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Glyoxal and Methylglyoxal Setschenow Salting Constants in Sulfate, Nitrate and Chloride Solutions: Measurements and Gibbs Energies Eleanor M. Waxman, Jonas Elm, Theo Kurtén, Kurt V. Mikkelsen, Paul Jeffrey Ziemann, and Rainer Volkamer Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b02782 • Publication Date (Web): 03 Sep 2015 Downloaded from http://pubs.acs.org on September 15, 2015

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Environmental Science & Technology

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Glyoxal and Methylglyoxal Setschenow Salting Constants in Sulfate, Nitrate and Chloride

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Solutions: Measurements and Gibbs Energies

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Eleanor M. Waxman,1,2 Jonas Elm,3,4 Theo Kurtén,5 Kurt V. Mikkelsen,3 Paul J. Ziemann,1,2 and

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Rainer Volkamer1,2*

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1. University of Colorado Boulder, Department of Chemistry and Biochemistry, UCB 215,

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Boulder, Colorado, United States

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2. CIRES, University of Colorado, UBC 216, Boulder, Colorado, United States

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3. University of Copenhagen, Department of Chemistry, Universitetsparken 5, 2100 København

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Ø, Denmark

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4. University of Helsinki, Department of Physics, P. O. Box 64, Finland

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5. University of Helsinki, Department of Chemistry, P. O. Box 55, Finland

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*Corresponding author. [email protected]. Telephone: +1 (303) 492-1843. Fax:

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+1 (303) 492-5894

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Abstract

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Knowledge about Setschenow salting constants, KS, the exponential dependence of Henry’s Law

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coefficients on salt concentration, is of particular importance to predict secondary organic

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aerosol (SOA) formation from soluble species in atmospheric waters with high salt

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concentrations, such as aerosols. We have measured KS of glyoxal and methylglyoxal for the

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atmospherically relevant salts (NH4)2SO4, NH4NO3, NaNO3 and NaCl, and find that glyoxal

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consistently ‘salts-in’ (KS of -0.16, -0.06, -0.065, -0.1 molality-1, respectively) while

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methylglyoxal ‘salts-out’ (KS of +0.16, +0.075, +0.02, +0.06 molality-1). We show that KS values

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for different salts are additive, and present an equation for use in atmospheric models.

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Additionally, we have performed a series of quantum chemical calculations to determine the

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interactions between glyoxal/methylglyoxal monohydrate with Cl-, NO3-, SO42-, Na+, and NH4+,

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and find Gibbs free energies of water displacement of -10.9, -22.0, -22.9, 2.09, and 1.2 kJ/mol

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for glyoxal monohydrate, and -3.1, -10.3, -7.91, 6.11, and 1.6 kJ/mol for methylglyoxal

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monohydrate with uncertainties of 8 kJ/mol. The quantum chemical calculations support that

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SO42-, NO3- and Cl- modify partitioning, while cations do not. Other factors such as ion charge or

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partitioning volume effects likely need to be considered to fully explain salting effects.

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1. Introduction

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Most atmospheric aerosols contain an inorganic fraction.1,2 This fraction is typically

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made up of salts containing sodium chloride (from sea spray) and ammonium, nitrate, and sulfate

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that result from the gas-phase processing of anthropogenic precursor gases such as NOx, SO2,

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and NH3. The hygroscopicity of these inorganic ions is responsible for the majority of the water

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taken up by the aerosols and the formation of an aerosol aqueous phase.3–5 Salt concentrations

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tend to be quite high in aerosols; between 3 M and 20 M.6

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Organic aerosol formation by aqueous-phase processing is a topic of recent interest.7–19

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Small, water-soluble species such as glyoxal, methylglyoxal, acetaldehyde, glycolaldehyde, or

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isoprene epoxydiols (IEPOX) partition to the aerosol aqueous phase by Henry’s law. They can

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then undergo further aqueous-phase reactions such as hydration, oligomerization, reactions with

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nitrogen-containing species, reactions with sulfate ions, and aqueous-phase oxidation reactions in

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the presence of OH radicals.20–24


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The presence of salts in the aqueous phase can significantly impact the solubility and

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activity of organics in the aerosol as well as modify bulk (as opposed to surface) reaction rate

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constants. When salts increase the solubility of the organic, this is called “salting in” with

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respect to the organic and when salts decrease the solubility of the organic, this is called “salting

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out”. This effect was first published in 1889 by Setschenow who described it in terms of an

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organic’s solubility in water:25

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log ( S0 / S ) = KS Csalt

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where S0 and S are the solubility of the organic in pure water and the salt solution respectively,

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KS (here, per molarity) is the salting constant or Setschenow constant, and Csalt (molarity) is the

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concentration of the salt solution, typically in molarity. Kampf et al.6 modified this equation to

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represent Csalt based on molality, and to describe salting in terms of Henry’s law coefficients:6

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 KH ,w log  K  H ,salt

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where KH,w is the Henry’s law constant of the organic in pure water, KH,salt is the Henry’s law

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constant in the salt solution, and csalt is the concentration of the salt in molality. Here, KS has the

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units of per molality (m-1). Therefore, this effect changes the partitioning of these species

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between the gas and aqueous phase. If the molecule salts in, partitioning towards the aqueous

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phase is favored which increases the amount of precursor molecule available for further reactions

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and SOA formation. If the molecule salts out, the reverse is true. The effective Henry’s law

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constant thus becomes a function of environmental conditions.

(Eq. 1)

  = KS csalt 

(Eq. 2)

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Recent measurements show that glyoxal salts in to ammonium sulfate aerosols.6 A series

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of quantum chemical calculations suggests that this is because the hydrated forms of aqueous-

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phase glyoxal, especially the mono-hydrate, bind to sulfate more strongly than water in the



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sulfate hydration shell (water displacement).26 This results in the loss of free glyoxal in the

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aqueous phase as part of the sulfate hydration shell. Therefore, additional glyoxal must partition

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in to the aqueous phase to maintain equilibrium thus increasing the effective Henry’s law

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constant. Here we present additional experimental data for glyoxal and methylglyoxal for the

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atmospherically-relevant salts ammonium sulfate ((NH4)2SO4), ammonium nitrate (NH4NO3),

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sodium nitrate (NaNO3), and sodium chloride (NaCl), as well as quantum mechanical

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calculations for interactions with salt ions.

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complicated than simply replacing the water in the anion hydration shell with exactly one

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organic molecule.

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2. Experimental

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2.1. Laboratory Salting Constant Measurements

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Salting constant measurements for glyoxal and methylglyoxal were performed using two

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different sets of instrumentation: (1) a novel inlet coupled with a quadrupole ion trap mass

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spectrometer (ITMS measurements), and (2) gas chromatography with flame ionization detector

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(GC-FID measurements).

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unreported, the intercomparison between the two methods serves as method validation.

We also show that salting behavior is more

Since salting constant values for these species are previously

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For both sets of measurements, 10.00 mL solutions of 0.1 M methylglyoxal (40% w/w

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solution, Sigma Aldrich) or 0.2 M glyoxal (40% w/w solution, Sigma Aldrich) were prepared in

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18 MΩ water (Millipore). Solution concentrations were chosen to minimize oligomerization27

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but give sufficient gas-phase signal. In addition to glyoxal or methylglyoxal, the solutions also

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contained 0, 0.5, 1.0, 1.5, 2.0., or 3.0 molarity salt solution (ammonium sulfate, ammonium

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nitrate, sodium nitrate, or sodium chloride, all Sigma Aldrich).

All measurements were


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performed at room temperature (22 ± 2°C). A list of experimental conditions and number of

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repeats are listed in Table S1.

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Notably, measurements of KS do not require knowledge of the absolute vapor pressures.

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Instead both the ITMS and GC-FID setup rely on relative measurements only, i.e., the signal

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measured by sampling the gas-phase above a solution containing both the organic and the salt is

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compared to the signal from sampling the gas-phase above a solution that does not contain the

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salt.

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2.1.1 Quadrupole Ion Trap Measurements

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A novel atmospheric pressure inlet was developed for these measurements. A diagram of

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this inlet is shown in Figure 1, panel A. Solutions were added to a 10 mL graduated cylinder

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outfitted to connect to a 0.5 inch UltraTorr. The system in front of the ion trap was then pumped

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down to house vacuum pressures (nominally 60 hPa) to remove excess O2 and N2. The air in

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equilibrium with the solution leaked through the pinhole (0.0007 inches, O’Keefe Controls).

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This ensured that the system was in an effusion regime which means that the water molecules

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diffused slightly faster than the organic solute molecules. The faster diffusion of the water

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molecules helped to pull along the organic molecules, thus shortening the time scales under

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which equilibrium was reached. The faster evaporation of water did not significantly alter the

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salt molarity of the solution. The valve to the quadrupole ion trap mass spectrometer (Thermo-

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Finnigan LCQ) was then opened, letting the sample in via the gas chromatography port. Data

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was collected for thirty minutes at which point signal in the mass spectrometer had stabilized

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(Figure S1). The last five minutes of the data was averaged and used in the analysis. No change

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in solution volume was observed so no change in solution temperature due to evaporation was

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expected to occur.



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A blank (18 MΩ water, Millipore) was run between each methylglyoxal solution to

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determine the extent of background methylglyoxal signal remaining in the ion trap. Signal was

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normalized to m/z 19 (H3O+, the strongest ion in the spectrum and not a methylglyoxal fragment)

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to account for any variability in ion trap signal. The water spectrum immediately preceding a

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methylglyoxal spectrum was then subtracted from the methylglyoxal spectrum to remove any

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residual background methylglyoxal, N2, O2, and instrument contaminants.

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methylglyoxal in the blank water spectra was between 1 and 2% of the maximum signal of the

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methyl glyoxl spectrum. A typical spectrum is shown in Figure 1, panel B. The standard NIST

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EI spectrum is shown in black on the top and the blank-corrected methylglyoxal spectrum

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measured in this experiment is shown on the bottom in red. There is excellent agreement

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between the two spectra. The signals at m/z 32 and around m/z 16 and 18 are due to incomplete

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water and air subtraction, and the extra signal at m/z 58 is due to background contamination from

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the ion trap but overall this method does an excellent job of correcting for background signal.

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To calculate salting constant values, we expand the Setschenow equation to include the

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explicit definition of Henry’s law:

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 Corg ,w  porg ,w log   Corg ,salt   porg ,salt

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Typical residual

  =K c S salt   

(Eq. 3)

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where Corg,w (molarity) is the concentration of the organic species in pure water, porg,w (atm) is

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the vapor pressure of the organic species over that solution, Corg,salt (molarity) is the

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concentration of the organic species in the salt solution with concentration csalt (here, molality

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but analysis can also be done with molarity) and porg,salt is the vapor pressure of the organic


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species over that salt solution. In this experiment, Corg,w and Corg,salt are the same (0.1 M

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methylglyoxal or 0.2 M glyoxal), so this reduces to:

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 porg ,salt log   porg , water 

  = K S csalt 

(Eq. 4)

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The signal at m/z 43 from the ion trap is taken to be a proxy for the vapor pressure

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because only relative measurements (rather than absolute pressure measurements) are necessary.

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Thus the salting constants can be calculated solely from the ion trap signal.

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2.1.2 Gas Chromatography Measurements

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Methylglyoxal and glyoxal salting constant measurements were also made using gas

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chromatography with flame ionization detection. GC-FID instruments are most sensitive to

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reduced carbon and signal is proportional to the number of reduced carbons that elute from the

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GC column at a given time.

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Therefore, to increase the number of reduced carbons and thus the sensitivity of the GC as well

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as lowering the vapor pressure of the molecules, they were derivatized with O-(2,3,4,5,6-

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Pentafluorobenzyl)hydroxylamine hydrochloride (PFBHA, Sigma Aldrich).

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reacts with carbonyl groups to form an oxime.

Methylglyoxal has one reduced carbon; glyoxal has none.

This molecule

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Solutions of organics and salt in water were added to 22 mL Supelco vials with Mininert

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valves with septa. A schematic of this is shown in Figure 1, Panel C. Solutions were allowed to

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equilibrate for 45 minutes (at longer time scales, imidazoles were formed in ammonium

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sulfate/glyoxal solutions and the aqueous phase concentration of glyoxal could no longer be

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assumed to be the same as the initial concentration of glyoxal). A solid phase micro extraction

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(SPME) fiber (65 µm poly(dimethylsiloxane)/divinylbenzene, Supelco) was first exposed to the

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headspace above a continuously stirred 17 mg/mL solution of PFBHA for two minutes. The



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fiber was then exposed to the headspace above the solution of organics and salt for one minute

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(methylglyoxal) or fifteen minutes (glyoxal). This method requires the assumptions that for salt

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and pure water solutions (1) the fiber is identically coated with PFBHA every time, (2) the gas-

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phase diffusion rates of the organic solute are the same, and (3) the derivatization reaction rates

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are the same. These assumptions are difficult to test, but we have exposed the fiber to the

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headspace above the PFBHA solution for the same amount of time to minimize variability in

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PFBHA coating, and we have no reason to believe that the diffusion rate for either glyoxal or

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methylglyoxal should vary based on the salt concentration since the salt is assumed to be non-

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volatile, nor should this affect the derivatization rates. Thus it is important to expose the fibers

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for a consistent period of time for a given organic solute. The fiber was then injected in to the

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GC and measured immediately. The GC program was as follows: 40°C for 2 minutes, 10°C/min

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to 110 °C, 5°C/min to 155 °C, 10°C/min to 280°C, 280°C for 5 minutes. The inlet was held at

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250°C. The GC was a Hewlett-Packard model 6890 equipped with a 30 m × 0.32 mm Agilent

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DB-1701 column with 1 µm thickness. PFBHA was always measured in significant excess to

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the oxime products, indicating that the amount of oxime formed was limited by the amount of

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gas-phase glyoxal or methylglyoxal, rather than the amount of PFBHA. Tests showed that after

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a run, all PFBHA and oxime products had been volatilized off of the SPME fiber so no blanks

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were run between measurements. For analysis, peak areas were used as the proxy for porg,w and

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porg,salt. All measurements were repeated; the number of repeats are given in Table S1.

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We additionally performed one set of measurements for glyoxal with both 1.6 molal

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ammonium sulfate and 1.6 molal ammonium nitrate to study the effects of salting in a solution

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with mixed salts.

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2.2 Computational Methodology


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Electrostatic interactions between the organics and salt ions were calculated using

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quantum chemical calculations. All geometry optimizations and frequency calculations were

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performed with Gaussian0928 using the M06-2X functional.29 The M06-2X functionality was

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chosen on basis of recent benchmarks showing its adequate performance in describing sulfur

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containing compounds and yielding reliable thermodynamics for sulfuric acid-water clusters.30–32

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For the initial conformational sampling solvent effects are taken into account using a Polarizable

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Continuum Model with the integral equation formalism variant (IEFPCM).33,34

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Methylglyoxal can exist in three different forms in aqueous solution: unhydrated,

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monohydrated and di-hydrated. Recently, we showed that glyoxal has an unfavorable interaction

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with sulfate, but that partitioning into sulfate aerosol could occur through the hydrates leading to

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a salting-in effect.26 Therefore only the methylglyoxal hydrates are further considered in this

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study. Methylglyoxal monohydrate exists in two different forms since water can be added either

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to the aldehyde or ketone moiety,35 which will be denoted monohydrate-a and monohydrate-k,

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respectively. For details on the conformational sampling of molecules and clusters, see the

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Supplemental Information. In Figure S2 the lowest identified free energy conformations are

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shown optimized at the M06-2X/6-31+G(d) level of theory in water. The relative stability of the

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monohydrates indicates that monohydrate-a is 17.9 kJ/mol more stable than monohydrate-k, and

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therefore the second monohydrate would only exist at lower concentrations.

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results from Nemet et al.35 show that the dihydrate can be present at significant concentrations

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(about 40% of the total methylglyoxal), however as our calculations show that the monohydrate

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is the most abundant species, we focus on the monohydrate for all further analysis.

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formation of the dihydrate will occur from adding water to monohydrate-a through the following

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reaction:

Experimental

The



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monohydrate-a + H2O ↔ dihydrate

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The formation free energy of the dihydrate is found to be +7.32 kJ/mol, and once appropriate

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water concentrations are accounted for, monohydrate-a and the dihydrate are expected to be

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present in similar concentrations within uncertainties.

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monohydrate is more prevalent than the dihydrate; therefore only the monohydrate-a

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conformation needs to be taken into account when exploring the salting-in/out effect of

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methylglyoxal. From here on in, we refer to monohydrate-a as simply monohydrate.

Experimental results show that the

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To investigate potential salting-in/out effects of methylglyoxal with various ions, the

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following reactions between the methylglyoxal monohydrate and the ions Cl–, NO3–, SO42−, Na+

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and NH4+ are studied:

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Monohydrate(aq) + Cl−(aq) ↔ (Cl−)(monohydrate)(aq) (1)

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monohydrate(aq) + NO3−(aq) ↔ (NO3−)(monohydrate)(aq) (2)

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monohydrate(aq) + SO42−(aq) ↔ (SO42-)(monohydrate)(aq) (3)

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monohydrate(aq) + Na+(aq) ↔ (Na+)(monohydrate)(aq) (4)

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monohydrate(aq) + NH4+(aq) ↔ (NH4+)(monohydrate)(aq) (5)

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The dianions are known to be unstable in the gas-phase and prone to electron spill: the

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exothermal and very rapid detachment of one electron to yield a radical monoanion and a free

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electron.36

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molecules significantly stabilize the dianion with respect to the monoanion and free electron, this

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issue presents severe technical problems for the quantum chemical modelling of aqueous-phase

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reactions of dianions. Continuum solvent models (e.g. IEFPCM), which describe most neutral

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solvent-phase reactions qualitatively correctly, are not alone able to describe reactions involving

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small dianions. In order to circumvent possible errors arising from electron detachment, as well

Though not directly relevant for solution-phase processes, where the solvent


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as to improve the overall description of the hydration of the ions and ion-molecule clusters, we

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have modeled hydration both implicitly (using the IEFPCM continuum solvent model) and

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explicitly, by adding water molecules to the simulation. In the case of sulfate, it has been shown

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that at least three water molecules are required to stabilize the dianion with respect to electron

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detachment.37,38 The combination of both implicit and explicit hydration was recently shown to

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provide a qualitatively correct description of salting out for glyoxal in sulfate solutions.26 By

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utilizing a similar approach for methylglyoxal, the cluster reactions can be modeled using the

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following displacement reactions:

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monohydrate(aq) + (Cl−)(H2O)3(aq) ↔ (Cl−)(monohydrate)(H2O)2 + H2O (6)

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monohydrate(aq) + (NO3−)(H2O)3(aq) ↔ (NO3−)(monohydrate)(H2O)2(aq) + H2O (7)

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monohydrate(aq) + (SO42−)(H2O)6(aq) ↔ (SO42−)(monohydrate)(H2O)5(aq) + H2O (8)

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monohydrate(aq) + (Na+)(H2O)3(aq) ↔ (Na+)(monohydrate)(H2O)2(aq) + H2O (9)

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monohydrate(aq) + (NH4+)(H2O)3(aq) ↔ (NH4+)(monohydrate)(H2O)2(aq) + H2O (10)

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Reactions 1-5 represent the fundamental chemical processes we wish to understand.

For

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technical reasons we represent these as reactions 6-10 as model systems to study reactions 1-5.

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Aside from the electron detachment issue, any potential errors in the entropy contribution are

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simultaneously reduced by keeping the number of molecules fixed in the displacement reactions.

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We have chosen to hydrate the singly charged ions by three water molecules and the doubly

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charged ions with six water molecules to be certain that there are no errors due to electron

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detachment. The structure of the ion-water clusters have in several cases previously been

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identified and were extracted from the following publications: (Cl–)(H2O)n from Nadykto et al.39,

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(NO3−)(H2O)n from Liu et al.38, (NH4+)(H2O)n from Pickard et al.40 and (SO42−)(H2O)n from

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Lambrecht et al.41 In case of the (Na+)(H2O)3 cluster, we were unable to find any literature



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structure and it was therefore manually constructed.

For details of the construction and

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configurational sampling of the monohydrate-ion-water clusters, the SI and the references cited

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therein.

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3. Results and Discussion

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The results of the salting constant measurements are shown in Figure 2, where the slopes

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of the lines correspond to the KS values. Salting constants for (NH4)2SO4 are shown in panel A,

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NH4NO3 in panel B, NaNO3 in panel C, and NaCl in panel D.

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measurements are shown with open circles in brighter colors.

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designated with solid lines. Methylglyoxal GC-FID measurements are shown in darker colors

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with open squares. Fits to the methylglyoxal GC-FID measurements are given with dotted lines.

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Glyoxal GC-FID measurements are shown in darker colors with solid diamonds. Fits to the

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glyoxal data are given with dashed lines. The grey lines are the average of the slopes of the GC-

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FID and ITMS measurements for methylglyoxal.

Methylglyoxal ITMS

Fits to the ITMS data are

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Salting constants for glyoxal and averages of methylglyoxal values are presented in Table

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1 along with the smallest, most water-soluble species measured in Wang et al.42 and Endo et al.43

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(converted to molality). All salting constant values are presented in Table S2 in both molarity

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and molality, with ITMS and GC-FID measurements for methylglyoxal separated for clarity.

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Excellent agreement is observed between the ITMS measurements and the GC-FID

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measurements for all the salts, except for ammonium nitrate, where measurements only agree

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marginally within error. The source of this discrepancy is presently unclear. Generally the good

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agreement for other salts confirms that both methods gave consistent results.


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Our results in Table 1 are presented in molality, as this is the more relevant and more

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easily calculated value for atmospheric aerosols.6 For consistency in Table 1, we have converted

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the values from Wang et al.42 from molarity to molality (see Table S1 for molarity units).

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Additionally, good agreement is observed between the glyoxal-(NH4)2SO4 salting

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constant as measured in Kampf et al.6 and in this work, given the very different salt

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concentration regimes (this work: 0 to 3.8 m ammonium sulfate in bulk solutions; Kampf et al:6

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4-15 m ammonium sulfate in metastable aerosols) and different methods of measuring both the

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gas and aqueous phase concentrations.

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Glyoxal very clearly salts in to aqueous salts, whereas methylglyoxal salts out. This

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indicates that molecular structure is critical for determining the salting behavior of a species, as

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glyoxal and methylglyoxal are fairly similar except for the methyl group. They are both small α-

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dicarbonyl species, have very high Henry’s law constants, have the ability to form hydrates in

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water, and have very high O/C ratios. However, the addition one methyl group converts an

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aldehyde group to a ketone; this change in structure switches the species from salting in to

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salting out. Additionally, even the smallest, most water-soluble species (2-butoxyethanol) tested

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in Wang et al.42 also salts out – more strongly than methylglyoxal. All of the species measured

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in Wang et al.42 and Endo et al.43 as well as methylglyoxal salt out; this suggests that molecules

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that salt in are likely a small subset of the molecules that occur in the atmosphere. Additionally,

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there is a large difference in the magnitude of the salting constants for methylglyoxal and for the

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species from Wang et al.42, where the larger species in Wang et al.42 were found to salt out of

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(NH4)2SO4 solutions much more strongly than methylglyoxal. Figure 3 shows that there is a

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clear trend with increasing salting out constants with carbon number for a series of 2-ketones (2-

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hexanone to 2-decanone were measured) and aldehydes.42 Interestingly, there seems to be an



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inverse correlation with expected number of OH groups and magnitude of the salting out

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constant measured in Wang et al.42 In particular, 1-hexanol (1 OH group), 2-hexanone (0 OH

294

groups as hydration is not expected), and heptanal (2 OH groups expected due to hydration) salt

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out with KS values of 0.364 m-1, 0.38 m-1 and 0.35 m-1, respectively. Despite the longer carbon

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chain heptanal has the lowest value. This is consistent with work by Kurtén et al.26 who found

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that the salting in from glyoxal is due to the interaction of the OH groups in glyoxal

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monohydrate and glyoxal dihydrate with the sulfate anion.26 Figure 3 also shows calculations for

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salting constants predicted using a poly-parameter linear free energy relationship (ppLFER; for

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details see Wang et al.42) for glyoxal and methylglyoxal with open triangles and open squares,

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respectively. ppLFER predictions of salting constants are shown for the unhydrated,

302

monohydrate, and dihydrate forms. The values for the unhydrated and monohydrate forms seem

303

to bracket the value backextrapolated from the ketone fit. The best estimate for the salting

304

constants calculated by the ppLFER method are shown as a solid triangle (glyoxal) and square

305

(methylglyoxal), accounting for the hydration equilibrium. The best estimate value for glyoxal

306

corresponds to that of the dihydrate form. For methylglyoxal, the best KS value represents

307

approximately 60% monohydrate and 40% dihydrate35 and falls in between both forms. The KS

308

measurements for glyoxal and methylglyoxal are given by the black triangle and square,

309

respectively. The best prediction from the ppLFER method shows a reasonable agreement for

310

methylglyoxal, but overestimates salting out by about 20%. For glyoxal, the ppLFER estimation

311

suggest salting out, while salting in is observed. Glyoxal is significantly smaller, more water-

312

soluble, and more highly oxygenated (i.e., has a larger number of OH groups) than the molecules

313

used during fitting of the ppLFER parameters; it is thus not expected to be well-represented by

314

this equation. This further underscores the importance of molecular structure, as the


Page 15 of 29


315

measurements are significantly lower (less salting out) than predicted by the correlation, in good

316

consistency with the salt interactions of glyoxal and methylglyoxal hydrates. This highlights the

317

need for additional measurements of water-soluble SOA precursors.

318

Our results are consistent with the Hofmeister series, which was initially developed to

319

explain the strength of ions on the salting out of proteins. This series is listed as CO32- > SO42- >

320

S2O32- > H2PO4- > F- > Cl- > Br- > NO3- > I- > ClO4- > SCN- where CO32- has the greatest effect

321

on protein salting and SCN- has the least effect.44

322

(NH4)2SO4 has the strongest effect, followed by NaCl. The nitrate salts have the weakest effect

323

on salting, but a non-zero effect. Moreover, Endo et al.43 measured salting constants for a

324

number of molecules in NaCl and Wang et al.42 measured salting constants for the same species

325

in (NH4)2SO4. In all cases, the molecules salted out of (NH4)2SO4 more strongly than NaCl.

326

Gordon and Thorne45 measured salting constants of naphthalene in sodium sulfate as well as a

327

number of chloride salts and found that sulfate caused naphthalene to salt out more strongly than

328

any of the chloride salts. Görgényi et al.46 measured salting constants for chloroform, benzene,

329

chlorobenzene, and anisole in a wide variety of nitrate, chloride, and sulfate salts.

330

chloroform, chlorobenzene, and anisole, sulfate salting constants were significantly higher than

331

the chloride salting constants which were higher than the nitrate salting constants. For benzene,

332

they found that sulfate had the strongest effect but nitrate and chloride salts had similar effects.46

For both glyoxal and methylglyoxal,

For

333

We also find a decrease in vapor pressure upon going from 1.6 m (NH4)2SO4 to 1.6 m

334

(NH4)2SO4 plus 1.6 m NH4NO3 with a set of six replicate measurements, which indicates that the

335

addition of NH4NO3 causes additional salting in beyond that caused by (NH4)2SO4. The decrease

336

is consistent with that observed upon going from 0 m NH4NO3 to 1.6 m NH4NO3. This suggests

337

that salting constants are additive since this additional decrease is what would be predicted based



Page 16 of 29

338

on the NH4NO3 salting constant. Therefore, the concentration of glyoxal that partitions to the

339

aerosol will be dependent on the concentration of all salts. We propose that the salting behavior

340

of glyoxal in a mixed (NH4)2SO4/NH4NO3 system can be calculated using:

341

 K H ,w log  K  H ,salt

342

This equation is consistent with our data, but the additivity of salting constants merits further

343

research.

  = −0.24 × c( NH4 )2 SO4 − 0.07 × cNH4 NO3 

(Eq. 5)

344

The fact that salting constants appear to be additive is consistent with the literature.

345

Gordon and Thorne45 performed a systematic series of salting constant measurements on

346

naphthalene in mixed salt solutions. They also found that the effects of salts are additive for

347

mixtures of sodium chloride with magnesium chloride, sodium sulfate, cesium bromide, calcium

348

bromide, and potassium bromide. Our results are thus in good agreement with this study since

349

we also find that mixture of (NH4)2SO4 and NH4NO3 is additive. However, this warrants further

350

investigation.

351

Results of quantum chemical calculations are given in Table 2. This lists the ∆G values

352

for the displacement of one water molecule with either a glyoxal monohydrate molecule or a

353

methylglyoxal monohydrate molecule. These values are all negative for anions and positive for

354

cations. This means that these monohydrates are expected to replace water in the hydration

355

shell, and in the case of glyoxal, replace the water molecules in the hydration shell surrounding

356

NO3- and SO42- quite significantly. We use the equation ∆G=-RTlnK to calculate the equilibrium

357

constant for the displacement of a water molecule with an organic, where R is the ideal gas

358

constant in J/mol K, T is the temperature in Kelvin, and K is the dimensionless equilibrium

359

constant. The ∆G values listed in Table 2 result in equilibrium constants between 88 (Cl-) and


Page 17 of 29


360

12000 (SO42-) for glyoxal and between 4 (Cl-) and 67 (NO3-) for methylglyoxal. However, the

361

organics we have studied do not significantly displace water molecules in the hydration shells of

362

the cations, as the equilibrium constants for these replacements are around 0.5 or less, which

363

means that only water molecules in the hydration shell is favored. The fact that the quantum

364

calculations predict replacement of water in the hydration shell for both glyoxal and

365

methylglyoxal, while both molecules behave quite different in terms of their overall salt

366

interaction indicates that other factors affect the interaction between the organics and the salt.

367

However, we note that the uncertainty on the quantum calculations is likely ±8 kJ/mol. In the

368

case of glyoxal, this results in an equilibrium constant greater than 1 within the entire range of

369

the ∆G calculations and thus glyoxal is always predicted to salt in. In the case of methylglyoxal,

370

the equilibrium constant can be slightly less than 1 for both Cl- and SO42- at the far end of the ∆G

371

range, allowing for the possibility of both salting in (all salts) and salting out (for Cl- and SO42-).

372

Thus weak salting in is within the error bars of the salting constant predictions for interactions

373

between methylglyoxal and these two anions. However, the trends predicted by the quantum

374

calculations also differ from those observed experimentally. This indicates that the reactions

375

represented by the calculations as well as other factors that are not yet well understood determine

376

the overall salting behavior of organic molecules. Other known factors that have been associated

377

to explain salting are ion charge and size effects47 as well as the dielectric properties and size of

378

the organic molecule.48

379

Quantum calculations can help assess whether salting in and out is likely to occur, but

380

should be used in combination with additional factors, such as the ion charge, and volume effects

381

that also need to be considered. Additional measurements need to be made for other water-

382

soluble organic molecules such as IEPOX and methyl tetrol, and further work on mixed salt



Page 18 of 29

383

solutions should be performed to confirm whether Eq (5) presents a good approximation over a

384

wider parameter space of mixed salt solutions.

385 386

4. Atmospheric Implications

387

The measured salting constants for glyoxal and methylglyoxal have implications for SOA

388

formation: at the high salt concentrations found in atmospheric aerosols, more glyoxal will

389

partition in to the aerosols than expected by just Henry’s law, while less methylglyoxal will

390

partition to the aerosols. This effect acts on top of the ~13 times higher effective Henry’s law

391

constant for glyoxal, and makes glyoxal relatively more available for further processing and

392

aerosol formation, but methylglyoxal less so.

393

Additionally, there is some evidence that salting constants are additive, both for benzene

394

in a variety of salts49 and for glyoxal in (NH4)2SO4 and NH4NO3. Glyoxal’s behavior in mixed

395

(NH4)2SO4/NH4NO3 aerosol can be calculated from Equation 5. This has significant implications

396

for predicting salting behavior in ambient aerosols as those typically contain a mixture of salts.1

397

For atmospheric modeling purposes we recommend using Eq. (5) to represent salting behavior

398

based on the molality of sulfate, nitrate, and chloride.

399 400

Acknowledgements

401

This work was supported by NSF-EAGER grant AGS-1452317 (RV). EMW is the recipient of a

402

CIRES Graduate Research Fellowship. JE and KVM thank the Danish Center for Scientific

403

Computing for computational resources and the Center for Exploitation of Solar Energy,

404

Department of Chemistry, University of Copenhagen, Denmark for financial support.

405

Furthermore, JE thanks and the Carlsberg Foundation for financial support.TK thanks the


Page 19 of 29


406

Academy of Finland for funding and the CSC IT Centre for Science for computer time. PJZ was

407

supported by NSF grant AGS-1219508. We would like to thank A. Ranney and M. Claflin for

408

technical assistance, and an anonymous reviewer for providing Abraham solvation parameters

409

among other helpful comments.

410

Supplemental Information: Supplemental information is available and contains two figures:

411

one of representative ITMS signals for methylglyoxal measurements and one of the lowest

412

energy methylglyoxal conformations and two tables: one for measurement conditions and one

413

for salting constant values in molarity as well as molality, and additional details about the

414

quantum chemical calculations. This information is available free of charge via the Internet at

415

http://pubs.acs.org/ .

416

References

417 418 419 420

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Zhang, Q.; Jimenez, J. L.; Canagaratna, M. R.; Allan, J. D.; Coe, H.; Ulbrich, I.; Alfarra, M. R.; Takami, A.; Middlebrook, A. M.; Sun, Y. L.; et al. Ubiquity and Dominance of Oxygenated Species in Organic Aerosols in Anthropogenically-Influenced Northern Hemisphere Midlatitudes. Geophys. Res. Lett. 2007, 34 (13), L13801.

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Philip, S.; Martin, R. V; van Donkelaar, A.; Lo, J. W.-H.; Wang, Y.; Chen, D.; Zhang, L.; Kasibhatla, P. S.; Wang, S.; Zhang, Q.; et al. Global Chemical Composition of Ambient Fine Particulate Matter for Exposure Assessment. Environ. Sci. Technol. 2014, 48 (22), 13060–13068.

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Waxman, E. M.; Dzepina, K.; Ervens, B.; Lee-Taylor, J.; Aumont, B.; Jimenez, J. L.; Madronich, S.; Volkamer, R. Secondary Organic Aerosol Formation from Semi- and Intermediate-Volatility Organic Compounds and Glyoxal: Relevance of O/C as a Tracer for Aqueous Multiphase Chemistry. Geophys. Res. Lett. 2013, 40 (5), 978–982.

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De Haan, D. O.; Corrigan, A. L.; Smith, K. W.; Stroik, D. R.; Turley, J. J.; Lee, F. E.; Tolbert, M. A.; Jimenez, J. L.; Cordova, K. E.; Ferrell, G. R. Secondary Organic AerosolForming Reactions of Glyoxal with Amino Acids. Environ. Sci. Technol. 2009, 43 (8), 2818–2824.

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De Haan, D. O.; Corrigan, A. L.; Tolbert, M. A.; Jimenez, J. L.; Wood, S. E.; Turley, J. J. Secondary Organic Aerosol Formation by Self-Reactions of Methylglyoxal and Glyoxal in Evaporating Droplets. Environ. Sci. Technol. 2009, 43 (21), 8184–8190.

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De Haan, D. O.; Hawkins, L. N.; Kononenko, J. A.; Turley, J. J.; Corrigan, A. L.; Tolbert, M. A.; Jimenez, J. L. Formation of Nitrogen-Containing Oligomers by Methylglyoxal and Amines in Simulated Evaporating Cloud Droplets. Environ. Sci. Technol. 2010, 45 (3), 984–991.

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Hawkins, L. N.; Baril, M. J.; Sedehi, N.; Galloway, M. M.; De Haan, D. O.; Schill, G. P.; Tolbert, M. A. Formation of Semisolid, Oligomerized Aqueous SOA: Lab Simulations of Cloud Processing. Environ. Sci. Technol. 2014, 48, 2273–2280.

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Powelson, M. H.; Espelien, B. M.; Hawkins, L. N.; Galloway, M. M.; De Haan, D. O. Brown Carbon Formation by Aqueous-Phase Carbonyl Compound Reactions with Amines and Ammonium Sulfate. Environ. Sci. Technol. 2013, 48 (2), 985–993.

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Galloway, M. M.; Chhabra, P. S.; Chan, A. W. H.; Surratt, J. D.; Flagan, R. C.; Seinfeld, J. H.; Keutsch, F. N. Glyoxal Uptake on Ammonium Sulphate Seed Aerosol: Reaction Products and Reversibility of Uptake under Dark and Irradiated Conditions. Atmos. Chem. Phys. 2009, 9 (10), 3331–3345.

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Woo, J. L.; Kim, D. D.; Schwier, A. N.; Li, R.; McNeill, V. F. Aqueous Aerosol SOA Formation: Impact on Aerosol Physical Properties. Faraday Discuss. 2013, 165, 357–367.

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Sareen, N.; Schwier, A. N.; Shapiro, E. L.; Mitroo, D.; McNeill, V. F. Secondary Organic Material Formed by Methylglyoxal in Aqueous Aerosol Mimics. Atmos. Chem. Phys. 2010, 10 (3), 997–1016.


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563 564



Molecule

KS (NH4)2SO4

KS NH4NO3

Glyoxal

-0.24±0.02 M-1 -0.16±0.02 M-1

--0.06±0.02 M-1

Methylglyoxal

0.16±0.03 M-1

0.075±0.03 M-1

KS NaNO3

Page 24 of 29

KS NaCl

Reference

---0.065±0.006 M-1 -0.10±0.02M-1 0.02±0.02 M-1

0.06±0.02 M-1

Kampf (2013)6 this work this work

-1

-1

2-hexanone

0.38±0.01 M --

---

---

0.18±0.02 M Wang (2014)42 -1 0.198±0.004 M Endo (2012)43

heptanal

0.35±0.03 M-1 --

---

---

1-hexanol

0.364±0.006 M-1 --

---

---

0.21±0.01 M-1 Wang (2014)42 0.221±0.004 M-1 Endo (2012)43

2-butoxyethanol

0.31±0.02 M-1 --

---

---

-Wang (2014)42 -1 0.211±0.009 M Endo (2012)43

-0.24±0.01 M-1

Wang (2014)42 Endo (2012)43

565

Table 1: Compilation of KS values for small oxygenated species in aerosol-relevant salts (all in

566

molality-1). Good agreement is observed between the literature value and the value measured in

567

this work for glyoxal in (NH4)2SO4 despite the different salt regimes and different methods used

568

for measuring the gas and aqueous phase concentrations. All species except for glyoxal are

569

determined to salt out of solution.

570


Page 25 of 29


Ion

∆G (kJ/mol)

ClNO3SO42Na+ NH4+

-10.9 -22.0 -22.9 2.1 1.2

ClNO3SO42Na+ NH4+

-3.1 -10.3 -7.91 6.11 1.6

∆G range (kJ/mol) Glyoxal -2.6 to -19.3 -13.6 to -30.4 -14.6 to -31.3 10.5 to -6.28 9.54 to -7.20 Methylglyoxal 5.23 to -11.5 -1.9 to -18.6 0.46 to -16.3 14.5 to -2.3 9.96 to -6.78

K

K range

88 8400 12000 0.42 0.62

2.8 to 2700 270 to 260000 390 to 380000 0.014 to 13 0.020 to 19

3.6 67 26 0.081 0.52

0.12 to 110 2.2 to 2100 0.83 to 800 0.0026 to 2.5 0.017 to 16

571 572

Table 2: Calculated ∆G, ∆G range assuming ±8 kJ/mol, K, K range assuming ±8 kJ/mol for

573

glyoxal and methylglyoxal. ∆G values are calculated for the replacement of a water molecule in

574

the hydration shell of the anion in the Ion column by either a glyoxal monohydrate molecule or a

575

methylglyoxal monohydrate molecule. The corresponding K values are calculated from the

576

equation ∆G=-RTlnK.

577



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Figure 1: A) Salting constant inlet for mass spectrometer. Solutions of known organic and salt concentration are added to graduated cylinder. System is pumped down to remove oxygen and nitrogen. Water and organics leak through the pinhole to create an effusion regime and then enter the mass spectrometer. B) Mass spectrometric proof of methyl glyoxal detection in EI mode. The standard NIST EI spectrum of methyl glyoxal is shown on the top in black and the spectrum recorded in the lab is shown on the bottom in red with air and water subtracted out. The very good agreement between the spectra is proof of methyl glyoxal detection. C) Schematic of SPME and GC-FID measurements. SPME fiber samples the headspace above the solution of organic and salt, and results in a two-peak chromatogram. 190x254mm (96 x 96 DPI)

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Figure 2: Salting constants for glyoxal (solid diamonds, dashed fit) and methylglyoxal calculated from both ITMS data (open circles, solid fit) and GC-FID data (open squares, dotted fit). Slopes of data correspond to KS values, which are given in Table 1 and Table S1. Solid grey lines: average of KS from ITMS and GC-FID data for methylglyoxal. Excellent agreement is observed between both methylglyoxal measurements except for NH4NO3. The positive slopes for methylglyoxal indicate that it salts out. The negative slopes for glyoxal indicate that it salts in. 100x110mm (220 x 220 DPI)



Figure 3: Relationship between KS values for (NH4)2SO4 and carbon number. Measurements from Wang et al.42 are given by red circles (2-ketones) and blue circles (aldehydes); fit the 2-ketone data given by the red dashed line. KS values for glyoxal (squares) and methylglyoxal (triangles) measured in this work (solid black symbols) are compared with predictions from the poly-parameter linear free energy relationship (ppLFER, Wang et al.42) for unhydrated, monohydrate, and dihydrate forms, respectively (open green symbols). The best estimations from the ppLFER are the solid green symbols. For glyoxal, this corresponds to the dihyrate value; for methylglyoxal to a linear combination of 60% monohydrate and 40% dihydrate. 116x102mm (220 x 220 DPI)


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254x190mm (96 x 96 DPI)


Glyoxal and Methylglyoxal Setschenow Salting ... - ACS Publications

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