J. Phys. Chem. B 2003, 107, 10829-10839
10829
Phase Transitions in Magnesium Nitrate Thin Films: A Transmission FT-IR Study of the Deliquescence and Efflorescence of Nitric Acid Reacted Magnesium Oxide Interfaces Hind A. Al-Abadleh and V. H. Grassian* Departments of Chemistry and Chemical and Biochemical Engineering, UniVersity of Iowa, Iowa City, Iowa 52242 ReceiVed: NoVember 26, 2002; In Final Form: June 17, 2003
In this study, transmission FT-IR spectroscopy is used to investigate the role of water in nitric acid uptake on MgO(100) and water uptake on nitric acid reacted MgO(100) at 296 K. Under dry conditions, nitric acid uptake is limited to the topmost surface layer and saturates at a nitrate coverage of (2.3 ( 0.1) × 1015 ions cm-2 to form a single layer of magnesium nitrate. In the presence of water vapor at 25% relative humidity (RH), subsurface layers can react and the extent of nitric acid uptake and the formation of magnesium nitrate is significantly enhanced on MgO(100) without evidence of saturation. Following reaction with nitric acid, water adsorption/desorption isotherms have been measured in the presence of 0.2-20 Torr water vapor pressure corresponding to 1-95% RH on surfaces with varying nitrate coverages. The infrared spectra clearly show two phase transitions in these thin nitrate films as a function of increasing RH. One transition occurs at low relative humidity, 1016 ions cm-2, the ERH shifts to a higher value for the thickest film. These differences may be related to the density of nucleation sites on the nearly flat singlecrystal surfaces of MgO(100) upon reaction with HNO3. The thinner nitrate film undergoes deliquescence but not efflorescence, most likely because of the large energy barrier associated with crystallization in the absence of nucleation sites. In a study by Lightstone et al.,31 the effect of inclusion of solid succinic acid on the deliquescence and efflorescence behavior of ammonium nitrate/water microdroplets was examined. Increasing the diameter of the succinic acid inclusions increased the efflorescence rate and thus decreased the supersaturation necessary for efflorescence. The increase in the number of potential nucleation sites is proportional to the increase in surface area as a result of increasing the diameter of succinic acid inclusions. Nucleation sites on the succinic acid surface catalyze the formation of the critical embryo by decreasing the nucleation barrier height. For the NH4NO3/succinic acid system, it was found that the number of molecules in the critical embryo calculated using the classical nucleation theory increases as the diameter of succinic acid inclusions increases. A similar type of analysis for the efflorescence of magnesium nitrate hexahydrate measured in this study is presented here. The relative number of sites in the critical embryo can be determined from the efflorescence data of the two thicker films. The critical supersaturation, S*, of aqueous magnesium nitrate with respect to the crystalline magnesium nitrate hexahydrate can be calculated using31,33
ln S* ) -
xH2O
ERH ∫DRH xNO
d ln RH
(5)
3
where xH2O/xNO3 is the mole ratio of water to nitrate in the solution phase. The value of S* can be determined from the area under the curve of xH2O/xNO3 calculated from the surface density of water molecules for each nitrate film as a function of decreasing RH versus - ln RH. The calculated values of S* are listed in Table 1 for each nitrate film. The relationship between S* and the number of molecules in the critical embryo, g*, is
W* 1 ) g* ln S* kT 2
(6)
where W* is the nucleation barrier and is a function of the
Figure 11. Bulk crystal structure of monoclinic Mg(NO3)2‚6H2O (a ) 6.19, b ) 12.7, and c ) 6.6 Å) and rock salt MgO (a ) 4.2 Å).
surface tension between the solid and the solution, the molecular volume in the solid, the contact angle between the critical nucleus of the solid and the surface, the temperature, and the critical supersaturation, k is Boltzman constant, and T is the temperature. The ratio of the number of molecules in the critical embryo formed for one nitrate film relative to another will lead to the cancellation of most of the terms with the assumption that the contact angle is the same for the nuclei formed on both films. Therefore, the resulting expression will be a relationship between g* and S*, as shown in eq 7:
( )
g1* ln S2* ≈ g2* ln S1*
3
(7)
For the efflorescence observed for the SNO3 ) (2.4 ( 0.1) × 1016 ions cm-2 and SNO3 ) (9.7 ( 0.5) × 1016 ions cm-2 films, the ratio of the natural log of the critical supersaturation value indicates that the number of molecules formed in the critical embryo on the thickest film is about 3-7 times greater than that on the thinner film and at least 20 times greater than that of the thinnest film. These values can be taken as estimates for the increase in the number of nucleation sites observed for thick nitrate films formed on the expense of the host MgO surface. The efficiency of heterogeneous nucleation of ammonium sulfate from supersaturated solutions due to the presence of mineral oxide particles has been discussed by Martin et al.34 It was found that the efficiency of germ formation for different oxide particles could be correlated to the crystallographic surface structure of the mineral oxide on which the germ grows. Figure
10838 J. Phys. Chem. B, Vol. 107, No. 39, 2003 11 shows the bulk crystal structure of both Mg(NO3)2‚6H2O and MgO. The crystal structure of Mg(NO3)2‚6H2O is monoclinic with cell parameters a ) 6.19, b ) 12.7, and c ) 6.6 Å35 whereas the structure of MgO is rock salt with a cell parameter of 4.2 Å3. In this study here, it is observed that MgO(100) does not nucleate Mg(NO3)2‚6H2O for thin films but does so for thicker films despite having a different crystalline structure. Thus, we conclude that the efflorescence of Mg(NO3)2‚6H2O is controlled by the density of defects and therefore the density of nucleation sites on the oxide crystal. Thicker nitrate films form from reaction of subsurface layers of MgO. This process increases the roughness of the flat single-crystal surface, as seen in the AFM images shown in Figures 8 and 9. An increase in the surface roughness will result in an increase in the density of nucleation sites, which acts to lower the nucleation barrier and leads to an ERH within ∼6% of the DRH for the thicker films. 3. Quantifying Nitrate and Water Coverages. The adsorbed nitrate and water coverages were calculated using eq 1 given in the Results. In this section, approximations made in the calculations and the greatest uncertainty in these calculations are discussed. Nitrate coverages were calculated using a value of the integrated cross section, σ j , for the nitrate absorption in the spectral range extending from 1200 to 1500 cm-1 determined for nitrate ion using ATR FT-IR spectra recorded for Mg(NO3)2 solutions. In these calculations the nitrate integrated cross section is determined relative to the known integrated cross section for the bending mode of liquid water. In the spectral range extending j is 5.73 × 10-18 cm molecule-1 for from 1520 to 1770 cm-1, σ 36 liquid water at 298 K. From several measurements of j ) (2.4 ( Mg(NO3)2 at concentrations near 1 M, a value of σ 0.1) × 10-16 cm ion-1, is determined. This calculation is similar to that done by Sporleder and Ewing in their work on sodium j ) (2.4 ( 0.1) × 10-16 cm nitrate solutions.17 The value of σ -1 ion is within 30% of that calculated for nitrate in NaNO3 solutions and NaNO3 solid. The nitrate ion coverage was determined only from the spectra recorded at a RH above the deliquescence of the film when the nitrate ion was in an aqueous environment similar to that of the standard ATR spectra used to calculate σ j . From the approximations used in these calculations, the absolute nitrate coverage may be as much as a factor of 2 different from that reported here. Adsorbed water coverages were determined using the integrated absorbance of the bending mode, δ(H2O). The bending mode of adsorbed water is sensitive to the water environment and the strength of hydrogen bonding.37 As discussed by Irish and Chang,20 polarization of water by the Mg2+ ion in Mg(NO3)2 solutions increases the molar intensity of the bending mode and shifts it to higher frequency. Therefore, the integrated cross section of the bending mode of water will be higher in the Mg(NO3)2‚6H2O system than in liquid water. The integrated cross section of δ(H2O) in this study has been approximated from the fact that the first spectrum recorded after deliquescence corresponds to a saturated solution of Mg(NO3)2 with a water to a nitrate molar ratio of 3:1. Thus, water coverage in molecules cm-2 at the deliquescence point can be obtained from the nitrate coverage calculated at that point. The integrated absorbance of δ(H2O) in the spectra recorded after deliqusence and eq 1 can be used to calculate σ j of δ(H2O), which is found to be equal to (2.77 ( 0.3) × 10-17 cm molecule-1. Figure 12 shows a plot of the water uptake on nitric acid reacted at MgO(100) for different nitrate coverages, (a) (2.3 ( 0.1) × 1015, (b) (2.4 ( 0.1) × 1016, and (c) (9.7 ( 0.5) × 1016 ions cm-2, as a function
Al-Abadleh and Grassian
Figure 12. Water adsorption isotherms at T ) 296 K as a function of relative humidity on nitric acid reacted MgO(100) (increasing RH, filled circles; decreasing RH, open circles). The water coverages were calculated using the bending mode and eq 1. See text for further details. Phase transitions for deliquescence and efflorescence are marked by solid and dashed lines, respectively, for films of different nitrate coverages: (a) SNO3 ) (2.3 ( 0.1) × 1015 ions cm-2; (b) SNO3 ) (2.4 ( 0.1) × 1016 ions cm-2; (c) SNO3 ) (9.7 ( 0.5) × 1016 ions cm-2. The water adsorption/desorption isotherm on unreacted MgO(100) is also shown in (a), note that there is significantly less water taken up by the unreacted surface.
of relative humidity (increasing RH, filled circles; decreasing RH, open circles). For the thickest films, the region above 60% RH is not shown. This is because the films were so thick that the absorbance of the liquid film was beyond the linear range of Beer-Lambert’s law. The approximation made in calculating the water coverage is that the integrated cross section for the bending mode is constant as a function of relative humidity. The data shown in Figure 12 are plotted in terms of SH2O in units of molecules cm-2 and monolayers, θ. For surface coverage in monolayers, the amount of adsorbed water has been normalized to the MgO(100) ion pair density, 1.13 × 1015 ions cm-2. Water uptake on the unreacted MgO(100) is also shown in Figure 12a and is in agreement with published water adsorption isotherms on MgO(100).7 The data show that the water adsorption properties of the MgO(100) surface after reaction with nitric acid are significantly altered. Conclusions The heterogeneous reaction of nitric acid with MgO(100) under dry and wet conditions has been investigated using FTIR spectroscopy. The presence of gas-phase water enhances the uptake of nitric acid on the surface and results in further reaction of subsurface layers. The enhanced reactivity is most likely due to an increase in the ionic mobility of Mg2+ and NO3- in the presence of adsorbed water whereby magnesium nitrate can form
Phase Transitions in Magnesium Nitrate Thin Films crystallites on the host oxide exposing additional sites for further reaction. Water adsorption experiments on the reacted MgO surface as a function of increasing relative humidity reveal two phase transitions. These transitions correspond to the conversion of a metastable amorphous phase of magnesium nitrate and surface adsorbed water to a crystalline hydrate salt, Mg(NO3)2‚ nH2O (4 < n e 6). The hexahydrate salt forms for all films as the RH increases. The deliquescence of the hexahydate salt occurs at 49 ( 2% RH for thin films and 54 ( 2% RH for thicker films. A hysteresis is seen as a function of decreasing relative humidity, and the transition corresponding to efflorescence depends to an even greater extent on film thickness. Efflorescence is not observed for the thinnest magnesium nitrate films studied. This study shows that phase transitions in thin films, tens to hundreds of nanometers thick, have very different behaviors than found in the bulk phase. Information gleaned from the experiments reported here may be insightful in understanding surface reactions and surface properties of interfaces present at ambient conditions. Acknowledgment. We thank Charles Schutte for providing the ATR FT-IR data for magnesium nitrate solutions and Jean Ross and Randy Nessler for their help in collecting AFM images at the Central Microscopy Research Facility. We gratefully acknowledge the National Science Foundation for support of this work (CHE-9988434). References and Notes (1) Blesa, M.; Morando, P.; Regazzoni, A. Chemical Dissolution of Metal Oxides; CRC Press: Boca Raton, FL, 1993. (2) Stumm, W. Chemistry of the Solid-Water Interface: Processes at the Mineral-Water and Particles-Water Interface in Natural Systmes; John Wiley and Sons: NewYork, 1978. (3) West, A. R. Basic Solid State Chemistry, 2nd ed.; John Wiley & Sons Ltd.: New York, 2000. (4) Stefanovich, E. V.; Truong, T. N. J. Chem. Phys. 1995, 102, 5071. (5) Giordano, L.; Goniakowski, J.; Suzanne, J. Phys ReV. Lett. 1998, 18, 1271. (6) Lyden-Bell, R. M.; Site, L. D.; Alavi, A. Surf. Sci. 2002, 496, L1. (7) Foster, M.; Furse, M.; Passno, D. Surf. Sci. 2002, 502-503, 102. (8) Johnson, M. A.; Stefanovich, E. V.; Truong, T. N.; Gu¨nster, J.; Goodman, D. W. J. Phys. Chem. B 1999, 103, 3391.
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