Micro-Raman and FTIR Spectroscopic Observation on the Phase

May 25, 2010 - Two shoulders at ∼993 and ∼1002 cm−1 occurred in the v1-SO42− .... Fullerenes, Nanotubes and Carbon Nanostructures 2017, 25 (2)...
0 downloads 0 Views 1MB Size
Download PDF
6480

J. Phys. Chem. A 2010, 114, 6480–6486

Micro-Raman and FTIR Spectroscopic Observation on the Phase Transitions of MnSO4 Droplets and Ionic Interactions between Mn2+ and SO42Xin Guo, Han-Shuang Xiao, Feng Wang, and Yun-Hong Zhang* The Institute of Chemical Physics, Key Laboratory of Cluster Science, School of Science, Beijing Institute of Technology, Beijing 100081, China ReceiVed: October 31, 2009; ReVised Manuscript ReceiVed: May 12, 2010

Micro-Raman and FTIR spectroscopy have been used to investigate the micrometer-sized MnSO4 droplets. The concentration of solute within the droplet is controlled accurately by decreasing the relative humidity (RH) of the surroundings. According to the Raman spectra of MnSO4 droplets, when the RH decreased from ∼94% to ∼76%, the full width of the peak at half-maximum (fwhm) of the V1-SO42- band at 983 cm-1 initially increased from 95.7 to 104.2 cm-1. Two shoulders at ∼993 and ∼1002 cm-1 occurred in the V1SO42- band at ∼76% RH, while the V2-SO42- band at 450 cm-1 split into two bands at 442 and 462 cm-1, indicating the formation of monodentate and bidentate contact ion pairs (CIPs) in supersaturated MnSO4 droplets. From component band analysis of the V1-SO42- band, four peaks at 983, 993, 1002, and 1010 cm-1 were identified and assigned to the free SO42-, monodentate CIPs, bidentate CIPs, and more complex ion aggregates, respectively. Signatures of monodentate and bidentate CIPs reached their maximum values at ∼76% and 60% RH, respectively. With further decreasing the RH, great abundance of various ion pairs presented in the droplet, corresponding the V1-SO42- band steadily blue-shifted, and the fwhm increased continuously until it went through a plateau at ∼60% RH. When the RH was between ∼44% and 13%, the intensity of the signature from four species of ion pairs was almost invariant, corresponding to the formation of an amorphous phase of MnSO4 · 2.8H2O. Further decreasing the RH below 13% RH, monodentate CIPs changed to bidentate ones. In the meantime, MnSO4 · 2.8H2O was deduced to transform into another amorphous phase with a stoichiometry of MnSO4 · 1.7H2O. This transition was also supported by the observation of the FTIR spectra according to a sharp increase of the fwhm of the V3-SO42- band (at ∼1091 cm-1) because of the appearance of a shoulder at 1132 cm-1. I. Introduction In atmospheric aerosol systems, S(IV) can be oxidized in the presence of catalytically active transition metals such as Fe(III), Mn(II), and Cu(II) complexes.1-3 Graedel et al. pointed out that it would account for 30-55% of the S(IV) oxidation in droplets at pH 4, which significantly affects the formation of acidic rain.3 MnSO4 is a potential catalyst in the autoxidation of S(IV) oxides at atmospheric background conditions.3-5 However, the chemical removal rate catalyzed by MnSO4 aerosol is found to be greatly dependent on the relative humidity (RH). That is, the rate abruptly decreases with the reduction of the RH, particularly when the RH is lower than the deliquescence point of the metal complex particles.4,5 Berresheim and Jaeschke considered that the rate change is from the decrease of the diffusivity of the S(IV) components due to the increase of concentration or the formation of ion pairs in the metal aerosol droplets.4 Hygroscopic investigations of MnSO4 aerosol droplets under changing RH conditions are essential for better understanding this issue, since these studies can provide information on the formation and evolution of contact ion pairs (CIPs) in MnSO4 droplets. In dilute MnSO4 solution, Mn2+ is octahedrally coordinated with six water molecules.6-8 However, the inner-layered water molecules in Mn(H2O)62+ complex can be readily replaced by SO42-, leading to the formation of various ion pairs between Mn2+ and SO42-.6,9 For example, in the measurement of the * To whom correspondence should be addressed. Phone: 86-1086668406. Fax: 86-10-68912652. E-mail: [email protected].

activity and osmotic coefficients of MnSO4 solution, Mn2+ and SO42- were found to interact and form contact ion pairs (CIPs) even in dilute solution.10,11 However, no detailed studies about the evolution of CIPs in MnSO4 droplets have been reported in previous literature, although references have been frequently made to those of other sulfate particles.12-18 Moreover, in spite of the existence of three thermodynamically stable crystalline hydrates, i.e., MnSO4 · 5H2O, MnSO4 · 4H2O, and MnSO4 · H2O,6 the solids precipitated from the saturated MnSO4 solution were observed to be continuously changing with time and were assigned to be metastable hydrates.19 The metastable hydrates were expected to slowly lose water and finally change into MnSO4 · H2O.8 With decreasing the RH, the aerosol droplets levitated in an electrodynamic balance (EDB) cell were observed to effloresce into an unknown composition in the glassy state or with small regions of crystalline order, having an overall water-to-solute molar ratio (WSR) of 2.8, instead of growing into an equilibrium crystalline form. These particles deliquesced in the RH range of 51% to 55%, much lower than the deliquescence RHs of thermodynamically stable ones.20,21 Obviously, the phase transitions in the MnSO4 droplet are complicated, which deserves a more detailed investigation. Here we use micro-Raman and FTIR technique to study the phase transitions and complex associations that occur in MnSO4 droplets. The diameter of the deposited droplets is from 1 to 20 µm, and the mass is on the order of a nanogram. The droplets are forced to enter into high solute concentrations by decreasing

10.1021/jp9104147  2010 American Chemical Society Published on Web 05/25/2010

Phase Transitions of MnSO4 Droplets

J. Phys. Chem. A, Vol. 114, No. 23, 2010 6481

Figure 2. Schematic diagram of the FTIR-ATR experimental setup.

Figure 1. Sketch of the experimental apparatus for the micro-Raman measurements of droplets on quartz substrate.

the relative humidity (RH) of the environment, allowing accurate control over the concentration of the solute within the droplet. Micro-Raman and FTIR, two kinds of complementary spectroscopy, are found to be a powerful means to study the droplets from dilute to supersaturated states.12,13 They can provide the interaction information between ions on the molecular level. In addition, the phase transitions in droplets can also be identified according to the spectral evolutions as a function of RH. By using micro-Raman and FTIR, this investigation has deduced the evolution of CIPs in droplets and the formation of a new phase (MnSO4 · 1.7H2O) in MnSO4 droplets at low RHs. II. Experimental Section A stock solution of 0.5 mol · L-1 MnSO4 was prepared from analytical-grade MnSO4 · H2O and triply distilled water. A free SO42- ion has Td symmetry and nine modes of internal vibration spanning the representation Td ) A1 + E + 2F2. All modes are Raman active, only the F2 modes are IR active. In Raman spectra, the V1-SO42- presents a distinctive evolution, which will help us to make further analysis on the structural changes in the water evaporation process, but also the bending bands can provide us reliable information about the symmetry change of SO42- when it forms any kind of CIPs. The infrared active V3 vibration only presented a weak band, while it has better evolution in Fourier transform infrared (FTIR) spectrometers. Raman and FTIR spectrometers are complementary in the observation of molecular vibrations. Considering these facts, it is timely to make some Raman and FTIR spectroscopic investigations of the MnSO4 aerosols. Sample Preparation and Measurements by Micro-Raman Spectroscopy. Figure 1 shows the experimental apparatus used to study the droplets by micro-Raman spectroscopy. The technique used in this work has been described previously.12 The central part is a micro-Raman system (Renishaw Invia) for measuring Raman spectra and imaging morphological changes of the droplets deposited on the substrate. Before the droplet

measurement, a spectral calibration was made with the 520 cm-1 silicon band as a reference. Spectroscopic measurements were then made on droplets observed with a 50× objective of the Leica DMLM microscope after being stabilized for about 1 h at a chosen chamber RH. The 514.5 nm line of an argon-ion laser was used as the excitation source with a power of 20 mW. A notch filter was used to remove the strong Rayleigh scattering, and a charge-coupled device (CCD) was used to record backscattering signals dispersed by an 1800 g/mm grating. The Raman spectra of MnSO4 droplets from 100 to 4000 cm-1 were obtained with five spectral scans, and each with an accumulation time of 10 s. Droplets of this solution were injected onto the Teflon film that was fixed to the bottom of a chamber, using a syringe. The diameters of these droplets were from ∼5 to ∼20 µm. The chamber was then sealed by a thin Teflon film and the RH in the chamber was adjusted by mixing a stream of water-saturated N2 and another of dry N2 at controlled flow rates. A humidity/ temperature meter (Centertek Center310) was used to record the RH with an accuracy of (2.5% near the exit of the chamber. Sample Preparation and Measurements by FTIR Spectroscopy. Figure 2 shows a scheme for recording the FTIRATR spectra of aerosols which was described in previous work.13 The central part was a chamber composed of a baseline horizontal ATR accessory (Spectra-Tech Inc., USA) using a ZnSe crystal (refractive index: 2.4) as a substrate. The chamber was sealed with a thin transparent polyethylene film. The prepared MnSO4 solution was used to produce droplets with size of 1-5 µm through an ultrasonic humidifier. After that, the droplets were sucked into the chamber to deposit on the substrate by a pump. Then the droplets on the substrate were made into solid particle samples by passing dry N2 through the chamber for about 40 min. A FTIR spectrometer (Nicolet Magna-IR 560), equipped with a MCT/A (mercury cadmium telluride type A) detector cooled by liquid nitrogen, was used for the production of the final spectra with a resolution of 4 cm-1 in the range from 650 to 4000 cm-1 by accumulating 64 scans. The RH in the chamber was adjusted by mixing one stream of water saturated N2 and another of dry N2 at controlled flow rates. The RH and temperature were recorded by the T & RH meter ((2.5% RH, CENTER 313), which is placed near the exit of the chamber. III. Results and Discussion Hygroscopic Properties of MnSO4 Droplets in the Dehumidifying Process. Figures 3 and 4 display the Raman and IR spectra of MnSO4 droplets at various RHs, respectively, which

6482

J. Phys. Chem. A, Vol. 114, No. 23, 2010

Guo et al.

Figure 3. Raman spectra of MnSO4 droplets at various RHs in the dehumidifying process.

Figure 4. FTIR spectra of MnSO4 droplets at various RHs in the dehumidifying process.

Figure 5. Hygroscopic properties of the MnSO4 droplet in the dehumidifying process, as a function of RH: (a) Raman measurement; (b) FTIR measurement; (c) the water to solute molar ratios (WSRs) of MnSO4 droplets from ref 20.

were obtained in the dehumidifying process. For the Raman spectra, the V1-SO42- band in the 955-1045 cm-1 region and the O-H stretching band of water in the 3000-3800 cm-1 region provide much structural information on MnSO4 aerosols, while in the IR spectra, the V3-SO42- band in the 940-1300 cm-1 region and the water O-H stretching envelope in the range of 2500-3629 cm-1 can also be applied to investigate the MnSO4 droplets. With decreasing the RH, MnSO4 droplets became easily supersaturated after passing through the saturation point (82.2% RH).6 As the RH was decreased to ∼4%, MnSO4 droplets did not crystallize in this work. The area ratios of the water O-H stretching envelope to the V1-SO42- band in Raman spectra and to the V3-SO42- band in IR spectra are selected to reflect the relative water content and their changes with RH can reveal the hygroscopic properties of MnSO4 droplets. The hygroscopic curves of MnSO4 droplets in the dehumidifying process are given in Figure 5, panels a and b, corresponding to Raman and FTIR measurement, respectively. The water-to-solute molar ratios (WSRs) of MnSO4 droplets are also included in Figure 5c for comparisons, which are calculated from the solute weight fractions measured previously by Cohen et al. in the RH range of ∼86% to ∼10%.20 In our experiments, the observation was extended to a lower RH of ∼4%. As a result, complicated transitions were found in the dehumidifying process of MnSO4 droplets. Five regions can be identified, which read as region I (above ∼76% RH), region II (∼76% to ∼60% RH), region III (∼60% to ∼44% RH), region IV (∼44% to ∼13% RH), and region V (below ∼13% RH). The area ratio decreases very rapidly in region I. As the

RH reaches 76% (WSR ∼9.5), a slight transition is observed in Figure 5b, similar to that occurring in the hygroscopic curve of MgSO4 droplets, which was previously attributed to the formation of CIPs.12,17 The ratio still decreases rapidly as the RH varies from ∼76% to ∼60% (WSR ∼6.2), corresponding to region II. In region III, it becomes less sensitive to the RH, and then remains nearly constant in region IV. Further decreasing the RH below ∼13% in region V, the ratio is found to decrease sharply, especially for the hygroscopic curve in Figure 5b. For MnSO4, the viscosity of the concentrated solution is very high due to the high charge on both the cation and the anion. As a result, the molecules may not easily rearrange themselves into an ordered crystalline lattice.21 As echoed from the solubility measurements, the metastable solids were found to form in MnSO4 solution according to a continuous change of the precipitates with time.19 The individual MnSO4 droplet levitated in an EDB was also found not to crystallize into a hydrate, and the transition point at ∼43% RH was related to the formation of amorphous particles with an overall stoichiometry of MnSO4 · 2.8H2O.20 In region IV, the formation of amorphous MnSO4 · 2.8H2O particles was assumed according to the transition point at ∼44% RH in Figure 5a,b. However, the solids may change into a new phase at RHs below ∼13% in light of the sudden change of the area ratio in region V. By relating its area ratio to that of MnSO4 · 2.8H2O, the WSR of the supposed new phase was approximated to be ∼1.7. SO42- Bands of One Single Droplet at Various RHs. The spectral features of SO42- (i.e., the position and bandwidth of its bands) are very sensitive to microenvironments.12-15 The bands of SO42- are found to present distinctive changes in

Phase Transitions of MnSO4 Droplets

J. Phys. Chem. A, Vol. 114, No. 23, 2010 6483

Figure 6. Experimental and fitted spectra in the region of 955-1045 cm-1: (square symbols) the experimental Raman spectra; (dashed lines) the four fitted spectral components; and (solid lines) the sum spectra of the four fitted spectral components. R2 is the coefficient of determination.

supersaturated MnSO4 droplets with decreasing the RH in the Raman and IR spectra, respectively, according to Figures 3 and 4. The Raman band contour of the symmetric stretching vibration of the SO42- ion can be used to investigate interactions between Mn2+ and SO42- ions by using component band analysis. This approach has been applied previously in the analysis of spectral bands arising from nondegenerate vibrations of solutes in aqueous solutions. In this work, each band component analyzed in the range of 955-1045 cm-1 is described by a Gaussian function as follows:

y ) y0 +

2/w2 A e-(x-x0) w√π/2

where, y0 is the baseline offset, A is the total area, x0 is the center of the peak, and w is 2σ, approximately 0.849 the full width of the peak at half-maximum (fwhm). The spectral weights A of each component (i.e., the free SO42-, monodentate, bidentate, and more complex ion aggregates) are the sole parameters allowed to vary in the fitting process. In previous Raman investigations of sulfate droplets, the shoulders at 995 cm-1 in MgSO4 droplets,12,18 at 987 cm-1 in ZnSO4,17,22 and at 989 cm-1 in CdSO4 droplets17 were considered to be monodentate CIPs. It was further suggested that bidentate CIPs or even more complex chains built on CIPs be responsible for the bands at 1007 and 1021 cm-1 in MgSO4 droplets.12,18 Therefore, four component bands (i.e., the free SO42-, monodentate, bidentate, and more complex ion aggregates) were also assigned in our work. The shoulders at 993 and 1002 cm-1 at

∼76% RH (WSR ∼9.5) in Figure 3 can be reasonably attributed to the formation of monodentate and bidentate CIPs, respectively. With decreasing the RH to ∼60%, the intensity of the shoulder at 1002 cm-1 gets stronger than that at 993 cm-1, suggesting that bidentate CIPs increase rapidly as the solution is concentrated. Further decreasing the RH, the monodentate and bidentate CIPs may continuously lose water and associate with SO42-. Ultimately, they form the complicated chain or web structural CIPs like those occurring in MgSO4 droplets,12,18,23 so the 1010 cm-1 band should be attributed to the formation of more complex ion aggregates. Figure 6 shows the Raman spectra as the square markers and the corresponding fitted bands as solid lines at various RHs in the 955-1045 cm-1 region. And the four component peaks as dashed lines and the coefficient of determination (R2) are also included. In this fitting, the fwhm of each peak is almost constant at all RHs, with the values of 9.62 ( 1, 17.27 ( 1, 32.92 ( 0.5, and 17.06 ( 0.5 cm-1, respectively. The area ratios of each band component to the sum of the four fitted ones at various WSRs are calculated and displayed in Figure 7, which can reflect the relative content of each component at various concentrations in the droplets. It is obvious that the free SO42- ions are dominant initially, and some of the monodentate CIPs also coexist in the droplets. With the RH decreasing, the free SO42- ions transit gradually into the monodentate CIPs in region I, and reach its maximum value at WSR of ∼9.5 (76% RH). As the RH enters into region II, the free SO42- ions and monodentate CIPs continuously reduce, accompanied by the increase of the bidentate CIPs and more complex ion aggregates. The bidentate and complicated CIPs reach their maximum values at WSR of ∼6.2 (60% RH) and

6484

J. Phys. Chem. A, Vol. 114, No. 23, 2010

Guo et al.

Figure 8. The full width of the peak at half-maximum (fwhm) of (a) the V1-SO42- band in Raman spectra and (b) the V3-SO42- band in FTIR spectra, as a function of RH.

Figure 7. The relative contents of the four fitted components at various RHs in the dehumidifying process.

∼2.8 (44% RH), respectively, agreeing with the transition points of the regions III and IV. Further decreasing the RH, all the components in the droplets are almost constant, which is relevant to the formation of the amorphous MnSO4 · 2.8H2O. However, as the WSR is lower than 1.7 (∼13% RH), it seems that some of the monodentate CIPs are changing into the bidentate ones, corresponding to the transition of the amorphous MnSO4 · 2.8H2O to MnSO4 · 1.7H2O. The droplets of MnSO4 give evidence of four species present in varying proportions at different RHs. The behavior is similar to that observed for droplets of MgSO4 where the species are the free SO42-, monodentate CIP, bidentate CIP, and more complex ion aggregates. With a d5 structure, Mn2+ has a hydration energy of -437.8 kcal mol-1, smaller than that of Mg2+ (-455.5 kcal mol-1).24,25 Therefore, the water molecules in the first hydration layer of Mn2+ should be more readily replaced by SO42- to form CIPs. This fact could also be reflected in the different exchange rates of water molecules in the inner sphere, which are 6 × 105 and 2× 107 s-1 for Mg2+ and Mn2+, respectively.6 On measuring the activity coefficients, diversified CIPs have been found in dilute MnSO4 solutions.10 Extended X-ray absorption fine structure (EXAFS) investigations also showed that Mn[H2O]62+ is not the only form of Mn2+, MnBr(H2O)5+ and MnBr2(H2O)4 also account for significant quantities of Mn2+ in dilute MnBr2 solutions.26,27 By combining X-ray absorption near edge structure (XANES) with EXAFS, Chen detected that approximately one bromine entered the first hydration shell of Mn2+ for 6 mol · kg-1 MnBr2 solution (WSR ∼9.3).9 In the measurement of the osmotic coefficients of bivalent transition metal sulfates, the exceptional behaviors of concentrated MnSO4 solutions were explained by the formation of biden-

tate CIPs, rather than monodentate CIPs that were proposed in other concentrated solutions.11 Corresponding proofs can also be provided by the evolution of the V2 band in Figure 3. As the RH decreases to ∼76%, the V2-SO42- band, located originally at 450 cm-1, split into two peaks at 442 and 462 cm-1, respectively. The split is due to the distortion of the symmetry of SO42- through the formation of CIPs,12,28 in support of the occurrence of monodentate and bidentate CIPs in MnSO4 droplets. In the IR spectra shown in Figure 4, the infrared inactive V1-SO42- band is found to occur at 983 cm-1, due to the symmetry decrease of SO42- caused by the neighboring solvent water molecules and cations.23 The V1-SO42- band shows an obvious blue shift from 983 to 998 cm-1 as the RH decreases from ∼93% to ∼5%. For the V3-SO42- band, no shift can be observed with the decrease of RH, except for a shoulder occurring at 1132 cm-1 at ∼10% RH. The shoulder can be attributed to the suggested amorphous phase (MnSO4 · 1.7H2O), which began to form at this RH. The fwhm of V1- and V3-SO42- bands in the Raman and IR spectra are shown in Figure 8, panels a and b, respectively. Similarly, the fwhm curves of the V1- and V3-SO42- bands in Figure 8a,b also can be divided into five regions by the transition point of ∼76%, ∼60%, ∼44%, and ∼13% RH. In regions I and II (WSR J6), the fwhm continuously increase with the decrease of RH, and a transition point appears at ∼76% RH induced probably by the formation of CIPs, which is consistent with the observations mentioned above. The fwhm curves go through a plateau at ∼60% RH and then decrease as the RH reduces further in region III. This observation indicates the presence in great abundance of various ion pairs at ∼60% RH, and more complex ion aggregates become the dominant component with decreasing the RH further. The diversified ion pairs may become settled in fractions in region IV, according to the nearly unchanged fwhm for V1- and V3-SO42- bands, and the particles are probably of the amorphous phase (MnSO4 · 2.8H2O) as suggested above. Decreasing further the RH below ∼13% in region V, the fwhm increases very rapidly,

Phase Transitions of MnSO4 Droplets

J. Phys. Chem. A, Vol. 114, No. 23, 2010 6485 attributed to the formation of various CIPs between Mn2+ and SO42-. When the RH is lowered below ∼60% (WSR j6.2) in region III, the fast evaporation results in the scarcity of water for the formation of hydrogen bonds in droplets. Further decreasing the RH, MnSO4 droplets effloresce into amorphous solids MnSO4 · 2.8H2O, according to the nearly constant intensity ratio in region IV. Although the change is not obvious in region V in Figure 5a, A3210/A3385 reduces sharply as the RH decreases below ∼13% in Figure 9b, which can be attributed to the structural transformation of the amorphous solids (MnSO4 · 2.8H2O) as described above to MnSO4 · 1.7H2O. IV. Conclusions

Figure 9. The relative content of strong to weak hydrogen bond components (a) in Raman spectra and (b) in FTIR spectra, as a function of RH.

particularly in the V3-SO42- band of the IR spectra, with the value varying from 108.8 to 126.8 cm-1. The fwhm change in the IR spectra in Figure 8b also indicates that an abrupt structural transformation occurs in droplets, leading to the formation of a new phase (MnSO4 · 1.7H2O) in MnSO4 particles. Hydrogen Bond Structures. In Figures 3 and 4, the O-H stretching envelope can be observed to develop in the Raman and FTIR spectra as a function of RH. In the Raman spectra in Figure 3, the O-H stretching envelope peaks at 3420 cm-1 with a shoulder at 3245 cm-1. However, the shoulder weakens more rapidly than the main peak with the decrease of RH. In the IR spectra in Figure 4, the main peak of the O-H stretching envelope occurs at 3210 cm-1 with a shoulder at 3385 cm-1 at high RHs. The main peak decreases more rapidly with lowering the RH. As a result, the shoulder at 3385 cm-1 at high RHs becomes the main peak with decreasing the RH below 57%. Hydrogen bonds can be influenced by various species in aqueous solutions, whose strength qualitatively decreases with increasing the wavenumber.12-14,18 For Mn2+, it has a somewhat large charge-to-radius ratio and can polarize water molecules in the first hydration layer, reinforcing accordingly the hydrogen bonds between the first and second spheres.29 As a “structure maker”, SO42- also forms stronger hydrogen bonds with the neighboring water.30 As a result, both Mn2+ and SO42- are expected to strengthen hydrogen bonds in dilute solutions. However, the strong hydrogen bond component was found to decrease more rapidly than the weak one as the RH decreased, according to Figures 3 and 4. The formation of CIPs can weaken the polarization ability of Mn2+, leading to the depression of strong hydrogen bond component in droplets.12,14 In panels a and b of Figure 9, I 3245/I3420 and A3210/A3385 were used to indicate the relative abundance of strong and weak hydrogen bond components in the Raman and IR spectra of MnSO4 droplets, respectively. Similar to the hygroscopic curves in Figure 5, Figure 9 can also be divided into the five regions. In region I, both I3245/I3420 and A3210/A3385 decrease slowly with the RH. On entering region II (RH below ∼76%), the two ratios descend very rapidly, reflecting the reduction of strong hydrogen bond component with the decrease of RH. The transition is

In this work, MnSO4 droplets have been investigated in the dehumidifying processes by micro-Raman and FTIR, respectively. Upon decreasing the RH, Mn2+ and SO42- form monodentate and bidentate CIPs in the droplets at high RHs, which reach their maximum values at WSR of ∼9.5 (76% RH) and ∼6.2 (60% RH), respectively. Further decreasing the RH, one amorphous solid was observed to form at ∼44% RH. This solid was previously identified as MnSO4 · 2.8H2O by Cohen et al. With decreasing the RH below ∼13%, however, the amorphous phase was inferred to transform into another one with a stoichiometry of MnSO4 · 1.7H2O, showing as an abrupt fwhm increase of the V3-SO42- band in the FTIR spectra and the transition of monodentate CIPs to bidentate ones. These conclusions, especially the evolution of CIPs and the formation of a new phase, are useful not only for understanding the association states and kinetic behavior of metals ions, but also for the study of manganese pollutions as well as the relevant atmospheric reactions. Acknowledgment. This work was supported by the NSFC (20933001, 20903036, and 20873006). References and Notes (1) Manoj, S. V.; Mishra, C. D.; Sharma, M.; Rani, A.; Jain, R.; Bansal, S. P.; Gupta, K. S. Atmos. EnViron. 2000, 34, 4479. (2) Connick, R. E.; Zhang, Y. X. Inorg. Chem. 1996, 35, 4613. (3) Graedel, T. E.; Weschler, C. J.; Mandich, M. L. Nature 1985, 317, 240. (4) Berresheim, H.; Jaeschke, W. J. Atoms. Chem. 1986, 4, 311. (5) Grgic´, I.; Bercˇicˇ, G. J. Atmos. Chem. 2001, 39, 155. (6) Bock, C. W.; Katz, A. K.; Markham, G. D.; Glusker, J. P. J. Am. Chem. Soc. 1999, 121, 7360. (7) Loeffler, H. H.; Yagu1e, J. I.; Rode, B. M. J. Phys. Chem. A 2002, 106, 9529. (8) Othaki, H.; Radnai, T. Chem. ReV. 1993, 93, 1157. (9) Chen, Y. S.; Fulton, J. L.; Partenheimer, W. J. Solution Chem. 2005, 34, 993. (10) Malatesta, F.; Trombella, S.; Fanelli, N. J. Solution Chem. 2000, 29, 685. (11) Libus´, W.; Sadowska, T.; Libus´, Z. J. Solution Chem. 1980, 9, 341. (12) Wang, F.; Zhang, Y. H.; Zhao, L. J.; Zhang, H.; Cheng, H.; Shou, J. J. Phys. Chem. Chem. Phys. 2008, 10, 4154. (13) Zhao, L. J.; Zhang, Y. H.; Wei, Z. F.; Cheng, H.; Li, X. H. J. Phys. Chem. A 2006, 110, 951. (14) Zhang, Y. H.; Chan, C. K. J. Phys. Chem. A 2000, 104, 9191. (15) Rudolph, W. W. J. Chem. Soc., Faraday Trans. 1998, 94, 989. (16) Zhang, Y. H.; Chan, C. K. J. Phys. Chem. A 2002, 106, 285. (17) Gao, Y. G.; Chen, S. B.; Yu, L. E. J. Phys. Chem. A 2006, 110, 7602. (18) Dong, J. L.; Li, X. H.; Zhao, L. J.; Xiao, H. S.; Wang, F.; Guo, X.; Zhang, Y. H. J. Phys. Chem. B 2007, 111, 12170. (19) Rard, J. A. J. Chem. Eng. Data 1984, 29, 443. (20) Cohen, M. D.; Flagan, R. C.; Seinfeld, J. H. J. Phys. Chem. 1987, 91, 4563. (21) Cohen, M. D.; Flagan, R. C.; Seinfeld, J. H. J. Phys. Chem. 1987, 91, 4583. (22) Hayes, A. C.; Kruus, P.; Adams, W. A. J. Solution Chem. 1984, 13, 61.

6486

J. Phys. Chem. A, Vol. 114, No. 23, 2010

(23) Zhang, H.; Zhang, Y. H.; Wang, F. J. Comput. Chem. 2009, 30, 493. (24) Marcus, Y. J. Chem. Soc., Faraday Trans. 1987, 83, 339. (25) Burgess, M. A. Metal Ion in Solution; Ellis Horwood: Chichester, England, 1978. (26) Beagley, B.; Gahan, B.; Greaves, G. N.; McAuliffe, C. A. J. Chem. Soc., Chem. Commun. 1983, 1265.

Guo et al. (27) Beagley, B.; Gahan, B.; Greaves, G. N.; McAuliffe, C. A.; White, E. W. J. Chem. Soc., Chem. Commun. 1985, 1804. (28) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed.; Wiley & Sons: New York, 1986. (29) Bergstro¨m, P.; Lindgren, J. Inorg. Chem. 1992, 31, 1529. (30) Bergstro¨m, P.; Lindgren, J. J. Phys. Chem. 1991, 95, 8575.

JP9104147