Why Do Sulfuric Acid Coatings Influence the Ice Nucleation Properties

LETTER pubs.acs.org/JPCL

Why Do Sulfuric Acid Coatings Influence the Ice Nucleation Properties of Mineral Dust Particles in the Atmosphere? Zheng Yang, Allan K. Bertram, and Keng C. Chou* Department of Chemistry, University of British Columbia, Vancouver, BC V6T 1Z1, Canada

bS Supporting Information ABSTRACT: Laboratory studies with supermicrometer particles have shown that mineral particles coated with sulfuric acid are relatively poor ice nuclei. We investigated this phenomenon, which is of atmospheric relevance, by probing the structure of water at the mineral
aqueous acid interface as a function of the sulfuric acid concentration using sum frequency generation vibrational spectroscopy. We found that ordered water structures at water/mica interfaces drastically diminished at molarities of sulfuric acid equal to 0.5 M and totally disappeared when the molarities reached 5 M. The decrease in ordered water structures at the interface was caused by a combined effect of the decreased mica surface potential at low pH, the adsorption of sulfates on mica, and the lack of free water molecules in high concentrations of acidic solution. The good ice nucleation ability above liquid water saturation is correlated with the presence of structured water, suggesting that structured water at the interface may be needed for efficient heterogeneous ice nucleation. SECTION: Atmospheric, Environmental and Green Chemistry

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ineral dust particles are abundant in the atmosphere. Common minerals found in aerosolized dust include quartz, Illite, muscovite, chlorite, kaolinite, and calcite.1
11 During their lifetime in the atmosphere, mineral dust particles can become coated with inorganic material such as sulfuric acid.12
14 Recently, several studies have investigated the effect of sulfuric acid coatings on the ice nucleation properties of mineral dust particles.4,15
20 Laboratory studies with supermicrometer particles have shown that uncoated mineral particles such as Illite and kaolinite are good ice nuclei both below and above water saturation.21,22 However, once the supermicrometer particles are coated with sulfuric acid, they are only good ice nuclei at close to and above 100% relative humidity (RH) for temperatures around 235 K.16,19 Experiments with submicrometer mineral particles also show that particles coated with sulfuric acid are poor ice nuclei below water saturation but can act as ice nuclei above water saturation (although the ice nucleation ability may be reduced above water saturation compared to the uncoated case due to the permanent loss of some active sites).20 This difference between uncoated and coated mineral dust particles in terms of ice nucleation ability is illustrated in Figure 1a for the specific case of supermicrometer Illite particles from the work of Chernoff et al.19 Figure 1b and c shows the pH and H2SO4 concentration of the coating as a function of RH, calculated from the Aerosol Inorganic Model.23 A comparison of panel a, b, and c illustrates that coated mineral dust particles are poor ice nuclei when the coating material has a low pH (less than 1) and a high acid concentration (H2SO4 concentration > 0.1 M). r 2011 American Chemical Society

Despite the observation that sulfuric acid coatings influence the ice nucleation properties of mineral dust particles, a molecular-level understanding is lacking. A molecular-level understanding is preferred so that laboratory results can be confidently extrapolated to the atmosphere. In the following, we investigate why sulfuric acid coatings influence the ice nucleation properties of mineral dust particles at molarities > 0.1 M. We probed the structure of water at the mineral aqueous acid interface as a function of the sulfuric acid concentration using IR
visible sum frequency generation (SFG) vibrational spectroscopy, which is a highly surface specific optical technique for studying liquid/solid interfaces.24 We specifically focused on differences in water structures for concentrations less than and greater than 0.1 M. For these studies, mica was chosen for the mineral interface because clean flat mica surfaces are relatively easy to obtain by cleaving a mica sheet, and mica has a surface structure similar to Illite, which is reported to make up more than 50% of dust in some regions.25 Figure 2 shows the SFG vibrational spectra of deuterium oxide (D2O)/mica interfaces with D2SO4 concentrations of 0, 5  10
6, 5  10
5, 5  10
4, 5  10
3, 5  10
2, 5  10
1, and 5 M. Deuterated water and sulfuric acid were used to avoid the IR absorption peak of mica at 3620 cm
1.26 The spectra of D2O exhibit two peaks located near 2375 and 2550 cm
1, which represent the peaks at 3200 and 3400 cm
1, respectively, for Received: March 10, 2011 Accepted: May 2, 2011 Published: May 06, 2011 1232

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Figure 1. (a) Schematic showing the range of RH values over which uncoated and coated mineral dust particles are good ice nuclei. The figure is specifically created for Illite particles based on the work of Chernoff et al.19 A similar behavior was also observed for kaolinite. The temperature applicable for the ice nucleation studies was approximately 237 K. (b, c) pH and the molarity of the sulfuric acid coating, assuming that the coating is in equilibrium with the RH.

interfacial H2O.27 The red shift of the D2O peaks with respect to those of H2O is a result of the isotopic substitution. These two peaks are sometimes labeled as the “ice-like” and the “liquid-like” peaks, indicating that their peak positions are similar to those of bulk ice and liquid water, respectively.24,28 It has been proposed that the peak at 3200 cm
1 represents water in a more ordered hydrogen-bonding network, and the 3400 cm
1 peak represents a less ordered hydrogen-bonding structure.24 More recent studies have suggested that these two peaks originate from vibrational coupling between the stretching and bending overtone, rather than from structural effects.30 Although the detailed structure of the interfacial water is still under debate,29,30 it is generally accepted that the interfacial water molecules observed by SFG are more ordered than those in the bulk because SFG is largely depressed in a disordered medium. To obtain a more quantitative analysis, the spectra in Figure 2 were fitted with Lorentzian line shape I(ωSFG) µ |χ(2) NR þ ∑q [Aq/(ωIR
ωq þ iΓq)]|2, where χ(2) NR is the nonresonant contribution, Aq is the amplitude, Γq is the width, and ωq is the resonant wavenumber for the qth vibrational mode. The fitting curves are shown in Figure 2 (solid lines), and the fitting amplitudes and resonant wavenumbers are shown in Figure 3. The best fits were obtained with a negative Aq for the 2375 cm
1 peak and a positive Aq for the 2550 cm
1 peak. These assignments are consistent with the phase-sensitive SFG measurements at low pH reported by Ostroverkhov et al.31 As shown in Figures 2 and 3a, when the concentration of D2SO4 increased, the intensities of both peaks decreased. With a D2SO4 concentration of 5 M (pH ≈
0.7), both peaks completely vanished, indicating that the ordered hydrogen-bonding network of water

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Figure 2. SFG spectra of D2O/mica interfaces with D2SO4 concentrations of (a) 0, (b) 5  10
6, (c) 5  10
5, (d) 5  10
4, (e) 5  10
3, (f) 5  10
2, (g) 5  10
1, and (h) 5 M. The inset shows the schematic layout of the spectroscopic setup. The beams were s-, s-, and p-polarized for SFG, visible, and IR, respectively.

Figure 3. The fitted amplitudes (a) and frequencies (b) of water peaks in the SFG spectra (Figure 2) with various D2SO4 concentrations.

no longer existed. Figure 3b shows that the frequencies of the water peaks are pH-dependent. In general, both the surface 1233

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The Journal of Physical Chemistry Letters potential of mica and dipole
dipole coupling between water molecules can shift the frequency,32 but studies of the frequency shifts are beyond the focus of the current study. The drastic reduction of the ordered water structures with D2SO4 concentrations of 0.5 M is consistent with previous observations by Eastwood et al. and Chernoff et al. indicating that H2SO4-coated mineral surfaces with H2SO4 concentrations > 0.1 M are poor ice nuclei (see Figure 1).16,19,22 The results suggest that the ordered surface water structures may have played an important role in the ice nucleation process. Although the drastic reduction and eventual disappearance of the ordered water structures offers a microscopic explanation for why H2SO4coated mineral surfaces are poor ice nuclei, the question that remains to be answered is why the ordered surface water structure is drastically reduced with a high acid concentration. On the basis of our current understanding, there are three possible effects responsible for the disappearance of ordered water structures, (a) the decrease of mica surface potential, (b) the adsorption of sulfates on the surface, and (c) solvations of sulfates in water. These effects are discussed below. It is known that a decrease in the surface potential leads to a less-ordered water structure on the material surface.33
36 The surface potential on a mineral surface is pH-dependent. The structure of muscovite mica consists of octahedral hydroxylaluminum sheets lying between two silicon tetrahedral layers. One in four silicon atoms is substituted by an aluminum atom in the silicon tetrahedral layer, and the substitution results in a negative charge that is neutralized by Kþ ions located between the silicon tetrahedral layers. When mica is cleaved, a cleavage plane happens in the potassium layer. The Kþ ions are equally distributed between the two surfaces, and the surface is overall neutral.37 When placed in water, hydrated potassium ions dissociate from the mica surface,38,39 and the surface, which consists of Si, Al, and O connected by Si
O and Al
O bonds, become negatively charged and then partially neutralized by Hþ ions in water. The surface charge of mica is therefore dependent on the pH of the solution. Overall, the surface charge decreases when the pH value decreases. The phenomena have also been observed on other mineral surfaces.40 Measurements for the point of zero charge (PZC) of mica were not conclusive, but it is generally accepted that the PZC of muscovite mica is less than 3.41 One of the difficulties in measuring the PZC for mica is the increased solubility of lattice aluminum ions at low pH,42 which creates negative surface charges on the surface. However, it was clearly observed that the increased concentration of hydrogen ions at low pH decreased the surface potential of mica, which can be attributed to the protonation of the Si
O groups at the surface to form ionizable surface silanol groups.41 Therefore, the decrease of the surface potential on mica at lower pH is one of the mechanism responsible for the decrease of the water peaks in Figure 2. When the mica surface approaches neutral or positively charged, the adsorption of anions on the mica surface may also affect the structure of water. It is known that the adsorption of sulfate on a mineral surface is mainly associated with Al and Fe oxyhydroxides and with allophanic constituents.43,44 Sulfate adsorption generally increases when the pH decreases.43,45,46 The vibrational peaks of sulfate are too weak to be measured directly by the current optical setup. To further study whether the disappearance of the ordered water structure was related to any particular properties of sulfate, the same experiment with 5 M hydrochloric acid (HCl) was carried out for comparison.

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The experiment showed that the SFG peaks of water also disappeared with 5 M HCl solution. Therefore, the vanishing of the ordered water structures was likely not linked to any particular properties of sulfates. However, the anions interact with the mineral surface when the surface approaches neutral or becomes positively charged. With a high concentration of H2SO4, the solvations of sulfate anions (SO42
) become an important factor affecting the water structure on mica. Previous ab initio studies by Cannon et al. showed that 13 water molecules are present in the first solvation shell of SO42
.47 With a H2SO4 concentration of 5 M, the water
sulfate mole ratio is roughly 8:1. At this concentration, the mica surface must compete for water with the sulfate ions in the bulk solution. The phenomenon can affect ice nucleation processes in two ways. First, the number of water molecules available to the mica surface is reduced. Second, as the mica and sulfates are competing for water molecules, most water molecules are well captured either by the surface charges on mica or by the anions in the solution. The situation creates an energy barrier for ice nucleation because formations of ice nuclei would have to overcome the electrostatic interactions to rearrange the water molecules. When the concentration of sulfuric acid increases, the above three mechanisms all work against ordering of water molecules on the mica surface. When the pH decreases, the surface potential decreases, and consequently, the ordering of interfacial water decreases. As the surface potential decreases, the adsorption of anions on the mica surface becomes significant, and the adsorbed anions displace ordered water molecules on the surface. Finally, when the concentration of sulfuric acid reaches a critical concentration, in which the solvation of sulfate ions consumes a large amount of water, the ordered water structure disappears. At this stage, nearly all water molecules are captured by the anions, and few water molecules are freely available to the mica surface. The process is illustrated in Figure 4, showing water molecules and hydrated sulfate (bisulfate) anions at solution/mica interfaces with increasing concentrations of H2SO4. A possible alternative explanation for the ice nucleation results to the one discussed above involves the capillary effect. The presence of sulfuric acid may influence the capillary attraction between mineral particles, which may lead to a change in the total surface area of the mineral particles and the environment for ice nucleation. On the basis of surface area arguments, this seems unlikely. In the supermicrometer ice nucleation studies referenced above, the freezing results were independent of surface area, which was varied by a factor of 150.48 Hence a change in surface area from capillary action is not expected to change the freezing results significantly. However, it is difficult to completely rule out a capillary hypothesis based on the previous research. Studies that investigate ice nucleation on coated and uncoated single mineral particles (i.e., no agglomerates) would be useful for ruling this hypothesis. In the atmosphere, the sulfate coating on mineral dust particles may also be partially or completely neutralized. As a result, coatings of NH4HSO4 and (NH4)2SO4 are also relevant and may play a role in heterogeneous ice nucleation.15,16,49 Studies in the future should investigate the water structure at the mineral
aqueous interface for these coatings to gain a complete understanding of the effects of coatings on ice nucleation properties in the atmosphere. Furthermore, coatings of organics have been shown to influence the ice nucleation properties of mineral dust particles and are atmospherically relevant.50 Studies with these coatings would also be interesting. 1234

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Figure 4. Schematics of water molecules and hydrated sulfate (bisulfate) ions at solution/mica interfaces with different concentrations of H2SO4 solutions. (a) Pure H2O/mica interface with pH ≈ 6. The surface is highly negatively charged. Water molecules are more ordered near the charged surface. (b) With a low concentration of H2SO4, the surface charge decreases as the surface is more protonated, and the anions interact with the mineral surface. (c) With a high concentration of H2SO4 (for example, 5 M), water molecules are well captured by the sulfate (or anions) in the solution, and few water molecules are freely available for the mica surface. In (a) and (b), the dashed lines separate the ordered water molecules from the disordered water molecules in the bulk solution. Above the dashed lines, water molecules have good order because of the negative surface charges on the mica surface. In (b) and (c), Hþ adsorbs on the mica surfaces. The dashed circles represent hydrated sulfate ions (in low H2SO4 concentration) or hydrated bisulfate anion (in high H2SO4 concentration). Water molecules in the dashed circle are parts of the hydrated anions and move together with the core anions.

In summary, the structures of water on mica surfaces in the presence of H2SO4 with atmospheric-relevant concentrations were studied using SFG vibrational spectroscopy. We found that ordered water structures significantly decreased with 0.5 M H2SO4 and disappeared with 5 M H2SO4. The study provided a molecular-level understanding for previous laboratory studies showing that minerals particles coated with sulfuric acid are relatively poor ice nuclei. The observed phenomenon was explained by a combined effect of the decreased mica surface potential at low pH, the adsorption of sulfates on mica, and the lack of free water molecules in high concentration of acidic solution.

’ ASSOCIATED CONTENT

bS

Supporting Information. Experimental setup and procedures. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was financially supported by the Natural Sciences and Engineering Research Council of Canada (NSERC). ’ REFERENCES (1) Pruppacher, H. R.; Klett, J. D. Microphysics of Clouds and Precipitation. 2nd revision and Englihs ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands and Boston, MA, 1997. (2) DeMott, P. J.; Sassen, K.; Poellot, M. R.; Baumgardner, D.; Rogers, D. C.; Brooks, S. D.; Prenni, A. J.; Kreidenweis, S. M. African Dust Aerosols As Atmospheric Ice Nuclei. Geophys. Res. Lett. 2003, 30, 1732. (3) Hung, H. M.; Malinowski, A.; Martin, S. T. Kinetics of Heterogeneous Ice Nucleation on the Surfaces of Mineral Dust Cores Inserted into Aqueous Ammonium Sulfate Particles. J. Phys. Chem. A 2003, 107, 1296–1306.

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