Amorphous AmmoniaâWater Ice Deposited onto Silicate Grain

Article pubs.acs.org/JPCC

Amorphous Ammonia−Water Ice Deposited onto Silicate Grain: Effect on Growth of Mantles Ice on Interstellar and Interplanetary Dust Elizabeth Escamilla-Roa*,†,‡ and C. Ignacio Sainz-Díaz‡ †

Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain Instituto Andaluz de Ciencias de la Tierra (CSIC-UGR), Avenida de las Palmeras 4, 18100 Granada, Spain

‡

ABSTRACT: The interaction of molecules on the mineral surface is interesting in understanding the development of icy mantles on interstellar and interplanetary dust. The ice grains can freeze and cover the silicate cores, growing an amorphous ice mantle. In the interstellar medium, olivine is a silicate that has been found in many places in dust. Previously we have simulated the interaction between amorphous water ice and forsterite surface. In this work we describe a more realistic situation, by adding ammonia molecules in a model of amorphous dirty ice onto forsterite surface. The NH3 is a part of the volatile components of cometary and interstellar ices. We propose models that describe a mixture of amorphous ice (ammonia−water) and forsterite (100) surfaces (dipolar and nondipolar). Our quantum mechanical calculations show that the ammonia has a similar affinity (30 kcal/mol) to the forsterite surface as that of water (31 kcal/mol). We calculated also the infrared frequencies to characterize the most reactive sites in the chemisorption processes. We observed important frequency shifts related to the position of the main vibrational modes of the NH3 moieties, which react chemically with the mineral surface. infrared know as stoichometric water−ammonia solid: NH3· H2O (monohydrate), NH3·2H2O (dihydrate), and 2NH3·H2O (hemihydrate). These hydrates have crystalline ordering, and other nonstoichometric NH3·H2O ratio complexes can exist.19,20 Besides, in the interstellar medium (ISM), previous authors postulated that the water−ammonia mixture can be close to a 3:1 ratio with the presence of dust grains.4 Olivine is the most abundant of the minerals that constitute the dust grains and has been detected spectroscopically showing up as prominent emission bands in the mid-infrared.21 Olivine consists of magnesium, iron, and silicate forming a complete solid solution between two end members: forsterite (Mg-rich olivine) and fayalite (Fe-rich olivine). Forsterite has an orthorhombic structure in the Pbnm space group with the crystallographic parameters of a = 4.756, b = 10.207, c = 5.980 Å with γ = β = α = 90°22 and consists of one independent SiO4 tetrahedron linked by divalent cations with octahedral coordination. Forsterite has been investigated experimentally and theoretically.22−25 There is evidence that the (100) surface is reactive for water adsorption25,26 and the sites with Mg atoms with a low coordination have been associated with high reactivity.27 The aim of this work is to understand the interactions that occur on the surface of dust grains with ammonia and water molecules to generate an amorphous ice. Several theoretical

1. INTRODUCTION The interstellar and interplanetary ice grains are formed by CO2, CO, NH3, H2O, CH3OH, and silicates. These ice grains are frozen in silicate cores as amorphous ice. The interaction of the molecules on the mineral surface leads to the development of icy mantles in which the mineral surface can be consider as ice nuclei. These molecules can be adsorbed directly onto the mineral surface or form first a mixture of multicomponent ice and then be adsorbed onto the mineral surface. Several astrophysical and theoretical models have considered that the first layer (gas−surface mineral interphase) has great importance in forming the icy mantle.1−4 Ammonia was the first polyatomic molecule detected in space5 and may be an important repository of nitrogen in interstellar and cometary dust.6 This molecule was detected in the nuclei of comets and represents the 1% level relative to water ice,7 and it is possible that its content can be enhanced in deeper areas of ice bodies.8 In the solar system it has been found in atmospheres of Jupiter,9 Saturn,9 Uranus,10 Neptune, and Titan.11 Also it has been detected by near-infrared (NIR) in various ice bodies such as Enceladus,12 Miranda,13 and Charon.14 Hydrated ammonia was also observed in Kuiper Belt objects. 15 Amorphous ammonia ice can be formed at temperatures below 50 K16 by deposition of the vapor on a cold substrate. Spectral signature has been used to identify the environment in which ammonia is formed. A great number of experiments have been used to study several mixtures of ammonia with other molecules of interest in astrophysics.17,18 There are three ammonia hydrate types detected in the mid© 2014 American Chemical Society

Received: October 24, 2013 Revised: January 23, 2014 Published: January 25, 2014 3554

dx.doi.org/10.1021/jp4105147 | J. Phys. Chem. C 2014, 118, 3554−3563

The Journal of Physical Chemistry C

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matrix. In order to estimate the contribution of the weak van der Waals interactions in the adsorption complexes, the correction with the semiempirical formalism of Grimme32 for the weak dispersion forces was included in some optimizations. The boxes of amorphous mixtures of ammonia/water ices were generated using the amorphous cell builder modulus based on Monte Carlo simulations with geometry optimization for refinement, using the compass force field at 298 K and a density of 1 g/cm3 implemented in the Materials Studio.31 The surface energy per area unit (γ) was calculated as a measure of the thermodynamic stability of the surfaces as follows33,34

astrophysical models have studied the possible interaction of ice mantle and grain from a macroscopic viewpoint.3,4 We propose to study any possible scenarios to grow amorphous ammonia− water ice in the solid−gas interphase. In the first model we consider the growth as a layer stacking, in which the interaction of H2O and NH3 with grain is through a thin layer of amorphous ice. In a second model we consider a mixture of water−ammonia that interacts at the same time with grain surface. Our models are based on electronic structure calculations by means of the density functional theory (DFT), which allows us to describe the interaction in the interphase gas−solid. Also we can interpret and compare the infrared spectroscopic (IR) features with experimental values.

γ=

2. MODELS AND METHODOLOGY The mineral dust grain will have different possible surfaces. Besides, within one surface several terminations can also be exposed in the external surface. However, we consider the surface (100) as a representative model; there is experimental and theoretical evidence that shows that this surface is one of the most reactive on forsterite mineral.23 We consider that any amorphous ice will interact faster with this surface. After the cleavage, different terminations can exist. Also we could consider the existence of other terminations and defects that probably will exist in the real dust grains, but this exhaustive study is out of the scope of this work. One of the main differences between terminations of the (100) surface is the polarity. Hence, we have chosen the difference in polarity as example models for reactivity, and we can consider two possible and different (100) surfaces terminations: a dipolar one and a nondipolar one. The dipolar surface has a dipole moment perpendicular to the surface.25 In our previous work we studied adsorption processes onto dipolar and nondipolar (100) surfaces of forsterite, and we detailed the pristine mineral surface and main geometrical features, such as Mg−O and Si− O bonds, and the different Mg atoms that occur when the surface is created. These undercoordinated atoms have a strong relationship with reactivity on mineral surface.28,29 We propose two models to study a more realistic scenario with amorphous water−ammonia ice and two forsterite (100) surfaces. In this model we use molecules of ammonia and water to simulate the effect of amorphous ice adsorbed on mineral surface. We analyze the adsorption energy from one to five ammonia molecules. In the second model, we assume that a mixture of amorphous water−ammonia ice is set to interact with the mineral surfaces. To optimize the geometry parameters and energies of the bulk, clean surface and adsorbed complex, we used first principles calculations, based on density functional theory (DFT) methods with the generalized gradient approximation (GGA) and the PBE exchange correlation functional30 and the Dmol3 program, implemented in the Materials Studio (MS) package.31 The electronic calculations were made with a double-ζ basis set augmented with polarization functions (DNP). The orbital cutoff quality was determined with a 0.1 eV atom−1 of energy accuracy threshold. We used DFT semicore pseudopotentials (DSPP). The convergence criterium for the self-consistent field was 1 × 10−6. The geometry optimizations of different structures were performed at 0 K. This methodology has been previously used to describe the adsorption process successfully.28,29 The harmonic vibration frequencies for the principal complex adsorbed were calculated diagonalizing the mass-weighted second-derivative Hessian

Eclean‐surface − E bulk A

(1)

where γ is the surface energy, Eclean‑surface is the energy of the surface of the crystal, Ebulk is the energy of an equivalent number of bulk ions, and A is the surface area. In the adsorption processes, the geometry of the adsorbed molecule (ammonia and water molecules) was optimized alone within a periodical box with the same size of the adsorption complex periodic system at constant volume. This molecule was also optimized with the periodical surface of mineral at constant volume with the same crystal cell parameters. Analogously, in the surface with adsorbate, the surface energy γads will be γads =

E surface+adsorbate − Ebulk − nE adsorbate A

(2)

where Esurface+adsorbate is the total energy of the relaxed surface with the adsorbate, Ebulk refers to the energy of the bulk crystal with equal number of atoms and chemical species to the surface model, and nEadsorbate is the energy of the n adsorbate molecules. To obtain the adsorption energy, we use the following equation ads Ex/forst = −(E x/forst(100) − E forst(100) − nEadsorbate) x/forst(100)

forst(100)

(3)

adsorbate

where E ,E , and nE are the total energies of the forsterite (100) surface with the X-chemical species (complex adsorbed) on the surface, the pristine forsterite surface, and the total electronic energies of the n chemical species involved, respectively. All chemical species were adsorbed on a 1 × 2 × 1 supercell of the clean dipolar and nondipolar surfaces of forsterite (100).

3. RESULTS AND DISCUSSION 3.1. Surface Models of the Silicate Grains. The (100) surface was created by cleaving parallel to the axis a of the fully optimized forsterite bulk crystal lattice, atom positions, and crystal lattice, yielding crystal cell parameters of a = 4.81, b = 10.35, and c = 6.06 Å with α = β = γ = 90°. These parameters are very close to the experimental values.22 In the simulation of (100) surfaces we considered a 1 × 1 × 2 supercell. After the cleavage of the surfaces, a certain vacuum was generated by enlarging the axis perpendicular to the surface (the axis a of the pristine crystal) until reaching a constant surface energy. In our previous work we reported the geometrical parameters for the surface stacking sequences which produce two surfaces: dipolar and nondipolar terminations of the (100) surface. According to the work of de Leeuw et al. (2000), in the cleavage to form these surfaces tetrahedra should be maintained intact. Then the breaking of the Si−O bonds produces a less stable surface. Both 3555



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urations of the NH3 molecule with respect to the surface: with the H atoms oriented to the vacuum, to the surface, and parallel to surface. In all cases for the optimized complex, the ammonia molecule is adsorbed through the interaction of electron lone pair of N atom with the Mg atom of surface forming a coordination bond whereas the H atoms are oriented to the vacuum zone (Figure 2). In the dipolar surface, the N atom was

surfaces are in a 3-D box and have a thickness of 32.18 Å on the axis that is normal to the surface including a vacuum spacing of 20.0 Å. The initial surface corresponds to a surface area 125.44 Å2 and the crystalline parameters u = 10.35 Å, v = 12.12 Å for both surfaces. The slab generated for these surfaces contains five SiO4 horizontal planes with alternate Mg atoms. After optimizations, the surface energies were 2.27 and 2.59 J/m2, for dipolar and nondipolar surfaces, respectively. These values are in good agreement with the previous theoretical values of 2.25 and 2.57 J/m2 reported for Watson et al. (1997) and de Leeuw et al. (2000) for this mineral. The slab of the dipolar surface has several undercoordinated Mg atoms in the exposed surface: fourfold (4f), threefold (3f), and twofold (2f) (Figure 1a). In the nondipolar relaxed surface, the Mg atoms

Figure 2. Adsorption of one molecule of NH3 on (a) dipolar D-fors/ 1NH3 and (b) nondipolar fors/1NH3 surface.

coordinated to the twofold Mg site forming H bonding interactions with the surface O atoms at 2.31 and 2.47 Å (Figure 2a). With this adsorption, an arrangement of atoms of the mineral surface was observed and the fourfold Mg atom changed to a fivefold coordination. The strong H bond of ammonia with the surface O atom affects the Si−O bonds and one Si−O−Mg oxygen coordinated with another deeper 4f Mg, forming a 5f Mg. In the nondipolar surface, the N atom was coordinated to the fourfold Mg site forming a 5f Mg cation. In this complex, the interaction of the ammonia H atoms with the surface O is weaker with distances of 3.04−3.11 Å (Figure 2b). Notice that the adsorption energy of this ammonia molecule is different for each surface, being 29.3 kcal/mol higher in the dipolar surface than in the nondipolar one. This difference suggested to us to explore the effect of the adsorption site of the top Mg atoms on the adsorption process. This study helps us to map the surface in the different reactive sites for the adsorption. Then, three sites of surface Mg with different coordination were chosen for the adsorption. In Table 1 we show the adsorption energy and the Mg−N bond distances for the ammonia adsorbed complexes on different Mg sites for both types of surface. Dipolar surface yields strong adsorption energy when NH3 is adsorbed on more undercoordinated Mg atom (2f); this suggests that this site is energetically more favorable for the adsorption process. On the other hand, the adsorption energy corresponds to a chemisorption where a

Figure 1. Pristine forsterite (100) surface: (a) dipolar and (b) nonpolar. O, Si, and Mg atoms are displayed as red, yellow, and green color, respectively. The coordination of the Mg atoms on surfaces is as follows: fourfold (4f and 4f*), threefold (3f), and twofold (2f).

have different coordinations: fourfold (4f), threefold (3f), and fourfold on top (4f*) (Figure 1b). For both surfaces, the geometrical parameters of Mg−O and Si−O distances28 are in good agreement with previous results of de Leeuw et al. (2000) and Watson et al. (1997). 3.2. Mineral Surface Covered with a Thin Layer of Ammonia Amorphous Ice. For the adsorption studies, the (100) surfaces were optimized for a relaxed slab of three SiO4 planes, and the rest of the plane was fixed, yielding a relaxed slab of the surface. We started with the study of the interaction of one ammonia molecule onto mineral surface (dipolar and nondipolar) to determine the adsorption energy and the interaction sites on surface. One ammonia molecule was placed randomly over the surface at 4 Å from the surface and after the whole complex was optimized. We explored several config3556



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Table 1. Adsorption Energy (kcal/mol) for One Ammonia Molecule per Simulation Cell and Mg−N Bond Coordination of the Sites Mg Undercoordinated on the Mineral Surface adsorption site

dipolar surface

Mg-coordinated

Eads

Mg−N (Å)

2f 3f 4f 4f*

59.3 (43.2)a 37.0

2.15 2.16

52.7

2.15

a

nondipolar surface Eads

Mg−N(Å)

33.5 30.0 (34.3)a 26.1

2.16 2.22 2.18

Optimized with Grimme correction.

chemical bond Mg−N is formed, in which the Mg−N distance is the shortest one. When the NH3 is adsorbed on the 4f-Mg site which is on the top of surface 4f*, the adsorption energy is also high but lower than the former one, owing to the fact that in this site there are O atoms exposed that form hydrogen bonds with the ammonia H atoms. Owing to the coordination of the N atom, the Mg cation loses the coordination with one O atom due to the atom arrangement during the adsorption. However, when the ammonia molecule is adsorbed on a 3f Mg site, the sorption energy is lower than the former ones, being less reactive for the sorption. In this case there is no significant interaction between the ammonia H atoms and the surface O atoms, explaining the low adsorption energy. In the nondipolar surface the higher adsorption energy occurs on the 3f Mg site where the Mg is more undercoordinated. The ammonia molecule forms H bond with a top O atom at 1.990 Å. When the adsorption is on 4f Mg sites, the adsorption energy is lower than on 3f Mg atom. Nevertheless, all three adsorption sites do not show high energy differences in adsorption energy, and they are always lower than in the dipolar surface. There is not a direct relationship between the adsorption energy and the Mg−N distance because the H bonds are also significant in some cases. For both surfaces, our results are consistent with previous theoretical works where the low-coordinated sites are highly reactive.27−29 However, when applying the Grimme correction for the weak dispersive forces, the adsorption energy was smaller in the dipolar surface and greater in the nondipolar than without this correction (Table 1). This can indicate that this semiempirical correction cannot be applied in our systems probably because the adsorption energy is very large. To estimate the number of molecules adsorbed on the surface, we employed the adsorption energy criterion. In Figure 3 we show the adsorption energy from one to seven ammonia molecules onto surfaces, in both cases dipolar and nondipolar surfaces, and the energy tends to converge to a constant value beyond the addition of the fifth NH3 molecule. Similar behavior was obtained in the adsorption of water molecules on forsterite surface previously.29 For both surfaces, the energy/coverage plots are similar, the values of the dipolar surface being greater. Consequently, as in our previous work on amorphous water ice,29 we consider five ammonia molecules that correspond to a coverage of four molecules/nm2 for dipolar and nondipolar surface, respectively, which is consistent with previous theoretical and experimental works.23,25,35,36 To determine the stability of the surface with the NH3 adsorbed, we calculated the surface energy of the adsorption complexes as eq 2. Previous theoretical works reported that this measure has a relationship with the adsorption processes (physisorption and chemisorption).23,33,37−39 We observed that

Figure 3. Adsorption energy profile of the ammonia molecules that cover the (100) surfaces of forsterite (dipolar and nondipolar).

in both surfaces the coverage with ammonia molecules stabilized the surface energy in a similar way as the hydration processes (dipolar and nondipolar surfaces).23,29 When the adsorption energy increases, the energy surface decreases (Figure 4). For the dipolar surface, the surface energy decreases from 2.27 J/m2 in clean surface to 1.45 J/m2 when five NH3 are adsorbed (Figure 4a); this value is similar to our previous work of five water molecules adsorbed in associative mode in dipolar surface (1.3 J/m2).29 In the case of the nondipolar surface, the surface energy decreases from 2.59 to 1.99 J/m2. Comparing both surfaces covered with five NH3, we observed that energy surface is different in each surface, i.e., dipolar surface is stabilized at 0.82 J/m2, while nondipolar surface is stabilized at 0.60 J/m2. This difference is in good agreement with the adsorption energy when the dipolar surface is the most reactive one. In both surfaces a linear relationship between the surface energy and the adsorption energy was observed with high correlation constant, the slope being higher for the dipolar surface. With the coverage of five NH3 molecules, we simulated the first monolayer of amorphous ammonia ice adsorbed on two mineral surfaces, dipolar (D-fors/5NH3) and nondipolar (fors/ 5NH3). The NH3 molecules are adsorbed in an associative process; i.e., the NH3 molecules remained complete and are adsorbed by the lone pair electrons of N atom (Figure 5). Onto the dipolar surface, when five NH3 molecules are randomly placed simultaneously above the surface at 4 Å and optimized, we found a configuration of minimum energy in which two nitrogen atoms of the NH3 molecules interact with the surface through a coordination bond Mg−N at 2.12 Å with 2f and 3f Mg sites (Figure 5c), and the rest of the NH3 molecules are physisorbed by hydrogen bonds between the H atoms of ammonia and the O atoms of surface and N atoms of vicinal adsorbates (D-fors/5NH3a). The ammonia adsorbed on the 2f Mg site forms two H bonds with the O atoms of surface at 1.96 and 2.24 Å and another one with the N atom of a vicinal physisorbed ammonia at 2.06 Å with an NHN angle of 166°. This adsorption changes the atomic arrangement of the surface where the pristine 2f Mg is displaced coordinating with another O atom obtaining a three-coordination that, with the adsorbate, becomes a 4f Mg (Figures 1 and 5c). The ammonia adsorbed on the 3f Mg site does not form significant H bonds with the surface O atoms; however, one of its H atom forms a strong H 3557



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Figure 4. Effect of the coverage of NH3 molecules on the adsorption energy and surface energy: (a) dipolar and (b) nondipolar surface.

Figure 5. Most stable positions of five ammonia molecules adsorbed onto mineral surfaces: (a) dipolar D-fors/5H2O (1 × 2 × 1 view), (b) nondipolar fors/5H2O surface adsorbed sequentially one by one, and (c) dipolar surface randomly adsorbed (D-fors/5H2Oa).

bond with one N atom of a vicinal adsorbate at 1.85 Å with an NHN angle of 166°. This indicates that the adsorption on a reactive Mg site enhances the adsorbate to be a good H donor for hydrogen bond. From the rest of the adsorbates, two molecules have no significant contact with the surface but only with the vicinal adsorbates forming strong H bond with the chemisorbed molecule at 2.06 Å and with another ammonia molecule at 2.13 Å. The remaining physisorbed ammonia molecule forms a weak H bond with the surface O atom at 2.38 Å and a strong H bond with the H atom of the another chemisorbed adsorbate at 1.85 Å. The geometrical parameters give an average Mg−N distance of 2.12 Å. However, adding the ammonia molecules sequentially one by one as in our calculations described in Figure 5a, an adsorption complex with five ammonia molecules can be obtained where all adsorbates are coordinated with the surface Mg sites at an Mg−N distance of 2.13−2.21 Å (D-fors/5NH3). One adsorbed ammonia forms H bonds with the top O atoms of Si−O groups at 1.86−2.16 Å. These O atoms are 2f coordinated with Mg and Si in the pristine surface. However, with this adsorption this oxygen loses the coordination with Mg and becomes more

coordinated with ammonia molecules. This adsorption complex is much more stable than the above one (Table 2) due to the higher number of ammonia coordinated with Mg atoms with a higher adsorption energy (151 kcal/mol) than in the above one (94 kcal/mol) (Table 2). In the case of nondipolar surface (Figure 5b), all NH3 molecules were adsorbed on the 3f, 4f, and 4f* Mg sites (fors/5NH3). The conformations of the ammonia molecules tend to balance the repulsive forces between the vicinal H atoms and attractive forces between the H atoms and the top surface O atoms. The adsorption energy is lower than in the dipolar surface (Table 2). This result is in good agreement with previous theoretical result in the coordinated system Mg− N.28,40−45 Previous work with only water showed associative and dissociative interaction with surface and hence a higher adsorption energy (124.2 kcal/mol)29 than with only five ammonia adsorption (108.1 kcal/mol). 3558



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Table 2. Adsorption Energy of the Models of Ammonia and Amorphous Water−Ammonia Ice Absorbed on Two (100) Forsterite Surfaces: Dipolar (D-fors) and Nondipolar (fors) system

adsorption energy (kcal/mol)

D-fors/5NH3 fors/5NH3 D-fors-5H2O/1NH3 fors-5H2O/1NH3 D-fors-5NH3/1H2O fors-5NH3/1H2O D-fors/3H2O-NH3 fors/3H2O-NH3 fors/6H2O-2NH3

150.8 (94.1)a 108.1 20.7 23.2 24.9 22.6 132.8 106.0 207.0

a Value in brackets correspond to the (D-fors/5NH3a) complex with a simultaneous adsorption.

3.2.1. Sequential Adsorption of Amorphous Ammonia/ Water Ice onto Mineral Surfaces. In the interstellar medium (ISM), the presence of ammonia has been detected.5 However, the ammonia molecules can be in gas phase, adsorbed on ice− water molecules or onto mineral surfaces present in interstellar dust. There are several theories that suggest that the NH3 can interact with dust grain or with amorphous water ice. Two possible scenarios in the ISM can occur: that the ammonia molecule interacts by layers or as a mixture water−ammonia ice. In the “layer” models we consider two mechanisms: one ammonia molecule adsorbing onto mineral surface (dipolar and nondipolar) covered by a thin layer of amorphous water ice or one water molecule adsorbing onto a thin layer of amorphous ammonia−ice that covers a mineral surface. In order to describe the interaction of the ammonia molecule with a thin water monolayer, we optimized a model with five H2O molecules adsorbed onto mineral surface.29 We placed five water molecules randomly distributed in a layer at 4 Å over the clean forsterite surfaces, a dipolar and a nondipolar one, and both complexes were optimized. In both kinds of surface, chemisorption was produced, where the negatively charged O atoms of water try to bind to the Mg cations of the surface. In the dipolar surface all water molecules are adsorbed by an associative way, where the water O atoms are bonded to the surface Mg cations with an average O−Mg distance of 2.09 Å. Some H bonds between the water molecules keep the adsorption complex stable. In the nondipolar surface, the water molecules are adsorbed by both mechanisms, associative and dissociative. Three water molecules are bonded to surface Mg cations, and another water molecule is not directly adsorbed onto the surface but is bound by H bonds to the vicinal water molecules. Another water molecule is dissociated, where the H atom protonates a SiOMg bond and the OH group is bonded to a Mg cation. One ammonia molecule was placed at 4 Å over the water layer adsorbed onto the forsterite surfaces, dipolar and nondipolar, and these complexes were also optimized. In the dipolar surface the ammonia is adsorbed onto mineral surface forming a Mg−N bond of 2.2 Å and H bonds with vicinal water molecules and topmost surface O atoms (Figure 6a) (D-fors5H2O/1NH3). On the other hand, in the nondipolar surface the ammonia adsorbate is bonded to the water molecules forming H bonds between the N atoms and the water H atoms. The arrangement of the water molecules on the mineral surface did not change during the ammonia adsorption (Figure 6b) (fors-5H2O/1NH3). The adsorption energies of one ammonia

Figure 6. Sequential adsorption of amorphous water ice and one NH3 molecule: (a) dipolar D-fors-5H2O/1NH3 and (b) nondipolar fors5H2O/1NH3 surface.

molecule on the dipolar and nondipolar hydrated surface are exothermic (Table 2). Within this sequential mechanism, the adsorption of one water molecule on the mineral surface previously covered by amorphous ammonia ice can be also considered. In the ammonia covered dipolar surface, the water adsorbate is bonded directly to one Mg (D-fors-5NH3/1H2O) (Figure 7a), whereas in the nondipolar surface the water molecule is joined by H bonds to the surface O atoms and the ammonia molecules (fors-5NH3/1H2O) (Figure 7b). In both cases, the adsorption energy is similar to that of ammonia onto the hydrated surfaces, being slightly higher in D-fors-5NH3/1H2O (Table 2). This suggests that the interactions between the ammonia molecule and the amorphous water ice covered substrate and vice versa are weaker (20.7−24.9 kcal/mol) (Table 2) than when this ammonia or water molecule is adsorbed directly onto the clean mineral surface (26.1−59.3 kcal/mol) (Table 1), because the new adsorbate belongs to a second layer that is more weakly bonded to the mineral surface than the first layer. This is consistent with previous theoretical and experimental studies.23,29,46−48 3.2.2. Simultaneous Adsorption of Amorphous Ammonia/ Water Ice onto Mineral Surfaces. Another possible mechanism for the adsorption of ammonia and water onto mineral surfaces is the simultaneous adsorption. Previous radioactive transport models studies suggested that the grain coating can be formed by a mixture of water and ammonia molecules3,4 whose relative proportion can be close to 3:1.4 Hence, we modeled a mixture of water and ammonia with the ratio (3H2O + NH3) previously built as an amorphous structure in a periodical box onto both mineral surfaces. We observed that in both surfaces all NH3 3559



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Figure 7. Sequential adsorption of amorphous ammonia ice and one H2O molecule: (a) dipolar D-fors-5NH3/1H2O and (b) nondipolar fors-5NH3/1H2O surface.

molecules are adsorbed in associative mode (Figure 8). In the adsorption of water molecules, the associative process is a behavior expected in this mineral surface.23,29 The oxygen atoms of all water molecules are bonded to the topmost Mg atoms of the dipolar surface to produce a hydrated surface, the average Mg−OH2O bond distance being 2.03 Å (Figure 8a). Analogously, the N atom of ammonia adsorbate is also bonded to one Mg atom at 2.16 Å (D-fors/3H2O-1NH3). When the mixture is adsorbed on the nondipolar surface, the water and ammonia molecules are adsorbed in associative processes, although only two water molecules are adsorbed at Mg sites (Figure 8b), and the Mg−OH2O bonds have an average distance of 2.1 Å (fors/3H2O-1NH3). The adsorption energy is higher in dipolar surface than in nondipolar one. The energetic difference is ΔE = 26.7 kcal/mol; this increase of adsorption energy can be due to the fact that in the dipolar surface all molecules are adsorbed at the sites of Mg atom through associative adsorption. Taking in to account that in the above adsorption complexes some Mg sites on the surface remain unbound to the adsorbates, we duplicate the mixture of amorphous water/ ammonia ice (6H2O + 2NH3) previously built as an amorphous structure in a periodical box onto the nondipolar surface. In Figure 8c, we can observe that the water molecules are adsorbed through both associative and dissociative processes. This behavior is similar to previous adsorption process of water molecules in this mineral surface (associative/dissociative). This adsorption produced hydroxylated and hydrated sites; i.e.,

Figure 8. Most stable positions of the simultaneous adsorption of amorphous ammonia−water ice onto mineral surface: (a) dipolar surface (D-fors/3H2O-NH3), (b) nondipolar (fors/3H2O-NH3), and (c) nondipolar (fors/6H2O-2NH3) surface.

in associative process water remained completely. In this case, four water molecules were adsorbed at Mg sites, and two water molecules are dissociated. Nevertheless, in the dissociated water molecules, the water OH group is bonded to 3f Mg atoms of top surface (clean surface) and the water H atom protonates the oxygen at the SiOMg site to created silanol groups (SiOH). Hence hydroxyl groups appear on the surface (MgOH and SiOH). This behavior was also observed in previous theoretical and experimental works.23,27,29,46,47 Another important aspect is that the hydroxyl and hydrated groups are adsorbed in specific sites of surface; these sites can be absorbed in Figure 8c. The NH3 and H2O-associated have a preference for sites with higher coordination Mg atom (4f). However, OH is preferred for low coordination sites of Mg (3f). As a previous theoretical result, the unsaturated sites of the surface have a strong relationship with the surface reactivity.27−29 The geometrical parameters indicated that the four water molecules’ Mg−OH2O bonds have an average distance of 2.07 Å. For the two dissociated water molecules, the OH groups are bonded to two Mg atoms with a Mg−OH average bond length 3560



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Table 3. Main Calculated Vibration Frequencies (in cm−1) of NH3 and Water Molecules Adsorbed on (100) Forsterite Surfacesa 3H2O:NH3/min.surf. mode υ(HNH2)as υ(NH3)s δ(NH)as δ(NH3) umbrella υ(OH) a

NH3 gas

NH3/mineral surfacec

3H2O:NH3

dipolar

nondipolar

3512 (3403−3374)b 3389 (3310−3210)b 1638 1628b 1069 1066b

3515 3404 1641 1266

3513 3394 (3395)d 1673 (1627)d 1121 (1130−1102)d

3482

3523 3445 1627 1186

3086−3806

2930−3746

b

1645 1212

3721−3774

exptl. work on surface 3340e, 3240e, 1600e, 1140e,

3393f 3195f 1600f 1225f

theor. work 3478g 3350g, 3227h 1618g 1115g 3728−3015h

17 c

Values in parentheses correspond to experimental data. Experimental data NH3:N2 ice. Nondipolar forsterite surface, fors/1NH3. Experimental data NH3:H2O ice. 51 eLow coverage adsorption of ammonia on Pt(111) surface. 53 fAdsorption of ammonia on TiO2 surface.54 g Calculations of ammonia adsorption on hydroxylated TiO2 anatase surface.55 hCalculations of water and ammonia clusters.52 d

and the dipolar moment (μ) will also change, changing the frequency. On the contrary, the vibration atomic movements in δ(NH)as maintain the H atoms at almost constant distances to the surface, and Δμ is small. This result is in a good agreement with experimental evidence that shows that this mode has a strong relationship with the chemisorption process in which ammonia molecule is bonded to metallic atom by N atom that transfers electrons toward the surface through lone pair electrons.50 For the amorphous water−ammonia ice model, which is a mixture of three molecules of water surrounding one ammonia molecule, the frequencies of the main vibration modes were also calculated. The ν(HNH2)as mode appears at a similar frequency as that in the gas phase. The ν(NH3)s mode appears at slightly higher frequency than in the gas phase, though this shift is smaller than in the adsorption onto mineral surface. On the other hand, the δ (scissoring and umbrella) modes are amenable to hydrogen bond interactions observing a higher frequency shift in δ(NH3)s. These frequency shifts effect follows the same trend as in the adsorption of ammonia onto mineral surface but in a smaller level owing to the lower strength of the adsorption interaction than in the mineral surface. Then this δ(H−NH2)as frequency is higher than that of forst/1NH3 adsorption complex on the mineral surface (Table 3). The umbrella mode of our water−ammonia ice appears at lower frequency than in the forst/1NH3 adsorption complex. Nevertheless, our results are consistent with experimental values of water/ammonia ices observed at 90−100 K51 and previous theoretical study of water−ammonia clusters.52 In order to analyze the most stable complexes in the “mixture model” of amorphous water/ammonia ice, adsorbed onto (100) dipolar and nondipolar mineral surfaces, we calculated the main vibrational frequencies (Table 3). In dipolar surface (D-fors/3H2O-NH3), all water molecules are adsorbed in an associative process. The OH stretching υ(OH) mode appears in the range 2930−3746 cm−1 for the water adsorbed. In the nondipolar surface (fors/3H2O-NH3) this υ(OH) mode appears at higher frequency probably due to the free H2O placed out of the mineral surface. In the dipolar D-fors/3H2ONH3 complex, the ν(HNH2)as mode appears at a frequency similar to the gas phase, ice−ammonia mixture. However, the ν(NH3)s mode has not been assigned as a pure normal mode, but it appears as a coupling of several vibration modes. This fact is probably due to chemisorption process of NH3 and to the hydrogen bonds with surface O atoms and with the vicinal adsorbed water molecules. These stretching ν(NH) modes appears clearly in the fors/3H2O-NH3 nondipolar complex,

of 1.99 Å. Hydrogen of the dissociated water molecules is also transferred to the surface forming a SiOH group with a Si−OH bond length of 1.65 Å, and the other proton is bonded to Si and Mg (SiOHMg) with a bond length of 1.99 Å. With respect to ammonia molecules, one is adsorbed directly to one Mg atom of the top surface forming a coordinated Mg−N bond with a distance of 2.13 Å, and the other ammonia molecule is trapped out of the mineral surface forming H bonds with the vicinal water molecules. These parameters are in good agreement with previous theoretical works of the adsorption process in this surface.25,27−29,47 The adsorption energy for this process is 207.0 kcal/mol. Taking into account that this adsorption is for a double number of adsorbates as compared to that in fors/3H2O-NH3, both adsorption energy values are of the same order.

4. SPECTROSCOPIC PROPERTIES OF AMORPHOUS AMMONIA−WATER ICE ONTO (100) FORSTERITE SURFACES To validate our theoretical approach, we calculated the frequencies of the normal vibration modes of isolated ammonia molecule (Table 3). Compared with experimental data,17 we reproduce the experimental values of ammonia gas with a linear relationship with a good correlation coefficient (R = 0.9993). The slope of this correlation is close to the unity, and we consider that our calculated values do not need any scaling to be compared with experimental data. The experimental absolute values of ν(HNH2)as and ν(NH3)s modes appears at lower frequency than our calculated ones, because the experiment refers to ammonia ice where there are NH3−NH3 interactions, which do not exist in our calculations. The δ(NH3)as and δ(NH3)s (umbrella) modes are close to experimental values.17 The umbrella mode is important in astronomical observations (1070 cm−1), because it is the only band that is not completely blended with water bands, and then it is used to determine the abundance of NH3 in amorphous NH3/H2O ice.49 When one ammonia molecule is adsorbed on the pristine nondipolar forsterite surface, some frequency shifts are observed. The ν(HNH2)as and δ(NH)as modes appear at similar frequency as that in the gas phase. The ν(NH3)s mode appears at a higher frequency (15 cm−1) than in the gas phase. The δ(NH)s(umbrella) mode shows a significant shift to higher frequency with respect to the gas phase. This fact can be explained by the electrostatic interactions of these H atoms with the O atoms of the surface. The atomic displacement during the vibration of this mode will alter these interactions, 3561



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and their frequencies are similar to those in gas phase, water− ammonia mixture, and the fors/1NH3 complex, indicating that the ammonia adsorption mechanisms are similar in both nondipolar complexes. Our results are consistent with previous experimental data about ammonia adsorption onto other minerals53,54 and previous calculations.52,55 Scissoring mode δ(NH)as appears at similar frequency in both surfaces, being also close to the fors/1NH3 complex. The δ(NH3)umbrella mode shows similar frequencies for both complexes and presents a significant shift to higher frequencies with respect to the gas phase. These values are in good agreement with experimental works of NH3 adsorbed on TiO2 and Pt.50,53,54 Experimental works consider that this umbrella mode is very important because it indicates the adsorption site of the NH3 chemisorbed where N is coordinated to metallic atom.50,53

amorphous dirty ices, which can be present in comets, meteorites, planets, and interstellar dust.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to Prof. Fernando Moreno for his fruitful discussions, to the Supercomputational Center of CSIC, to the Spanish MEC and European FEDER for grants AYA2009-08190, AYA2011-30613-C02-01, and to the Andalusian Government for grants FQM-4555 and RNM-363. E.E.́ R. is thankful to D. Rafael Esteso and Elisa G. Martin-Vivaldi for their help with graphics.

4. CONCLUSIONS We present a detailed computational study of the adsorption process in the mineral surfaces and ammonia−water molecules interphase, to produce amorphous ammonia−water ice adsorbed onto forsterite surface. Our results show that the affinity of ammonia molecule for the mineral surface is similar to that of the water molecule. The adsorption mechanism implies the Mg−N interaction through lone pair electron of the N atom. With respect to the reactivity of the mineral surface, our calculations conclude that the dipolar surface of forsterite covered with a thin layer of ammonia amorphous ice shows a higher adsorption energy than the nondipolar surface. This difference is attributed to low coordination of Mg atoms of the forsterite (100) surface. The adsorption energy is related to the low coordination state of the surface Mg atoms and the H bonds between ammonia and the surface O atoms. The main interactions correspond to ammonia coverage of four molecules/nm2 of the forsterite (100) surface. The calculation of the models that simulate the amorphous ammonia−water ice indicates that in the “layer model” the interaction between layers is weak. The most stable interaction is when ammonia and water molecules are mixed and simultaneously are adsorbed onto mineral surface. This result is in good agreement with the astrophysical results that suggested that the ammonia can be in a mixture with water molecules. Hence, the adsorption of a premixed water/ ammonia amorphous ice is more exothermic (33 and 26.5 kcal/mol per adsorbate molecule for dipolar and nondipolar surfaces, respectively) than in the layers mechanism (20.7−24.9 kcal/mol per adsorbate). It is remarkable that water molecule is dissociated only when the ratio of H2O:NH3 is increased (6H2O:2NH3). On the other hand, the ammonia molecule is not dissociated in all adsorption models studied. The infrared vibration frequencies show a good agreement with experimental values on astrophysics ice and chemisorption process on other mineral surfaces. Our results suggest the effect of ammonia molecule that is strongly adsorbed onto mineral surface through a chemisorption process. There is a significant shift to higher frequency of the umbrella bending mode of ammonia group with the adsorption onto the forsterite surface. This can be very important because this band is less overlapped with bands corresponding to water, and it can be explored by astrophysics in Space. This study can be very interesting for the experimental community because this methodology can be applied to other surfaces and other minerals and also to other kinds of

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REFERENCES

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