Water Droplets Affected

Enrique Rodríguez-Castellón , Samuel Tehuacanero-Cuapa , Paulo T.C. Freire , Josué M. Filho , Gilberto D. Saraiva , Francisco F. de Sousa , Rossan...
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Hygroscopicity of Mixed Glycerol/Mg(NO3)2/Water Droplets Affected by the Interaction between Magnesium Ions and Glycerol Molecules Yang Wang, Jia-Bi Ma,* Qiang Zhou, Shu-Feng Pang, and Yun-Hong Zhang* The Institute of Chemical Physics, School of Chemistry, Beijing Institute of Technology, Beijing 100081, People’s Republic of China ABSTRACT: Tropospheric aerosols are usually complex mixtures of inorganic and organic components, which can influence the hygroscopicities of each other. In this research, we applied confocal Raman technology combined with optical microscopy to investigate the relationship between the hygroscopic behavior and the molecular interactions of mixed glycerol/Mg(NO3)2/water droplets. Raman spectra provide detailed structural information about the interactions between glycerol molecules and Mg2+ ions, as well as information about the interactions between glycerol and NO3− ions through electrostatic interaction and hydrogen bonding. The change of the CH2 stretching band of glycerol molecules in mixed droplets suggests that the backbone structures of glycerol mainly transform from αα to γγ in the dehumidifying process, and the additional Mg2+ ions strongly influence the structure of glycerol molecules. Because the existence of glycerol suppresses the crystallization of Mg(NO3)2·6H2O in the dehumidifying process, Mg(NO3)2 molecules in mixed droplets form an amorphous state rather than forming crystals of Mg(NO3)2·6H2O when the relative humidity is lower than 17.8%. Moreover, in mixed droplets, the molar ratio of NO3− to glycerol is higher in the center than in the outer region.

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electrodynamic balance,15 optical tweezers, and Raman spectroscopy.16−22 Reid and co-workers18,20,21 explored several mixed organic/inorganic droplets by using optical tweezers coupled with Raman spectroscopy. For the soluble organic acids mixed with sodium chloride, the efflorescence relative humidity (ERH) and deliquescence relative humidity (DRH) of the mixtures were found to be lower than those of pure sodium chloride droplets.22 However, the vapor pressure of the pure liquid organic acid did not change in the presence of sodium chloride within experimental uncertainty.22 DennisSmither et al. revealed that the addition of insoluble organic acid did not affect the ERH and DRH of the inorganic components.17 Chan and co-workers investigated the effects of different organic species on the hygroscopic behavior of NaCl and (NH4)2SO4.15,23−28 They found that the grow factors of mixed organic/inorganic particles were lower than those of pure inorganic species. The presence of organic species in the mixed particles reduced the water absorption of NaCl but enhanced that of (NH4)2SO4.15 Several mixed organic acid/ (NH4)2SO4/water particles showed different hygroscopic and phase transition properties. For instance, mixed malonic acid/ (NH4)2SO4/water droplets crystallized partially at RH 16%, and mixed glutaric acid/(NH4)2SO4/water and succinic acid/ (NH4)2SO4/water droplets formed completely dry crystals about RH 30%. The presence of organic acids also caused the deliquescence of mixed particles to be more gradual. Water uptake spanned from RH values of less than 10% to 79% for mixed malonic acid/(NH4)2SO4/water particles, 70% to 80%

INTRODUCTION The composition of atmospheric aerosols is complicated, and atmospheric aerosols usually include both inorganic and organic compounds. Atmospheric organics are ubiquitous and abundant, generally accounting for 20−60% of total aerosol mass.1 Field research indicates that 20−70% of condensedphase organic matter is water-soluble.2 Kanakidou et al. showed that organic particles were totally mixed with inorganic species in the troposphere.1 The incorporation of organic compounds into inorganic aerosols changes their hygroscopic properties.3 Middlebrook et al. denoted that aerosols could contain various ratios of inorganic to organic species,4,5 and these ratios depend on factors such as location, season, and other environmental conditions.6 It was estimated that 104−105 different atmospheric organic species with a range of chemical and physical properties were identified in atmospheric aerosols.7,8 The hygroscopicities of atmospheric aerosols depend on the aerosols’ chemical compositions and determine the sizes and the water content of aerosols. Hygroscopicity has an influence on aerosols’ environmental impacts, including those on global climate. It is generally acknowledged that the inorganic constituents alone cannot fully explain the observed hygroscopicities of atmospheric aerosols in laboratory and field measurements.9,10 Organic compounds constitute a significant mass fraction of fine atmospheric aerosols. The role of organic compounds in the hygroscopicity of atmospheric aerosols has been the subject of a number of investigations.11,12 Saxena and co-workers pointed out that the amount of water absorbed by aerosol particles at subsaturated relative humidity (RH) could be significantly altered by the presence of organics.13,14 Several research groups also investigated the deliquescence and crystallization of mixed organic/inorganic particles using an © 2015 American Chemical Society

Received: January 16, 2015 Revised: April 9, 2015 Published: April 10, 2015 5558

DOI: 10.1021/acs.jpcb.5b00458 J. Phys. Chem. B 2015, 119, 5558−5566

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The Journal of Physical Chemistry B

Figure 1. (a) Several backbone conformations of the glycerol molecules. (b) Newman projections of the glycerol backbone conformations.

that the conformation of glycerol molecules in mixed glycerol/ NaNO3/water droplets transforms from αα mainly to γγ and partly to αγ with decreasing RH. Na+ and NO3− ions can combine with glycerol molecules by electrostatic interaction and hydrogen bonding.47 In the mixed glycerol/Na2SO4/water droplets, the glycerol molecular conformation transforms from γγ to αα in the dehumidifying process, which is similar to the conformational transformation obtained in the glycerol/water droplet.48 Recently, Goursot et al. used DFT-optimized doubleζ and Born−Oppenheimer molecular dynamics to analyze a 550 ps trajectory cutout.49 They found around 15 backbone conformational changes (αγ−γγ and γγ−αγ) in the depicted trajectory cutout of 550 ps, which indicates that glycerol conformation changes, on average, every 40 ps. The time for transitions between the αα and γγ orientations of the glycerol molecules should be in the same magnitude. In this work, the mixed glycerol/Mg(NO3)2/water droplets with different organic-to-inorganic molar ratios (OIRs) deposited on a hydrophobic quartz substrate were chosen as a typical system of mixed organic/inorganic aerosols. Raman spectra of the droplets were acquired and compared during the dehumidifying process. The changes of the CH2 stretching band of glycerol (ν-CH2) in the mixed glycerol/Mg(NO3)2/ water droplets and the glycerol/water droplets were analyzed to infer the glycerol conformational transformation and its interaction with Mg2+ and NO3− during the dehumidifying process. Moreover, to understand how glycerol molecules influence the hygroscopicity of Mg(NO3)2, we identified ions pairs in mixed droplets through the analysis of the symmetric stretching vibration band of NO3− (ν1-NO3−).

for mixed glutaric acid/(NH4)2SO4/water particles, and 80% to more than 90% for mixed succinic acid/(NH4)2SO4/water particles.23 Although many studies on the hygroscopicities of mixed organic/inorganic particles were reported, the mechanism for how the interaction between organic and inorganic molecules affects the hygroscopic properties of mixed organic/ inorganic particles is still not well known.29 Magnesium nitrate (Mg(NO3)2) results from the reactions of Mg-containing mineral dusts and sea salt particles with nitrogen oxides such as NO2, NO3, N2O5, and HNO3 at various RHs.30,31 The relationship between the hygroscopic property of Mg(NO3)2 and the formation of ion pairs were well explored, both experimentally32,33 and theoretically.34 The phase transition of Mg(NO3)2 during the dehumidifying process was found to have several paths: (1) the aqueous droplet formed an amorphous state directly; (2) the aqueous droplet crystallized to Mg(NO3)2·6H2O between RHs of 30% and 5%; (3) the aqueous droplet crystallized to Mg(NO3)2·6H2O between RH of 30% and 5% first, then formed an amorphous state when the RH was below 5%. Glycerol is one of the most major components of sugars and sugar polyols in the atmosphere.35,36 It is widely used in industry and at home for many applications.37 It exists as a liquid with low vapor pressure and does not show ERH or DRH over its entire humidity range, although it is watermiscible.38 Glycerol molecules have six different backbone structures: αα, ββ, αγ, αβ, βγ, and γγ.39 Some of these backbone conformations of glycerol molecules are shown in Figure 1a, and Newman projections of the glycerol backbone conformations are given in Figure 1b. Chelli found that in the condensed phases of glycerol, the majority of backbone structures were αα (∼74%); the other two conformations (γγ and αγ) accounted for ∼19% and ∼7%, respectively.40−42 In pure droplets, glycerol molecules trend to self-aggregate by intermolecular hydrogen bonds and exist as open and cyclic dimers, trimers, and oligomers, but no linear chains.43,44 In contrast, for glycerol/water droplets, water−glycerol hydrogen bonds would form at the expense of the glycerol−glycerol intermolecular hydrogen bonds. Therefore, the degree of glycerol aggregation diminishes, and the glycerol molecules form dimers and monomers upon dilution from a statistical point of view.45,46 In glycerol/water droplets, the conformation of the glycerol molecules transforms from γγ to αα with decreasing RH.47 The conformation of the glycerol/water droplet is largely determined by the environmental RH and will become more complex with the addition of inorganic compounds such as Na2SO4 and NaNO3.47,48 It was reported

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EXPERIMENTAL SECTION The experimental setup used in this work for the Raman measurements of aerosol particles has been previously described elsewhere50,51 and only a brief description of the experimental setup is given here. It consists of a confocal Raman system and a sample cell. The confocal Raman system is equipped with a Leica DMLM microscope, a 514.5 nm Ar-ion laser (LS-514 model, Laserphysics), an 1800 g/mm grating, and a charge-coupled device (CCD). The sample cell is softly sealed with a piece of polyethylene (PE) film. The RH in the sample cell is adjusted by mixing a stream of water-saturated N2 with a stream of dry N2 at controlled flow rates. The RH and temperature are recorded by a hygrometer (Cemtertek Center 310) with an accuracy of ±2.5% for RH and ±0.7 °C for temperature, respectively. A Mg(NO3)2 solution with a concentration of 1 mol·L −1 is prepared from crystal 5559

DOI: 10.1021/acs.jpcb.5b00458 J. Phys. Chem. B 2015, 119, 5558−5566

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Figure 2. Raman spectra of the (a) Mg(NO3)2/water droplet and mixed glycerol/Mg(NO3)2/water droplets with OIRs of (b) 0.5, (c) 1, and (d) 2 on the Teflon substrate in the dehumidifying process.

Table 1. Molecular Vibration Assignments of Mg(NO3)2/Glycerol/Water System wavenumber (cm−1) a

compound

(a)

magnesium nitrate Mg(NO3)2 glycerol C3H6O3

1058

water H2O

(b) 854 922 1116 1254 1300 1470 2752 2887 2946

3147 3422

(c)

(d)

(e)

1054 851

1052 851 918 1103 1229 1301 1467 2764 2904 2969 3270 3450

1050 852 920 1100 1232 1301 1463 2756 2904 2967 3270 3450

1219 1301 1467 2907 2972 3270 3450

ref 33 43 43 43, 43, 43, 43, 43, 43, 43, 43, 43,

54 54 54 54 54 54 54 54 54

assignment NO3 stretch C−C stretch CH2 rock C−O stretch CH2 twist CH2 twist CH2 deformation C−H stretch symmetric CH2 stretch asymmetric CH2 stretch symmetric OH stretch asymmetric OH stretch

a Key: (a) Mg(NO3)2/water droplet, (b) glycerol/water droplet, (c)−(e) mixed Mg(NO3)2/glycerol/water droplets with different OIRs (0.5, 1, and 2, respectively) at a RH of 1.5%.

Mg(NO3)2·6H2O (analytically pure) and triply distilled water. The mixed glycerol/Mg(NO3)2 solutions with different OIRs are obtained by adding a designated amount of analytically pure glycerol into Mg(NO3)2 solutions. Mg(NO3)2·6H2O and glycerol are provided by Xilong Chemical Co., Ltd. and Sinopharm Chemical Reagent Beijing Co., Ltd., respectively. The molar ratios between the glycerol and the magnesium nitrate, i.e., the OIRs, are 0.5, 1, and 2. Droplets of solutions are injected by syringe onto the Teflon film fixed to the bottom of the chamber. The diameters of all of these droplets are about 50 μm. The droplets are excited by an Ar-ion laser with an output power of 20 mW. The backscattering light signals, after passing through an 1800 g/mm grating, are detected by a CCD. A 514.5 nm notch filter is used to remove strong Rayleigh

scattering. Calibration is made with respect to the silicon band at 520 ± 0.05 cm−1 prior to measurements. The spectra are reproducible within ±0.2 cm−1. The Raman spectral coverage is from a Stoke’s shift of 400−4000 cm−1 with an accumulation time of 10 s. Spectroscopic measurements are then made on droplets observed by the 50× objective of a Leica DMLM microscope. All measurements are made at room temperatures about 20 °C, and the spectra are obtained by the Wire 2.0 program supplied by Renishaw. Using the micro-Raman technique, the laser beam was highly focused twice on the outer region of and in the center of the spherical droplets.52,53 Every experiment was repeated at least three times. The changes of the nitrate peak positions and full width at half 5560

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Figure 3. Raman shifts of (a) the νa-CH2 band and (b) the νs-CH2 band of the glycerol/water droplet and the mixed glycerol/Mg(NO3)2/water droplets at various RHs in the dehumidifying process.

evaporation process, which confirms that molecular conformation tends to form the more regular αα conformation.55 It is noteworthy that without the perturbation of hydrogen bonds from water molecules, the αα conformation in the pure glycerol droplet is more stable. In mixed glycerol/Mg(NO3)2/water droplets, the blue shifts of the ν-CH2 bands are determined by the OIRs. With decreasing RH, the νs-CH2 band of the mixed glycerol/Mg(NO3)2/water droplet with OIR = 0.5 shifts from 2894 to 2907 cm−1, and the νa-CH2 band shifts from 2952 to 2973 cm−1. In the mixed glycerol/Mg(NO3)2/water droplets with OIRs of 1 and 2, the peak positions of the νs-CH2 band shift from 2896 to 2904 and 2905 cm−1, respectively, and the shifting of the νa-CH2 band is from 2954 to 2969 and 2966 cm−1, respectively. In comparison, the shifts of the νa-CH2 and νs-CH2 bands in the glycerol/water droplet are from 2892 to 2887 and from 2951 to 2947 cm−1, respectively.

maximums (FWHMs) caused by solvation were corrected when the peak areas were calculated.

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RESULTS The Raman spectra of the Mg(NO3)2/water and mixed glycerol/Mg(NO3)2/water droplets with different OIRs (OIR = 0.5, 1, and 2) are presented in Figure 2. In addition, the peak positions of the glycerol/water and Mg(NO3)2/water43,54 droplets as well as the mixed glycerol/Mg(NO3)2/water droplets at RH of 1.5% are shown in Table 1. For the Mg(NO3)2/water droplets in the dehumidifying process, as shown in Figure 2a, the ν1-NO3− band shifts from 1048.3 to 1052.1 cm−1 with RH decreasing from 90% to 15%; the peak then changes to a sharp one at 1059.1 cm−1 at 6% RH and to a broad one at 1059.0 cm−1 at 3% RH. The results suggest that the concentration of the Mg(NO3)2/water droplet increases in a dehumidifying process; when RH decreases from 15% to 6%, the droplet crystallizes to Mg(NO3)2·6H2O. Other repeated experiments indicate that the ERH region of Mg(NO3)2·6H2O is 30%−6%, which is consistent with previous research.33 After the RH further decreases to 3%, some portion of the Mg(NO3)2·6H2O crystal loses water and forms an amorphous state. However, for the mixed glycerol/Mg(NO3)2/water droplet with OIR = 0.5 (Figure 2b), the position of the ν1NO3− band blue shifts from 1048.3 to 1054.0 cm−1. In Figure 2c (OIR = 1) and Figure 2d (OIR = 2), the ν1-NO3− band shifts from 1048.3 to 1052.2 and 1050.0 cm−1, respectively. No formation of Mg(NO3)2·6H2O crystal is observed in the mixed droplets, and there is only a transformation from the liquid state to the amorphous state for the mixed droplets, without the crystallization of Mg(NO3)2·6H2O. In previous research, the coexistence of various ion pairs and the amorphous state in the Mg(NO3)2/water droplet appears at water-to-solute ratios (WSR) less than 6 (RH < 17.8%).33 However, in our experiments, the FWHM of the ν1-NO3− band of the Mg(NO3)2/water droplet with an amorphous state is 17.0 cm−1. Therefore, the FWHM of the ν1-NO3− band >17.0 cm−1 is applied as a criterion of the existence of an amorphous state.33 Other bands shown in Table 1 have no obvious differences among the investigated droplets. Figure 3 presents the peak positions of the symmetric (panel a) and the asymmetric bands (panel b) of the glycerol/water droplet and the mixed glycerol/Mg(NO3)2/water droplet in the dehumidifying process. Both the νa-CH2 and νs-CH2 bands in the glycerol/water droplet red shift during the water

Figure 4. Area ratios of the νa-CH2 band to the νs-CH2 band (Aνa‑CH2/ Aνs‑CH2) of the glycerol/water droplet and the mixed glycerol/ Mg(NO3)2/water droplets at various RHs in the dehumidifying process.

Figure 4 presents the area ratios of the asymmetric to the symmetric bands (Aνa‑CH2/Aνs‑CH2) of the glycerol/water droplets and the mixed glycerol/Mg(NO3)2/water droplets in the dehumidifying process, which are usually used to investigate the conformational structure of glycerol molecules.56,57 For mixed glycerol/Mg(NO3)2/water droplets with OIR = 0.5, 1, 5561

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Figure 5. (a) Raman shifts and (b) FWHMs of the ν1-NO3− band of mixed glycerol/Mg(NO3)2/water and Mg(NO3)2/water droplets at various RHs in the dehumidifying process. The contrast data (×) are adapted from ref 33.

Figure 6. (a) Area ratios of the ν1-NO3− band to the ν-CH2 band (Aν1‑NO3/ACH2) in mixed glycerol/Mg(NO3)2/water droplets with different OIRs on the outer region and in the core; (b) expanded view of the droplet with OIR = 2.

band blue shifts from 1048.3 to 1050.0 cm−1, and the FWHM grows from 12.1 to 21.3 cm−1 in the RH range 88%−0.2%. The area ratios of the ν1-NO3− band to the ν-CH2 band (Aν1‑NO3/ACH2) in the mixed glycerol/Mg(NO3)2/water droplets with different OIRs are given in Figure 6a, and an expanded view of the mixed droplets with OIR = 2 is shown in Figure 6b. The area ratios refer to the molar ratios of NO3− to glycerol in droplets, and they are larger in the core than on the outer region for mixed droplets with OIR = 0.5 and 1 in the whole RH range. However, the ratio of the mixed droplet with OIR = 2 shows an obvious difference at RH < 10%. The Aν1‑NO3/ACH2 of the droplet outer region decreases from 0.21 to 0.17, and the value obtained in the core increases from 0.21 to 0.22. The difference between the area ratios of the core and of the outer regions indicates that the glycerol molecules accumulate on the outer region, which is consistent with previous work.19

and 2 in the dehumidifying process, the values of Aνa‑CH2/Aνs‑CH2 are increased from 2.0, 1.7, and 1.7 to 5.9, 3.8, and 2.4, respectively. For comparison, the Aνa‑CH2/Aνs‑CH2 value of the glycerol/water droplet decreases from 1.0 to 0.7 in the whole RH range. Figure 5 presents the peak positions of the ν1-NO3− band and FWHMs of the Mg(NO3)2/water and mixed glycerol/ Mg(NO3)2/water droplets in the dehumidifying process. The data adapted from ref 33 are also shown in Figure 5. In Figure 5a, as the RH decreases, the ν1-NO3− bands of all investigated droplets blue shift; for mixed droplets, the amount of shift is dependent upon the OIR. For the Mg(NO3)2/water droplet, the ν1-NO3− band has a blue shift of 3.8 cm−1 from 1048.3 to 1052.1 cm−1, and the FWHM changes from 11.8 to 16.7 cm−1 as the RH decreases from 90% to 15% before droplet efflorescing. At 6% and 3% RH, although the peak positions of the ν1-NO3− bands are the same (1059.0 cm−1), the FWHMs are different (7.1 and 34.1 cm−1, respectively). For the mixed glycerol/Mg(NO3)2/water droplet with OIR = 0.5, the ν1-NO3− band blue shifts from 1048.3 to 1054.3 cm−1, and the FWHM increases from 11.5 to 29.9 cm−1 in the RH range between 89% and 0.6%. In a similar RH range (88%−0.6%), the ν1-NO3− band of the mixed droplet with OIR = 1 blue shifts from 1048.3 to 1052.2 cm−1, and the FWHM raises from 11.6 to 23.5 cm−1. For the mixed droplet with OIR = 2, the ν1-NO3−

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DISCUSSION Conformation of Glycerol Molecules in Mixed Glycerol/Mg(NO3)2/Water Droplets. It is interesting and instructive to address the conformation of glycerol molecules in mixed droplets. As shown in Figure 3, the blue shifts of the νaCH2 and νs-CH2 bands in the glycerol/Mg(NO3)2/water droplets (8−21 cm−1) are much higher than those of the glycerol/water droplets, glycerol/NaNO3/water47 droplets (1− 4 cm−1), and glycerol/Na2SO4/water48 droplets in the whole 5562

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tend to combine with Mg2+ and NO3− ions through electrostatic interaction and hydrogen bonding, respectively, instead of intermolecularly interconnecting with glycerol and water molecules. To further understand the effect of Na+ ions, we compared the glycerol/Na2SO4/water and glycerol/water droplets, as shown in Figure 7. In the dehumidifying process, the ν-CH2 bands in both mixed glycerol/Na2SO4/water and glycerol/ water droplets red shift; however, the amount of the former is less than that of the latter.48 It is known that SO42− ions have very little influence on the droplet environment.65,66 Therefore, the differences between these two systems result from the effect of the Na+ ions. The ν band shifts further prove that, compared to Mg2+ and NO3−, Na+ ions have little influence on glycerol conformation. Influence of Glycerol on the Hygroscopicity of Mg(NO3)2 in Mixed Droplets. In the dehumidifying process, the influence of glycerol on the hygroscopicity of Mg(NO3)2 is different from that on NaCl, (NH4)2SO4, NaNO3, and Na2SO4.15,47,48 As shown in Figure 2a, the Mg(NO3)2/water droplet effloresces first; then, with the RH decreasing to 3%, the crystallized Mg(NO3)2·6H2O transforms into an amorphous state. However, the mixed glycerol/Mg(NO3)2/water droplets with different OIRs do not effloresce. This can be attributed to the additional glycerol molecules, which effectively suppress the crystallization of Mg(NO3)2. Glycerol molecules in the mixed droplets inhibit the heterogeneous nucleation and form multiple ion pairs with Mg2+ and NO3−, resulting in the formation of an amorphous state.34 Such a phenomenon was also observed by Marcolli67 and Yu,47 and they pointed out that some atmospheric aerosols exist in the liquid phase even under very low RH conditions. The mixed glycerol/Mg(NO3)2/water droplet may serve as a model for this kind of aerosol. Peak positions of the ν1-NO3− band are sensitive to the droplet microenvironment.68 According to the Raman spectra of Mg(NO3)2/water droplets, the structures of NO3− anions transform into solvent-separated ion pairs, solvent-shared ion pairs, and contact ion pairs, and the coexistence of various ion pairs with ambient RHs decreases from 92.0 to 1.8%, respectively.33,69 Our results suggest that glycerol molecules suppress the transformation of ion pairs in a glycerol/ Mg(NO3)2/water droplet and also hamper the crystallization; in a similar dehumidifying process, the blue shift of the ν1NO3− band of a mixed glycerol/Mg(NO3)2/water droplet decreases with increasing amounts of glycerol. It is noteworthy that the FWHM of the ν1-NO3− band is related to the NO3− structure multiplicity in droplets.70 In the mixed glycerol/Mg(NO3)2/water droplets, the FWHM should be largely determined by the multiplicity of the ion pairs among the Mg2+ and NO3− ions and the glycerol molecules. As shown in Figure 5b, the FWHMs of the ν1-NO3− band for the mixed droplets are the same as those of the Mg(NO3)2/water droplets before the RH decreases below 17.8%. When the RH is lower than 17.8%, the FWHMs of Mg(NO3)2/water droplets increase due to the coexistence of various ion pairs of Mg2+ and NO3− ions. However, at RH < 17.8%, interactions between the glycerol and the NO3− ions through hydrogen bonding in the glycerol/Mg(NO3)2/water droplet could result in the formation of glycerol−NO3− complexes and reduce the multiplicity of the NO3− ion pairs, which further reduces the FWHMs. Therefore, at RH values below 17.8%, the FWHMs of these investigated droplets vary such that the value for the Mg(NO3)2/H2O droplet is greater than that of the mixed

RH range. In addition, the values of Aνa‑CH2/Aνs‑CH2 of the ν-CH2 band in the glycerol/Mg(NO3)2/water droplets are much higher than those of other droplets.47,48 The shifts and area ratios of the ν-CH2 band in the mixed glycerol/Mg(NO3)2/water droplets are different from those in the mixed glycerol/NaNO3/water droplets due to the property differences of the Mg2+ and Na+ ions. The charge-to-radius ratio (Z/r) could be used to determine the hydration capacity of ions. The radius of a Mg2+ ion is 0.66 Å, and the Z/r(Mg2+) is 3.58 Mg2+ ions tend to hold stable hydration structures because of the larger charge-to-radius ratio and higher hydration energy (−1920 kJ·mol−1)59 compared to those of Na+ ions, which are 1 and −400 kJ·mol−1, respectively.50,60 In addition, one Mg2+ ion tightly attracts six water molecules in its primary hydration shell,61,62 and they have a strongly structuremaking effect on the hydrogen-bonded environment of water molecules;33 however, Na+ ions have no obvious effect on the hydrogen-bonded environment.63 In other words, Na+ ions tend to form contact ion pairs with NO3− ions rather than clinging strongly to water or glycerol molecules; Mg2+ ions have the reverse trend.50 The peak positions and areas of ν-CH2 bands can be used to investigate the glycerol molecular structures and the droplet microenvironment because they are sensitive to these factors. It was reported that the stretching of CH2 in glycerol molecules with regular αα conformation was observed at a lower wavenumber than those with γγ conformation.64 As discussed above, the values of Aνa‑CH2/Aνs‑CH2 in mixed glycerol/Mg(NO3)2/water droplets are larger than those reported in previous research47 and those of the glycerol/water droplet. For pure glycerol droplets, glycerol molecules tend to form aggregates with the αα conformation as the dominant component, as shown in Figure 7.40−42 Water molecules can

Figure 7. Changes of glycerol conformations in different droplets.

form hydrogen bonds with glycerol molecules and convert part of αα into γγ, resulting in the increase of the Aνa‑CH2/Aνs‑CH2 and the blue shifts of the νa-CH2 and νs-CH2 bands in the glycerol/ water droplet. However, glycerol molecules with the αγ conformation could form the most stable ion pairs (αγNO3−) with NO3− ions through hydrogen bonds as the hydrogen donors in density functional theory calculations.47 Compared to the glycerol/water droplet at the same RH, the mixed glycerol/Mg(NO3)2/water droplets showed a large increase in the area ratios and efficient blue shifts of the νCH2 band that indicate the αα dominating conformation transforms into the γγ dominating conformation caused by the addition of Mg(NO3)2. While the RH decreases, the values of Aνa‑CH2/Aνs‑CH2 in the mixed glycerol/Mg(NO3)2/water droplets are still rising, which is the same trend seen with the mixed glycerol/NaNO3/water droplets. This fact indicates that, compared to Na+ ions, Mg2+ ions would strongly influence the structure of glycerol molecules. Generally, in mixed glycerol/Mg(NO3)2/water droplets, the glycerol molecules 5563

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hygroscopicity of Mg(NO3)2. Moreover, the molar ratio of Mg(NO3)2 to glycerol is higher in the center than on the outer region. Especially for the mixed glycerol/Mg(NO3)2/water droplet with OIR = 2, glycerol accumulates on the outer region at RHs less than 5%. Understanding the hygroscopicity and conformation transformation of the mixed glycerol/Mg(NO3)2/water droplets is a crucial step toward obtaining a comprehensive insight into the heterogeneous aging of aerosolcontaining glycerol and Mg(NO3)2.

droplet with OIR = 0.5, which is greater than that of the mixed droplet with OIR = 1, which is greater than that of the mixed droplet with OIR = 2. In sharp contrast, the FWHMs of the ν1NO3− bands for the mixed glycerol/NaNO3/water droplets are larger than those for the NaNO3/water droplets47 and are smaller than those for the mixed glycerol/Mg(NO3)2/water droplets. As we know, the typical ion pair relaxation time is on the order of 100−200 ps in water. However, the relaxation time of water in the first shell of Mg2+ was observed by 17O NMR to be more than 1.7 μs in Mg(ClO4)2 solution.71 The same time magnitude for contact ion pair formation observed in ultrasonic relaxation measurements72,73 of aqueous MgSO4 solutions has been correlated with the water exchange rate around the cation. Jiao et al. revealed that the lifetime of Mg2+−water coordination was 228 ps by calculation,74 which was much longer than that of Na+−water coordination (1.8 ps).75 The interaction of Mg2+ with NO3− is weaker than that with SO42−. In this case, the kinetics may be the key factor in determining the transformation of ion pairs. It is known that the kinetics of aerosols are usually controlled by viscosity when they are very viscous. The relaxation times of aerosols in such a viscous state can be investigated by several measurements such as optical tweezers.76,77 The maximum viscosities of saturated inorganic salt solutions [2.2 mPa·s for a NaNO3 solution with a 40% mass concentration, 2.5 mPa·s for a Na2SO4 solution with a 22% mass concentration, and 7.0 mPa·s for a Mg(NO3)2 solution with 5.3 mol·kg−1 concentration] are less than 0.01 Pa·s,38,78 and the maximum viscosity of glycerol solution with a 96% mass concentration upon reduction in relative humidity is 780 mPa·s, which is less than 1 Pa·s.36 According to the model involving relaxation time and viscosity depicted in ref 79, the relaxation times of the investigated droplets herein should be much less than 1 min and also much less than the equilibration time (30 min for every RH) in our study. This result indicates that viscosity may not be a key factor in the kinetics of solutions in our study. In addition to RHs, other atmospheric conditions such as temperature would affect the presented results. In our group’s previous research,80 we investigated the effect of temperature on the ion association structures in NaNO3 droplets, and the results indicate that Na+ and NO3− ions tend to aggregate with increasing temperatures. In the mixed droplets investigated herein, when temperatures are increased, the aggregation of ions as well as the volatilization of glycerol molecules are expected, which will complicate the hygroscopicities of droplets. How these atmospheric conditions, especially temperature, affect the results will be further investigated in the future.

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AUTHOR INFORMATION

Corresponding Authors

*J.-B.M. Tel: 86-10-68913596. E-mail: [email protected]. *Y.-H.Z. Tel: 86-10-68913596. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the NSFC (41175119, 21373026, and 21473009), 111 project B07012, 2011YQ04013615, and the Beijing Natural Science Foundation (2144055).

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REFERENCES

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CONCLUSION In this work, the Raman spectra of mixed glycerol/Mg(NO3)2/ water droplets with different OIRs are investigated. Several interesting findings are obtained by analyzing the changes of the ν1-NO3− and ν-CH2 bands of glycerol with decreasing RH values. The conformation transformation of glycerol molecules in the mixed glycerol/Mg(NO3)2/water droplets is observed and compared with that in mixed glycerol/NaNO3/water, glycerol/Na2SO4/water, and glycerol/water droplets. In mixed glycerol/Mg(NO3)2/water droplets, the conformation of glycerol molecules changes from αα to γγ conformation. This is because the additional Mg2+ ions would strongly influence the hydrogen bonding of glycerol molecules. In addition, glycerol molecules suppress the transformation of ion pairs and heterogeneous nucleation in mixed droplets, thus affect the 5564

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