximately 0.87 in the absence of iodide. Although we know the concentration of DS- is sufficient to saturate the surface of the majority of these droplets, it is possible that the interfacial structure is not uniform under the conditions of the experiment. Micelle formation may play a



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Figure 4. The photoemission yield as a function of iodide concentration for three cationic surfactant covered aerosol particles: C12TAC (£), C14TAC (¯), C16TAC (¡). The solid PQ & lines are scaled Langmuir fits using Δ𝐺#$% = −21 . The RSC dashed horizontal line represents the maximum photoemission yield from the surfactant free particle surface. The vertical line at 0.067 mol/L represents the approximate monolayer concentration for C12TAI for the geometric mean particle diameter.

role in determining the morphology of these particles, as well. A third possibility is the PQ

& ( )) 𝛥𝐺#$% 𝐼 is less negative than −15. RSC , and the coverage fraction of DS- is smaller than 0.87. PQ

& ( )) 𝐼 of −9.5 RSC. A limiting value of 𝜃_`D = 0 yields a lower limit for 𝛥𝐺#

%$Figure 4 shows the results of the same experiments as in Figure 3, but with tetradecyltrimethylammonium chloride (C14TAC) and hexadecyltrimethylammonium chloride (C16TAC) as the surfactant. The figure also contains a subset of the C12TAC data for comparison. The y-axis scale for these datasets is not normalized. Instead, it shows the photoemission yield which, as described earlier, is significantly higher for these data than for the surfactant-free and SDS-coated particles. The figure shows that for C14TAC covered particles, the photoemission signal varies with iodide concentration in the same way that it does for the C12TAC particles in


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& this range of concentration. These two data sets are consistent with a Δ𝐺#$% (𝐼 ) ) of

approximately -21 kJ/mol, represented by the solid lines in Figure 4. As stated above, our expectation is that iodide associates with the cationic surfactant headgroups on the surface, creating an ion pair with substantially higher packing density on the surface than hydrated iodides. This higher packing density at least partly accounts for the increase in signal, approximately a factor of six for coverages near 𝜃 = 1, compared to the surfactant free particles. It is possible that the change in chemical environment also alters the ionization mechanism, which can affect the overall photoemission yield. As noted above, the presence of cationic surfactants is known to affect the lifetime of solvated electrons generated by CTTS excitation.17 The low solubility of the resulting tetradecyl- and, especially, hexadecyltrimethylammonium iodide species (C14TAI and C16TAI, respectively) limits the range of iodide concentration we study for these species. For both surfactants, higher concentrations of iodide than shown in Figure 4 result in noticeable precipitation in the atomizer solutions. The large surface area to volume ratio of sub-micron aerosol particles causes the bulk solute concentration to be significantly less than the nominal concentration for surface-active species. For example, the maximum number of C16TAI molecules that may be accommodated on the surface of a 340-nm aqueous droplet, assuming the 0.44 nm2 head group area of hexadecyltrimethylammonium bromide,43 would produce a concentration of 6.7 × 10), mol/L were they all dissolved in the bulk solution. Figure 4 shows that for C12TAI and C14TAI the signal is at a maximum at 6.7 × 10), mol/L, corresponding to a saturated monolayer coverage. These results are consistent with particles where essentially all of the iodide is involved in iodide surfactant ion pairs at the surface for nominal concentrations up to approximately 6.7 × 10), mol/L. Additional iodide partitions to the bulk phase rather than the saturated interface. Because



the maximum coverage occurs at a surface density comparable to monolayer coverages of bulk solutions, we can conclude that the residence time of the aerosol flow system is sufficient to produce equilibrated surfaces prior to the ionization step. The solubility of C16TAI is only 2 × 10)d mol/L.44 As a result, C16TAI precipitation in the atomizer occurs even before the concentration of iodide is large enough to produce one monolayer for the particles in this study. Figure 4 illustrates this effect. While the C12TAI and C14TAI systems both reach a maximum ionization yield and level off, the yield for the C16TAI system does not reach a plateau value prior to the formation of solids in the atomizer. Because the surface-area-to-volume ratio is inversely related to the particle diameter, the degree of the shortfall diminishes with increasing particle size. The bulk solubility of C16TAI and the equivalent nominal concentration of surface sites (surface sites / particle volume) are roughly equal for a 1 mm diameter droplet. The solubility of C12TAI is higher, allowing us to atomize solutions that sample a larger range of iodide concentration, as shown in Figure 3. As noted above, the high concentration data points do not follow trend implied by Langmuir adsorption. While C12TAI does not precipitate in the atomizer, it may do so in aerosol phase when the RH drops to 60%, increasing the solute concentration to over 1000 times that of the atomizer solution. That change in morphology may be responsible for the decrease in signal for large iodide concentration. As noted above, iodide reduces the critical micelle concentration for these surfactants,41 and micelle formation may also alter the morphology of the particle.

4. Conclusion These results presented here show that the concentration of iodide on aqueous sea spray aerosol particle surfaces is enhanced relative to the bulk, even in the presence of other surface-


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active species and for ionic strengths in the supersaturated regime. They also demonstrate how the presence of charged surfactants either suppresses (if negatively charged) or enhances (if positively charged) the concentration of interfacial iodide. Finally, these measurements may also provide an important experimental touchstone for modelers interested in representing very high ionic strength environments.

Supporting Information Particle size distribution and surfactant coverage, dependence of photoemission signal on laser power, estimating the iodide concentration

ACKNOWLEDGMENT The authors acknowledge the support of NSF grant CHE-1507880.



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Photoemission of Iodide from Aqueous Aerosol Particle Surfaces - The

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