A Strategy for Single Supersaturated Droplet Analysis: Confocal

We report a new strategy for single supersaturated droplet analysis, i.e., the complicated hygroscopic properties of. MgSO4 aerosols under supersatura...
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Anal. Chem. 2005, 77, 7148-7155

A Strategy for Single Supersaturated Droplet Analysis: Confocal Raman Investigations on the Complicated Hygroscopic Properties of Individual MgSO4 Droplets on the Quartz Substrate Feng Wang, Yun-Hong Zhang,* Su-Hui Li, Liang-Yu Wang, and Li-Jun Zhao

The Institute for Chemical Physics, Beijing Institute of Technology, Beijing, China 100081

We report a new strategy for single supersaturated droplet analysis, i.e., the complicated hygroscopic properties of MgSO4 aerosols under supersaturated state were studied through the micro-Raman observation on an individual MgSO4 droplet deposited on a quartz substrate in a relative-humidity-controlled chamber. Upon reduction of the ambient relative humidity (RH), MgSO4 droplets with tiny volume lost water but did not effloresce. Thus, a detailed spectral evolution of the symmetric stretching vibration band (v1-SO42-) from free ions (at ∼983 cm-1) to monodentate (∼995 cm-1) and then to bidentate contact ion pairs (CIPs) or more complex chain-structural compositions (∼1021 cm-1) was observed with the high signal-to-noise (S/N) confocal Raman spectra of the droplet with a diameter of ∼80 microns. Such a transition process could be well-described by the changes of relative intensity at 983, 995, and 1021 cm-1. Four steps, i.e., concentrated step, monodentate CIPs step, bidentate CIPs step, and gel step, were roughly observed in the dehumidifying-humidifying cycle according to the intensity ratios of I995/I983 and I1021/I983. Even though the area ratio of the O-H stretching band of water molecules to the v1-SO42- band seemed reversible in the dehumidifying and humidifying processes, the intensity ratios of I995/ I983 and I1021/I983 showed a hysteresis in the decomposition of CIPs in the humidifying process with the RH < 40%. The O-H stretching envelope of the MgSO4 droplet was also observed to be sensitive to the structural changes of the hydrogen bonding of water molecules in the four steps. The intensity ratio of Raman scattering for the components with strong hydrogen bonds to those with weak ones, i.e., I3224/I3431, was used to understand the effects of CIPs on the water structures of the first hydration layer of Mg2+. Good consistency on the hysteresis in the humidifying process was also observed from the ratio of I3224/I3431 changing with RH.

INTRODUCTION The efflorescence and deliquescence processes of aerosols attract much interest of chemists, aerologists, physicists, and * Corresponding author: [email protected].

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biochemists1-8 because of the achievement of a supersaturated state of salt solutions and the formation of metastable solid phases in aerosols. How to understand the efflorescence and deliquescence processes at the molecular level, including the formation of various ion pairs with different structures, is a focus of atmospheric science, solution chemistry, and crystallography.4-7,9-11 Metastable solidification and supersaturation have been considered as the results of the suppression of both heterogeneous and homogeneous nucleation in the wall-less aerosols with tiny volume. As an excellent levitation technique, the electrodynamic balance (EDB) can trap single droplets and made it possible to study the supersaturated state of the “wall-less” droplets.12 Combining EDB with Raman technique, Tang and Fung13 studied the phase transformation and metastability of supersaturated nitrate microparticles. With the Raman observation on levitated droplets, Zhang and Chan4,10 have found the relationship between the hygroscopic properties and the formation of contact ion pairs (CIPs) for supersaturated sulfate and nitrate solutions.4,10 They also observed the existence of water monomers in highly supersaturated perchlorate droplets.5 Other levitation techniques such as acoustical levitation14-16 have been used to study reaction and crystallization processes in the area of airborne chemistry. (1) Colberg, C. A.; Krieger, U. K.; Peter, Th. J. Phys. Chem. A 2004, 108, 27002709. (2) Braban, C. F.; Carroll, M. F.; Styler, S. A.; Abbatt, J. P. D. J. Phys. Chem. A 2003, 107, 6594-6602. (3) Choi, M. Y.; Chan, C. K. J. Chem. Eng. Data 2002, 47, 1526-1531. (4) Zhang, Y. H.; Choi, M. Y.; Chan, C. K. J. Phys. Chem. A 2004, 108, 17121718. (5) Zhang, Y. H.; Chan, C. K. J. Phys. Chem. A 2003, 107, 5956-5962. (6) Wang, L. Y.; Zhang, Y. H.; Zhao, L. J. J. Phys. Chem. A 2005, 109, 609614. (7) Krueger, B. J.; Grassian, V. H.; Iedema, M. J.; Cowin, J. P.; Laskin, A. Anal. Chem. 2003, 75, 5170-5179. (8) Martin, F. P.-D.; Dumas, R.; Field, M. J. J. Am. Chem. Soc. 2000, 122, 76887697. (9) Rudolph, W. W.; Irmer, G.; Hefter, G. T. Phys. Chem. Chem. Phys. 2003, 5, 5253-5261. (10) Zhang, Y. H.; Chan, C. K. J. Phys. Chem. A 2002, 106, 285-292. (11) Pilinis, C.; Seinfeld, J. H.; Grosjean, D. Atmos. Environ. 1989, 23, 16011606. (12) Bogan, M. J.; Agnes, G. R. Anal. Chem. 2002, 74, 489-496. (13) Tang, I. N.; Fung, K. H. J. Chem. Phys. 1997, 106, 1653-1660. (14) Leopold, N.; Haberkorn, M.; Laurell, T.; Nilsson, J.; Baena, J. R.; Frank, J.; Lendl, B. Anal. Chem. 2003, 75, 2166-2171. (15) Santesson, S.; Johansson, J.; Taylor, L. S.; Levander, L.; Fox, S.; Sepaniak, M.; Nilsson, S. Anal. Chem. 2003, 75, 2177-2180. 10.1021/ac050938g CCC: $30.25

© 2005 American Chemical Society Published on Web 10/13/2005

Magnesium sulfate is a major component of seawater9 and is ubiquitously present in atmospheric aerosols.10 Different from atmospheric hygroscopic aerosols, which have been expected to quickly equilibrate with the environment in the water evaporation and condensation processes,11 MgSO4 droplets were found to exhibit a significant delay in water evaporation at high concentrations.17 The growth of MgSO4 droplets in the humidifying process also showed significant mass transfer limitations at low relative humidity (RH).18 Buchner et al.19 recently found that there is a complex interaction between Mg2+ and SO42-, even in a dilute state, according to their measurement of dielectric relaxation spectroscopy (DRS) of bulk aqueous MgSO4 solutions. Such complexity was considered as the result of the formation of ion pairs in the MgSO4 solutions, including double solvent separated (2SIP), solvent-shared (SIP), and contact (CIP) ion pairs.20-22 Raman spectroscopy has been extensively used to study the structures of ion pairs because it can provide molecular information on the interactions between cations and anions as well as the hydrogen bonding between water molecules.23,24 However, only a small asymmetry of the v1-SO42- band can be observed in the Raman spectra of bulk MgSO4 solutions.23 There had been a controversial issue about the explanation of the asymmetry in the v1-SO42- band, i.e., whether the small shoulder resolved at 995 cm-1 can be assigned to the formation of monodentate CIPs.23-29 Using the difference spectroscopy method to obtain spectral information of CIPs in the low-frequency region of bulk MgSO4 solutions, Rudolph et al.9 resolved a shoulder at 328 cm-1 from the v1-MgO6 stretching vibration band (at 358 cm-1) of the hexaaquo complex and a new polarized vibration band at 245 cm-1. They assigned them to the Mg(II)-OH2 stretching vibration of the CIP [(H2O)5Mg(II)OSO32-] and the Mg(II)-OSO32- stretching vibration band, respectively. With the combination of EDB and Raman technique, Zhang and Chan10,30 obtained the spectroscopic information of MgSO4 droplets under a supersaturated state.10,30 Rich monodentate CIPs in the levitated MgSO4 droplets at supersaturated concentrations showed a main peak at 995 cm-1.10,30 More complex compositions with bidentate structures at very dry states contributed a wide peak at 1007 cm-1.10,30 Bidentate structures were also observed in some sulfate31-33 and perchlor(16) Wood, B. R.; Heraud, P.; Stojkovic, S.; Morrison, D.; Beardall, J.; McNaughton, D. Anal. Chem. 2005, 77, 4955-4961. (17) Chan, C. K.; Ha, Z.; Choi, M. Y. Atmos. Environ. 2000, 34, 4795-4803. (18) Chan, C. K.; Elagan, R. C.; Seinfeld, J. H. J. Am. Ceram. Soc. 1998, 81, 646-648. (19) Buchner, R.; Chen, T.; Hefter, G. J. Phys. Chem. B 2004, 108, 2365-2375. (20) Eigen, M.; Tamm, K. Z. Elektrochem. 1962, 66, 93-106. (21) Eigen, M.; Tamm, K. Z. Elektrochem. 1962, 66, 107-121. (22) Atkinson, G.; Pstriucci, S. J. Phys. Chem. 1966, 70, 3122-3128. (23) Davis, A. R.; Oliver, B. G. J. Phys. Chem. 1973, 77, 1315-1316. (24) Pye, C. C.; Rudolph, W. W. J. Phys. Chem. A 1998, 102, 9933-9943. (25) Daly, F. P.; Brown, C.; Kester, D. R. J. Phys. Chem. 1972, 76, 3664-3668. (26) Rull, F.; Balarew, Ch.; Alvarez, J. L.; Sobron, F.; Rodriguez, A. J. Raman spectrosc. 1994, 25, 933-941. (27) Rudolph, W. W.; Brooker, M. H.; Tremaine, P. J. Solution Chem. 1997, 26, 757-777. (28) Rudolph, W. W.; Pye, C. C. J. Phys. Chem. B 1998, 102, 3564-3573. (29) Hayes, A. C.; Kruus, P.; Adams, W. A. J. Solution Chem. 1984, 13, 61-75. (30) Zhang, Y. H.; Chan, C. K. J. Phys. Chem. A 2000, 104, 9191-9196. (31) Nakamoto, K.; Fujuta, J.; Tanaka, S.; Kobayashi, M. J. Am. Chem. Soc. 1957, 79, 4904-4908. (32) Finholt, J. E.; Anderson, R. W.; Fyfe, J. A.; Caulton, K. G. Inorg. Chem. 1965, 4, 43-45. (33) Horn, R. W.; Weissberger, E.; Collman, J. P. Inorg. Chem. 1970, 9, 23672371.

ate34 crystals. Raman spectra of bulk aqueous MgSO4 solutions at high temperature (198 °C) also showed a peak at 993 cm-1 as a sign of monodentate CIPs.9 On the basis of the three-step process equilibrium suggested by Atkinson and Pstriucci as follows,22

Mg2+ (aq) + SO42- (aq) S [Mg(WW)SO4] S [Mg(W)SO4] S [MgSO4]

where SO42- (aq) represents the free ions, [Mg(WW)SO4] and [Mg(W)SO4] represent the 2SIP and the SIP, respectively, and [MgSO4] refers to the CIPs, Rudolph et al.9 obtained the ratio of CIPs to the total SO42- ions by considering that the shoulder at 995 cm-1 was from the monodentate CIPs. Even though the EDB-Raman system was considered as a unique combination to measure spectroscopic signatures of CIPs under a supersaturated state,10 which cannot be achieved for bulk solutions because of saturation point limitations, there are strong physical morphology-dependent resonances (MDRs) overlapping with the spectral changes of chemical interactions for the regular spherical droplets.10 So far, our analysis on the levitated MgSO4 droplets was only based on the strong v1-SO42- band.5,10,30 Much spectral information of other weak bands such as the v2-SO42band should be more sensitive to the structural symmetry of CIPs but has not been used because of the heavy disturbance of MDRs of the droplets. Especially, we have no method to make use of the Raman spectra of water molecules, since there is a big distortion for the O-H stretching vibration envelope from the strong MDRs. In this paper, we demonstrate the new utility of single supersaturated droplet analysis to study the hygroscopic properties of MgSO4 aerosols by deposition of MgSO4 droplets on a quartz substrate instead of levitation. Similar to the levitated droplets in EDB, the constraint of saturation for the tiny-volume MgSO4 droplets on the quartz substrate can be also surprisingly relaxed and supersaturated MgSO4 droplets could be easily prepared by simply decreasing the RH of the environment of the droplets. One more important advantage is that there are no MDRs for the half-ellipsoidal MgSO4 droplets in the process of water evaporation or condensation. A wide range of Raman spectra of MgSO4 droplets with high signal-to-noise (S/N) ratios can be obtained by micro-Raman spectroscopy. More detailed information on the chemical interactions related to the formation of monodentate and bidentate CIPs can be obtained not only from the strong v1-SO42- band but also from the reliable weak bands such as the two bending bands (v2- and v4-SO42-) and the asymmetric stretching band (v3-SO42-). Besides, we can also understand the hydrogen bonding of water molecules in the gel phase of supersaturated MgSO4 droplets through the well-defined O-H stretching envelope, which was rarely investigated before. The ultimate aim of this study is to prove the efficiency of the analysis of a single supersaturated droplet on substrate, by studying the hygroscopic properties of MgSO4 droplets, especially the detailed structural changes of CIPs in the dehumidifying and humidifying processes. (34) Miller, A. G.; Macklin, J. W. J. Phys. Chem. 1985, 89, 1193-1201.

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Figure 1. Schematic diagram of the experimental setup for the micro-Raman measurements of MgSO4 droplets on quartz substrate.

EXPERIMENTAL SECTION Droplets Preparation. The 0.5 M MgSO4 solution was prepared by dissolving analytical-grade magnesium sulfate in the triply distilled water without further purification. Magnesium sulfate droplets of the solutions were injected onto a quartz substrate by a syringe. Then the quartz substrate was fixed to the bottom of a chamber sealed with thin transparent PE (polyethylene) film. The RH in the chamber was adjusted by mixing a stream of water-saturated N2 and another of dry N2 at controlled flowrates. The RH was detected by the humidity temperature meter (Centertek Center310) with the accuracy of (2.5% and (0.7 °C. Micro-Raman Measurements for Single Supersaturated Droplet Analysis. Figure 1 shows the schematic diagram of the experimental setup for the micro-Raman measurements. The micro-Raman system (Renishaw Invia), including a Leica DMLM microscope, was used to obtain the Raman spectra of the droplets. Spectral calibration was made with the 520 cm-1 silicon band as the standard before the experiment. The spectral resolution was ∼1 cm-1 within the range of 200-4000 cm-1. The chamber was mounted on an automated motorized stage. A 50× objective was used to select a droplet with a diameter of ∼80 microns for the Raman measurement. A 514.5 nm line from an argon ion laser (LS-514 model, Laserphysics) with pinhole-in was adopted to excite the droplet at 20 mw output power. A 514.5 nm Raman notch filter was used to remove the strong Rayleigh scattering. The laser spot was ∼1 micron in diameter after being focused on the surface of the droplet. The backscattering signal, after passing through a 1800 g/mm grating, was detected by the CCD (charge-coupled device) with the confocality-in-high mode. Raman spectra of MgSO4 droplets in the range of 200-4000 cm-1 were obtained with repeated scanning of 5 times, each time with an exposure time of 10 s. To make sure that the droplet should completely equilibrate with ambient RH, 1 h was spent before each Raman measurement. All the measurements were made at ambient temperature of 22-24 °C. All the spectra were obtained by the Wire 2.0 program supplied by Renishaw. The morphological changes of the droplets were observed online with the Leica DMLM microscope. 7150 Analytical Chemistry, Vol. 77, No. 22, November 15, 2005

Figure 2. Raman spectra of the MgSO4 droplet on quartz substrate in the (a) dehumidifying and (b) humidifying processes. The weak peak at about 3000 cm-1 marked with an asterisk arises from the PE film.

RESULTS AND DISCUSSION SO42- Bands without Disturbance of MDRs at Various RHs. Since the droplet on the quartz substrate is a half-ellipsoid rather than a standard sphere, there is no MDRs disturbance on the chemical interactions of Raman spectra for the half-ellipsoidal MgSO4 droplet. Thus, the Raman spectra with high S/N ratios can be obtained, which are shown in Figure 2. A free SO42- ion has Td symmetry and nine modes of internal vibration spanning the representation Γvib ) A1 + E + 2F2. All modes are Raman active; only the F2 modes are IR active. In the dilute state (at 96% RH in Figure 2a), the v1-SO42- (A1), the v3SO42- (F2), the v2-SO42- (E), and the v4-SO42- (F2) appear at 983, ∼1114, 448, and 607 cm-1, respectively. In the dehumidifying process as shown in Figure 2a, the v1-SO42- presents a distinctive evolution, which will help us to make further analysis on the structural changes in the water evaporation process, and also the bending bands can provide us reliable information about the symmetry change of SO42- when it forms any kind of CIPs. The v1-SO42- band has been widely used to analyze the interactions between sulfate and cations since its position and full width at half-height (fwhh) are sensitive to the countercation

Table 1. Correlation Table for the SO42- Bands with Td, C3v, and C2v Symmetry point group

ν1

ν2

ν3, ν4

Td C3v C2v

A1 (R)a A1 (IR, R)b A1 (IR, R)

E (R) E (IR, R) A1 (IR, R) + A2 (R)

F2 (IR, R) A1 (IR, R) + E (IR, R) A1 (IR, R) + B1 (IR, R) + B2 (IR, R)

a R indicates a Raman-active mode. b IR indicates an infrared-active mode.

species, the salt concentration, and the temperature.24,26-28,35,36 Similar to dilute bulk solutions, the strong v1-SO42- band at 983 cm-1 in this work can be considered as the total contribution of pure hydrated SO42-, 2SIP, and SIP, which were called as “free ions” in order to distinguish the CIPs.19,23 When the RH decreases to 60%, a shoulder at 995 cm-1 can be clearly identified, causing the asymmetry in the Raman band of the v1-SO42-. Zhang and Chan10 studied highly supersaturated droplets with the combination of EDB and Raman technique and considered the shoulder as an indicator of the formation of monodentate CIPs. This argument is supported by the analysis of Rudolph and coworkers,24,37 who have observed corresponding changes of the stretching bands of both Mg(II)-OH2 and Mg(II)-OSO32- in the low-frequency region in addition to the shoulder at 993 cm -1. The v1-SO42- band gets weak at 983 cm-1, rises at 995 cm-1, and finally becomes a wide peak at 1021 cm-1 in the dehumidifying process. A similar observation of the wide peak at 1007 cm-1 was also reported by Zhang and Chan10,30 and was ascribed to the formation of bidentate CIPs.30 Thus, the relative intensity at 983, 995, and 1021 cm-1 implicates much structural information related to the transition of CIPs from the monodentate to the bidentate and then to more complex structures in the gel state. When the RH decreases from 96% to 4%, the fwhh of the v2SO42- increases from 38 to 63 cm-1. Beside the main peak at 448 cm-1, an obvious shoulder arises, indicating that there is a split for the v2-SO42- band when the free SO42- ions with Td symmetry change into CIPs with lower symmetry.38 Table 1 shows the correlation table for the SO42- bands with Td, C3v, and C2v symmetry. The monodentate CIPs have the symmetry of C3v, and thus, the v2-SO42- band remains the doubly degenerate mode according to Table 1. In our observation, the v2-SO42- band constantly appears at 448 cm-1 and no obvious change is observed for this band when the RH continually decreases from 96% to 53%, even though monodentate CIPs mainly start to form at RH ) 60%, as confirmed by the v1-SO42- band. However, with further decreasing RH, an obvious shoulder arises at ∼482 cm-1 starting at RH ) 53% and finally makes the v2-SO42- band into a trapezoidal shape, suggesting that the bidentate CIPs with the symmetry of C2v begin to form. A similar sensitivity of the v2ClO4- to bidentate contact association with C2v symmetry was also observed by Miller and Macklin34 in concentrated sodium perchlorate solution. (35) Rudolph, W. W.; Irmer, G. J. Solution Chem. 1994, 23, 663-684. (36) Chatterjee, R. M.; Adams, W. A.; Davis, A. R. J. Phys. Chem. 1974, 78, 246250. (37) Rudolph, W. W.; Mason, R. J. Solution Chem. 2001, 30, 527-548. (38) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination compounds, 4th ed.; Wiley & Sons: New York, 1986.

Figure 3. Area ratio of water envelope to the v1-SO42- band of the MgSO4 droplets at various RHs in the dehumidifying (solid square) and humidifying (open circle) processes. The molar water-to-solute ratio (WSR) values of the MgSO4 (solid triangle) droplet in the dehumidifying process are from ref 10.

The fwhh of the v4-SO42- band increases from 40 to 55 cm-1, which also results from the split of the triply degenerate band. The intensity of the band is too weak to provide useful information on the formation of CIPs. It is interesting to note that the Raman band of the v3-SO42is much broader when compared with the bands of the other vibration modes of SO42-, even in dilute solutions (see Figure 2a at 96% RH). Thus, the v3-SO42- is more sensitive to the environment of SO42- ions, even for different hydrated structures.4 That is to say, the surrounding water molecules have more obvious effects on the v3-SO42- band than other vibration bands. As the RH decreases, the fwhh of the v3-SO42- band increases from 90 cm-1 at 96% RH to 142 cm-1 at 4% RH because of the complexity of the environment of the SO42- ions at low RH. Continuous formation of ion pairs including SIPs and CIPs in the water evaporation process leads to the broadening of the v3-SO42- band. There is a slight overlap between the v3-SO42- band and the v1SO42- band at 4% RH. Figure 2b shows the Raman spectra of the MgSO4 droplet in the humidifying process. Compared with the dehumidifying process, the spectra in the humidifying process seem to change in a reverse way as the RH increases from 4% to 96%. However, a hysteresis of decomposition of CIPs in the humidifying process can be drawn out either from the v1-SO42- band or from the waterstretching envelope as discussed in the following parts. Hygroscopic Properties. Figure 3 shows the area ratio of the water envelope to the v1-SO42- band in terms of RH for the MgSO4 droplets in the dehumidifying and humidifying processes. The molar water-to-solute ratio (WSR) of the MgSO4 droplet in the dehumidifying process measured by Zhang and Chan10 is also included for comparison. The WSR continuously decreases as the RH decreases from 88% to 8%, but there is a transition point (point D in Figure 3) at 57% RH, which was related to the formation of CIPs.10 However, there is a complicated change in our research in the RH range of 40%-60%. Three transition points (points A, B, and C in Figure 3) are observed at RH ) 60%, 50%, and 40%, respectively. Between point A and point B, the area ratio is less sensitive to RH variation. However, it becomes very sensitive to RH variation from point B to point C. In addition, when the RH Analytical Chemistry, Vol. 77, No. 22, November 15, 2005

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Figure 4. (a) I995/I983 and (b) I1021/I983 in the Raman spectra of the MgSO4 droplets in the dehumidifying (solid square) and humidifying (open circle) processes. The error bars of the intensity ratios at various RHs are added in the graph.

decreases further below 40% (point C), the area ratio of the water envelope to the v1-SO42- band remains nearly constant. Although the area-ratio curves (Figure 3) show that there is not much difference in the dehumidifying and humidifying processes, further analysis on the relative intensity of the v1SO42- band and the water-stretching envelope will provide more useful and interesting information related to a hysteresis for the decomposition of CIPs in the humidifying process, which will be presented in the following discussion. Structure-Dependent Hysteresis of Water Absorption. From the high S/N spectra, reliable analysis on the cause of the asymmetry of the v1-SO42- band can be conducted in detail. Although it is difficult to do quantitative analysis on the compositional distribution of the CIPs, ratios of the intensities at 995 and 1021 cm-1 to that at 983 cm-1 can be used as an indicator of the relative abundance of monodentate or bidentate CIPs or more complex chain-structural compositions. Parts a and b of Figure 4 show the intensity ratio curves of the MgSO4 droplets in the form of I995/I983 and I1021/I983 versus RH, respectively. The dehumidifying-humidifying circle can be roughly divided into four regions with three transition points of A, B, and C as boundaries. In region a (RH > 60%) as shown in Figure 4, I995/I983 increases slightly as the RH decreases. A rapid increase of I995/I983 starts at 60% RH (point A in Figure 3). Zhang and Chan10 have already ascribed this change to the formation of monodentate CIPs. In region b 7152

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(60% > RH > 50%), I995/I983 increases with decreasing RH while I1021/I983 remains constant before the RH achieves 50% (point B in Figure 3), indicating that no bidentate CIPS form but mostly monodentate CIPs form in region b. However, in region c (50% > RH > 40%), I1021/I983 increases obviously as the RH decreases. This means that more and more monodentate CIPs have probably transformed to bidentate CIPs. A rapid increase of I1021/I983 in region d (RH < 40%) predicates that the bidentate CIPs or more complex structures such as the chain structures are forming continuously. It should be mentioned that, at lower RH, especially in region d, I995/I983 shows a pseudoincrease, since the peak at 995 cm-1 has been heavily masked by the wide peak at 1021 cm-1, as shown in Figure 2a. The four regions, in fact, correspond to the four steps in the dehumidifying process of the MgSO4 droplets, i.e. concentrated step, monodentate CIPs step, bidentate CIPs step, and gel step. In step a, the solution becomes concentrated without obvious formation of monodentate CIPs as RH decreases. In step b, a large amount of monodentate CIPs form and become the predominant component as RH decreases. In step c, monodentate CIPs form continuously while more and more monodentate CIPs transform to bidentate ones. In step d, the gel structure, including the chain-structural composition, gradually forms. In the humidifying process, it is very interesting to note that I1021/I983 remains constant till 40% RH, indicating that there is a hysteresis in the decomposition of CIPs in the humidifying process. Even though the equilibration time at the desired RH was extended to 6 h, the same results were also observed. To our best knowledge, no such observation was reported in previous research. The hysteresis in the humidifying process implies that there are some differences in structure in the MgSO4 droplets between the two processes. In the dehumidifying process, MgSO4 does not crystallize, even at low RH in this experiment. Instead, gel formation in concentrated MgSO4 droplets occurs continuously as the RH decreases. However, in the humidifying process, the gel structure may inhibit the penetration of the water molecules into the bidentate CIPs or the chain-structural composition till 40% RH, leading to the hysteresis in the humidifying process. A series of droplet images shown in Figure 5 also describes this interesting phase change in terms of the equilibration time at 40% RH. In the humidifying process, at 36% RH, the droplet is filmy and the surface of the droplet is smooth (Figure 5 at 36% RH). When the RH increases to 40%, there is no obvious change for the droplet in a few minutes. With further extending the equilibration time, interesting changes of the surface morphology of the droplet are observed. At the first step, the surface of the droplet becomes scraggly (Figure 5 parts 1-3 at 40% RH). Within 7 min, the droplet rapidly turns half-ellipsoidal with a smooth surface (Figure 5 parts 4-7 at 40% RH) and the size of the droplet increases with equilibration time. These macroscopic morphology changes observed above show the characteristic of the hygroscopic properties of the MgSO4 droplets in the gel state. Structures of Water Molecules under Supersaturated State. There are many reports on the hydrogen bonding of water using Raman spectroscopy and FTIR, because the O-H stretching vibrations of water molecules are sensitive to the interactions between the water molecules and the hydrated ions, the hydrogen bonds between water molecules, and the effect of ions on the

Figure 5. Morphologic change in the humidifying process of the MgSO4 droplet. From 1 to 7 at RH ) 40%, the corresponding time is 10, 12, 13, 15, 16, 18, and 20 min, respectively. The unique graph at RH ) 36% is used for comparison.

hydrogen bonds.9,23,39-49 However, few investigations on the supersaturated solution have been reported. With the combination of Raman spectroscopy and EDB technique, the O-H stretching vibration band of water molecules in supersaturated droplets of NaClO4, Mg(ClO4)2, and LiClO4 was studied by Zhang and Chan.5 However, the MDRs from the spherical droplet limit the investigation on the O-H stretching vibration band of water molecules in (39) Wei, Z. F.; Zhang, Y. H.; Zhao, L. J.; Liu, J. H.; Li, X. H. J. Phys. Chem. A 2005, 109, 1337-1342. (40) Ratcliffe, C. I.; Irish, D. E. Can. J. Chem. 1984, 62, 1134-1144. (41) Hartman, K. A. J. Phys. Chem. 1966, 70, 270-276. (42) Mitterbo ¨ck, M.; Fleissner, G.; Hallbrucker, A.; Mayer, E. J. Phys. Chem. B 1999, 103, 8016-8025. (43) Senior, W. A.; Verrall, R. E. J. Phys. Chem. 1969, 73, 4242-4249. (44) Zangmeister, C. D.; Pemberton, J. E. J. Phys. Chem. A 2001, 105, 37883795. (45) Chen, Y.; Zhang, Y. H.; Zhao, L. J. Phys. Chem. Chem. Phys. 2004, 6, 537542. (46) Liu, J. H.; Zhang, Y. H.; Wang, L. Y.; Wei, Z. F. Spectrochim. Acta, Part A 2005, 61, 893-899. (47) Tian, Z. Q.; Chen, Y. X.; Mao, B. W.; Li, C. Z.; Wang, J.; Liu, Z. F. Chem. Phys. Lett. 1995, 240, 224-229. (48) Zou, S. Z.; Chen, Y. X.; Mao, B. W.; Ren, B.; Tian, Z. Q. J. Electroanal. Chem. 1997, 424, 19-24. (49) Chen, Y. X.; Tian, Z. Q. Chem. Phys. Lett. 1997, 281, 379-383.

Figure 6. Intensity ratio of the shoulder at the lower wavenumber side (3224 cm-1) and the main peak (3431 cm-1) in the dehumidifying (solid square) and humidifying (open circle) processes. The error bars of the intensity ratios at various RHs are added in the graph.

the MgSO4 droplets. In this research, the MgSO4 droplet on a quartz substrate is a half-ellipsoid rather than a perfect sphere. MDRs are eliminated while the supersaturated state can also be Analytical Chemistry, Vol. 77, No. 22, November 15, 2005

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Figure 7. Some species existed in supersaturated solutions of the MgSO4 droplets in (a) concentrated step, (b) monodentate CIPs step, (c) bidentate CIPs step, and (d) gel step with the red balls as oxygen atoms, gray balls as hydrogen atoms, yellow balls as sulfate atoms, and green balls as magnesium atoms. The water molecules as hydrogen atom donors in hydrogen bonds are circled with the thin lines for clarity.

achieved. Without MDRs, detailed and accurate information on the O-H stretching vibration band of water molecules in the MgSO4 droplet can be obtained easily, as shown in Figure 2. In pure liquid water, the O-H stretching envelope generally contains four components, attributed to an ice-like component (C1) at ∼3230 cm-1, an ice-like liquid component (C2) at 3420 cm-1, a liquid-like amorphous phase (C3) at ∼3540 cm-1, and monomeric water molecules (C4) at 3620 cm-1.50,51 The existence of metal or sulfate ions will change the O-H stretching envelope, because they affect the hydrogen bonding between the water molecules.5 As Figure 2a shows, a main peak at ∼3431 cm-1 (weak hydrogen bonding) and a shoulder at 3224 cm-1 (strong hydrogen bonding) are observed changing with RH in the dehumidifying process. The weak peak at about 3000 cm-1 marked with an asterisk in Figure 2 arises from the PE film. Figure 2b shows the O-H stretching envelope of the MgSO4 droplet in the humidifying process. A hysteresis of decomposition of CIPs in the humidifying process can be drawn out from the water-stretching envelope similar to that from the v1-SO42- band. Since free Mg2+ has a contribution to the strong hydrogen bonding components,39 I3224/I3431 should be sensitive to the change of the concentration of free Mg2+ that resulted from the formation of CIPs. With close investigation on the O-H stretching vibration band of water molecules in the MgSO4 droplets, the four steps mentioned above can also be observed obviously. Figure 6 shows the intensity ratio of the shoulder at the low wavenumber side (3224 cm-1) to the main peak (3431 cm-1) in terms of RH in the dehumidifying and humidifying processes. In the dehumidifying process, I3224/I3431 decreases slightly as RH decreases. An abrupt decrease is observed at 60% RH (point A in Figure 3) where the monodentate CIPs start to form. Then I3224/I3431 decreases more rapidly at the RH range from 50% (point B in Figure 3) to 40% (point C in Figure 3), in which more and more bidentate CIPs form. When the RH decreases to 4%, the ratio decreases to a minimum at the gel state. The hysteresis step is also observed in the humidifying process in the RH range from 4% to 40%. As shown in Figure 6, I3224/I3431 remains constant in the humidifying process below 40% RH. As the RH increases further from 40% RH, the ratio changes reversibly with respect to the dehumidifying process. Figure 7 shows some structures present in the supersaturated MgSO4 droplets related to the four steps. In dilute solution (step a), there are 6 water molecules in the first hydration layer of Mg2+ ions. Such a structure has been studied by Pye and Rudolph with the ab initio method.24 Mg2+ has a strong polarization effect on the water molecules around it, and hence, the hydrogen bonds (50) Walrafen, G. E. J. Chem. Phys. 1970, 52, 4176-4198. (51) Carey, D. M.; Korenowski, G. M. J. Chem. Phys. 1998, 108, 2669-2675.

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between the water molecules in the first hydration layer and those in the outer layer are strengthened (Figure 7a). Although SO42is also considered as a “structure-making” species which forms hydrogen bonds with water molecules,52 the hydrogen bonds between SO42- ions and water molecules are even slightly weaker than those between water molecules.53-55 Therefore, only Mg2+ is expected to noticeably strengthen hydrogen-bonding structures of water molecules in dilute MgSO4 solution with increasing MgSO4 concentrations. As a result, the O-H covalent bonds in the first hydration layer are lengthened because of the polarization effect of Mg2+. As expected, there is a positive contribution of free Mg2+ ions to the intensity of the Raman band at 3224 cm-1 at high RH, corresponding to dilute solutions (Figure 6). FTIRATR spectra of MgSO4 bulk solutions also show an absorbance increase at 3136 cm-1 as increasing its concentration, which is considered as the contribution of the hexahydrated Mg2+.39 When monodentate CIPs form (step b), the polarization effect of Mg2+ is reduced because of the direct contact between Mg2+ and SO42(Figure 7b). Therefore, the hydrogen bonds between the first hydration layer and the outer layer are weakened, and the O-H covalent bonds in the first hydration layer are shortened compared with those of hexahydrated Mg2+. These changes are responsible for the decrease of the ratio (I3224/I3431). When bidentate CIPs form (step c), the polarization effect of Mg2+ is weakened further (Figure 7c), ultimately leading to a continuous decrease of I3224/ I3431. At the gel state (step d), the polarization effect of Mg 2+ must have been largely eliminated, which makes the ratio achieve a minimum. The hysteresis in the humidifying process should be ascribed to the formation of the chain structures (Figure 7d), which inhibit mass transfer and suppress crystallization.10 CONCLUSIONS In this paper, a detailed investigation of the MgSO4 droplets on the quartz substrate in a dehumidifying-humidifying cycle is made by micro-Raman spectroscopy. This method has been proved to be very effective and practical in obtaining Raman spectroscopic information of CIPs. The spectral evolution of four vibration modes of SO42- evolving from free ions to monodentate and bidentate CIPs or more complex chain-structural compositions is observed with the high S/N spectra. The water envelope with structural information of the gel state is also obtained without MDRs, which were frequently encountered as a drawback in the EDB measurements. Although it is difficult to draw a quantitative conclusion on the formation and composition distribution of the CIPs, a more (52) Fran, H. S.; Wen, W. Y. Discuss. Faraday Soc. 1957, 24, 133-140. (53) Walrafen, G. E. J. Chem. Phys. 1971, 55, 768-792. (54) Walrafen, G. E. J. Chem. Phys. 1962, 36, 1035-1042. (55) Walrafen, G. E. J. Chem. Phys. 1964, 40, 3249-3256.

authentic analysis of I995/I983 and I1021/I983 for MgSO4 droplets can be used to explain the formation of CIPs and the hygroscopic properties of MgSO4 droplets. Especially, we have drawn out structural information in the irreversible dehumidifying-humidifying circle at a low RH range, i.e., there is a hysteresis in the decomposition of chain-structural CIPs in the humidifying process when RH is < 40%. Three transition points at RH ) 40%, 50%, and 60% are used as the boundaries of the four steps marked as concentrated step, monodentate CIPs step, bidentate CIPs step, and gel step in the dehumidifying and humidifying processes. Because of the formation of monodentate and bidentate CIPs, the polarization ability of Mg2+ to its surrounding water molecules decreases, and thus, the intensity ratio of the Raman band at 3224 cm-1 (the strong hydrogen bonding) to that at 3431 cm-1 (the weak hydrogen bonding) reduces with decreasing RH. Supersaturated MgSO4 droplets with tiny volume could be easily prepared by this means. It can be concluded that the

supersaturation of the droplets on the quartz substrate is ubiquitous, which have been confirmed in other metal-inorganic systems. It will generate much interest in the area of atmospheric science and solution chemistry. ACKNOWLEDGMENT This work is supported by the NSFC (20073004, 20473012), and the Trans-Century Training Program Foundation for the Talents by the Ministry of Education of China is also gratefully acknowledged.

Received for review May 28, 2005. Accepted September 13, 2005. AC050938G

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