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Pneumatic Atomization of Liquid Water and Characterization of Submicrometer-Sized Droplets by Fourier Transform Infrared Spectroscopy Hideto Matsuoka,†,‡ Shinji Sekiguchi,† Naoto Yagi,§ Katsuaki Inoue,§ Noboru Ohta,§ and Toshinori Suzuki*,†,| Chemical Dynamics Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan, Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, Katahira-2-1-1, Aobaku, Sendai 980-8577, Japan, Japan Synchrotron Radiation Research Institute (JASRI)/SPring-8, 1-1-1 Kouto, Sayo 679-5198, Japan, and Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Kyoto 606-8502, Japan ReceiVed: March 5, 2009; ReVised Manuscript ReceiVed: June 23, 2009
Pure water was sprayed by a pneumatic atomizer into a low-pressure gas cell, and the droplets were interrogated by Fourier transform infrared (FTIR) extinction spectroscopy in the region of 700-5000 cm-1. The spectral analysis using Mie scattering theory revealed that the particles have an average diameter of 170 nm and their phases are either supercooled liquid or ice, depending on the condition of atomization. Application of pneumatic atomization to an aqueous solution of BSA was also demonstrated. Introduction The structure of a single water molecule is simple. However, its assemblages exhibit complex structures and unusual properties because of the hydrogen bonding. The order and disorder of hydrogen bonding around a protein molecule are of paramount importance in biological sciences, and studies on anomalies of pure water and the properties of aqueous solutions offer the basis for their understanding.1 During the past decade, extensive spectroscopic studies have been carried out on hydrogen-bonded clusters in a molecular beam.2-9 These studies provided a microscopic view of the hydrogen bonding around a small solute molecule; however, in these small clusters, almost all molecules were on the surface, making the properties of the clusters somewhat remote from those of the bulk. For bridging the gap between small-sized clusters and bulk solutions, it is interesting to study large clusters or liquid droplets. Large clusters have been successfully created by condensation of water vapor in a low-temperature collision cell. Devlin and co-workers created a water cluster 12 nm in diameter and discussed crystallization based on the spectral feature of the OH stretching band in the FTIR spectrum.4,9 The crystalline solid was also identified for water aerosols larger than 500 nm produced in the same manner.10 On the contrary, clusters can also be produced by atomization of bulk liquid. Electrospray is a well-known method to atomize liquid; however, it is only applicable to electrolyte solutions. Pneumatic atomization is advantageous in that it is applicable to any liquid. In our previous work, we atomized aqueous solutions of carbohydrates by a pneumatic atomizer and characterized their sizes and structures of particles by FTIR spectroscopy and differential mobility analysis.11 Since aqueous droplets sprayed into the ambient air quickly dry out, it is necessary to control ambient * To whom correspondence should be addressed. Phone: (+81) 48-4671411. Fax: (+81) 48-467-1403. E-mail:
[email protected]. † RIKEN. ‡ Tohoku University. § Japan Synchrotron Radiation Research Institute. | Kyoto University.
humidity and/or temperature to maintain water molecules in the droplets. In the present work, we atomize liquid water in a lowpressure gas cell and use evaporative cooling of droplets to prevent drying. In addition to the three standard phases of solid, liquid, and gas, water has various metastable forms such as supercooled or glassy state (amorphous ice).12,13 The supercooled state is at a precarious equilibrium that can be easily destroyed by weak perturbations from dissolved or suspended impurities. In the absence of perturbations, supercooled water spontaneously crystallizes at about 233 K (the homogeneous nucleation temperature).14 If liquid water is rapidly cooled below 100 K at rates on the order of 106 deg/s under ambient pressure, it becomes a glass.15,16 Various phase transitions have been discussed for supercooled and glassy water.17-23 The existence of two liquid phases was inferred from the corresponding two phases of amorphous solid,19 and the evidence for the lowdensity liquid was reported very recently.24 The existence of the high- and low-density liquid phases may explain enhanced anomalies of water in the supercooled region.13,25 Experimental Methods Distilled water (Kanto Chemical Co.) was sprayed from a homemade air assist atomizer made of stainless steel SUS316 (18% Cr, 12% Ni, 2% Mo). As described in the previous work,11 the atomizer has a liquid discharging nozzle in the center, surrounded by a circular gas slit. The liquid and gas streams collide inside the atomizer and expand together from the final exit orifice. The diameters of the liquid discharging orifice and the final exit aperture were 20 and 40-70 µm, respectively. The distance of the liquid discharging orifice from the final exit orifice, called recess, was about 40 µm. The flow rate of water was controlled by an HPLC pump. The atomizing gas (dry N2) flow rate and pressures were about 0.5-0.9 standard liters per minute (slm) at the stagnation pressure of 1 MPa. FTIR spectra were recorded at a resolution of 0.6 cm-1 over a range from 700 to 5000 cm-1 with a vacuum FTIR spectrometer (Bruker VERTEX 80v) equipped with a Globar light source, a KBr beam splitter, and an MCT detector. The
10.1021/jp902026a CCC: $40.75 2009 American Chemical Society Published on Web 07/21/2009
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Figure 1. Schematic view of our experimental setup. Figure 3. FTIR spectra in the OH stretching region of droplets produced by atomizing liquid water, where the spectrum of water vapor was already subtracted. The liquid flow rate was 50 µL/min, the nebulizing gas pressure was 1 MPa, and pressures inside the chamber were (a) 1, (b) 12, (c) 60, and (d) 200 Torr.
Figure 2. FTIR spectra in the OH stretching region of ice (black),26 supercooled water (blue),26 liquid water (red), and water vapor (green).
optical compartment of the spectrometer was evacuated to eliminate atmospheric moisture absorptions. We employed a commercial multipass cell (Specac Ltd.) with a path length adjustable up to 10.6 m, except that the borosilicate glass body was replaced with our own aluminum chamber accommodating the atomizer nozzle and an efficient vacuum pumping line. The path length was fixed to the maximum value of 10.6 m. As shown in Figure 1, droplets were sprayed into the cell in the direction perpendicular to the IR beam propagating in the cell. Therefore, the effective optical path length for sampling droplets was considerably shorter than 10.6 m. We have also carried out small-angle X-ray scattering of water droplets atomized in atmospheric pressure at the BL40B2 beamline of SPring-8, which is described in the Supporting Information. Results and Discussion FTIR Spectra of Gas, Liquid, and Solid Phases of Water. Figure 2 shows the imaginary part of the complex refractive indices of ice (at 235 K),26 supercooled water (at 240 K),26 liquid water, and water vapor. The last two spectra at room temperature were measured in our laboratory. In the case of water vapor, symmetric and antisymmetric OH stretching and bending of the OH bonds appear at around 3650, 3740, and 1590 cm-1, respectively.27-29 Because of large rotational constants of water, rotational lines are well resolved for water vapor. On the other hand, condensed water exhibits much broader spectral features and red-shifts of the OH stretching bands. Notice that ice exhibits the sharpest band shape among the three bulk phases and also shoulders on both low and high frequency sides. The band widths of supercooled water (240 K) and liquid at room temperature are almost the same, suggesting that the degrees of structural order in these media are similar to each other. FTIR Spectra of Sprayed Water under Various Pressures. Figure 3 shows the FTIR spectra of water droplets sprayed from a pneumatic atomizer into a multipass cell maintained at various pressures. The pressure in the cell was controlled by changing the conductance of a pumping line. The water flow rate was 50 µL/min, and the atomization gas pressure was 1 MPa. The spectrum of water vapor has been already subtracted to show the bulk features clearly. As seen in the figure, a broad feature
of the OH stretching band, which is characteristic to bulk water, is identified below ca. 10 Torr. When water droplets are injected into the cell, water molecules evaporate from the surface and cool the droplets. Previously, Fisenko et al. sprayed water droplets using a twofluid nozzle into a low-pressure chamber maintained from 20 to 80 Torr and measured the temperature of the droplets in a carrier gas using a thermocouple.30 It was demonstrated that the temperature diminished for a lower chamber pressure and a smaller ratio of the mass flows of water and the carrier gas. They also calculated the expected temperature of the droplets using differential equations derived by Fuchs.31 According to the theory, the temperature of the droplet depends on many parameters such as the co-flow velocity, the liquid density, the partial pressure of water vapor, the saturated vapor pressure, the distance from the nozzle, the specific heat capacity of the liquid, the latent heat of the phase transitions, the carrier gas pressure, and the mass of a carrier gas molecule.30,31 The lowest temperature is estimated as follows:30
ps(T0)pt ≈ ps(Tf) pa
T0 Tf
(1)
where ps, pt, and pa are the saturated vapor pressure at temperature T0, the total pressure in the chamber, and the atmospheric pressure, respectively. Equation 1 was derived by assuming that the lowest droplet temperature Tf is achieved when the droplet radius becomes the minimum. Assuming an initial temperature of 298 K, Tf is estimated to be about 230 K at a total pressure of 1 Torr. The emergence of the bulk feature of the OH stretching bands below ca. 10 Torr in our experiment is well explained by evaporative cooling of liquid droplets. Mie Analysis of FTIR Spectra. Spectra a and b of Figure 4 show the FTIR spectra in the OH stretching region observed at liquid flow rates of 50 and 10 µL/min, respectively. In both cases, atomization gas pressure was 1 MPa, and the chamber pressure was maintained at 1 Torr. Comparison of spectra a and b of Figure 4 reveals that the bandwidth decreases and the peak position red-shifts in spectrum b. In addition, Figure 4b exhibits shoulders on both the red side (3150 cm-1) and the blue side (3400 cm-1). As shown in Figure 4c, a narrow bandwidth and shoulders appear more clearly at a further lower cell pressure (0.05 Torr). The different spectral features in spectra a and c of Figure 4 indicate that two different states of water are formed depending on the atomization and evaporation conditions. Clearly, the spectrum in Figure 4c reminds us of the spectrum of ice shown in Figure 2.
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Figure 4. FTIR spectra in the OH stretching region of droplets produced by atomizing liquid water, where the spectrum of water vapor was already subtracted. The nebulizing gas pressure was 1 MPa, the liquid flow rates were (a) 50, (b) 10, and (c) 50 µL/min, and the pressures inside the chamber were (a) 1, (b) 1, and (c) 0.05 Torr.
Figure 5. Observed FTIR spectra of submicrometer-sized water particles (black lines) and calculated spectra (red lines). Since the spectral regions 1200-2100 and 3500-4000 cm-1 suffer from severe interference from water vapor, they are not shown. The particles were produced by atomizing liquid water at a flow rate of (a) 10 and (b) 50 µL/min. The nebulizing gas pressure was 1 MPa. In the simulation, the refractive index of ice at 235 K26 was used for spectrum a, and that of supercooled water at 240 K was used for spectrum b.
The spectra for a much wider frequency range are shown as black lines in Figure 5, which exhibit base lines rising toward higher frequencies of light. This is clearly a consequence of light scattering. Although the water vapor spectrum was already subtracted in spectra a and b of Figure 5, the interference from water vapor was too large in the regions of 1200-2100 and 3500-4000 cm-1. Therefore, the spectra in these regions were removed for the sake of clarity. We analyzed these spectra using Mie theory.11,32 The size distributions of the particles were unknown, and we assumed them as log-normal distributions. By modifying the BHMIE program of Bohren and Huffman, we calculated the frequencydependent extinction and scattering cross sections weighted with an assumed size distribution.11,32 First, we carried out the spectral fitting using the refractive indices for ice. Three sets of complex refractive indices were reported for 200, 210, and 235 K,26 and we obtained the best-fit spectrum shown by the red line in Figure 5a by using the index for 235 K. The assumed geometric mean diameter and standard deviation were 170 nm and 2.7. Next, we carried out spectral fitting using the refractive indices for supercooled water. Four sets of complex refractive indices were reported for supercooled water at 240, 253, 263, and 273 K,26 and the best fit was obtained with the index at 240 K, as shown by the red lines in Figure 5b. The assumed geometric mean diameter and standard deviation were 170 nm and 2.4. Notice that these temperatures, 235 and 240 K, are comparable with the minimal temperature estimated by using eq 1. In several recent publications, Mie theory was employed to retrieve the size distribution of aerosols with average particle sizes smaller than 500 nm.11,33-35 On the other hand, some workers claim that similar extinction spectra can be obtained for different particle size distributions and that the retrieval is not unique.10,36 In the previous work, we examined the unique-
Matsuoka et al.
Figure 6. FTIR spectrum of sprayed aqueous solution of 1% BSA in D2O. A liquid flow rate was 50 µL/min, and pressure inside the chamber was 50 Torr. The broad band at around 2500 cm-1 is due to OD stretch, and its band width clearly indicates that the droplets are in supercooled liquid phase. Other bands are ascribed to BSA. Overlaid in red color is the spectrum in a bulk D2O solution of 25% BSA.
ness of size distributions obtained by Mie theory in comparison with the results obtained by differential mobility analysis.11 We found that an assumption of a log-normal distribution is reasonable, and the initial droplet size prior to drying was ca. 600 nm. Although the average diameter of 170 nm estimated above for pure water particle in a low-pressure cell may have some errors, it is reasonable in comparison with the primary droplet size of 600 nm. From these results, we conclude that the phase of the water droplets can be controlled by properly selecting the liquid flow rate and the cell pressure. Application to Aqueous Solution of Biological Sample. Figure 6 shows the FTIR spectrum of a sprayed aqueous solution of BSA (Bovine serum albumin, 69323.4 Da). To reduce spectral interference from H2O, we employed BSA 1% in D2O. Similar results were obtained for liquid flow rates of 50 and 200 µL/ min. A broad band at ca. 2400 cm-1 is due to O-D stretch of D2O, and its width clearly indicates that the droplets are in the supercooled liquid phase. Other features appearing around 1700-1300, 3000, and 3300 cm-1 are all assigned to vibrational bands of BSA. The intensity ratios between the D2O and BSA features were almost the same for 5% and 1% samples. The amide I (C-O stretching), amide II (N-H bending), and amide III (C-N stretching) bands are located at 1653, 1550, and 1250 cm-1, respectively. For comparison, FTIR spectrum of 25% BSA in D2O is shown in red color. The spectrum of bulk aqueous solution is in excellent agreement with the spectrum of sprayed liquid, further confirming that the sprayed liquid is in the liquid phase. The FTIR spectra of BSA observed by the KBr method or Nujor method (not shown here) were totally different from these, indicating that BSA was denaturated in the latter two methods. Broad bands at 3300 and 3000 cm-1 were assigned to the hydrogen bond stretching vibrations (N-H and O-H) and the C-H vibration, respectively. Conclusion Submicrometer-sized water particles with an average diameter of a couple of hundred nanometers were produced by pneumatic liquid atomization, and their FTIR spectra were analyzed by Mie theory. The analysis revealed that these droplets were either supercooled liquid or ice depending on the spraying conditions. The method was also applied to an aqueous solution of BSA, and the liquid phase droplets were observed by FTIR. The atomization technique demonstrated in this work will be of use for injecting supercooled aqueous solution into vacuum and also
Pneumatic Atomization of Liquid Water bringing hydrated proteins into the gas phase for X-ray light scattering, absorption, emission, and photoemission studies. Supporting Information Available: A description of in situ measurements of the droplet sizes by small-angle X-ray scattering (SAXS) using the third-generation synchrotron radiation facility (SPring-8). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Prielmeier, F. X.; Lang, E. W.; Speedy, R. J.; Lu¨dermann, H.-D. Phys. ReV. Lett. 1987, 59, 1128–1131. (2) Paul, J. B.; Collier, C. P.; Saykally, R. J.; Scherer, J. J.; O’Keefe, A. J. Phys. Chem. A 1997, 101, 5211–5214. (3) Paul, J. B.; Provencal, R. A.; Chapo, C.; Roth, K.; Casaes, R.; Saykally, R. J. J. Phys. Chem. A 1999, 103, 2972–2974. (4) Devlin, J. P.; Joyce, C.; Buch, V. J. Phys. Chem. A 2000, 104, 1974–1977. (5) Buck, U.; Huisken, F. Chem. ReV. 2000, 100, 3863–3890. (6) Keutsch, F. N.; Saykally, R. J. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 10533–10540. (7) Ko¨ddermann, T.; Schulte, F.; Huelsekopf, M.; Ludwig, R. Angew. Chem., Int. Ed. 2003, 42, 4904–4908. (8) Steinbach, C.; Andersson, P.; Kazimirski, J. K.; Buck, U.; Buch, V.; Beu, T. A. J. Phys. Chem. A 2004, 108, 6165–6174. (9) Buch, V.; Baurecker, S.; Devlin, J. P.; Kazimirski, J. K.; Buck, U. Int. ReV. Phys. Chem. 2004, 23, 375–433. (10) Clapp, M. L.; Miller, R. E.; Worsnop, D. R. J. Phys. Chem. 1995, 99, 6317–6326. (11) Matsuoka, H.; Sekiguchi, S.; Nishizawa, K.; Suzuki, T. J. Phys. Chem. A 2009, 113, 4686–4690. (12) Debenedetti, P. G.; Stanley, H. E. Phys. Today 2003, 56, 40–46. (13) Ludwig, R. Angew. Chem., Int. Ed. 2006, 45, 3402–3405. (14) Speedy, R. J. J. Phys. Chem. 1982, 86, 982–991. (15) Mayer, J. E. Appl. Phys 1985, 58, 663–667. (16) Bruggeller, P.; Mayer, E. Nature 1980, 288, 569–571.
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