Simulations of Ice Nucleation by Kaolinite (001) with Rigid and

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Simulations of Ice Nucleation by Kaolinite (001) with Rigid and Flexible Surfaces Stephen A Zielke, Allan K. Bertram, and Grenfell Norman Patey J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b09052 • Publication Date (Web): 02 Nov 2015 Downloaded from http://pubs.acs.org on November 12, 2015

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The Journal of Physical Chemistry

Simulations of Ice Nucleation by Kaolinite (001) with Rigid and Flexible Surfaces Stephen A. Zielke, Allan K. Bertram, and G. N. Patey∗ Department of Chemistry, University of British Columbia, Vancouver, BC (Dated: October 29, 2015)

Abstract Nucleation of ice by airborne particles is a process vital to weather and climate, yet our understanding of the mechanisms underlying this process is limited. Kaolinite is a clay that is a significant component of airborne particles and is an effective ice nucleus. Despite receiving considerable attention, the microscopic mechanism(s) by which kaolinite nucleates ice is not known. We report molecular dynamics simulations of heterogeneous ice nucleation by kaolinite (001) surfaces. Both the Al-surface and the Si-surface nucleate ice. For the Al-surface, reorientation of the surface hydroxyl groups is essential for ice nucleation. This flexibility allows the Al-surface to adopt a structure which is compatible with ice Ih at the atomic level. On the rigid Si-surface, ice nucleates via an unusual structure that consists of an ordered arrangement of hexagonal and cubic ice layers, joined at their basal planes where the interfacial energy cost is low. This ice structure provides a good match to the atomistic structure of the Si-surface. This example is important and may have far reaching implications because it demonstrates that potential ice nuclei need not be good atomic-level matches to particular planes of ice Ih or ice Ic . It suggests that surfaces can act as effective ice nuclei by matching one of the much larger set of planes that can be constructed by regular arrangements of hexagonal and cubic ice.

Keywords: Atmospheric Aerosols, Kaolinite, Heterogeneous Ice Nucleation, Molecular Dynamics

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I.

INTRODUCTION

Airborne particles, or atmospheric aerosols, are a vital yet poorly understood component of Earth’s climate and weather. Aerosols are responsible for cloud formation, initiate a significant portion of precipitation1 , and represent one of the largest uncertainties in climate forcing2 . Ice, which in the atmosphere does not nucleate homogeneously until approximately −38◦ C3 , is often the beginning of precipitation. Nucleation of ice at warmer temperatures requires the aid of heterogeneous ice nuclei4 . Every year approximately 1000-4000 Tg of dust is carried up into our atmosphere2 . This dust is believed to be a large source of atmospheric heterogeneous ice nuclei5,6 , yet a good understanding of how dust particles nucleate ice is lacking7 . Experimental studies have revealed certain types of dust such as illite, feldspar, and kaolinite to be effective ice nuclei5,6,8 . However, the microscopic mechanisms responsible for ice nucleation on these surfaces have not been determined. Airborne particles of these dusts no doubt contain a large variety of surface structures and active sites, with varying ice nucleating ability9 . Using computer simulations it may be possible to identify the surfaces that are most effective at nucleating ice, and determine the microscopic mechanism of ice nucleation. Recently, success in this area has been demonstrated for silver iodide10,11 and graphene12 . In this paper we report molecular dynamics simulations of heterogeneous ice nucleation on two kaolinite (001) surfaces. Our simulations show that both the kaolinite surface structure and the structure of ice can adapt to facilitate ice nucleation. Kaolinite, Al2 Si2 O5 (OH)4 , is a clay which has been reported to nucleate ice up to −10◦ C13,14 , although it is most active as an ice nucleus below −20◦ C5,6 . The primary cleavage plane of kaolinite is the 001 plane15 which has two different surfaces. One surface, here referred to as the Al-surface, is covered by hydroxyl groups, attached to Al atoms, arranged in a hexagonal pattern. The other surface, referred to as the Si-surface, consists of silicon atoms and bridging oxygens atoms forming hexagonal rings. The Al-surface is considered hydrophilic and the Si-surface hydrophobic16,17 . Two related phases of ice can exist under atmospheric conditions. Both phases share a common layer of water molecules arranged into chair conformed hexagonal rings. Differences in the stacking pattern of this layer produces the two phases of ice18 : hexagonal ice, Ih , where the layer containing the chair conformed hexagonal rings forms the basal or (0001) plane; ACS Paragon 2 Plus Environment

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cubic ice, Ic , where the layer containing the chair conformed hexagonal rings forms the (111) plane. However, in the absence of a surface or electric field controlling the stacking pattern, upon freezing the layers of chair conformed rings stack randomly19–21 producing a metastable stacking disordered ice structure, denoted as ice Isd . Previous Monte Carlo simulations suggest that a rigid Al-surface of kaolinite (001), based on crystallographic coordinates, is not capable of organizing water into ice-like configurations22,23 .

Instead, it was suggested that trenches or surface defects may be

responsible for ice nucleation as they can produce electric fields that order the water within them24,25 . Quantum mechanics based calculations have shown the Al-surface of kaolinite (001) to be amphoteric as the hydroxyl groups rotate allowing water to both donate and receive hydrogen bonds from the surface17,26–31 . No bulk ice nucleation was observed in these studies, partially due to the small size of of these simulations, although Hu et al.29 were able to observe the formation of an ice-like monolayer on the Al-surface. Cox et al.32 recently simulated ice nucleation in the presence of the Al-surface. However, the ice nucleation was caused by the small size of the simulation cell employed, and did not appear to be directly caused by the structure of the Al-surface.

II.

SIMULATION METHODS

Molecular dynamics simulations were carried out employing the GROMACS 4.5.5 and 4.6.2 programs33 . The leap-frog integrator with a time step of 2 fs was used, and the temperature was regulated with the N´ose-Hoover thermostat34,35 . Both the six-site36 (melting point 289±3 K37 ) and TIP4P/Ice38 (melting point 270±3 K) water models are considered, and the CLAYFF force field39 is used to model kaolinite. The Lorentz-Berthelot mixing rules are used to specify water-kaolinite interactions. Electrostatic forces are handled with the smooth particle mesh Ewald method40 , and the LINCS algorithm41 is used to constrain bond lengths and angles. The CHILL algorithm42 was employed to detect and classify ice structures. Kaolinite was constructed from crystal structures43,44 to create an infinite surface with the use of periodic boundary conditions. Simulation cells were constructed as in previous studies10,22,23 by positioning two kaolinite slabs (each one unit cell thick) such that they mirror each other in order to eliminate spurious electric fields, due to the finite thickness ACS Paragon 3 Plus Environment


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of the slabs combined with periodic boundary conditions23 . The kaolinite slabs were 6 nm apart in the Z direction and 0.5 nm from the top and bottom of the simulation cell, both measured from the boundaries of the unit cell, and giving a total Z dimension of 8.42661 nm, unless otherwise stated. Water was placed between the slabs. A summary of the XY dimensions of the simulation cells, the number of water molecules used, and the bulk water densities is provided in Table 1. The bulk densities given in Table 1 are estimated from water density profiles in the middle of the simulation cell (obtained at 300 K), where perturbations induced by the surfaces have disappeared. We note that our simulations are carried under N V T conditions, at densities a little lower than those of liquid water to provide sufficient space for ice to grow. We did carry out N P T simulations of bulk liquid water at the simulation temperatures used for ice nucleation (230 K for TIP4P/Ice and 240 K for the six-site model), and in both cases the average bulk density at 1 bar was less than 950 kg/m3 , which is only a little larger than the estimated densities of our ice nucleating simulations (Table 1). Therefore, although the lower densities used in the N V T simulations might have some influence on nucleation rates (which we do not measure), we would not expect any significant influence on the nucleation mechanism. For each surface considered, a simulation was performed at 300 K and frames were taken every 0.5 ns to produce initial configurations for subsequent simulations. Initial configurations were then given random velocities from a Maxwell-Boltzmann distribution, and the systems were further relaxed for 1 ns at 300 K before being cooled to the simulation temperature over a period of 1 ns. TIP4P/Ice simulations were usually run at 230-240 K and six-site simulations at 240-250 K. Complete lists of the simulations performed and their temperatures are given in the supporting information (Tables S1-S9). Several variations of the Al-surface are considered: (1) Simulations with kaolinite held completely fixed with all atoms in their crystallographic positions are referred to as “rigidkaolinite”. (2) The hydrogen atoms of the Al-surface hydroxyl groups are allowed to move, but still remain bonded to their oxygen atoms with a fixed bond length. This is referred to as the “free-H” surface. These conditions give the effect of the hydroxyl group pivoting on its oxygen atom such that the OH bond adopts various orientations. (3) In addition to the free-H conditions, we also sometimes allow the hydroxyl oxygen atoms to vibrate within a harmonic potential of 1000 kJ mol−1 nm−2 , similar to the potential employed by Fraux and Doye11 for silver iodide. This is referred to as the “free-OH” surface. We attempted ACS Paragon 4 Plus Environment

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to simulate a layer of kaolinite with all atoms completely free to move by supporting it from below with two fixed layers45 , but the free layer failed to remain intact. Allowing the hydroxyl oxygen atoms to vibrate imparts some of the motion expected in an entirely free surface, without the risk of the surface falling apart. In the free-OH case, the OH bond length is again held fixed. (4) Lastly, several rigid Al-surface structures with non-crystallographic, or “prepared”, hydrogen atom coordinates are considered.

III. A.

RESULTS AND DISCUSSION Varying Flexibility of the Al-Surface

Figure 1 shows ice growth as a function of time in several simulations for Al-surfaces with varying degrees of flexibility. Initially, there is little ice in the free-H simulation, but ice begins to form after 75-250 ns and continues to grow until the simulation cell is full, and the plots level off. The final state of a free-H simulation with six-site water is shown in Figure 2. Ice Ih is the dominant form of ice formed in these simulations, and the figure is oriented so that one is looking along the c-axis (into the basal plane) of ice Ih . A few layers of ice Ic also formed in this simulation, but do not make contact with the surface, and are the water molecules visible within the “holes” of the ice Ih lattice. The other simulations shown in Figure 1 did not result in ice nucleation. These include the rigid-kaolinite case, where the kaolinite hydrogen atoms are fixed onto their crystallographic coordinates and are not able to adapt to the presence of water molecules, and the freeOH surface. In the free-OH simulations the hydroxyl hydrogen atoms are free to move, as in the free-H case, the oxygen atoms are also free to move, but are restrained to their crystallographic positions by means of a harmonic potential. The free-OH construction is meant to approximate an unconstrained kaolinite surface, while avoiding the possibility of the surface layer breaking apart (atoms leaving the surface etc.) during the simulation. However, as noted above (also see discussion below), we did not observe ice nucleation on a free-OH surface. The three simulations, rigid-kaolinite, free-H, and free-OH reveal that flexibility (but not too much) of the kaolinite surface results in ice nucleation. The structure of water that forms on the the crystallographic Al-surface is not compatible with ice, and hence nucleation does ACS Paragon 5 Plus Environment


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not occur in the rigid-kaolinite case, consistent with earlier simulation results22,23 . On the other hand, if the surface atoms are allowed too much freedom, as in the free-OH case, we did not observe ice nucleation on simulation timescales. This might be what prevented ice nucleation in earlier work32 . We carried out a set of simulations to investigate if the free-OH surface is somehow incompatible with ice, or if some kinetic aspects of nucleation are inhibited by motion of the oxygen atoms. These simulations were initiated in free-H mode, then at a later time switched to free-OH conditions (hydroxyl oxygen atoms are allowed to vibrate about their lattice positions as described above) and continued to see if ice formed during the initial free-H period would continue to grow, or possibly melt. The results (number of ice molecules versus time) are shown in Figure 3. The free-H portions of the simulations are shown in grey, and the free-OH parts as colored curves. Configurational snapshots showing ice growth (or not) for these simulations are shown in Figures S1 and S2. We note that if some ice forms (approximately 2 surface layers or 250-500 ice molecules are sufficient) during the free-H period of the simulation, then ice continues to grow after switching to free-OH conditions. This suggests that ice is compatible with the free-OH surface, but that the vibration of the hydroxyl oxygen atoms increases the time required for nucleation beyond the timescale of our simulations. Further insight into the water (or ice) structure near the three Al-surface variations can be obtained from the density profiles and snapshots presented in Figure 4 for the TIP4P/Ice model. Similar results for the six-site water model are given in Figure S3. The four panels in Figure 4 show results for rigid-kaolinite (4A), free-OH (4B), and free-H before (4C) and after (4D) ice nucleation. Density profiles for the oxygen and hydrogen atoms of water, and in some plots those of the kaolinite hydroxyl groups, are shown. The zero point of the y-axis of the density plots is set at the crystallographic position of the kaolinite oxygen atoms. A configurational snapshot is positioned adjacent to each density profile, and a snapshot of the first water layer on the particular kaolinite surface is shown below each profile. First we consider Figure 4 (Panel D) which shows the density profiles for ice that has grown on the free-H surface. We note that the profile for the water oxygen atoms is a series of doublets, reflecting the bilayers that comprise the hexagonal rings of ice. The water hydrogen atom profile has strong peaks lying between the oxygen atom doublets that are due to hydrogen atoms hydrogen bonded to the oxygen atoms. On both sides of the oxygen ACS Paragon 6 Plus Environment

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doublets are weaker hydrogen peaks formed by hydrogen atoms that hydrogen bond to the kaolinite surface and/or to bilayers above and below. The density profile of the kaolinite hydroxyl hydrogen atoms shows that they are usually directed away from the surface, but ∼ 1/4 lie parallel to the surface. The snapshot below the density profile shows that the water molecules form hexagonal rings on the surface. Results for liquid water near the freeH surface are shown in Panel C. We note that the surface imprints ice-like structure in the nearby liquid. This is similar to our earlier observations for some surfaces of silver iodide, which also induced ice-like structure in the liquid near the surface, prior to bulk freezing10 . The free-OH profile (Panel B) is very similar to the free-H liquid profile shown in Panel C, and also resembles results of a previous DFT study17 , suggesting that the force fields employed here give a reasonable picture of the behavior of the kaolinite surface. The strong similarity of the profiles in Panels B and C suggest that the free-OH surface (Panel B) is compatible with ice, and, as discussed above, longer simulations might lead to ice nucleation. Note that the kaolinite hydroxyl oxygen atoms (green curve in the density profile) have only a narrow range of oscillation. The rigid-kaolinite profile (Panel A) has some similarity to Panel C (free-H), but there are apparently crucial differences. In Panel C, the first water oxygen peak lines up with the leading edge of the second water hydrogen peak, whereas this is not the case for rigid kaolinite. Additionally, for rigid-kaolinite (Panel A) the first hydrogen peak lies closer to the surface. These differences may appear small at first sight, but they indicate a less ice-like structure in the adjacent water. This is likely why ice nucleation was not observed for the rigid-kaolinite surface in the present and earlier simulations22–25 . Figure 5 shows the amounts of ice Ih and ice Ic formed during simulations employing the free-H surface. We see that for both the six-site and TIP4P/Ice models, hexagonal ice is by far the dominant structure. For the six-site model one or two layers of cubic ice formed towards the end of the simulation, but we note (Figure 2) that these cubic layers do not make contact with the kaolinite surface. Calculation of the lattice mismatch clearly shows that the Al-surface of kaolinite is expected to favor ice Ih . The lattice mismatch compares an ice lattice with that of a potential heterogeneous ice nucleus via the equation ACS Paragon 7 Plus Environment


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mismatch =

n × an − m × aI × 100% , m × aI

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(1)

where an and aI are the lattice constants of the ice nucleus, and of ice, respectively, and n and m are integers chosen to minimize the mismatch4 . Mismatch percentages are given in Table 2. A good match between kaolinite and ice will have ice matching kaolinite in both directions of the unit cell, with mismatches of only a few percent. The results given in the table show that kaolinite (001) is a good match only for the prism face of ice Ih (5.7%, 0.2%), consistent with what we see in the simulations. There is no good match for ice Ic , and hence we see little or no cubic structure in ice nucleated on the free-H surface. This result agrees with previous simulation work of Cox et al.32 who observed that ice nucleating in contact with a kaolinite surface was exclusively ice Ih . The results given in Table 2 were obtained using experimental values for the unit cells of ice. Ice Ih unit cell values for six-site water have been reported36 , and give mismatches within 1-2% of those in Table 2. It is important to note that while a good lattice match explains why the prism face of ice Ih nucleates on kaolinite (001), it is the reorientation of the surface hydroxyl groups that allows ice to nucleate at all. The hydrophilic16 hydroxide surface of kaolinite (001) is covered with hydroxyl groups that take one of two primary orientations. Hydroxyl groups can point away from the surface and donate hydrogen bonds to water, or lie parallel to the surface, such that the oxygen atoms can accept a hydrogen bond from water. This behavior allows the Al-surface to be amphoteric and led previous workers28–30 to ascribe kaolinite’s ice nucleating ability to this behavior.

B.

Non-crystallographic Rigid Al-surfaces

Further insight into the role of the orientation of the surface hydroxyl groups in ice nucleation can be gained by examining several rigid surface structures prepared in different ways. We consider four such rigid surfaces. Two were obtained by taking surface configurations from free-H simulations (at 230 K for TIP4P/Ice and 240 K for six-site) before and after freezing, which we label liquid-prepared and ice-prepared, respectively. A third “ideal” surface was constructed based on our observations for the ice-prepared surface, and a fourth surface was obtained by vacuum annealing (no water present) the kaolinite surface at 25 K. The surface produced by vacuum annealing closely resembles the surfaces predicted ACS Paragon 8 Plus Environment

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by DFT simulations28–30 . Examples of these prepared rigid surfaces are shown in Figure 6 and in Figures S4 and S5. In these figures only the oxygen atoms of the surface hydroxyl groups and the hydroxyl hydrogen atoms that are directed away from the surface are shown. Also, the hydrogen atoms are enlarged such that they obscure the underlying oxygen atoms. This surface view allows one to easily observe any pattern formed by the orientations of the hydroxyl groups. The results obtained for the four surfaces are summarized in Table 3. Of the four surfaces considered, only the ice-prepared surface and the “ideal” surface nucleated ice in our simulations. The ice-prepared surface nucleated ice immediately upon cooling, and did so up to 255 K (15 K below the melting temperature) for the TIP4P/Ice model and up to 260 K (29 K below the melting temperature) for six-site water. Ice nucleation at these temperatures also occurred when ice-prepared configurations from one water model were used with the other model, showing that the favorable surface configurations are not model specific. The fact that the rigid ice-prepared surface readily nucleates ice suggests that it is the surface structure, rather than some dynamical effect related to the motion of surface hydrogen atoms, that is important for ice nucleation. Once an appropriate structure is achieved nucleation occurs whether or not the hydroxyl groups are allowed to reorient during the simulation. The liquid-prepared surface bears some resemblance to the ice-prepared case (Figure 6) but did not nucleate ice in our simulations. This suggests that the final surface structure complementary to ice is established during the freezing process. Upon close examination, ice-prepared surfaces appear to have a little more order than liquid-prepared surfaces. In particular, ice-prepared surfaces have a repeating pattern of clusters of upward pointing hydrogen atoms, highlighted with yellow hexagons in Figure 6. These clusters consist of a hexagonal ring of six hydrogen atoms surrounding a central hydrogen atom. Despite some obvious defects, these clusters appear to be regularly positioned to form a larger hexagonal lattice. Based on these observations, we constructed the “ideal” ice nucleation surface shown in Figure 6. This surface nucleated ice Ih , but nucleation required more time (∼ 200 ns) than ice-prepared surfaces. Thus, the pattern of the ideal surface clearly traps the essential features required for ice nucleation on the Al-surface, but the slower nucleation time suggests that some finer details might also facilitate ice nucleation. The final rigid surface considered is the vacuum-prepared surface shown in Figure 6. As noted above, this surface closely resembles the structure obtained in DFT calculations28–30 ACS Paragon 9 Plus Environment


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(providing some confirmation of the accuracy of the CLAYFF force field39 ), but it did not nucleate ice. We note that for the vacuum-prepared surface 1/3 of the hydroxyl hydrogen atoms are lying parallel to the surface, compared to ∼ 1/4 in the ice-prepared case. Sarupria and coworkers also report the loss of nucleation ability when the surface hydroxyl groups are lying parallel to the surface46 .

C.

Si-surface

As previously mentioned, Kaolinite (001) has two different faces, both with the same crystallographic match to ice, but with different composition and structure at the atomic level. We also carried out simulations attempting to nucleate ice on the hydrophobic16 Sisurface shown in Figure 7 (Panel A). Unlike the Al-surface discussed above, the Si-surface was held rigid in all simulations. All simulations carried out with the Si-surface are summarized in Table S9. For the Si-surface, we observed ice nucleation with both water models. Ice nucleation on the Si-surface is interesting in that it followed a path different from any that we had previously observed. Specifically, the first layers of ice to form on this surface do not correspond to any bulk phase of ice of which we are aware. This result was repeated in several simulations with both models. A series of snapshots showing ice growth on the Si-surface is shown in Figure 8. Initially, an ordered layer of water forms on the surface composed of five, six, and eight membered rings. Examples of six and eight membered rings are highlighted in Figure 7 (Panel B); five member rings stand perpendicular to the surface and can be seen in Figures 8 and 9. Figure 8 shows that unlike the Al-surface, the basal planes of ice are not stacked perpendicular to the Si-surface. Also, unlike the single hexagonal ice phase that nucleates on the Al-surface, for the Si-surface the CHILL algorithm detects layers of both hexagonal and cubic ice. However, this is not a random mixture of hexagonal and cubic ice layers, such as was observed in previous experiments and simulations19–21 , and known as ice Isd . Rather, in the present case, the ice that grows is adapted to “fit” the surface structure with ordered layers of hexagonal and cubic ice (as reported by CHILL). Close inspection of snapshots (Figure 9), with the aid of a molecular model (Figure S6), reveals that the stacking of hexagonal (h) and cubic (c) layers follows the pattern hhhhcchhhhcc· · · . Also, we note that the basal planes lie at an angle of 13 − 15◦ , with respect to the surface normal. ACS Paragon10 Plus Environment

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A view of the structure is given in Figure 9. This is a close up of the 700 ns configurational snapshot shown in Figure 8, highlighting the orientation of the hexagonal planes. If we consider the ice Ih structure, prism planes are perpendicular to the basal plane, and are denoted by (01¯10) in Figure 9. The (03¯3¯1) plane lies at an angle to (01¯10), and is parallel to the Si-surface. If we follow the red arrow indicating the (03¯3¯1) plane, we note that it passes through the bottom of two hexagonal rings (as we would expect for ice Ih ), but through the middle of the next ring, which lies at junction of two sets of basal planes, and is a ring from ice Ic . The blue arrow parallel to the surface passes though the bottom of two hexagonal rings, followed by the bottom of a five member ring. The five membered ring is a hexagonal ring disrupted by the surface. Thus, the layer of ice in contact with the Si-surface bears some resemblance to the (03¯3¯1) plane ice Ih , but contains regular defects. To be sure that our Si-surface results are not strongly influenced by finite-size and/or the fixed cell volume, we carried out a simulation of one system approximately four times larger, and with a free surface, essentially corresponding to zero pressure (see large Si-surface in Table 1 and Figure S7). The system was equilibrated at 300 K for 1 ns after which the top slab was pulled back 1 nm from the water (with a corresponding 1 nm extension of the simulation cell), and the system was cooled to 250 K. Ice nucleation proceeded in a manner similar to the other Si-surface simulations, verifying the absence of any significant dependence on system size or confinement. As noted above, earlier work has shown that mixtures of hexagonal and cubic ice occur quite commonly, which is not surprising as both phases of ice contain basal planes that bond to each other with only a small interfacial energy47 . The difference here is that on the Si-surface the stacking pattern of hexagonal and cubic layers is controlled by the surface to produce a regular structure. As a bulk phase, we believe that the ice formed on the Si-surface would have a monoclinic unit cell. A interesting outcome of the Si-surface result is the realization that the number of ice planes that can match a particular surface is not limited to the planes of ice Ih or ice Ic , but extends to what must be a much larger number of potential matches, given the myriad of possible stacking patterns involving combinations of hexagonal and cubic layers. This appears to be a hitherto unrecognized possibility for heterogeneous ice nucleation. ACS Paragon11 Plus Environment


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IV.

CONCLUSIONS

In this paper we report simulations of heterogeneous ice nucleation on two kaolinite (001) surfaces. The simulations reveal interestingly different ice nucleation processes for each face. Surface adaptation of the Al-surface allows nucleation of ice Ih , whereas nucleation occurs via a unique surface-adapted ice structure on the Si-surface. Ice nucleation on the Al-surface did not occur with the hydroxyl hydrogen atoms fixed in their crystallographic positions. Ice nucleation on this surface required that the surface hydroxyl groups reorient to adopt a configuration compatible with ice. In the configuration favorable to ice nucleation, the surface hydroxyl groups act both as donors and acceptors in hydrogen bonds with water molecules in the ice lattice. This is consistent with recent observations made by Sarupria and coworkers46 . On the Al-surface, ice Ih is the dominant ice structure observed, consistent with expectations based on lattice mismatch calculations. We speculate that the adaptive surface mechanism may be relevant to ice nucleation by other “flexible” ice nuclei. We also obtained ice nucleation on the rigid Si-surface. Ice nucleation at this surface occurs by a different and interesting mechanism. The atomic structure of the Si-surface is not a good atomistic match to any plane of ice Ih or ice Ic , and a pure ice Ih or ice Ic phase does not grow at this surface. Rather, one observes an ice structure where layers of hexagonal and cubic ice form a repeating pattern at a particular angle to the Si-surface. This ice structure is imposed by the detailed atomic structure of the Si-surface. The ice formed is similar to ice Isd in that the hexagonal and cubic layers join at basal planes where the interfacial energy cost is low. However, it is unlike ice Isd in that the stacking is not disordered, but forms a regular pattern dictated by the surface structure. This mechanism of nucleation opens the possibility for ice nucleation on surfaces that may be good atomic-level matches to a variety of planes which do not exist in either ice Ih or ice Ic , but can be found in some regular combination of these two structures.

ACKNOWLEDGMENTS

The financial support of the Natural Science and Engineering Research Council of Canada is gratefully acknowledged. This research has been enabled by the use of WestGrid and ACS Paragon12 Plus Environment

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Compute/Calcul Canada computing resources, which are funded in part by the Canada Foundation for Innovation, Alberta Innovation and Science, BC Advanced Education, and the participating research institutions. WestGrid and Compute/Calcul Canada equipment is provided by IBM, Hewlett Packard and SGI.

V.

SUPPORTING INFORMATION

Supporting information is available free of charge at http://pubs.acs.org. This contains: lists of the simulations performed; initial and final configurational snapshots of selected simulations; density profiles and snapshots for the six-site model; additional snapshots of liquid-prepared and ice-prepared surfaces; a photograph of a physical model of ice on the Si-surface; a series of snapshots showing ice nucleation on the large Si-surface.

∗

[email protected] 604-822-2996

1

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TABLE 1. Simulation parameters and system properties. For each system considered, the XY cell dimensions, the number of water molecules, and the bulk water densities estimated as described in the text are given. The first number in the density column is for the six-site model, and the second is for TIP4P/Ice. The Z cell dimension is 8.42661 nm in all cases, except for the large Si-surface, which is 1 nm larger. System

X (nm) Y (nm) Water molecules Density (kg/m3 )

Al-surface

5.66885 6.2593

7100

940, 943

Si-surface

5.66885 6.2593

6800

943, 945

17700

930

large Si-surface 10.307 10.73016

TABLE 2. Lattice mismatch percentages from equation 1. Numbers in square brackets give the values of m and n ([m:n]) used. Numbers in rounded brackets are the lattice parameters. The only good match is highlighted in bold. lattice parameter Kaolinite-a (5.15 ˚ A)43 Kaolinite-b (8.94 ˚ A)43 Ih -a (4.48 ˚ A)48

[1:1]15 %

[2:1]0.2 %

Ih -c (7.31 ˚ A)48

[2:3]5.7 %

[1:1]22.3 %

Ic (6.37 ˚ A)48

[1:1]19.1 %

[3:2]6.4 %

TABLE 3. Prepared rigid surfaces. Yes and No indicate if ice nucleated or not. Preparation

six-site TIP4P/Ice

ideal

Yes

Yes

vacuum

No

No

ice (same model)

Yes

Yes

ice (different model)

Yes

Yes

liquid

No

No


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FIG. 2. A configurational snapshot of ice grown on the Al-surface (free-H) with the six-site model. The simulation cell is oriented such that one is looking along the c-axis (into the basal plane) of ice Ih . Water molecules visible within the hexagons formed by the ice Ih lattice are from a few sheets of ice Ic . Mirrored kaolinite slabs are at the top and bottom of the simulation cell. Hydrogen, aluminium, silicon, and oxygen atoms are black, pink, yellow, and red, respectively, except for oxygen atoms reported as ice, which are blue.


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Simulations of Ice Nucleation by Kaolinite (001) with Rigid and

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