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J. Phys. Chem. C 2007, 111, 8030-8036
Interaction of NaF, NaCl, and NaBr with Amorphous Ice Films. Salt Dissolution and Ion Separation at the Ice Surface Jung-Hwan Kim, Young-Kwang Kim, and Heon Kang* Department of Chemistry, Seoul National UniVersity, Gwanak-gu, Seoul 151-742, Republic of Korea ReceiVed: January 8, 2007; In Final Form: April 11, 2007
We have studied the adsorption and dissolution phenomena of sodium halide salts (NaF, NaCl, and NaBr) at the surface of amorphous D2O-ice films for the temperature range 105-140 K by the techniques of lowenergy sputtering, reactive ion scattering, and temperature-programmed desorption mass spectrometry. These salts readily dissociate to ions at the ice surface. The dissociated Na+ and F- ions migrate from the surface to the interior at temperatures above 125 K, while Cl- and Br- ions stay at the surface, leading to the spatial separation of ions near the ice surface. Br- shows a slightly higher propensity for residing at the surface than Cl-, most likely due to its less efficient solvation by surface water molecules. The ion separation process is driven by thermodynamic forces specific to the ions. The speed of ion migration depends strongly on ice temperature, indicating that the ion motion is limited by the diffusion of water molecules.
1. Introduction Understanding the distribution of electrolyte ions near the surface of aqueous solutions is important for industrial and environmental heterogeneous processes. Reactions at the surface of sea-salt aerosols and snow are considered to play a significant role in the production of active halogen species in the lower atmosphere.1,2 Traditionally, the surface of aqueous salt solutions was considered to be devoid of ions, an interpretation based primarily on surface tension measurements.3 This conventional view has faced severe challenges from recent investigations of aqueous salt solutions using molecular dynamics simulations4-10 and various surface-sensitive experimental techniques,11-17 as well as the studies of water clusters in the gas phase.18 These research endeavors have greatly improved our understanding of aqueous solution surfaces, from which a consensus appears to have been reached for the refined molecular picture:7 small, nonpolarizable ions such as Na+ and F- exhibit a classical behavior in which the concentration increases from the ionfree surface toward the bulk. On the other hand, monovalent large anions such as Br- and I- have an enhanced concentration at the interface. For ice surfaces, however, comparable studies have been relatively scarce until very recently.19-26 An interesting question is whether the ion specificity observed for liquid water surfaces exists for ice surfaces as well. Liquid water and ice crystals have very different solubilities for foreign ions, and therefore the distribution of ions near the surface of crystalline ice may differ from that for liquid water. On the other hand, an amorphous ice formed by vapor deposition at low temperature, often called amorphous solid water, has structural similarity to liquid water, and its certain thermodynamic properties are at metastable extension from liquid water.27 It is also speculated that the solvation and diffusion processes of simple ions in an amorphous ice might take similar routes to those in liquid water.20 Ion distribution near the surface[s] of amorphous ice has been studied by Kempter and co-workers21-25 for alkali * To whom correspondence should be addressed. Phone: +82 2 875 7471. Fax: +82 2 889 8156. E-mail:
[email protected].
halide salts (NaCl, NaI, CsF, CsCl, and CsI) using the techniques of metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy (UPS). They observe that the I- ion segregates to the ice surface,23-25 but alkali ions and Fand Cl- ions do not.21-25 In our previous study,19,20 we examined the adsorption states of NaCl on amorphous ice films using the reactive ion scattering (RIS) and low-energy sputtering (LES) methods and observed that Na+ is depleted from the ice surface at temperatures above 120 K, while Cl- stays at the surface. Experimentally, amorphous ice films offer some advantages over liquid water for the investigation of the salt dissolution process and the resulting ion distribution. An amorphous ice film can be prepared in a vacuum, and its surface can be examined with various surface science tools. In the lowtemperature environment of amorphous ice, the ion motion inside the film is extremely retarded and can even be virtually “frozen” during the experimental measurements.28 Under such circumstances, it is possible to directly analyze the composition of ice film surfaces with surface spectroscopic techniques and to take a snapshot of the progress of ion movement. In the present work, we adsorb alkali halide salts (NaF, NaCl, and NaBr) on amorphous ice films and examine their dissolution and ion segregation processes with LES and RIS techniques, which measure the chemical composition of the ice surfaces by mass spectrometric detection. 2. Experimental Section The experiment was carried out in an ultrahigh vacuum (UHV) surface analysis chamber28,29 equipped with instrumentations for LES, RIS, temperature programmed desorption (TPD), and Auger spectroscopy (AES). Ice films were prepared on the (0001) face of a Ru single crystal mounted onto the temperature control stage of a sample manipulator. The substrate temperature was variable in the range of 90-1500 K, which was monitored by a thermocouple spot-welded to the crystal. The Ru surface was cleaned by repeated cycles of surface oxidation in an O2 environment (5.0 × 10-7 Torr, 10 min, 700 K), Ar+ sputtering at 2 keV, and annealing at 1200 K. The surface cleanness was checked by AES and RIS. A nonporous, amorphous ice film
10.1021/jp0701587 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/15/2007
NaF, NaCl, and NaBr and Amorphous Ice Films was deposited onto the Ru surface maintained at 130 K by backfilling the chamber with a D2O pressure of 1.0 × 10-8 Torr, as read by an ionization gauge without calibration for specific gases. The liquid D2O sample was degassed by freezevacuum-thaw cycles. The thickness of the ice films was deduced from the area of the water desorption peak in the TPD experiment.30 Sodium halide salts were deposited on the ice film from thermal evaporators charged with powders of NaF (Aldrich, 99.99% purity, mp ) 1266 K), NaCl (Aldrich, 99.999% purity, mp ) 1074 K), or NaBr (Aldrich, 99.99+ % purity, mp ) 1028 K). The operating temperature of the evaporator was 850 (NaF), 700 (NaCl), and 650 K (NaBr). The salt deposition time was controlled by a shutter installed between the evaporator and the substrate. The surface coverage of deposited salts was estimated from RIS and TPD measurements. For the LES and RIS experiments, a Cs+ beam from a lowenergy ion gun (Kimball Physics) was scattered at the ice film surface with its incident energy chosen between 20 and 35 eV and a beam current density of 2-4 nA/cm2. Both positive and negative ions emitted from the surface were detected by a quadrupole mass spectrometer (QMS; ABB Extrel) with its ionizer filament switched off. The background noises in the spectra were effectively eliminated by switching off the stray ion sources in the chamber. The beam incidence and detector angles were both fixed at 45° with respect to the surface normal. The detected ions were composed of reflected Cs+ primaries, RIS products that were association products of Cs+ with neutral molecules at the surface, and LES ions which were the preexisting ions ejected from the surface by the Cs+ impact. The mechanisms of RIS and LES processes on thin ice films have been explained.28,31 LES and RIS have a sampling depth of one bilayer (BL) of ice surface in the employed energy range.32 Surface contamination by incident Cs+ ions was suppressed by minimizing the spectral acquisition time and, whenever necessary, by preparing a fresh sample for the measurement. The TPD experiment was done with a QMS that was used also for LES and RIS. 3. Results 3.1. Adsorption and Dissolution of Sodium Halides on Amorphous Ice Films. An amorphous ice film was prepared by condensing D2O vapor on a Ru (0001) substrate at 130 K to the thickness of 4 BL, and a sodium halide salt (NaX, where X ) F, Cl, or Br) was deposited onto the ice surface to a submonolayer coverage after the film temperature was lowered to 120 K. The chemical states of the adsorbed salts were then examined by LES and RIS. LES spectra in Figure 1 show the ionic species present at the surface of D2O-ice films adsorbed with NaF (parts a and b of Figure 1), NaCl (Figure 1c), and NaBr (Figure 1d). In the positive ion spectrum of Figure 1a, Na+ (m/z ) 23 amu/charge), NaD2O+ (m/z ) 43), and NaD4O2+ (m/z ) 63) peaks represent the emission of Na+ and its hydrated species from the surface. These ions are due to the emission of preexisting species at the surface, formed by spontaneous dissociation and ionization of NaF to Na+ and F- at the ice surface at 120 K. Cs+ impact may also induce the salt ionization and the secondary ion emission, but this contribution to the spectra should be negligible for the present beam energy.28 The negative ion spectrum from the same surface (Figure 1b) shows the emission of F- (m/z ) 19) and a small amount of FD2O(m/z ) 39). This result confirms the ionic dissociation of NaF at the surface. The strong emission of Na+ and F- suggests that the surface populations of these ions are substantial at 120 K.
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Figure 1. (a) LES spectrum of positive ions emitted from a D2O-ice film on which NaF was deposited at 120 K to a coverage of 0.8 monolayer (ML). (b) Negative ion LES spectrum from the same film as in (a). Negative ion LES spectra from D2O-ice films on which NaCl (c) or NaBr (d) was deposited to a coverage of 0.8 ML at 120 K. The salt coverage was determined by TPD measurement, to be described in section B, and the coverage represents the sum of the amounts of both ionized and molecular salt forms. The ice films were 4 BL thick and prepared in an amorphous structure by condensing D2O vapor at 135 K. The ice film temperature was then lowered to 120 K for the deposition of salt and the LES measurement. The time interval between the LES measurement and the salt adsorption was 3 min. The Cs+ beam energy was 35 eV.
The spectra in parts c and d of Figure 1 show the negative ions ejected from ice films adsorbed with NaCl and NaBr, respectively, at 120 K. The strong Cl- and Br- peaks indicate the spontaneous ionization of these salts upon adsorption, and their residence at the surfaces. The positive ion spectra from these films (not shown) have features basically identical to those in Figure 1a. Na+ shows a distinctly higher propensity for being ejected as hydrated cluster ions than the halide anions in LES (Figure 1). Among the halide anions, F- shows a somewhat higher intensity of hydrated ions than Cl- and Br-. The hydrated ion signals for Cl- and Br- are almost invisible on the intensity scale of Figure 1. The yield of hydrated ions must be related to the binding energy of ions to water molecules as well as to the degree of ion solvation at the ice surfaces. The binding energy between these ions and a water molecule in the gas phase is 86.6 (Na+), 105.0 (F-), 54.8 (Cl-), and 48.1 kJ/mol (Br-) according to theoretical calculations.33,34 Also, the solvation efficiencies of these ions differ at the ice surface, as will be shown in section B; Na+ and F- are more efficiently solvated than Cl- and Br- at the ice surfaces, such that Na+ and Foccupy subsurface positions. Figure 2 shows an RIS spectrum obtained from an NaFadsorbed D2O-ice film, prepared under the same conditions as that for parts a and b of Figure 1. The peak at m/z ) 133 amu/charge indicates the reflected Cs+ primaries, and the peaks at m/z ) 133 + 20n (n ) 1-4) are due to the pickup of surface
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Figure 2. Cs+ RIS spectrum from a D2O-ice film on which NaF was deposited to a coverage of 0.8 ML at a temperature of 120 K. The ice film conditions were the same as those given in the caption of parts a and b of Figure 1. The CsNaF+ peak is shown in the magnified intensity scale.
D2O molecule(s) by Cs+. The Cs(D2O)n+ peaks are accompanied by satellite peaks appearing at masses lower by 1-2 amu/charge, which are due to adsorption of H2O molecules in the residual gas. A small peak appearing at m/z ) 175 amu/charge, which is visible only in the magnified intensity scale in Figure 2, indicates the presence of molecular NaF at the surface. The surface population of this molecular NaF is about 0.01 ML, as estimated from the relative intensities of CsNaF+ and CsD2O+ signals assuming the same RIS detection efficiency for NaF and D2O. This result indicates that most of the adsorbed NaF dissociates to Na+ and F- on the surface, in agreement with the observation of strong Na+ and F- peaks in the LES spectra of Figure 1. The CsNaF+ signal disappears when the ice temperature is raised above 125 K or when the amount of NaF deposition is small (1 BL) will be called the “interior”, in contrast to the “surface” which indicates only the first bilayer of ice films. In Figure 3b, which is obtained from NaBr-adsorbed ice films, the behavior of the sodium ion intensity is similar to that in
Figure 3. (a) Temperature-dependent variation of LES intensities for sodium (O) and fluoride ions (b) measured from NaF-adsorbed D2Oice films. (b) The behavior of sodium (O) and bromide ions (2) measured from NaBr-adsorbed D2O-ice films. The coverage of the salts was 0.8 ML on D2O-ice films with 4 BL thickness. The LES measurements were made at the indicated temperatures of salt adsorption, 3 min after the sample preparation. The Cs+ beam energy was 35 eV. The error bars indicate the fluctuations in repeated measurements.
Figure 3a, showing a decrease with increasing temperature. In contrast, the bromide ion intensity remains at the same level for all temperatures (105-140 K). This indicates that Br- stays at the surface without inward migration even at high temperatures, resulting in separation of the Na+ and Br- ions near the ice surface. A similar behavior has been observed from the NaCl adsorption experiment on ice films,20 in which Cl- stays at the surface, whereas Na+ disappears from the surface. Apparently, F- behaves differently from Cl- and Br-, penetrating through the surface layer as Na+ does. It might be suspected that the reduction in the intensities of Na+ and F- in Figure 3a was due to the desorption of NaF from the ice film at the high temperatures. To check this possibility, we removed water molecules from the NaF-deposited ice film by thermal desorption and examined the species left on the Ru surface. Figure 4a shows the RIS spectrum measured after heating the substrate to 300 K, well above the temperature at which water molecules completely desorb from a Ru(0001) surface (200 K)30 or from a NaCl(100) surface (270 K).35 The RIS spectrum shows a CsNaF+ peak at m/z ) 175 amu/charge, indicating that NaF remains as a molecule on the Ru surface after water desorption. The peaks at m/z ) 156 (CsNa+) and 174 amu/charge (CsNaOD+) indicate the presence of neutral Na and NaOD, respectively. These species can be formed when Na+ is neutralized to a Na atom upon contact with a Ru surface, and the neutralized Na adsorbate reacts with D2O to form NaOD.20,36 The very strong Cs+ signal in the spectrum relative to RIS products is characteristic of the RIS process on metal surfaces.32,37 The amount of salt deposited on an ice film was estimated from the TPD experiment. Figure 4b shows a TPD spectrum
NaF, NaCl, and NaBr and Amorphous Ice Films
Figure 4. (a) RIS spectrum obtained after removing D2O molecules from a NaF-deposited D2O-ice film by heating the sample to 300 K. The original ice film was prepared in the same way as in parts a and b of Figure 1. The Cs+ peak shown is reduced by the factor indicated. (b) Top (solid curve): TPD curve for NaF desorption measured from an ice film adsorbed with 0.8 ML of NaF. Middle (dotted curve): TPD measurement from a Ru(0001) surface on which NaF is directly adsorbed to the coverage of 0.8 ML. Bottom (dashed curve): TPD measurement from a Ru(0001) surface with a NaF coverage of 1.3 ML. The desorption peak from the NaF multilayer appears around 810 K, and the first-layer desorption peak, around 970 K. The temperature ramping rate was 3 K s-1.
(solid curve, top) for NaF desorption from a D2O-ice film on which NaF is adsorbed to a specific coverage (which was determined to be 0.8 ML by the procedure described below). NaF desorption is monitored by the Na+ signal of QMS, instead of the NaF+ signal which is extremely weak due to the instability of the molecular ion.38 The TPD spectrum shows NaF desorption at 950-990 K, well above the temperature of water desorption (
