Variation in pH of Model Secondary Organic Aerosol during Liquid

Apr 15, 2016 - Using confocal microscopy and pH sensitive dyes, the pH of internally mixed model aerosols consisting of polyethylene glycol 400 and am...
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Variation in pH of Model Secondary Organic Aerosol during Liquid− Liquid Phase Separation Magda A. Dallemagne,† Xiau Ya Huang, and Nathan C. Eddingsaas* School of Chemistry and Materials Science, Rochester Institute of Technology, 85 Lomb Memorial Drive, Rochester, New York 14623, United States S Supporting Information *

ABSTRACT: The majority of atmospheric aerosols consist of both organic and inorganic components. At intermediate relative humidity (RH), atmospheric aerosol can undergo liquid−liquid phase separation (LLPS) in which the organic and inorganic fractions segregate from each other. We have extended the study of LLPS to the effect that phase separation has on the pH of the overall aerosols and the pH of the individual phases. Using confocal microscopy and pH sensitive dyes, the pH of internally mixed model aerosols consisting of polyethylene glycol 400 and ammonium sulfate as well as the pH of the organic fraction during LLPS have been directly measured. During LLPS, the pH of the organic fraction was observed to increase to 4.2 ± 0.2 from 3.8 ± 0.1 under high RH when the aerosol was internally mixed. In addition, the high spatial resolution of the confocal microscope allowed us to characterize the composition of each of the phases, and we have observed that during LLPS the organic shell still contains large quantities of water and should be characterized as an aqueous organic-rich phase rather than simply an organic phase.

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INTRODUCTION Secondary organic aerosols (SOA) in the troposphere have a significant impact on terrestrial and atmospheric earth systems. SOA provide feedback mechanisms toward climate change, links to the biogeochemical cycles of various compounds, and have an effect on regional visibility and human health.1 SOA comprise internally mixed organic and inorganic species.1−5 Ammonium sulfate (AS) is considered one of the most important and abundant compounds found in the inorganic fraction of tropospheric aerosols,6,7 whereas the organic fraction is composed of a complex mixture of hundreds to thousands of different compounds of which only about 10−20% have been identified.3,5,8 SOA remain aloft for several days to weeks,9,10 during that time they are subjected to both physical and chemical change through gas−particle transitioning and heterogeneous chemistry.3,5,11 One such physical change that SOA undergo is liquid−liquid phase separation (LLPS) along with efflorescence and deliquescence as a function of relative humidity (RH) and temperature.1,3,7,11−18 In internally mixed organic−inorganic aerosols, LLPS occurs because of a salting-out effect as the aerosol’s water content decreases.14 The effect that several factors, primarily size and composition, have on LLPS has been well studied.1,4,13,14,17−20 It has been demonstrated that the smaller an aerosol is, the less likely it is to undergo LLPS.16,21 Size also helps to determine whether the LLPS configuration is core− shell or partially engulfed, though the typical conformation is an inner inorganic core surrounded by an organic shell.16,21 The effect of composition on LLPS has been studied in various ways in terms of oxygen to carbon ratio (O/C) of a © 2016 American Chemical Society

compound, and organic to inorganic ratio (OIR). In aerosols with an O/C ratio of less than 0.56, LLPS was found to always occur, while LLPS has not been observed with ratios higher than 0.8. In aerosols with O/C ratios between 0.56 and 0.8, LLPS is dependent on the organic functional groups present.3,14 The typical O/C ratio measured in both ambient urban and marine atmospheres is between 0.2 and 1.0, so LLPS is considered a common occurrence in tropospheric aerosols.3,14 The organic to inorganic ratio has an observable effect on the different mechanisms of LLPS.13 Aerosols with an OIR between 8:1 and 2:1 LLPS are formed by nucleation and growth (spontaneous formation of inorganic-rich phases within the aerosol followed by growth of the inorganic-rich phases by aggregation and segregation). Spinodal decomposition occurred in aerosols with an OIR between 1.5:1 and 1:1.5. A second phase grew at the surface of the particle in aerosols with an OIR between 1:2 and 1:8.13 While there have been many investigations into why LLPS occurs and the dynamics and thermodynamics of the process, there are still a number of parameters that have not been well studied. One in particular is the effect that LLPS has on the pH of the aerosol, especially the organic fraction. When the organic and inorganic fractions of the aerosol segregate from each other during LLPS it is believed that other factors will also vary, including the pH of each fraction. Received: January 10, 2016 Revised: April 6, 2016 Published: April 15, 2016 2868

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second is the shape of the excitation spectrum due to the protonated and deprotonated forms of the dye (Figure 1).

The acidity of aerosols is important to understand as numerous compounds have been discovered in aerosols that are a result of acid catalyzed reactions.22−28 Products of acid catalyzed reactions such as organosulfates, aldols, hemiacetals/ acetals, esters/amides, and products of epoxide ring-opening mechanisms have all been discovered in significant concentration in the atmosphere.29−34 Along with laboratory experiments, theoretical thermodynamic and kinetic properties have been used to create aerosol models to enhance the study of these mechanisms.22,26,31,35−38 While pH is an important characteristic of SOA, there have been no direct measurements of the pH of aerosols, and what happens to the pH of an aerosol under conditions in which LLPS occurs has not been studied. Currently, the pH of aerosols is determined by means of a bulk methods, that is, the balancing of ionic composition or through the use of models such as AIM or AIOMFAC.23,24,38−42 However, during LLPS the organic and inorganic components occupy different fractions of the aerosol, and the pH of these separated fractions could change and no longer be accurately represented by the bulk system. The use of the bulk pH measurements could lead to inaccurate kinetic and thermodynamic models of the acid catalyzed reactions. Because of these limitations and the desire to better understand the kinetics and thermodynamics of SOA aging, the ability to determine the pH of individual aerosols as well as points within an aerosol will be of great benefit. Through the use of pH sensitive dyes and the high spatial resolution of confocal microscopy, we have characterized the pH of polyethylene glycol 400/ammonium sulfate aerosols in 1:1 weight fraction both at high RH when a homogeneous single phase is observed as well as at moderate RH where LLPS occurs and the organic and inorganic fractions are segregated from each other. To our knowledge, this is the first direct measurement of the pH of any SOA. When the aerosol was in single phase the pH was found to be 3.8 ± 0.1, while during LLPS (at 80% RH) the pH of the organic fraction was observed to increase to 4.2 ± 0.2 due to the segregation of ammonium sulfate away from the organic-rich phase. In addition, we obtained time-resolved micrographs allowing for characterization of the dynamics of LLPS formation and dissolution. Finally, we have determined concentrations of species within the aerosols as a function of RH and observe that at RH where LLPS occurs, the organic-rich shell still contains a substantial fraction of water, and should be characterized as an aqueous organic-rich phase instead of as simply an organic phase.

Figure 1. (A) Fluorescence excitation spectrum of Oregon Green 488 as a function of pH. Each spectrum is normalized at 465 nm to highlight the effect of pH on the excitation spectrum. Arrows indicate change in spectral shape with increasing pH. The vertical gray lines are the wavelength available for excitation in the confocal microscope. (B) Ratio of integrated fluorescence intensity of Oregon Green 488 from 500−550 nm from excitation at 488 and 476 nm as a function of pH as obtained by confocal microscopy of pH buffered aqueous droplets. The datum was fit with a standard sinusoidal curve with all variables allowed to float to find the best fit represented by minimized variance.

Due to various changes in volume, the concentration of dye is expected to vary throughout the LLPS experiments. To obtain accurate pH measurements, the calibration curve must be unaffected by the concentration of the dye itself. This is accomplished by using the ratio of emission intensity (500−550 nm) from excitation at a number of different wavelengths. The excitation wavelengths used (405 nm, 458 nm, 476 nm, 488 nm) were chosen based on the Oregon Green 488 excitation spectrum and the excitation wavelengths available on the confocal microscope. Calibration curves were constructed of emission ratio from excitation at 488 nm/405 nm, 488 nm/458 nm, and 488 nm/476 nm. Each point on the calibration graphs is a buffered solution containing the Oregon Green 488 dye; the pH was checked before and after fluorescence imaging with a pH meter to ensure consistency. Oregon Green 488 was also calibrated using the confocal microscope. The objective and laser intensity were varied between 10× and 50× and 5 and 10%, respectively. Droplets of the buffered solutions were dispensed into a sealed cell at 100% relative humidity and were analyzed in the same manner as the model aerosols were. In addition, on each day an LLPS experiment was performed, Oregon Green 488 was also calibrated on the confocal microscope, ensuring reproducible calibration of the dependency of pH. The solutions for this

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MATERIALS AND METHODS Chemicals. A model organic aerosol was prepared using a one-to-one weight ratio of polyethylene glycol 400 (PEG) [Sigma-Aldrich] to ammonium sulfate (AS) [ ≥ 99.00% SigmaAldrich] with trace Oregon Green 488 carboxylic acid, succinimidyl ester [99% Life Technologies] fluorescence dye. All solutions were diluted using nanopure water (Thermo Scientific Barnstead GenPure 18 MΩ resistance). All chemicals were used as received with no further purification. Calibration of pH Sensitive Fluorescent Dye, Oregon Green 488. The initial calibration of the fluorescence of Oregon Green 488 as a function of pH was performed on a Horiba Scientific FluoroMax-4P spectrofluorometer using standard buffer solutions. The fluorescence spectrum is affected in two ways as a function of pH. The first effect is on overall intensity of emission, which increases as pH is increased. The 2869

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The Journal of Physical Chemistry A calibration were the same as those used in the fluorimeter calibration, as were the ratios of emission intensity. Relative Humidity. A closed system which utilized varying amounts of humidified and dry nitrogen gas was constructed to control the RH. Nitrogen gas was fed from a single source through two separate lines and mass flow controllers (MFC) (200 sccm each, Omega FMA5410A-ST with Omega FMI100Totalizer). One MFC controlled the flow of nitrogen through a series of water bubblers, creating humidified nitrogen. The other MFC controlled a flow of nitrogen that bypassed the bubblers, providing dry nitrogen. The MFCs were used to control the amount of dry and humidified nitrogen gas in a ratio such that the total amount of nitrogen passing through the system remained constant at 200 sccm. All ratios of the two gas lines are expressed as percent of humidified gas. After the bubblers, the two lines of nitrogen were combined and fed through the microscope perfusion cell [Chamlide CF-T for 25 mm round coverslip] and then to an RH meter (Omega iTHX-W3 Temperature and Humidity iServer Microserver). Microscope Cell and Aerosol Generation. A sealed perfusion microscope cell with a Teflon-coated bottom coverslip was used in the LLPS experiments. The contact angle of a water drop on the Teflon coverslips was measured to be 120° using a Ramé-Hart Standard Goniometer. Single aqueous aerosols were dispensed onto the Teflon coated coverslip using a droplet generator (The Lee Co. IKTX0322000AC Micro-Dispensing application development kit) with nitrogen back-pressure using 1 ms pulses at 1 Hz rep rate. Optical Microscope Studies. We first studied phase separation in PEG/AS with a Nikon Eclipse E600 POL optical microscope. Images were obtained using Huvitz Panasis 2011 software with a Huvitz Lusis Camera microscope camera. Initially, a solution consisting of 0.37 M PEG and 1.1 M AS was prepared; this is a solution that is a 1:1 ratio by weight of the organic and inorganic components. This solution was determined to be the saturation limit of PEG in the 1:1 PEG/AS mixture and displayed no change in volume under the optical microscope when subjected to 100% humidified nitrogen gas flow for over 2 h. A 4-fold dilution of the saturated solution was prepared to reduce the saturation volume of the droplet to approximately 200 μm in diameter. Under the optical microscope, the LLPS was characterized as a function of percent hydrated nitrogen gas flow. The aerosols behaved as reported in the literature with LLPS occurring at ∼85% RH.13 Confocal Microscope Studies. The pH of single phase and LLPS of PEG/AS aerosols was studied using a Leica SP5 confocal laser scanning microscope and the RH system. The excitation wavelengths used were 405, 458, 476, and 488 nm with fluorescence detected at a range of 500 nm−550 nm using the same Leica HyD detector. Two lasers were used, a 405 nm blue diode laser and an argon laser for the 458, 476, and 488 nm excitation wavelengths. The laser intensity was set between 5% and 10% maximum intensity, and variations in laser intensity had no observable effect on the ratio of fluorescence intensity and the experimental pH. The data were collected in the sequential scanning mode along the x−y plane with the standard pinhole of 1 Airy at either 400 or 700 Hz scan rate with single frame and line accumulation. Scans were collected in the x−y plane at 50 z-stacks (height) to facilitate 3-D mapping. Each full 3-D scan took an average of nine and a half minutes. Individual laser scans for an x−y plane of 405, 458, 476, and 488 nm took an average of 1.9, 2.2, 3.5, and 3.4 s

respectively, for a total of 11.1 s for a single z-stack. To collect time-elapsed data as the aerosol underwent LLPS or its reverse, data were gathered on an x−y plane at a single z-stack located in the middle of the PEG/AS aerosol. These time-elapsed data were collected while the model was subject to both increasing and decreasing RH; the 476 and 488 nm individual laser scans took an average of 3.5 s with a total time of 7.1 s between each sequence. Data were collected while the model was in the single phase, LLPS, and in the process of formation and dissolution of LLPS. The model was in one phase at RH greater than 85% and phase separated at RH less than 85%. Since Oregon Green 488 is a sparingly soluble organic molecule, when the PEG/AS underwent LLPS the dye was exclusively confined to the organic shell, and only the pH of this fraction could be directly measured.

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RESULTS AND DISCUSSION pH Calibration. The pH of aerosols cannot be directly measured with a probe on the nano- or microscale. However, there are spectroscopic methods to experimentally determine the pH of a system without direct probing. Confocal microscopy in conjunction with pH sensitive dyes has been used to determine the pH of small biologic systems and cells, such as microspheres.43 With a pKa of ∼4.6, Oregon Green 488 is often used to study pH gradients in biologic systems under acidic conditions.44−52 When the concentration of Oregon Green 488 is held constant, the dye has a steady increase of emission intensity from pH 4−6.44 Unfortunately, under experimental conditions to study LLPS within aerosols, the concentration of all species vary as volume changes due to water loss and addition with changing RH. Therefore, a spectroscopic technique unaffected by concentration is necessary for directly measuring the pH of aerosols. The excitation profile of Oregon Green 488 varies as a function of pH as the ratio of the protonated and deprotonated forms vary (Figure 1a). The emission profile does not change with pH, though the intensity increases with pH. This allows for consistent measurements for each sample at the maximum emission wavelength range (500−550 nm). These measurements are then used in a ratio to omit the effects of concentration, similar to the technique Haggie et al.53 used to study lysosomal acidification of cystic fibrosis cells. On the basis of the excitation profile of the dye and the excitation wavelengths available on the confocal microscope, 405, 458, 476, and 488 nm were chosen as the excitation wavelengths. Using these different excitation wavelengths, three individual calibration curves were constructed using a ratio of emission intensity at 488 nm excitation to the emission intensity of the other three excitation wavelengths: 405 nm, 458 nm, 476 nm. A representative calibration curve can be seen in Figure S1 of the Supporting Information. The concentration of dye did not have an effect on the ratios of fluorescence when the concentration of Oregon 488 was varied between 1.88 × 10−5 M and 2.87 × 10−7 M. In addition, varied concentrations of PEG (0−0.15 M) and AS (0−1.0 M) had no observable effect on the calibration curve. Data from all of the varied concentrations are included in Figure S1. The difference in the spectral shape as a function of pH is due to the difference in fluorescence of the protonated and deprotonated form of Oregon 488. The ratio of the protonated form to deprotonated form as a function of pH is a sigmoidal 2870

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resulting in an aqueous organic-rich phase and not merely an organic coating around an aqueous AS core. Confocal microscope images of a typical PEG/AS aerosol at high RH (>95%) and during LLPS (80% RH) are shown in Figure 2. Displayed are the bright field image, the fluorescence

function with the point of greatest tangent at the pKa. The ratio of the two forms can be fit using the Hill equation: R pH =

10n(pH − pKa) 1 + 10n(pH − pKa)

(1)

where RpH is the ratio of deprotonated to protonated form and n = 1 is valid for a protonation reaction involving a single proton. To account for the fact that the fluorescence at each wavelength is due to both the protonated and deprotonated form, two constants must be included for the fit; a y-offset (a) to account for the ratio of fluorescence of the fully protonated form and a multiplication constant (b) to account for the ratio of fluorescence of the fully deprotonated form. R pH = a +

10n(pH − pKa) 1 + 10n(pH − pKa)

b

(2)

These constants were optimized using Microsoft Excel program, Solver, to create the best-fitted calibration curve for each emission ratio holding n = 1. The best fit for the ratio of the fluorescence at 488 to 476 nm using the confocal microscope was derived when a = 3.95, b = 3.65, n = 1, and a pKa of 4.5. Using the confocal microscope, a calibration curve of the ratio of emission intensity as a function of pH demonstrated the same Henderson−Hasselbalch behavior (Figure 1b), though the y-offset and max ratio are 4.8 times that from the spectrofluorometer. The difference between the spectrofluorometer and the confocal microscope is presumably due to the difference in sensitivities of the detector to a given wavelength. The robustness of the calibration was checked during every confocal run. All four excitation wavelengths were used during confocal runs. On the basis of the observed results, the 488 nm/476 nm ratio showed the greatest discrimination between pH values and so this calibration curve (Figure 1b) was used to determine the pH of PEG/AS. Dynamics of Liquid−Liquid Phase Separations. The confocal microscope allowed for the capture of high resolution images of PEG/AS aerosols during LLPS and the reverse process, from two liquid phases to a single liquid phase. As Oregon Green 488 is an organic dye, it is retained in the organic fraction, thus allowing for clear delineation between the organic-rich phase and inorganic-rich phase. The clear delineation between the organic and inorganic fraction also allows us to measure the diameter of the aerosol as well as the different phases, and therefore, allows the calculation of the volume of the different components of the aerosol as a single phase and in two phases. At high RH, after the PEG/AS aerosol has equilibrated for 2 h, the aerosol is at its saturation point. The saturation point of a 1:1 solution of PEG/AS was experimentally determined to be at the concentration of 0.37 and 1.1 M, respectively. Marcolli and Krieger have previously reported that the solubility of PEG and AS are suppressed in the presence of each other as AS is a very strong salting out agent for polyols.4 The solubility of AS without PEG present is much greater (5.8 M), and it was believed that under conditions when LLPS occurs, the concentration of AS within the inorganic-rich phase will approach this value prior to the efflorescence point. Similarly it was believed that the concentration of PEG in the organicrich phase would also become more concentrated. While the concentration of PEG will increase in the organic shell during LLPS, it was believed that plenty of water will still be present

Figure 2. Bright field (a, d), fluorescence (c, f), and overlay (b, e) confocal microscope images of PEG/AS aerosol at 95% RH (a−c) and at 80% RH (d−f). The scale bar in each section is identical and represents 100 μm. Also included is the measured radius of the aerosol as well as the internal, core, phase of the aerosol during LLPS.

image, and the overlay of the two. The concentrations of PEG and AS at saturated RH were assumed to be 0.37 and 1.1 M, respectively, based on the saturation experiments discussed above. At 80% RH, the radius of the PEG/AS aerosol was observed to decrease from 113 to 95 μma 1.7-fold reduction of volume. On the Teflon-coated slide the aerosol is not completely spherical, but instead is a capped sphere with a contact angle of 120°. A capped sphere with a contact angle of 120° results in a ∼33% reduction in volume compared to a perfect sphere of the same radius. From previously reported results and from the confocal fluorescence datum obtained here, the aqueous ammonium sulfate core during phase separation is either not in contact with the bottom slide or in minimal contact and therefore in this discussion was assumed to be spherical.18,20 At 80% RH, the AS core had a radius of 66.5 μm, resulting in a volume ratio between the organic shell and the inorganic core of ∼1:1. Assuming no evaporative loss of PEG or AS and complete segregation of the organic and inorganic fractions, the concentration of ammonium sulfate in the core is 3.9 M and PEG in the shell is 1.2 M. The concentration of AS is still well below its solubility, thus it is still in an aqueous state. Using the literature value for the density of PEG (1.13 g mL−1) and ammonium sulfate (1.77 g mL−1) the volume of each within their respective fractions can be calculated. The PEG in the shell is not in high concentration and considering the volume that would be taken up by the PEG, over 50% of the volume in the shell is still water. A summary of radius and concentration values can be found in Table S1. At RH of 65% the total and core radii decreased by almost the same fraction resulting in both the ammonium sulfate and PEG increasing in concentration to 5.6 and 1.8 M, respectively. At 1.8 M concentration of PEG in the shell, roughly 35% of the volume will still be water. The presence of water in the organic phase confirms the model predications by Zuend and Seinfeld and others.2,54 The AIOMFAC model developed by Zuend and Seinfeld uses thermodynamics to predict aerosol composition, phase, and 2871

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The Journal of Physical Chemistry A uptake capabilities.2 Using their model they simulated the phase state and composition of SOA from the ozonolysis of α-pinene, a system that will have a similar O/C ratio as the one here. At 60% RH they predict that the aerosols would be phase separated and that the organic-rich phase will consist of around 50% water by volume and at 40% RH the water content of the organic-rich phase will be 38%. The model predicts slightly higher water content of the organic phase then was observed here but the same conclusion is drawn that substantial water is still present. The concentration values presented here are estimates, as not all scans were of the same individual aerosol. While they are estimates, the data are robust as the droplet generator produces reproducible aerosols. While the shell is organic, it is aqueous organic, and the water content of the phase must be taken into account when characterizing it. The fact that a large amount of water is present within the outer organic-rich phase will have an effect on characteristics such as viscosity, solution reactivity, and uptake of gaseous compounds. The dynamics of LLPS formation have been studied, most notably by Ciobanu et al. on the PEG/AS system and by Song et al. on dicarboxylic acid/AS systems.13,20 In a 1:1 by weight ratio of PEG/AS, Cioubanu showed that the process involves first schlieren formation followed by spinodal decomposition with subsequent cluster growth and cluster aggregation or coalescence. This process differs from the other possible phase separation mechanism, nucleation-and-growth by the fact that no activation barrier must be overcome, and that the process will occur throughout the solution instead of at a small number of nucleation sites. The fact that the fluorescent dye exclusively partitions to the organic-rich phase, and the ability to take timeelapsed video of the interior of the aerosol using confocal microscopy together provide an excellent way to study the formation as well as the dissolution of LLPS in greater detail. Time-elapsed images of LLPS formation and dissolution were obtained by continuously scanning the x−y plane with the 476 and 488 nm lasers while remaining at one height near the center point of the aerosol. Runs were performed while the RH was decreased at a constant rate, resulting in the formation of LLPS, as well as increasing RH to observe the dissolution of LLPS. Representative stills of the process of LLPS formation and dissolution can be seen in Figures 3 and 4, focused at RHs near where LLPS sets in. Representative movies of each process can be found in Supporting Information Movies S1−S4. Figure 3 shows the process of LLPS onset. Upon decrease of RH, water evaporates from the aerosol and the aerosol shrinks in size (not shown in the time-elapsed images). Once enough water has evaporated, spinodal decomposition is clearly visible as inclusions are observed to form throughout the aerosol. Interestingly, it is not just inorganic inclusions that are formed but both inorganic inclusions in the organic-rich phase (dark inclusions with no fluorescent organic dye) and organic inclusions in the inorganic-rich phase (bright inclusions with organic dye). This can be readily seen in Figure 3, and even more clearly in Movies S1 and S2. In fact, it appears that most of the inclusions are organic-rich that eventually migrate to the outer shell of the aerosol. Once all of the PEG has been removed from the ammonium sulfate-rich phase, the standard core−shell LLPS aerosol is observed, typical of a 1:1 PEG/AS aerosol under intermediate RH. The dynamics of the dissolution of LLPS in atmospheric aerosols has not been as well documented. Figure 4 shows time-elapsed dynamics of the dissolution of LLPS. As the RH

Figure 3. Bright field (top) and fluorescence (bottom) images of a 1:1 PEG/AS aerosol as the RH was decreased at a rate of 0.75% per minute starting at ∼92% RH. Description of the dynamics can be found in the text. The scale bar represents 100 μm.

Figure 4. Bright field (top) and fluorescence (bottom) images of a 1:1 PEG/AS aerosol as the RH was increased at a rate of 2% per minute starting at ∼84% RH. The inorganic core is observed to migrate to the edge of the aerosol prior to rapid mixing as the organic shell envelops the inorganic core. The scale bar represents 100 μm.

increases, the aerosol is observed to take up water, growing in size and diluting the concentration of both the PEG and AS. As the RH continues to increase, and just prior to the dissolution of the two phases, the AS core is observed to migrate toward the edge of the aerosol. Once the AS core reaches the edge, it is engulfed by the organic-rich shell, starting from the point opposite where the core reached the side of the aerosol droplet. Within seconds after the core reaches the edge, there is only one internally mixed phase consisting of both PEG and AS. It is of interest that even in a core−shell LLPS structure, the dissolution results in a partially engulfed structure, if only for a brief time, before the aerosol becomes single-phase. The 2872

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facilitate the prediction of pH, bulk solutions of a mixed PEG/ AS system representing saturation, 0.37 M PEG and 1.1 M AS, as well as solutions representing concentrations of individual phase at relevant RH values were prepared. The pH of each solution was measured using a standard pH probe with the results displayed in Table 1. Note that the pH of the saturated

process of LLPS formation and dissolution results in a great deal of mixing within the aerosol. The model aerosols presented here are much larger, diameters of ∼200 μm, than ambient atmospheric aerosols. For atmospheric aerosols there are two classifications that are typically measured, PM 10 and PM 2.5 which refers to particulate matter 10 μm and smaller and 2.5 μm and smaller, respectively. Organic aerosols are typically much smaller than this with average size of SOA typically on the order of tens to hundreds of nanometers in diameter.55,56 There have only been a few studies that have looked at LLPS of aerosols that are less than 200 nm, and these studies have shown that while LLPS is observed for larger aerosols it does not always occur for smaller ones.16,21 In particular, aerosols consisting of high O/C ratio organics did not show phase separation below ∼200 nm in diameter; however, aerosols with low O/C ration organics of 0.66 to 0.44 did show LLPS at all sizes. The O/C ratio of PEG is 0.5, and therefore it is predicted that LLPS will be observed in aerosols of atmospherically relevant sizes and we predict that similar dynamics would be observed. pH Determination. Ratios of the fluorescence data gathered from the confocal microscope were used to determine the pH of the PEG/AS aerosol. Confocal microscopy provides full 3D mapping of the PEG/AS aerosol, thus allowing for pH determination at individual points throughout the aerosol. Confocal fluorescence measurements were taken while the PEG/AS aerosol was at constant RH in a single phase at >90% RH and in LLPS at 80%, 78%, 75%, and 65% RH. The pH levels determined using the ratio of fluorescence intensity in a single phase aerosol or in the organic-rich phase while in LLPS can be seen in Figure 5. For each aerosol, a number of regions

Table 1. pH of Ammonium Sulfate and Poly(Ethylene Oxide) Solutions Representing a Number of Conditions Found in the Model Aerosol solution

pH

0.37 M PEG, 1.1 M AS 0.37 M PEG 1.1 M AS 3.9 M AS 1.2 M PEG 1.8 M PEG

4.7 4.1 5.6 5.6 4.1 4.8

solution is nearly a full pH unit above what was determined from the aerosol using the fluorescent dye, but the pH of the outer shell at 80% RH, where the PEG concentration is ∼1.2 M, is nearly identical. We cannot explain the discrepancy between the pH measured of the bulk solution and that of the fluorescent measurement. One possible explanation is that the ammonium or sulfate ions is affecting the fluoresce measurement; however, we did not observe any effect of ionic strength on the fluoresce measurements as stated above up to a concentration of 1.0 M AS. It appears that the pH observed by fluorescence is influenced more by the PEG or that there are some unaccounted effects of the mixed aerosol system on the overall pH. From bulk measurements the pH of the core, consisting mainly of AS and water, will be more basic than the shell by greater than a pH unit. Finally, the bulk pH measurements show that as the PEG concentration increases the pH also increases. The fluorescence data indicate no change in pH of the organic-rich shell as the RH decreases while the aerosol is in a LLPS state. This suggests that the concentration of PEG in the shell might be less than calculated, especially as the RH decreases which would mean that there is even more water present than indicated from volume measurements alone. The pH of the PEG/AS aerosols was also evaluated in real time using the time-elapsed fluorescence data. Figure 6a shows the ratio of fluorescence of λex 488 nm/476 nm as the RH was increased, resulting in the dissolution of LLPS. At the start of the experiment the aerosol was in stable LLPS where the ratio of fluorescence remained constant at 5.2 ± 0.2 during stable LLPS. As the RH was increased and reached the transition point, the inorganic core crashed out forming one phase (middle points in Figure 6a). The ratio of fluorescence fluctuated greatly during the transition to single phase, mainly because the concentration of fluorescent dye within the region sampled varied widely. Upon formation of a stable single phase, the ratio of fluorescence once again stabilized to a ratio of 4.7 ± 0.1. Similarly, the fluorescence was measured from an aerosol as the RH was decreased, resulting in the formation of LLPS (not shown). The resulting pH determined from the time-elapsed measurements resulted in nearly identical values observed from stable aerosols held at constant RH (Figure 6b). To our knowledge this is the first direct experimental measurement of the pH within an aerosol, especially within an individual phase during LLPS.

Figure 5. pH of individual 1:1 PEG/AS aerosols held at a constant RH. pH is of the entire aerosol (RH > 90%) and that of the organicrich phase (80−65% RH). The pH of a single phase PEG/AS aerosol is statistically different from one in LLPS while no statistical different is observed during LLPS as the RH is decreased.

within the x−y plane were measured throughout the entire height of the aerosol resulting in 30−50 data points per pH measurement. The pH of the single phase aerosols was 3.8 ± 0.1 while the organic-rich phase during LLPS at 80% RH was found to be 4.2 ± 0.2. The pH of the organic-rich phase at RH down to 65% RH was determined, providing roughly the same value: 4.2 ± 0.1 at 78% RH and 4.1 ± 0.1 at both 75% and 65% RH. The pH of a single phase aerosol was found to be statistically different than the pH during LLPS. The model aerosol used here is a simple three component system and since we have estimated the concentration of PEG and AS at each RH, either mixed or isolated into individual phases, we can predict the pH under the given conditions. To 2873

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1.8 M or less, and the organic fraction is still 35% water or more. This indicates that the outer shell should be thought of as an aqueous organic-rich phase rather than strictly an organic phase. In addition, this amount of water will prevent glass transition at these relative humidities. Treating the shell of an aerosol in LLPS as aqueous organic will have implications in a number of aspects or the aerosol. First, when modeling the uptake of gases into an aerosol, uptake into an aqueous medium can occur, and Henry’s Law constants should be considered. Second, viscosity of the outer shell will be determined in part by the concentration of organics and amount of water retained in the organic shell. The water retention in the organic shell will be influenced by the hygroscopicity of the organic components, with highly hygroscopic compounds retaining a great deal of water and thus reducing viscosity. Diffusion will also be influenced by these parameters and as well diffusion within the shell is related to both viscosity and water content. The pH of a model atmospheric aerosol consisting of a 1:1 by weight mixture of PEG and AS was experimentally determined using confocal microscopy. To our knowledge, this is the first direct measurement of the pH of individual, tens to hundreds of micrometer-sized aerosols. This provides the most detailed view of the pH within an aerosol and the dynamics as the phase state of the aerosol changes. On the basis of measurements of the pH of the organic shell and bulk measurements of solutions representing the core and shell of a PEG/AS aerosol during LLPS, the shell will have a pH that is more acidic than the core by a pH unit or more. This indicates that knowing the bulk pH of an aerosol might not be sufficient to fully characterize it. The variation in pH observed here are statistically different; however, the differences are small and would probably not be atmospherically relevant. Where the variation in pH of the shell and core might be important atmospherically would be when carboxylic acids are present. For instance, if PEG was replaced with 3-methylglutaric acid, pKa of 4.24, and the shell resulted in similar concentration of organic, 1.2 M or greater, the pH of the shell would be 2 or less while the core consisting mainly of ammonium sulfate and water would have a pH of greater than 5. One route to the aging of aerosols is reactions within liquid aerosols where many of the reactions are acid catalyzed, and therefore understanding the pH of the aerosol where the organics reside is necessary to determine if and at what rate these reactions will proceed. In the case of 3-methylglutaric acid producing a pH of 2 in the organic-rich phase, many acid catalyzed reactions would be kinetically and thermodynamically favored. The studies presented are a simple, three component model aerosol system of 1:1 by weight PEG/AS in water. Atmospheric aerosols consist of hundreds to thousands of different components, and this complexity must be taken into account. While this is a simple model system, the results are an important step toward understanding the pH of aerosols, the dynamics of LLPS formation and dissolution, and the water content of aerosols.

Figure 6. (A) Ratio of fluorescence intensity from 500−550 nm from excitation at 488 and 476 nm as the RH is increased. The ratios obtained during LLPS and in single phase are self-consistent while the ratio varies widely during the transition time between the stable states. Also included is the pH value represented by each fluorescence ratio. (B) pH of homogeneous single phase 1:1 PEG/AS aerosol (red) and organic-rich phase during LLPS (blue) obtained during time-elapsed studies as LLPS formed (left) and dissolved (right).

The pH of the aerosol where the organic component resides was observed to vary under conditions of LLPS. This is most likely due to the exclusion of the ammonium sulfate from the organic-rich fraction. The pH of the single phase aerosol was found to be 3.8 by fluorescence but 4.7 by bulk pH measurements. While there is a discrepancy between these two measurements, the conclusion that the pH varies from single-phase to LLPS still holds true. In addition, during LLPS the organic-rich phase is predicted to be about pH unit more acidic than the inorganic core. The ratio of florescence for a single phase of aerosol in LLPS from the time-elapsed experiments was almost identical to those determined from the steady state experiments.

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CONCLUSIONS AND IMPLICATIONS Using pH-sensitive dyes and confocal microscopy we have experimentally observed the pH of individual, tens to hundreds of micrometers in diameter, aerosols both of internally mixed aerosols and the organic fraction while the aerosol is in LLPS. In addition to observing the pH, we have also characterized the concentration of species within each phase, as well as observed the dynamics of LLPS formation and dissolution. The results provide new details on how aerosols behave when cycling through different relative humidities including the nature of the organic shell during LLPS as well as pH changes under conditions of LLPS. The concentration of PEG in the organic shell was calculated under a number of different relative humidities while the aerosol was in LLPS. At 80% RH the shell is over 50% water and at 65% RH, the concentration of PEG in the shell is only

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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.jpca.6b00275. 2874

DOI: 10.1021/acs.jpca.6b00275 J. Phys. Chem. A 2016, 120, 2868−2876

Article

The Journal of Physical Chemistry A

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Calibration data from spectrofluoremeter (Figure S1), calculated radius and concentration of species within model aerosol (Table S1) (PDF) Time elapsed confocal images of decreasing relative humidity resulting in the onset of LLPS. This movie is an overlay of the bright filed and fluorescence confocal data (MPG) Time elapsed confocal images of decreasing relative humidity resulting in the onset of LLPS. This movie is only the fluorescence confocal data to better show the distinction between the organic and inorganic fractions of the aerosol (MPG) Time elapsed confocal images of increasing relative humidity resulting in the dissolution of LLPS. This movie is an overlay of the bright filed and fluorescence confocal data (MPG) Time elapsed confocal images of increasing relative humidity resulting in the dissolution of LLPS. This movie is only the fluorescence confocal data to better show the distinction between the organic and inorganic fractions of the aerosol (MPG)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address †

(M.A.D.) EPA Region 6, Dallas, TX 75202.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to greatly acknowledge Evan Darling for help with operating the confocal microscope. The authors would like to acknowledge financial support from School of Chemistry and Materials Science and College of Science at Rochester Institute of Technology as well as the Dean’s Research Initiation Grant.

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