Influence of Collecting Substrates on the Characterization of

To evaluate the hydrophilic or hydrophobic nature of the collecting substrates, contact angle measurements were performed three times for each substra...
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Influence of Collecting Substrates on the Characterization of Hygroscopic Properties of Inorganic Aerosol Particles Hyo-Jin Eom,† Dhrubajyoti Gupta,† Xue Li, Hae-Jin Jung,‡ HyeKyeong Kim, and Chul-Un Ro* Department of Chemistry, Inha University, 253 Yonghyun-dong, Nam-gu, Incheon 402-751, Republic of Korea S Supporting Information *

ABSTRACT: The influence of six collecting substrates with different physical properties on the hygroscopicity measurement of inorganic aerosol particle surrogates and the potential applications of these substrates were examined experimentally. Laboratory-generated single salt particles, such as NaCl, KCl, and (NH4)2SO4, 1−5 μm in size, were deposited on transmission electron microscopy grids (TEM grids), parafilmM, Al foil, Ag foil, silicon wafer, and cover glass. The particle hygroscopic properties were examined by optical microscopy. Contact angle measurements showed that parafilm-M is hydrophobic, and cover glass, silicon wafer, Al foil, and Ag foil substrates are hydrophilic. The observed deliquescence relative humidity (DRH) values for NaCl, KCl, and (NH4)2SO4 on the TEM grids and parafilm-M substrates agreed well with the literature values, whereas the DRHs obtained on the hydrophilic substrates were consistently ∼1−2% lower, compared to those on the hydrophobic substrates. The water layer adsorbed on the salt crystals prior to deliquescence increases the Gibb’s free energy of the salt crystal−substrate system compared to the free energy of the salt droplet−substrate system, which in turn reduces the DRHs. The hydrophilic nature of the substrate does not affect the measured efflorescence RH (ERH) values. However, the Cl− or SO42− ions in aqueous salt droplets seem to have reacted with Ag foil to form AgCl or Ag2SO4, respectively, which in turn acts as seeds for the heterogeneous nucleation of the original salts, leading to higher ERHs. The TEM grids were found to be most suitable for the hygroscopic measurements of individual inorganic aerosol particles by optical microscopy and when multiple analytical techniques, such as scanning electron microscopy-energy dispersive X-ray spectroscopy, TEM-EDX, and/or Raman microspectrometry, are applied to the same individual particles.

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spectrometry (ATOFMS),29,30 to determine the chemical compositions of individual ambient particles. In addition, the hygroscopic properties of single particles can be measured using levitation techniques.8,10−13,31−38 To obtain the chemical compositions along with the hygroscopic properties of single particles, the levitation techniques have been used in combination with nonintrusive analytical techniques, such as electro-dynamic balance (EDB) with Raman microspectrometry (RMS),38 laser induced fluorescence,39 or Mie-scattering40,41 and optical levitation with RMS.42 On the other hand, many hygroscopic studies have been performed for individual or bulk aerosol particles collected on a range of substrates. For example, measurements of the waterto-solute ratio of salt particles deposited on TEM grids,43 silicon wafer,20 ZnSe window,44 and Si3N4 window45 have been performed by micro-FT-IR in a flow-cell or static mode chamber (SMC). The hygroscopicity of submicrometer salt particles were studied by scanning transmission X-ray

tudies on the hygroscopic properties of aerosol particles can provide important insights into (i) alteration of the particle aerodynamic properties, (ii) cloud-droplet nucleation efficiency, and (iii) optical properties that contribute to direct and indirect radiative forcing on climate change.1−8 In addition, they are also valuable for understanding the physicochemical change that aerosols may experience through heterogeneous chemical reactions with atmospheric gas phase species,4−6,9−13 resulting in numerous studies of their hygroscopic behavior using various analytical techniques.8,14 For example, rapid single particle mass spectrometry15−18 and online Fourier transform infrared (FT-IR) spectroscopy19−21 were used to examine the hygroscopic properties of aerosol particles, depending on their chemical compositions or mixing state. Hygroscopicity-tandem differential mobility analyzer (H-TDMA) has also been used widely for the hygroscopic studies of size-segregated monodisperse aerosol particles (typically submicrometer range).22−25 Because H-TDMA cannot provide chemical compositional information on aerosol particles, the H-TDMA system has sometimes been combined with other techniques, such as scanning or transmission electron microscopy (SEM/ TEM),26,27 single-particle laser-ablation time-of-flight mass spectrometry (SPLAT-MS),28 or aerosol time-of-flight mass © 2014 American Chemical Society

Received: December 5, 2013 Accepted: February 10, 2014 Published: February 10, 2014 2648

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these interfacial energies is quite complicated and becomes even more complicated for real atmospheric aerosols comprised of a range of chemical species. Therefore, as a first step, it is imperative to examine the effects of substrates experimentally for single salt particles, which serve as inorganic aerosol particle surrogates. In this study, the hygroscopic behavior of three atmospherically relevant single salt particles [i.e., NaCl, KCl, and (NH4)2SO4] with known hygroscopic properties10,65,89 was examined by optical microscopy after depositing laboratorygenerated particles on six commonly used substrates with different surface properties. The six substrates were TEM grids, parafilm-M, Al foil, Ag foil, silicon wafer, and cover glass. The contact angle and surface roughness of each substrate were measured to examine its hydrophilic or hydrophobic character. The influence of the six substrates on the hygroscopic behavior of the single salt particles deposited on them was examined by obtaining humidifying and dehydration curves, DRHs, and ERHs. An attempt was made to establish a qualitative correlation between the experimental observations, expected changes in interfacial energies depending on the different physical properties of the substrates, and the corresponding changes in free energy during deliquescence or the critical free energy barrier for efflorescence of the single salt particles. To the best of the authors’ knowledge, this study is the first experimental report on the influence of collecting substrates in the hygroscopicity measurements of single aerosol particles.

microscopy/X-ray absorption spectroscopy (STXM/XAS) and noncontact environmental atomic force microscopy (e-AFM), using a Si3N4 X-ray transmission window46,47 and oxidized silane-treated silicon wafer48 as the substrate, respectively. The liquid water content of aerosol particles deposited on Fluoropore and Teflon filters was determined by gravimetric analysis and ion chromatography49 and by gas chromatography with a thermal conductivity detector,50−52 respectively. The phase changes of individual particles deposited on substrates against relative humidity (RH) changes were observed through high-resolution imaging by environmental scanning electron microscopy (ESEM) and environmental transmission electron microscopy (ETEM).53−58 For ETEM measurements, either conductive- or lacey-carbon-coated TEM grids were reported to yield consistent deliquescence RHs (DRHs) and efflorescence RHs (ERHs) for NaCl and (NH4)2SO4 particles.55 For ESEM measurements, a stainless steel crucible, Formvar film with Cu grids (uncoated TEM grids), and Pt, Ti, and Cu electron microscope plates were used as substrates for the analysis of inorganic salt particles,53 soot particles,53 and coarse dust particles,59 respectively. The chemical compositions of individual particles were examined either in situ using energy dispersive X-ray analysis (EDX)59 or off-line using standalone SEM-EDX or TEM-EDX.54,56−58,60−62 Optical microscopy has also been used to examine the hygroscopicity of aerosol particles collected on cover glass coated with hydrophobic organo-silane or polytetrafluoroethylene (PTFE)63,64 or TEM grids.65−67 As optical microscopy cannot provide chemical compositional information, standalone SEM-EDX was used for chemical analysis of the same individual particles collected on TEM grids after the hygroscopic study.65,67 Three coarranged substrates, such as Si3N4-coated silicon wafer chips, TEM grids, and silicon wafers with a Si3N4 window, were used to collect anthropogenic and marine aerosols for the study of water uptake and ice-nucleation efficiency by optical microscopy, chemical compositions by SEM-EDX and speciation/mixing state by STXM/XAS analysis, respectively.68 On the other hand, RMS is a good analytical technique when an environmental cell (EC) or an ice nucleation cell (INC) is installed inside because a built-in optical microscope can be used for the hygroscopic or ice-nucleation measurements, and Raman spectroscopy can provide chemical compositional information. Individual single salt, mixed, or ambient aerosol particles collected on Teflon/FEP (fluorinated ethylene propylene) films,69,70 quartz,71−73 and hydrophobically coated microscopic glass slides74,75 were examined by RMS. Up until now, studies on a range of substrates for single particle analysis by SEM-EDX and RMS have been performed,76−83 and TEM grids with conductive carbon coating were reported to be the most suitable compared to Al foil, Ag foil, Nucleopore filter, carbon tape, Be disc, and silicon wafer.84,85 However, the possible influence of substrate on the hygroscopicity measurements of aerosol particles has never been investigated systematically. Although a theoretical estimation indicates that the substrates do not influence the DRH value of NaCl particles larger than 0.5 μm,86 it was claimed that the substrate surfaces are likely to affect the efflorescence process by acting as seeds for heterogeneous nucleation.8 The interfacial energies between a substrate and salt crystal and between a substrate and nucleating salt molecules can affect the change in Gibbs free energy during deliquescence and the free energy barrier for nucleation during efflorescence, respectively.8,86−88 A quantitative estimation of

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EXPERIMENTAL SECTION Deposited Particle Samples. Droplets generated using an atomizer (HCT4810, single jet atomizer) from pure aqueous solutions of NaCl, KCl, and (NH4)2SO4 (1 M, Aldrich, >99.99% purity) were deposited as dry salt particles onto six substrates by passing through a silica packed diffusion dryer (residence time of ∼2 s, HCT4920 Diffusion dryer). The substrates were (i) TEM grids (Ted Pella Inc., Formvar/ Carbon 200 mesh Cu grid, 35−70 nm thickness), (ii) parafilmM (Pechiney Plastic Packaging Company, 127 μm thickness), (iii) Ag foil (Goodfellow Inc., 99.95% purity, 0.025 mm thickness), (iv) Al foil (Goodfellow Inc., 99.0% purity, 0.025 mm thickness), (v) silicon wafer (MTI Corp., 99.999% purity, 0.275 mm thickness), and (vi) cover glass (Menzel-Gläser, 0.13−0.16 mm thickness). Table S1 of the Supporting Information lists the general properties of the six substrates. Hygroscopicity Measurement System. Hygroscopicity measurements of pure salt particles were performed using a “see-through” inertial impactor apparatus equipped with an optical microscope. The experimental setup is described in detail elsewhere.65 Briefly, the apparatus was composed of three parts: (A) see-through impactor, (B) optical microscope, and (C) humidity controlling system. The collecting substrates onto which the particles were deposited had been mounted on the impaction plate in the see-through impactor. Dry N2 gas was passed through a bubbler containing the DI water, allowing the N2 to become saturated with water vapor. The saturated and dry N2 gases were mixed at different flow rates controlled using two mass flow controllers, resulting in a N2 gas flow with a controlled RH entering the impactor. The RH, which was monitored by a digital hygrometer (Testo 645), was varied from ∼4 to ∼95% in 0.1−0.3% steps. The digital hygrometer was calibrated using a reference dew-point hygrometer (M2 Plus RH, GE), providing RH readings of ±0.3% reproducibility. Each humidity condition was sustained for at least two minutes 2649

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Figure 1. Optical images obtained during the (A−D) humidifying process and (E−H) dehydration process for generated NaCl particles on the (a) TEM grid and (b) Ag foil.

to achieve a steady state for condensing or evaporating water. The particles on the impaction plate were observed through the nozzle throat by optical microscopy (Olympus, BX51M). Optical images of the particles were recorded at each RH during the humidifying (increasing RH from ∼4% to ∼95%) and dehydration (decreasing RH from ∼95% to ∼4%) experiments using a digital camera (Canon EOS 5D, full frame, Canon EF f/3.5 L macro USM lens). The optical images of the particles were processed using image analysis software (Matrox, Inspector v 9.0). The particle size was determined by measuring the two-dimensional (2D) projected surface area of the particle in an optical image. All the experiments were carried out under ambient room temperature (T = 22 ± 2 °C). Measurements of the Contact Angle and Surface Roughness of the Collecting Substrates. To evaluate the hydrophilic or hydrophobic nature of the collecting substrates, contact angle measurements were performed three times for each substrate using a computer-controlled contact angle analyzer (Phoenix 300, Seo Company Ltd., Republic of

Korea), where the contact angles were obtained using ImageXp from the images captured for deionized water droplets (1−2 mm sized) sitting on the substrates. The surface roughness of the substrates except TEM grids was examined by atomic force microscopy (AFM, Nanoscope multimode Iva, Digital Instruments, NS4A) in tapping mode on 10 × 10, 5 × 5, and 1 × 1 μm scan scales. These scales were chosen because the deposited inorganic salt particles on the collecting substrates were in the size range of 1−5 μm. For each scan scale, five different regions on each substrate were examined to obtain the root-mean-squared roughness (RMS or Rq) and average roughness (Ra).90 The surface roughness of the TEM grids could not be measured by AFM because of their thin, fragile surfaces.

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RESULTS AND DISCUSSION Physical Properties of Collecting Substrates. Table S2 of the Supporting Information lists the measured contact angles of deionized water droplets on the substrates (except TEM 2650

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Figure 2. Plots of the area ratio as a function of relative humidity (humidifying process: Δ triangles; dehydration process: ● solid circles) for typical NaCl particles collected on (a) TEM grid, (b) parafilm-M, (c) Al foil, (d) Ag foil, (e) silicon wafer, and (f) cover glass. The transition relative humidities in both humidifying and dehydration processes are marked with arrows in each plot.

grids). The contact angle of 106.8° ± 0.4° for the parafilm-M substrate, which is larger than 90°, suggests that it is hydrophobic, whereas Al foil, Ag foil, silicon wafer, and cover glass are hydrophilic because their contact angles are smaller than 90°, in the order of Al foil < Ag foil < silicon wafer < cover glass. Although TEM grids are generally considered to be hydrophobic due to the thin carbon layer over Formvar film,55,84 the measured contact angle was less than 90°, suggesting that the deionized water droplets (1−2 mm size) were too large to properly measure the contact angle of the very thin carbon layer, which also explains why their contact angle is not available in the literature. These observed contact angles are from a net effect of surface roughness and inherent hydrophobic or hydrophilic nature of the substrates. In accordance with the Cassie and Wenzel models,91,92 the interfacial contact area between a droplet and substrate depends on the surface roughness of the substrate. If the hydrophilic substrate surface is rough, the interfacial contact area between the droplet and substrate increases, resulting in an increase in the interfacial energy and, thus, the rough surface becomes more hydrophilic than a smooth surface. If the surface roughness of the hydrophobic substrate increases, the gap between the droplet and substrate fills with air, resulting in a decrease in the interfacial contact area and interfacial energy, and thus the rough surface becomes more hydrophobic. The models also suggest that the contact angle of the hydrophobic, rough surface is larger than that of the hydrophobic, smooth surface. Table S2 of the Supporting Information also lists the measured surface roughness of the five substrates except for the TEM grids. On the basis of the Rq and Ra values for the five substrates, their surface is rough in the following order: parafilm-M > Al foil > Ag foil > cover glass > silicon wafer (Table S2 of the Supporting Information).

The six substrates examined in this study can be used for a hygroscopicity study of particles by optical microscopy. After the hygroscopic properties of individual particles collected on a substrate are investigated by optical microscopy, it is sometimes inevitable to apply other standalone analytical techniques in combination, such as SEM-EDX, TEM-EDX, attenuated total reflection-FT-IR (ATR-FT-IR) imaging, and/or RMS, for their chemical compositional information.43,65,70,75,93 For TEM-EDX measurements, TEM grids are the only useful substrate.56,58,61,93 Nonconductive substrates, such as parafilm-M and cover glass, need to be coated with conductive metal or carbon before SEM-EDX measurements, where the conductive coating may interfere with X-ray analysis. Although major elements from the substrates as listed in Table S1 of the Supporting Information may interfere with X-ray spectral analysis of particles in SEM-EDX measurements, the possible interference can be reasonably corrected for.84 For the ATRFT-IR imaging measurements, TEM grids cannot be used as substrates because they are too fragile to withstand mechanical contact with an internal reflectance element crystal. Parafilm-M and cover glass cannot be used in ATR-FT-IR imaging because the chemical compositions of the polyolefin/paraffin wax and SiO2, respectively, produce strong IR peaks. On the other hand, TEM grids,82 Al and Ag foils,94 and cover glass69,70,74 were reported to be suitable for RMS measurements. After the hygroscopic properties of the particles are examined by optical microscopy, the relocation of the same individual particles on a substrate is essential for analysis using other techniques. This can be done relatively easily on TEM grids because the marked pattern on TEM grids can provide a coordinate system, which helps correctly relocate the individual particles (Table S1 of the Supporting Information). Deliquescence Behavior of Single Salt Aerosol Particles on the Six Different Substrates. Laboratory2651

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estimated value of 75.3%, from the extended aerosol inorganics model (E-AIM; http://www.aim.env.uea.ac.uk/aim/aim. php).89,95 On the other hand, for the hydrophilic cover glass, silicon wafer, Ag foil, and Al foil, the DRHs of the NaCl particles were 73.5(±0.5)%, 73.6(±0.5)%, 73.8(±0.6)%, and 73.9(±0.3)%, respectively, which were consistently lower than the literature values. This suggests that TEM grids behave like the hydrophobic parafilm-M substrates. For the KCl and (NH4)2SO4 particles, the same observation was made (i.e., the use of the two hydrophobic substrates resulted in DRHs that match the experimental and theoretical values reported in literature), whereas the four hydrophilic substrates showed DRHs ∼1−2% less than the reported values (Table S3 of the Supporting Information). From the general condition of thermodynamic equilibrium,86,96 when a nonvolatile salt crystal is suspended in air at a constant temperature and pressure, the Gibb’s free energies, G1 and G2, for the crystal prior to deliquescence and its aqueous droplet at deliquescence, respectively, can be represented as

generated NaCl, KCl, and (NH4)2SO4 aerosol particles were used to examine the influence of the six different substrates on the hygroscopic behavior of common inorganic aerosol particle surrogates. Figure 1a presents several optical images of NaCl particles deposited on a TEM grid obtained at different RHs during the humidifying and dehydration processes. The 2dimensional (2D) images clearly show the change in size, physical state, and morphology of the particles with the change in RH. The humidifying process was started at RH = 3.3%, where all seven particles appear as angular or rectangular solids (Figure 1a-A). When the RH was increased in the 0.1− 0.3% RH steps, the shapes of the seven particles remained unchanged until RH = 75.2%, where the particles began to appear more compact (Figure 1a-B). At RH = 75.3%, all the particles appeared spherical because water was absorbed in the particles but undissolved NaCl was still in the core of the particles (Figure 1a-C). Above RH = 75.3%, all the particles began to absorb water rapidly and at RH = 75.5%, NaCl was dissolved completely, resulting in aqueous NaCl droplets (Figure 1a-D). Above RH = 75.5%, the aqueous NaCl droplets exhibited continuous hygroscopic growth until RH = 95.0%. Similarly, the deliquescence transitions of NaCl particles on the Ag foil are shown in Figure 1b (panels A−D) and on the other four substrates in Figure S1a−S1d (panels A−D) of the Supporting Information. Figure 2 presents the humidifying and dehydration curves for the particle/droplet area ratios against RH (displayed as △ and ●, respectively) for typical NaCl particles on the six substrates. The area ratio of a particle was obtained by dividing the area of the particle at different RHs by that of the same dry particle at the starting RH of the humidifying process. The NaCl particles appeared angular on their 2D optical image obtained at the starting RH of the humidifying process. During the humidifying process, their size and shape remained unchanged up to certain RHs, such as 72.8% (cover glass), 72.9% (silicon wafer), 73.0% (Ag foil), 73.3% (Al foil), 74.6% (parafilm-M), and 75.0% (TEM grid). The particle 2D areas were decreased by 2−5% at RH = 73.0% (cover glass), 73.1% (silicon wafer), 73.2% (Ag foil), 73.4% (Al foil), 74.7% (parafilm-M), and 75.2% (TEM grid), where their shapes appeared spherical because water was absorbed in the particles but a small amount of NaCl solids remained inside. The initial shrinkage occurred immediately before the deliquescence transition began because of the structural rearrangement of NaCl crystals with the initial uptake of moisture.18,23 This phenomenon appeared to occur at lower RHs for the hydrophilic substrates. The deliquescence transitions began with the increase in RH from 75.2 to 75.5% for the TEM grid, from 74.9 to 75.2% for parafilm-M, from 73.6 to 73.9% for Al foil, from 73.2 to 73.8% for Ag foil, from 73.1 to 73.6% for silicon wafer, and from 73.0 to 73.5% for cover glass, as the NaCl particles dissolved completely to become aqueous NaCl droplets at their DRHs. Above their DRHs, their areas increased several times, showing hygroscopic growth. The droplets appeared to be most distorted in shape on the Ag foil, making 2D area measurements somewhat difficult. The DRHs for each substrate were obtained for all NaCl particles in the optical image field. Table S3 of the Supporting Information lists the measured DRHs of NaCl, KCl, and (NH4)2SO4 particles on the different substrates. For TEM grids and parafilm-M, the DRHs of NaCl particles were 75.2(±0.3)% and 75.5(±0.3)%, respectively, which are within the range of the reported experimental literature values (75− 76%)14,27,31,33,43,52,55 and quite close to the theoretically

G1 = Nμ1V + n2μ2C + [σ CVACV ]

(1)

G2 = n1μ1 + n2μ2 + (N − n1)μ1V + [σ LVALV ]

(2)

where N, n1, and n2 are the number of water vapor molecules, aqueous water molecules, and salt molecules, respectively, μV1 , μC2 , μ1, and μ2 are the chemical potentials of water vapor, solid crystal, aqueous water, and solutes in the aqueous droplet, respectively, and Aij and σij denote the interfacial area and tension, respectively, between phases i and j which can be L (droplet), V (gas), or C (crystal). In the right-hand side of eqs 1 and 2, the parts within the brackets represent the mechanical interfacial energy, whereas the remainder is the chemical energy part. At DRH, G1 = G2.86,97 The DRHs of the suspended single and multicomponent inorganic aerosol systems were derived from thermodynamic models, such as E-AIM,89,95 based on the above principle. On the other hand, an additional interface is introduced when the crystal particles or droplets sit on the collecting substrates. Hence, the Gibb’s free energies of an inorganic crystal (G′1) and its aqueous droplet (G′2) can be expressed as86 G1′ = Nμ1V + n2μ2C + [σ CVACV + σ CSACS + σ SVASV ] (3)

G2′ = n1μ1 + n2μ2 + (N − n1)μ1V + [σ LVALV + σ LSALS + σ SVASV ]

(4)

where the terms for the chemical energy part remain the same as in eqs 1 and 2, but the mechanical interfacial energies differ according to Aij and σij, denoting the interfacial area and tension, respectively, between phases i and j, which can be L (droplet), V (gas), C (crystal), or S (substrate). When G1′ = G2′ , the deposited particles deliquesce. This additional factor can affect the DRH of the particles on the substrate, which is illustrated schematically in Figure 3. This plot of Gibb’s free energy as a function of the RH shows how the free energies, G′1 and G′2, of the salt crystals (e.g., NaCl) and droplets deposited on the substrates, respectively, are expected to vary with RH.86 At low RH, the salt crystals are thermodynamically stable with G1′ < G2′ . On the other hand, G2′ keeps decreasing with 2652

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efflorescence is driven by the homogeneous nucleation of salt molecules, which is a kinetic and random process, the ERHs for the single salt droplets can be over a wide range,14,103 as shown in Figure 1 for the NaCl droplets. As shown in Figure 2, during the dehydration process, a typical NaCl droplet on each substrate effloresced at 47.8− 47.7%, 47.3−47.0%, 46.7−46.3%, 72.0−71.6%, 46.3−46.0%, and 45.7−45.5% for the TEM grids, parafilm-M, Al foil, Ag foil, silicon wafer, and cover glass, respectively. The ERHs were obtained for all NaCl, KCl, and (NH4)2SO4 droplets in the optical image field of each substrate (Table S4 of the Supporting Information). With the exception of Ag foil (ERH = 69.5−72.7%, Table S4 of the Supporting Information), the ERHs (44.2 − 48.0%, Table S4 of the Supporting Information) of NaCl droplets for the other substrates were within the range of experimental values reported in literature (42−48%)14,43,46,52,55 and match the theoretically estimated value of 46.3−47.5%.104 The measured ERHs of KCl and (NH4)2SO4 droplets were also within the range of literature values and matched the theoretical values (see references in Table S4 of the Supporting Information), except for the Ag foil. The ERHs of NaCl, KCl, and (NH4)2SO4 droplets on Ag foil were observed at 69.5−72.7% (∼21−24% higher than those of the other substrates), 71.0−74.6% (∼16−19% higher), and 46.8−49.4% (∼12−15% higher), respectively. It was reported that higher ERH (∼70%) for NaCl droplets deposited on Ag substrate was also observed by an ESEM.105 Furthermore, as shown in Figure 2 (panels c−d), the area ratio for the droplets does not change smoothly with the RH change because the droplets can flow and spread over the Al and Ag foils with hydrophilic and rough surfaces, suggesting that Al and Ag foils are unsuitable when full hygroscopic behavior is to be determined. To qualitatively explain the ERH values obtained for single salt droplets on the substrates, the Gibb’s free energies, G1 and G2, were first considered for the suspended supersaturated droplets prior to nucleation and when the solute nuclei form at ERH, respectively, which are expressed as106

Figure 3. Schematic plot of Gibbs free energy (arbitrary values) for a deposited droplet (G′2) and those for a deposited salt particle on hydrophobic substrate (G1′ ) and hydrophilic substrate (G1″) against relative humidity. DRHs of NaCl on TEM grids (75.3%) and Ag foil (73.8%) are taken as an example.

increasing RH. At DRH, G′2 = G′1, and above DRH, G′2 < G′1, thus the droplets become thermodynamically stable. The initial adsorption of water on the crystal surface was reported to occur immediately before the deliquescence of salt crystals.18 For a hydrophilic substrate, this adsorbed water layer can condense and/or spread within the interface between the substrate and salt crystal,98 which is more probable if the crystal and/or substrate have rough surfaces. This leads to an additional interfacial energy term in eq 3 for the interaction between the hydrophilic substrate and adsorbed water (i.e., σWSAWS, where the superscript, W, represents the condensed water layer). σWSAWS should be larger than σCSACS,99 leading to an increased free energy of the system (G″1 in Figure 3), whereby G1″ becomes equal to G2′ at a lower RH than that on hydrophobic substrates (e.g., DRH = 75.5% and 73.8% for the hydrophobic TEM grid and hydrophilic Ag foil substrates, respectively). The change in the droplet-substrate interfacial free energy, σLSALS, appears to balance the change in dropletvapor interfacial free energy, σLVALV.100 Hence, the total droplet free energy (G2′ ) does not change among the different substrates, particularly for supermicrometer particles (1−5 μm),86,101 which is also supported by the observation of no substantial change in contact angle above the DRH during the humidifying process.102 Efflorescence Behavior of Single Salt Aerosol Particles on the Six Different Substrates. The dehydration process began after decreasing the RH from ∼95.0%, during which the sizes of all the seven NaCl droplets on a TEM grid substrate decreased until RH = 48.0% where droplets nos. 1 and 2 effloresced (Figure 1a-E). Droplets nos. 4 and 5 crystallized at RH = 47.7% (Figure 1a-F), and the remaining three droplets, nos. 3, 6, and 7, crystallized at RH = 47.0% (Figure 1a-G). When RH was decreased further to 3.1%, the size and shape of the effloresced NaCl particles remained the same (Figure 1aH). Similarly, Figure 1b, panels E−H, and Figure S1a−S1d (panels E−H) show the efflorescence transitions of NaCl droplets on the Ag foil and the other four substrates, respectively. Except for the Ag foil substrate, the observed ERHs of the NaCl droplets were in the same range. As the

G1 = μ1N + σ LVALV

(5)

G2 = μ2 n + μ1(N − n) + σ LVALV + σ LNALN

(6)

where μ1 and μ2 are the solute chemical potentials in the droplet and nucleus, respectively, n is number of solute molecules in the nucleus, σij and Aij are the interfacial tension and area between phases i and j, respectively, which can be L (droplet), V (vapor or air), and N (nucleus). The free energy barrier for nucleation is given by ΔG = G2 − G1, which represents the work required for the inception of crystal germs from the solution.106 The contribution of the interfacial energy between the droplet vapor (σLV) in the free energy change (ΔG) is considered to be negligible.106 The supersaturated single salt droplets effloresce through classical homogeneous nucleation, for which the critical free energy barrier is derived from eqs 5 and 6 to be eq 7.88,106 3

ΔGh* =

16πσ LN vc 2 3(KBT ln S)2

(7)

where vc is the volume of a solute molecule. The difference in chemical potential can be expressed as Δμ = KBT ln S with S = a/a0 being the supersaturation ratio, where a is the solute activity in the supersaturated solution, a0 is the solute activity of 2653

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the saturated solution, and KB is the Boltzmann constant. Equation 7 shows that along with the supersaturation ratio, the interfacial energy between the salt nuclei and solution droplet (σLN) is also a key factor for defining the barrier for critical nucleation and in turn the rate of critical nucleation which eventually determines the ERH.33,88,106 In the case of supersaturated droplets deposited on substrates, the modified free energy terms, G1′and G2′, prior to nucleation and when the solute nuclei form at ERH, respectively, can be expressed as G1′ = μ1N + σ LVALV + σ SLASL

modified the Ag foil surface and in turn act as seeds for heterogeneous nucleation of the original solute molecules of NaCl, KCl, or (NH4)2SO4, resulting in higher ERH values. Owing to the chemical reaction with Ag foil, there should be a change in the chemical potentials of the products, which affect the Gibb’s free energies. On the other hand, the free energy barrier for nucleation is also expected to decrease due to the decreased interfacial energy between the solid AgCl or Ag2SO4 formed on the substrate and the nucleating salt molecules of NaCl and KCl or (NH4)2SO4, respectively, which facilitates heterogeneous nucleation.88,109 The elevation of the ERHs for (NH4)2SO4 was less than that for NaCl or KCl (Table S4 of the Supporting Information), which may be because Ag2SO4 is not as efficiently formed by reaction on the Ag foil surface and/or as good a heterogeneous nucleation seed as AgCl.

(8)

G2′ = μ2 n + μ1(N − n) + σ LVALV + σ SLASL + σ LNALN ( +σ SNASN )

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(9)

where all terms are the same as those in eqs 5 and 6, except for the extra interfacial energy term between the droplet and substrate (σSLASL). σSL and ASL represent the interfacial tension and area between the droplet and substrate, respectively. More importantly, if the crystal germ (nucleus) forms at the interface with the substrate, another interfacial energy term (σSNASN) needs to be included in eq 9, where σSN and ASN are the interfacial tension and area between the substrate (S) and nucleus (N), respectively. In this case, the free energy barrier for nucleation is given by ΔG′ = G2′ − G1′. As in the case of suspended droplets, the contribution of σLV to the change in the free energy barrier (ΔG′) can be considered negligible during dehydration for the deposited droplet. Because the ERHs of all three salt droplets [i.e., NaCl, KCl, and (NH4)2SO4], measured on the hydrophobic (TEM grids and parafilm-M) and hydrophilic (cover glass, silicon wafer, and Al foil) substrates, were within the range of the literature values, it appears that the interfacial energies between the salt nuclei-substrate (σSNASN) and substrate-droplet (σSLASL) do not substantially affect the critical nucleation barrier. A possible explanation for this observation is that the number of nucleation events at the substrate interface might not be high enough, compared to those in the volume of the droplet, to cause any observable changes in the ERHs. With the range of the ERHs observed for each salt being so wide (Table S4 of the Supporting Information), it is possible that any ERH deviation due to the changes in the nucleation site and interfacial energy caused by the use of different substrates would not be observed on the timescale of these measurements (i.e., optical images were recorded every 2 min between each step of the RH decrease). On the other hand, the ERHs of the NaCl, KCl, and (NH4)2SO4 droplets on the Ag foil were significantly higher than those on the other substrates. Although the contact angles for deionized water droplets on the Ag and Al foils were more or less similar (Table S2 of the Supporting Information), it was observed that there was an unusual spreading/wetting of aqueous NaCl droplets on the Ag foil (Figure 1b) compared to the Al foil (Figure S1b of the Supporting Information) at the end of the humidifying process. This indicates that after the humidifying process, the contact angles for salt droplets on the Ag foil were much less than those on the Al foil, which can be attributed to some physicochemical change on the Ag foil surface. This is probably because Cl− and SO42− ions in aqueous droplets react with Ag foil to form AgCl and Ag2SO4, respectively.105,107 AgCl and Ag2SO4 with low solubility at 0.00019 and 0.84 g per 100 g H2O at 25 °C, respectively,108

CONCLUSIONS

This study experimentally examined the influences and feasibility of widely used substrates for particle collection on the measurements of the hygroscopic properties of aerosol particles. Contact angle and surface roughness measurements showed that parafilm-M was hydrophobic, and four substrates, namely cover glass, silicon wafer, Al foil, and Ag foil, were hydrophilic. The TEM grids behaved like a hydrophobic substrate. The DRHs obtained for the hydrophilic substrates, such as Al and Ag foils, silicon wafer, and cover glass, were consistently ∼1−2% lower for NaCl, KCl, and (NH4)2SO4 compared to those of the hydrophobic TEM grids and parafilmM substrates and from the literature. The water layer adsorbed on the salt crystals prior to deliquescence condensed and spread over the hydrophilic substrate, resulting in additional interfacial energy between the substrate and condensed water. This leads to an increase in the Gibb’s free energy of the salt crystal−substrate system compared to the expected free energy of the salt droplet−substrate system, resulting in the observation of consistently lower DRHs. Therefore, it is necessary to avoid the use of hydrophilic substrates for hygroscopicity measurements. When hydrophilic substrates are needed, it is necessary to make their surface hydrophobic by coating with siloxane or polymer.48,64,88,110 The hydrophilic nature of the substrate does not affect the measured ERH values. On the other hand, Ag foil can strongly change the ERHs of inorganic salt droplets. The Cl− or SO42− ions in aqueous droplets react with Ag foil to form AgCl or Ag2SO4, respectively, which in turn act as seeds for heterogeneous nucleation of the original salts, leading to the observed early efflorescence or higher ERHs. However, to give a more comprehensive quantitative overview of these experimental observations, some further experiments like in situ measurement of changes in contact angles with changes in RH, on a preferably modified experimental setup,102 are required to get the necessary thermodynamic and kinetic parameters. Among the six different substrates studied, TEM grids and parafilm-M were found to be most suitable for hygroscopicity measurements because (i) they behave like hydrophobic substrates and reproduce well the deliquescence and efflorescence phase transitions during the humidifying and dehydration processes, respectively, and (ii) DRHs and ERHs measured on them are in good agreement with the theoretical and experimental literature values for suspended particles. TEM grids have an additional merit because they allow the relatively easy relocation of particles if multiple analytical techniques, 2654

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such as SEM-EDX, TEM-EDX, and/or RMS, can be applied to the same individual particles.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +82 32 860 7676. Fax: +82 32 867 5604. Present Address ‡

Air Quality Research Division, National Institute of Environmental Research, 42, Hwangyeong-ro, Seo-gu, 404-170, Incheon, Korea Author Contributions †

H.-J.E. and D.G. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (Grants 2011-0022911 and 2012R1A2A1A05026329), by Metrology Research Center funded by Korea Research Institute of Standards and Science (KRISS − 2013 − 13011055), and by Inha University.

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