J. Phys. Chem. C 2008, 112, 14495–14500
14495
Two-Dimensional Ordering of Water Adsorbed on a Mica Surface at Room Temperature Artur Meleshyn Center for Radiation Protection and Radioecology (ZSR), Leibniz UniVersita¨t HannoVer, Herrenha¨user Str. 2, 30419 HannoVer, Germany ReceiVed: May 9, 2008; ReVised Manuscript ReceiVed: July 23, 2008
Monte Carlo simulations of water films on muscovite mica at room temperature reveal the formation of a strongly bound and two-dimensionally ordered water layer dependent on the arrangement and concentration of adsorbed K+ ions. This first water layer has polygonal habits in accordance with those of mica crystals, and its formation within clusters with monolayer K+ ion coverage is proposed as a prerequisite for the nucleation and growth of the polygonal water islands observed recently with scanning polarization force microscopy (SPFM) on the muscovite surface at room temperature. The saturation of this water layer in the simulation corresponds very well to the threshold for the nucleation of polygonal water islands observed in experimental studies. The saturation of the first hydration shells of the K+ ions adsorbed within clusters with monolayer K+ ion coverage results in the formation of a water film with a water-air interface of ∼2.6 Å above the first water layer in very good agreement with a height contrast of 2.5 ( 0.5 Å observed for polygonal water islands in SPFM experiments. 1. Introduction Recent experimental studies of molecularly thin water films adsorbed on the mica surface at room temperature revealed the formation of two-dimensional water islands with polygonal edges in registry with the mica lattice directions.1,2 The polygonal appearance of these islands was found to be governed by the relative humidity (RH) of the air in contact with mica and was observed from ∼20% up to ∼70% RH.1-3 Their unusual stability motivated a first principle study of the mica-water interface,4 which revealed that at monolayer coverage, water forms a fully connected two-dimensional hydrogen bond network on the mica surface. The formation of such an ice-like water structure was verified by a subsequent spectroscopic study.3 Despite this progress in the understanding of the observed phenomenon, there are still remaining issues to be resolved for its consistent explanation. First of all, the formation of a monolayer water film on mica takes place only at RH values exceeding ∼90%.3 In order to be able to explain the existence of polygonal water islands at significantly lower RH values from 20% up to 70%, the nucleation of monolayer water patches at RH as low as ∼20% was conjectured.3 However, the existence of the nucleation threshold at ∼20% RH as well as of the two very distinct regimes of interaction of the atomic force microscope (AFM) tip with the water film below and above this RH value were not explained by this approach. In addition, the simulated water monolayer is suggested to be especially stable due to high barriers for water diffusion at the mica surface,4 whereas water islands were found to be mobile above 45% RH in the experiment.2 Moreover, these islands transform into narrow channels interconnected at angles of 120° after several hours of exposition to air at this humidity regime, which provides strong evidence for a participation of some diffusion processes. Furthermore, the structure of the simulated water monolayer is exclusively determined by the basal oxygen atoms at the mica surface.4 This, however, would result in the absence of preferred growth directions and circular appearance of water islands instead of the observed polygonal one. Indeed, the
polygonal habits of mica crystals are due to the growth of periodic bond chains with the mica layer stoichiometry, Al2(Si3Al)O10(OH)2, and are related to Al/Si ordering in the tetrahedral sheet of mica,5,6 whereas the surface arrangement of basal oxygens is independent of it. To resolve these inconsistencies, Monte Carlo (MC) simulations of water films adsorbed on the surface of 2M1-muscovite mica with the formula unit KAl2(Si3Al)O10(OH)2 were undertaken in this study as described in the next section. 2. Simulation Details A muscovite layer consists of two tetrahedral sheets with one out of four Si atoms substituted by Al, which sandwich an octahedral sheet with two out of three octahedrally coordinated positions occupied. The Al substitutions in the tetrahedral sheet are arranged in accordance with the Lo¨wenstein’s rule of avoidance of Al-O-Al linkages so that hexagonal rings of Si4Al2 and Si5Al1 compositions are equally represented in the modeled muscovite layer. Six basal oxygen atoms bridging Si and Al atoms of the same hexagonal ring are in the vertices of the two equilateral triangles with side lengths of ∼4 Å and ∼5 Å featuring a ditrigonal cavity in the mica surface. Parameters of the muscovite unit cell were taken from the X-ray reflectivity study by Schlegel et al.7 The simulation cell consists of the two muscovite layers of a total thickness of 20.059 Å separated from the next two layers through a cleavage along the plane of the interlayer K+ ions and pulling apart the cleaved surfaces to 100 Å. Hence, whereas the interlayer space between the two muscovite layers within the simulation box is identical with the bulk muscovite and contains K+ ions at a coverage corresponding to 2 K+ ions per unit cell area (Auc; Auc ) 46.72 Å2), their two external surfaces have a coverage of 1 K+/Auc as a result of cleavage. Lateral dimensions of the simulation cell of ∼20.75 Å by ∼18.01 Å enclose eight unit cells. To simulate the structure of water films adsorbed on muscovite surface, 16, 24, and 48 water molecules (or 2, 3, and 6 H2O/Auc, respectively) were randomly distributed in a slab of a thickness of 5 Å near one of the cleaved muscovite
10.1021/jp804124r CCC: $40.75 2008 American Chemical Society Published on Web 08/26/2008
14496 J. Phys. Chem. C, Vol. 112, No. 37, 2008
Meleshyn
Figure 1. Lateral atomic densities for water hydrogen, water oxygen, and potassium (color boxes with scale atoms/Å2) at the muscovite-water interface for monolayer K+ coverage and water coverages of (a) 2 H2O/Auc and (b) 6 H2O/Auc. Lateral atomic densities were sampled for water oxygens and water hydrogens within the first peak of the corresponding atomic densities as well as for monovalent ions within 7 Å from the mica surface. The circles are silicon atoms, the crossed circles are aluminum atoms, and the triangles are basal oxygen atoms (only structural ions and basal oxygens of the tetrahedral sheet at the interface with water film are shown). Water adsorption sites S1 and S2 are positioned above basal oxygens, bridging structural ions and above the centers of Si5Al1 hexagonal rings, respectively, as defined by Odelius et al.4 S4 sites are positioned above the centers of Si4Al2 hexagonal rings.
surfaces in the first series of simulations. The K+ ions at this surface, remaining after cleavage and amounting to one monolayer coverage (1 K+/Auc), were moved to positions 3.5 Å away from it. In the second series of simulations, 278 water molecules were randomly distributed in a slab of a thickness of 30 Å near a cleaved muscovite surface. Three different K+ coverages of 0.5, 1, and 2 K+/Auc with K+ ions in the positions 3.5 Å above the surface were considered. Removal of 0.5 K+/Auc from the simulation cell at the former K+ coverage was compensated by the addition of 0.5 H3O+/Auc (positioned 3.5 Å above the same cleaved surface) in accordance with the experimental observations for muscovite in contact with deionized water or water solution at lower KCl concentrations.7,8 Similarly, addition of 1 K+/Auc at the K+ coverage of 2 K+/Auc was compensated by the addition of 1 Cl-/Auc (positioned 7.65 Å above the same cleaved surface). The Monte Carlo simulations were carried out in canonical, constant-NVT ensemble with a temperature fixed at 298 K. Three-dimensional periodic boundary conditions were applied to the simulation cell to model the interface between a muscovite platelet and water. Mineral layers were considered as rigid bodies with atomic charges assigned according to Skipper et al.9 The imposed rigidity of the mica layers is a reasonable approximation considering that mineral atoms show relaxations of 150 Å and decreased to the height contrast of 2.5 ( 0.5 Å within several minutes.2 Accordingly, the second series of MC simulations was carried out with a water film thickness of ∼25 Å, which is large enough to take into account possible effects of water wetting in addition to those of water adsorption.19 The results of this series reveal that the formation of the first strongly bound water layer depends strongly on the K+ ion concentration in or at the interface with this layer. Namely, the number of water molecules in the first water layer equals ∼1.2 H2O/Auc at monolayer K+ coverage (1 K+/ Auc, Figure 4a) and only ∼0.5 H2O/Auc at K+ coverages of 1.375 K+/Auc (Figure 4b) and 0.5 K+/Auc. The data for the cumulative electron density profiles for water presented in Figure 5 emphasizes that such a dependence of water density on the K+ ion concentration is most profound within the first water layer, but is not limited to it (and actually extends over the whole thickness of ∼25 Å of the adsorbed water film). Again, whereas the presence of linear edges intersecting at angles of 120° and of corresponding polygonal shapes is apparent for the monolayer K+ coverage (Figure 4a), there are no such edges at higher (Figure 4b) or lower K+ coverages. At lower than monolayer K+ coverages, the loss of stability of water adsorption within the first water layer is due to the adsorption of H3O+ ions instead of K+ ions above Al substitutions. Since H3O+ and K+ ions have very different hydration properties, this adsorption leads to the loss of the water ordering established by K+ ions in the first water layer and to a desorption of water molecules from it. At higher than monolayer K+ coverages, a displacement of K+ ions from their positions above Al substitutions toward the center of the ditrigonal cavities takes place at (x,y) ∼ (10,1), (16,9) as well as, to a smaller degree, at (x,y) ∼ (8,15), (11,10) (Figure 4b). The completion of this displacement for K+ ions at (x,y) ∼ (10,1), (16,9), (20,10) (Figure 4b) results in a K+ ion shift by ∼0.45 Å from ∼2.15 Å to the position ∼1.7 Å above the mica surface (Figure 5). As a result of these structural changes and of the increased K+ concentration within or at the interface with the first water layer, the two-dimensional water ordering established by K+ ions in this layer is lost and water molecules are pulled out of it. The increasing displacement of K+ ions into the positions ∼1.7 Å above the mica surface upon K+ ion
Two-Dimensional Ordering of Water
J. Phys. Chem. C, Vol. 112, No. 37, 2008 14499
Figure 4. Lateral atomic densities for water hydrogen, water oxygen, potassium, and chloride (color boxes with scale atoms/Å2) at the muscovite-water interface for a water coverage of ∼35 H2O/Auc and K+ coverages of (a) 1 K+/Auc and (b) 1.375 K+/Auc. Although initial K+ and Cl- coverages in the system presented in (b) were 2 K+/Auc and 1 Cl-/Auc, respectively, 0.625 K+/Auc and 0.625 Cl-/Auc desorbed from the muscovite-water interface in the course of the simulation run. Cl- ions in (b) are ∼4 Å above the surface. Other details and symbols are as described in Figure 1.
Figure 5. K+ density profiles (lines) and cumulative water electron density profiles (symbols, right axis) at K+ coverages of 0.5 K+/Auc (dotted line, squares), 1 K+/Auc (dashed line, circles) and 1.375 K+/ Auc (solid line, triangles) for a water coverage of ∼35 H2O/Auc as functions of the distance from the muscovite surface. Electron density profiles for the potassium ion were calculated from the sampled atomic density profiles by multiplication with the corresponding number of electrons (18). Electron density profiles for water were calculated as the sum of the sampled atomic density profiles for oxygen and hydrogen multiplied by the corresponding numbers of electrons (8 and 1, respectively).
adsorption in excess of its monolayer coverage (1 K+/Auc) agrees well with the experimental observations. In bulk muscovite mica, where K+ coverage equals 2 K+/Auc, all dehydrated K+ ions are positioned in the center of the ditrigonal cavities at a distance of ∼1.7 Å from the two sandwiching mica layers.7 The above-described dependence of the water density and water binding energies on K+ ion arrangement and concentration suggests the preferential adsorption and diffusion of water molecules into surface clusters with monolayer K+ ion coverage and the diffusion of K+ ions triggered by the AFM tip contact at RH > 20% as reasons for the observed nucleation, growth and transformations of polygonal water islands.1-3 Indeed, K+ ions are generally not equally distributed between cleaved mica layers and characterized by inhomogeneities in their lateral distribution on a cleaved surface.24 The same effect takes place due to local deviations in the tetrahedral Al/Si ratio from its average 1:3 value resulting in considerable short-range order but no long-range order in Al/Si ordering in muscovite.25 As a consequence of this effect, water adsorption on the mica surface at RH e 20% can be expected to lead to a formation of surface clusters with different local coverages of K+ ions (for example, compare Figures 4a and 4b). As discussed above, the water
density is maximal in the clusters with monolayer K+ coverage (1 K+/Auc) due to water adsorption in the S4 adsorption sites. Since these sites are characterized by the strongest binding energies, it is straightforward to suggest that water adsorption in these clusters or water diffusion into these clusters from the clusters with nonmonolayer K+ coverage is favored. As the RH increases above ∼20% and the second water layer begins to form, these preformed polygonal clusters with monolayer K+ coverage are favored for water adsorption as well due to the resulting stronger total energy of binding of adsorbed water molecules to the mica surface as discussed for Figure 3. This allows a prediction of nucleation and growth of nanoscale polygonal water clusters characterized by a height contrast of 2.5 ( 0.5 Å as observed in experimental studies.2,3 The requirement of a minimal interfacial energy can favor clusters with larger areas at this stage. Since the edges of these clusters are determined by K+ ion arrangement which in turn is governed by the Al/Si ordering in the underlying mica layer, the growing clusters can be expected to remain polygonal in accordance with the polygonal habits of mica crystals. Furthermore, it is reasonable to assume that a contact of the AFM tip with the water film at RH > 20% leads to a preferential build-up of a capillary meniscus in the areas with the minimal distance between the tip and the water film or, accordingly, in the polygonal water clusters with the most developed structure of adsorbed water layers. It is possible at this stage that nonmonolayer K+ ion arrangement around the nucleated polygonal water clusters can change to a monolayer one due to K+ ion diffusion, whereas no change in K+ ion arrangement and two-dimensional water ordering within the polygonal water clusters occurs as discussed above for Figure 4. As a result of the subsequent redistribution of water adsorbed in excess of the first three water layers from the capillary meniscus to mica surface, the strongly enlarged polygonal water islands are left behind, which are resolvable by force microscopy.1-3 Apparently, such a growth mechanism can eliminate only those inhomogeneities in lateral distribution of K+ ions that originate from the cleavage process. In contrary to that, the inhomogeneities due to local deviations in Al/Si distribution from a stoichiometric one in mica layer cannot be eliminated this way and represent defects in the two-dimensional polygonal water structure. Therefore, the evaporation of water from the island surface is favored in the areas with such defects and can lead to a formation of holes within the water islands as observed in
14500 J. Phys. Chem. C, Vol. 112, No. 37, 2008 experimental studies.1,2 As the evaporation proceeds, the requirement of the minimal interfacial energy can effect that the most contiguous surface clusters with monolayer K+ coverage remain interconnected, so that the observed channel structure can exist for some period of time as described by Xu et al.2 4. Conclusions This work suggests the preferential adsorption and structural delocalization of water molecules into surface clusters with monolayer K+ ion coverage as well as structural delocalization of K+ ions triggered by the AFM tip contact at RH > 20% as reasons for the observed nucleation, growth and transformations of polygonal water islands on the mica surface.1-3 The formation of the first strongly bound and two-dimensionally ordered water layer within these clusters is a prerequisite for the nucleation and growth of the water islands. This water layer reaches saturation at 20% RH or, correspondingly, at a water coverage of 2 H2O/Auc. The edges of these clusters are determined by K+ ion arrangement which in turn is governed by the Al/Si ordering in the underlying mica layer. This determines their polygonal appearance in accordance with the polygonal habits of mica crystals. The present study further suggests that the observed stability and height of polygonal water islands are due to the saturation of the first hydration shells of K+ ions adsorbed at the mica-water interface within surface clusters with monolayer K+ ion coverage. Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft (DFG) under project No. ME 3128/ 1-1. I thank three anonymous reviewers for helpful comments and Dr. C. Bunnenberg and H. Wicke for reading the manuscript.
Meleshyn References and Notes (1) Hu, J.; Xiao, X.-d.; Ogletree, D. F.; Salmeron, M. Surf. Sci. 1995, 344, 221. (2) Xu, L.; Liu, A.; Hu, J.; Ogletree, D. F.; Salmeron, M. J. Phys. Chem. B 1998, 102, 540. (3) Miranda, P. B.; Xu, L.; Shen, Y. R.; Salmeron, M. Phys. ReV. Lett. 1998, 81, 5876. (4) Odelius, M.; Bernasconi, M.; Parrinello, M. Phys. ReV. Lett. 1997, 78, 2855. (5) White, G. N.; Zelazny, L. W. Clays Clay Miner. 1988, 36, 141. (6) Meunier, A. Clay Miner. 2006, 41, 551. (7) Schlegel, M. L.; Nagy, K. L.; Fenter, P.; Cheng, L.; Sturchio, N. C.; Jacobsen, S. D. Geochim. Cosmochim. Acta 2006, 70, 3549. (8) Cheng, L.; Fenter, P.; Nagy, K. L.; Schlegel, M. L.; Sturchio, N. C. Phys. ReV. Lett. 2001, 87, 156103. (9) Skipper, N.; Chang, F.-R.; Sposito, G. Clays Clay Miner. 1995, 43, 285. (10) Jorgensen, W. L.; Chandrasekhar, J.; Madura, J. D.; Impey, R. W.; Klein, M. L. J. Chem. Phys. 1983, 79, 926. (11) Gertner, B. J.; Hyne, J. T. Faraday Discuss. 1998, 110, 301. (12) Jorgensen, W. L.; Maxwell, D. S.; Tirado-Rives, J. J. Am. Chem. Soc. 1996, 118, 11225. (13) Yeh, I.-C.; Berkowitz, M. L. J. Chem. Phys. 1999, 111, 3155. (14) Park, S.-H.; Sposito, G. Phys. ReV. Lett. 2002, 89, 085501. (15) Cantrell, W.; Ewing, G. E. J. Phys. Chem. B 2001, 105, 5434. (16) Beaglehole, D.; Radlinska, E. Z.; Ninham, B. W.; Christenson, H. K. Phys. ReV. Lett. 1991, 66, 2084. (17) Ewing, G. E. Chem. ReV. 2006, 106, 1511. (18) Rakhmatkariev, G. Clays Clay Miner. 2006, 54, 402. (19) Beaglehole, D.; Christenson, H. K. J. Phys. Chem. 1992, 96, 3395. (20) Beaglehole, D. Physica A 1997, 244, 40. (21) Wang, J.; Kalinichev, A. G.; Kirkpatrick, R. J.; Cygan, R. T. J. Phys. Chem. B 2005, 109, 15893. (22) Cygan, R. T.; Liang, J.-J.; Kalinichev, A. G. J. Phys. Chem. B 2004, 108, 1255. (23) Kuwahara, Y. Phys. Chem. Miner. 2001, 28, 1. (24) Giese, R. F. Nature. 1974, 248, 580. (25) Palin, E. J.; Dove, M. T.; Redfern, S. A. T.; Bosenick, A.; SainzDiaz, C. I.; Warren, M. C. Phys. Chem. Miner. 2001, 28, 534.
JP804124R
