Atomic-Scale Surface Roughness of Rutile and Implications for

Article pubs.acs.org/Langmuir

Atomic-Scale Surface Roughness of Rutile and Implications for Organic Molecule Adsorption Kenneth J. T. Livi,*,† Bernhard Schaffer,‡,§,& David Azzolini,∥ Che R. Seabourne,⊥ Trevor P. Hardcastle,⊥ Andrew J. Scott,⊥ Robert M. Hazen,# Jonah D. Erlebacher,% Rik Brydson,⊥ and Dimitri A. Sverjensky∥,# †

HRAEM/IIC Facility, Department of Earth & Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218, United States ‡ SuperSTEM Laboratory, STFC Daresbury, Keckwick Lane, WA4 4AD Warrington, U.K. § SUPA, School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, U.K. ∥ Department of Earth & Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218, United States ⊥ Institute for Materials Research, SPEME, University of Leeds, Leeds LS2 9JT, U.K. # Geophysical Laboratory, Carnegie Institute Washington, Washington, D.C. 20015, United States % Department of Materials Sciences, Johns Hopkins University, Baltimore, Maryland 21218, United States ABSTRACT: Crystal surfaces provide physical interfaces between the geosphere and biosphere. It follows that the arrangement of atoms at the surfaces of crystals profoundly influences biological components at many levels, from cells through biopolymers to single organic molecules. Many studies have focused on the crystal−molecule interface in water using large, flat single crystals. However, little is known about atomic-scale surface structures of the nanometer- to micrometer-sized crystals of simple metal oxides typically used in batch adsorption experiments under conditions relevant to biogeochemistry and the origins of life. Here, we present atomic-resolution microscopy data with unprecedented detail of the circumferences of nanosized rutile (α-TiO2) crystals previously used in studies of the adsorption of protons, cations, and amino acids. The data suggest that one-third of the {110} faces, the largest faces on individual crystals, consist of steps at the atomic scale. The steps have the orientation to provide undercoordinated Ti atoms of the type and abundance for adsorption of amino acids as inferred from previous surface complexation modeling of batch adsorption data. A remarkably uniform pattern of step proportions emerges: the step proportions are independent of surface roughness and reflect their relative surface energies. Consequently, the external morphology of rutile nanometer- to micrometer-sized crystals imaged at the coarse scale of scanning electron microscope images is not an accurate indicator of the atomic smoothness or of the proportions of the steps present. Overall, our data strongly suggest that amino acids attach at these steps on the {110} surfaces of rutile. scanning tunneling microscopy (STM);10,11 however, STM requires very clean surfaces and high temperatures to ensure conductivity for insulators and can image only small portions of the surface of relatively large crystals. For single crystals of oxides and silicates in water, X-ray studies have been the most informative about the atomic structures of the mineral−water interface.12,13 However, these rely on large, flat single crystal surfaces. It has been assumed that the results of these studies are directly transferable in order to help interpret batch adsorption studies of nanometer- to micrometer-sized crystals. Atomic force microscopy of single crystals of relatively soluble minerals such as carbonates and sulfates in water has also provided important information about the structure of the crystal−water interface.14−16 Particularly relevant to the present

1. INTRODUCTION For decades, nanometer- to micrometer-sized crystals of simple metal oxides such as gibbsite, goethite, and rutile in water have been used in batch adsorption experiments as analogues for the interaction of dissolved organic and inorganic species with mineral surfaces in aqueous environments.1−3 It has long been recognized that the surface structures of these crystals may play a critical role in determining the amounts and locations of bound species and, in particular, the binding strength, orientations, assemblies, and abundances of adsorbates.4−9 However, although many batch adsorption experiments and theoretical models have been published in which the surface structures on oxide particles have been postulated, no atomic scale characterization of oxide mineral surfaces has been demonstrated in the laboratory. Most information on the atomic-scale structure of the surfaces of crystals comes from single crystal studies. Atomicimaging studies of crystal surfaces have often employed © 2013 American Chemical Society

Received: February 7, 2013 Revised: May 14, 2013 Published: May 15, 2013 6876

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study is the documentation of the importance of steps in the surface chemistry of these minerals in the presence of organic molecules such as amino acids. However, comparable information is not available for oxide minerals. We present here an application of atomic-resolution scanning transmission electron microscopy (STEM) capable of resolving the position of every heavy atom on the circumference of small (2 nm. 6878

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surface (Figure 5B,D). In the high-resolution image there are many surfaces with a high proportion of “/\” steps. The proportions of the three types of steps on the {110} edges of the above crystals (C, D) and an additional crystal (E) with smooth {110} edges can be seen in Table 1. A striking consistency of proportions of steps emerges from the HAADF images in all the crystals investigated. At the atomic scale made possible by using the high resolution of the STEM, all the edges exhibited the same approximate proportion of /−\ steps regardless of their roughness on the nanometer scale. The rutile {110} edges consist of approximately two-thirds of the “−” steps and approximately one-sixth of each of the “/\” steps or a 4:1:1 ratio. We are currently investigating the relationship between /−\ step surface energies and proportions in order to develop a model that explains this phenomenon. 3.2. Results of STEM Imaging on the Edges of the {111} Prism. STEM HAADF imaging of the {111} edges also reveals a systematic proportion of steps present (Table 2). Again, at the atomic scale, these edges are not smooth. They contain steps not parallel to the expected orientation (Figure 6), and the overall shape of the edges is curved. It can be seen in Table 2 that, within one crystal, the symmetry-related face edges contain nearly identical proportions of the three step typeseven though the lengths of the edges are different. Thus, for crystal C, the dominant step comprises ∼64.5% of the edge regardless of the edge orientation. The proportion of the three steps is somewhat different between crystals C and D. However, the average proportion of the dominant step is about 2/3 with the {110}-type step constituting approximately 1/3. Thus, the edge traces a vector approximately in the [012] direction.

simulations indicate that there is a contrast reversal at thicknesses around 2 nm, proving that the edge thickness is less than this value. On the basis of the HAADF image simulations, we employed one-dimensional profiles of HAADF intensity in the raw images at the edge of the crystals in order to locate the outermost Ti atom (Figure 4), corresponding to the last peak visually

Figure 4. STEM-HAADF image analysis for determining the outermost Ti atoms along the rutile {110} edge. The four plots of intensity versus distance represent one-dimensional profiles of HAADF intensity in the raw images at the edge of the crystals. The peaks represent Ti atoms in the line profiles as the perimeter of the rutile crystal is approached. The last significant peak common to all the profiles locates the position of the terminal Ti atom on the crystal.

4. DISCUSSION Our primary result is a confirmation of the inference from the adsorption and surface complexation modeling of glutamate on this rutile that {110} faces must incorporate {111}-type steps, which could incorporate ⟩TiO2-type sites. In fact, we see significant numbers of such steps as documented in Table 1. The STEM data demonstrate the presence of / and \ steps on the edges of the {110} nanorutile crystals. Such steps could contain the ⟩TiO2-type functional groups postulated for the adsorption of glutamate at high surface loadings; i.e., most of the adsorption could be taking place on these steps provided that they extend over the {110} surfaces. If, instead, the steps are only concentrated along the edges, it is unlikely that the bulk of the adsorption of glutamate could be accounted for. Our suggestion that the steps extend over the {110} surfaces and may be responsible for the glutamate adsorption is not only supported by the He-ion microscope images but is a very similar conclusion to those deduced concerning the importance of steps for the adsorption of amino acids and peptides on growing crystals of calcite.15 STM studies have also revealed the presence of atomic steps on the surface of {110} faces on rutile.27−30 STM images of {110} faces annealed at 800 K contain many steps,28 while fewer steps are apparently observed at 1000 K annealing.29 The STM data show that atomic steps run mainly parallel to the [111] and [001] directions. These directions produce half unitcell faces that are in the same orientation as the {111} and {110} macrofaces. Therefore, the “/−\” type steps inferred from our STEM images are reasonable representations of those present on rutile. The [111] direction steps will have an expression on the {110} and {111} edges, while the [001] step

estimated to be greater than three standard deviations above the background in these intensity profiles. The loci of terminal Ti atoms represent line profiles of the atomic heights of the Ti atom along the perimeter of the rutile crystal. About 15 STEM HAADF images were sufficient to cover the entire outline of rutile nanocrystals, which represents ∼2000 Ti atoms per crystal perimeter. STEM HAADF images reveal that the {110} edges are not atomically smooth, but rather incorporate numerous small steps (Figure 5). Titanium atom pairs were found to form three specific configurations: one parallel to the {110} edge and two parallel to {111} edges, but in mirror-related positions. Because most of the latter configurations were half unit-cell steps (one TiO2 octahedron), classic Miller indices cannot be assigned to them. We therefore used the simple convention of the symbols “−” for the type parallel to {110} and “/” and “\” for the two types parallel to {111} faces. Given this convention, it was a simple (yet time-consuming) task to count all the steps present on the {110} and {111} edges proceeding around as much of each crystal as possible (Tables 1 and 2). Crystals with both smooth and rough edges were analyzed. Examples of smooth and rough rutile {110} edges are illustrated in Figure 5. The first image is of a relatively smooth region on a well-formed doubly terminated crystal (Figure 5A,C). Although the {110} faces appear to be nearly flat at the scale of several nanometers, there are numerous atomic-scale steps on the surface. The second crystal has a relatively rough 6879

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Figure 5. Examples of rutile surface roughness. (A) A STEM-HAADF image of the {110} edge from a crystal with relatively flat rutile edges on crystal shown in (C). Each terminal Ti atom is marked by a red circle. Steps can be seen where the red circles drop down or jump up and can be represented by various combinations of the symbols −,/ and \ (see text). (B) A STEM-HAADF image of the {110} edge from a crystal with relatively rough edges in (D). (C, D) Low-magnification images of two rutile crystals viewed down [100]. The white boxes represent the positions of (A) and (B).

Table 1. Tabulation of Numbers of Ti−Ti Atomic Column Pair Steps on Various {110} Crystal Face Edges and Their Proportions

a

crystala (roughness, length)

− (no. pairs)

%

/ (no. pairs)

%

\ (no. pairs)

%

total no. Ti pairs

C (smooth, 260 nm) D (top) (rough, 118 nm) D (bottom) (smooth, 143 nm) E (not shown) (smooth, 167 nm) average (%)

704 359 437 539

68.8 67.6 63.6 66.2 66.6

163 91 123 135

15.9 17.1 17.9 16.6 16.9

156 81 127 140

15.2 15.3 18.5 17.2 16.5

1023 531 687 814

See Figure 5.

Table 2. Tabulation of Numbers of Ti−Ti Atomic Column Pair Steps on Various {111} Crystal Face Edges and Their Proportions

a

crystala

− (no. pairs)

%

/ (no. pairs)

%

\ (no. pairs)

%

total no. Ti pairs

C (right side) C (left side) D (top right) D (bottom right) average (%)

100 64 49 92

28.8 28.6 38.9 35.4 31.9

24 145 10 148

6.9 64.7 7.9 56.9 34.2

223 15 67 20

64.3 6.7 53.2 7.7 34.0

347 224 126 260

See Figure 5.

vacancies,29,30 none of which have been observed in single crystal X-ray reflectivity studies of rutile surfaces in water.2 Our observation of steps on the edges of the {110} faces can be further used to make a direct comparison with the site density inferred from surface complexation modeling of glutamate adsorption. More specifically, the data from the STEM images can be used to directly estimate the density of step sites along the edges. From this data, and knowing the total length of the {110} edges and the unit cell dimensions, we

direction will have an expression only on the {111} edges. It is important to note that our STEM images of steps did not reveal any significant amounts of {001} structures. Of the thousands of steps counted, fewer than five such steps were identified. In addition, the imaging conditions used for the STM studies (i.e., sputtered, cleaned, annealed, and under ultrahigh vacuum) will likely produce surface structures different from that of a fully oxidized rutile in an aqueous environment. For example, STM models of rutile surfaces include 5-fold coordinated Ti and O 6880

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bottom right is twice as long as D-top right), is likely due to energy minimization considerations. Although the details of this are not known for our sample, it seems likely that the addition of the lower energy “−” {110}-type steps on {111} would lower the total energy of the termination. Another interesting result of the quantification of step densities in Tables 1 and 2 is that the proportions of steps are independent of surface roughness. Crystals that have smooth edges at low magnifications have the same proportions of steps as crystals with rough edges. Whether this is a general feature of all rutile crystals is not yet known. Many more samples grown in different ways would need to be analyzed in detail. However, preliminary calculations of the relative surface energies of the {110} and {111} faces are very suggestive. Hardcastle et al.32 have calculated the ratio of the surface energies of a fully oxidized rutile by density functional theory (DFT) to be γ111/ γ110 = 3.21. In our samples, crystal surfaces are not fully equilibrated (in the sense of having attained the ideal Wulff shape, i.e., atomically flat faces), but using the atomic scale ratios of steps and terraces, we calculate γ111/γ110 = 3.16, in excellent agreement with the surface energy prediction. As such, the rutile system studied here supports the notion that crystal surface morphological equilibration occurs rapidly on the nanoscale.

5. CONCLUSIONS Through analysis of detailed STEM HAADF images of rutile at the nanoscale, we have demonstrated the existence of steps around the edges of the {110} and {111} faces of a sample widely used for batch studies of proton, cation, and amino acid adsorption. We have quantified the density of steps by counting the directly observable Ti atoms at the thin edges of the crystals. For example, on the most abundant crystal faces, the {110} faces, the proportion of steps is approximately 33% of the faces. This proportion appears to be invariant regardless of the macroscopic smoothness or roughness of the crystals. Our results lead to estimates of a site density of 3.3 sites/nm2 assuming that the steps extend across the {110} surfaces. This site density agrees closely with the value of 3.0 sites/nm2 previously obtained from surface complexation modeling of batch adsorption of acidic amino acids over a wide range of surface coverages.17,19,20 In turn, this strongly supports the suggestion that the acidic amino acids mainly adsorb to steps on the {110} surface of rutile. In contrast, proton adsorption is not expected to be restricted to steps. Our results suggest a minimum proton site density of 9.9 sites/nm2. If some sites can accept more than one proton, this minimum is consistent with the tritium site density of 12.5 sites/nm2 long used in surface complexation modeling of rutile. Overall, these results strongly suggest that organic molecules such as amino acid anions adsorb to a different set of sites, much smaller in number, than protons or cations. This conclusion is consistent with the results of triple-layer surface complexation models of the adsorption of cations and oxyanions on the same mineral.31,33 Cations typically are best modeled with a much larger site density than coadsorbing oxyanions. Taking all of this together, from a practical standpoint, in applying surface complexation models, our results suggest the use of at least a two-site model, one site with a low abundance for organic anions and one site with a much higher abundance for cations, with protons permitted to adsorb on all the sites. Using this approach for the rutile studied here,

Figure 6. STEM-HAADF image of a tip of a rutile pyramid termination defined by the intersection of {111} edges. Each terminal Ti atom is marked by a red circle.

calculated a step density of 3.3 sites/nm2, which is in close agreement with the value of 3.0 sites/nm2 obtained from the surface complexation modeling of glutamate batch adsorption data over a wide range of surface coverages.17 Interestingly, these values are significantly less than the value of 12.5 sites/ nm2 derived from experimental measurement of tritium exchange and successfully used to model the adsorption of protons and both mono- and divalent electrolyte cations on rutile.31 This result suggests that organic molecules such as amino acids are much more specific about the sites needed for adsorption compared to protons or cations. Proton and cation adsorption on our rutile are not expected to be confined to steps. Because the step density of 3.3 sites/ nm2 on our rutile represents about 33% of the {110} surfaces (Table 1), the total site density based on Ti atom counts should be about 3.3 × 3 = 9.9 sites/nm2. This result is almost certainly a minimum site density for proton adsorption as more than one proton may be able to adsorb at some of the sites. Consequently, the minimum value of 9.9 sites/nm2 is consistent with the tritium exchange result of 12.5 sites/nm2 (obtained on a different sample of rutile). At the pyramidal terminations of the nanorutile, the STEM data illustrate that there are considerable proportions of {110}type steps. This again suggests that, as with the prism faces, crystal terminations cannot be modeled by a single type of face. The reason for the consistency of step proportions, in spite of the fact that the {111} edges are of different lengths (edge D6881

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(5) Ridley, M. K.; Machesky, M. L.; Wesolowski, D. J.; Palmer, D. A. Modeling the surface complexation of calcium at the rutile-water interface to 250°C. Geochim. Cosmochim. Acta 2004, 68, 239. (6) Hiemstra, T.; van Riemsdijk, W. H. A surface structural approach to ion adsorption: The charge distribution (CD) model. J. Colloid Interface Sci. 1996, 179, 488. (7) Hiemstra, T.; van Riemsdijk, W. H.; Bolt, G. H. Multisite proton adsorption modeling at the solid/solution interface of (hydr)oxides: A new approach I. Model description and evaluation of reaction constants. J. Colloid Interface Sci. 1989, 133, 91. (8) Ridley, M. K.; Machesky, M. L.; Wesolowski, D. J.; Palmer, D. A. Surface complexation of neodymium at the rutile-water interface: A potentiometric and modeling study in NaCl media to 250°C. Geochim. Cosmochim. Acta 2005, 69, 63. (9) Villalobos, M.; Cheney, M. A.; Alcaraz-Cienfuegos, J. Goethite surface reactivity: II. A microscopic site-density model that describes its surface area-normalized variability. J. Colloid Interface Sci. 2009, 336, 412. (10) Majzik, Z.; Balázs, N.; Berkó, A. Thermally activated reconstruction of TiO2 (110)-(1 × 1) surface in the presence of potassium: An STM study. Catal. Today 2012, 181, 89. (11) Li, S.-C.; Losovyj, Y.; Diebold, U. Adsorption-site-dependent electronic structure of catechol on the anatase TiO2 (101) Surface. Langmuir 2011, 27, 8600. (12) Brown, G. E., Jr.; Sturchio, N. C., An overview of synchrotron radiation applications to low-temperature geochemistry and environmental science. In Applications of Synchrotron Radiation in LowTemperature Geochemistry and Environmental Science; Fenter, P. A., Rivers, M. L., Sturchio, N. C., Sutton, S. R., Eds.; Mineralogical Society of America: Washington, DC, 2002; Vol. 49, pp 1−115. (13) Fenter, P.; Rivers, M. L.; Sturchio, N. C.; Sutton, S. R. Applications of Synchrotron Radiation in Low-Temperature Geochemistry and Environmental Science; Mineralogical Society of America: Washington, DC, 2002; Vol. 49, p 579. (14) Dove, P. M.; Han, N.; De Yoreo, J. J. Mechanisms of classical crystal growth theory explain quartz and silicate dissolution behavior. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 15357. (15) Teng, H. H.; Dove, P. M. Surface site-specific interactions of aspartate with calcite during dissolution: Implications for biomineralization. Am. Mineral. 1997, 82, 878. (16) Stack, A. G.; Grantham, M. C. Growth rate of calcite steps as a function of aqueous calcium-to-carbonate ratio: Independent attachment and detachment of calcium and carbonate ions. Cryst. Growth Des. 2010, 10, 1409. (17) Jonsson, C. M.; et al. Attachment of L-glutamate to rutile (αTiO2): A potentiometric, adsorption, and surface complexation study. Langmuir 2009, 25, 12127. (18) Parikh, S. J.; Kubicki, J. D.; Jonsson, C. M.; Jonsson, C. L.; Hazen, R. M.; Sverjensky, D. A.; Sparks, D. L. Evaluating glutamate and aspartate binding mechanisms to rutile (α-TiO2) via ATR-FTIR spectroscopy and quantum chemical calculation. Langmuir 2011, 27, 1778. (19) Jonsson, C. M.; Jonsson, C. L.; Estrada, C.; Sverjensky, D. A.; Cleaves, H. J.; Hazen, R. M. Adsorption of L-aspartate to rutile (αTiO2): Experimental and theoretical surface complexation studies. Geochim. Cosmochim. Acta 2010, 74, 2356. (20) Bahri, S.; Jonsson, C. M.; Jonsson, C. L.; Azzolini, D.; Sverjensky, D. A.; Hazen, R. M. Adsorption and surface complexation study of L-DOPA on rutile (α-TiO2) in NaCl solutions. Environ. Sci. Technol. 2011, 45, 3959. (21) Smith, D. J. Progress and problems for atomic-resolution electron microscopy. Micron 2012, 43, 504. (22) Egerton, R. F.; Li, P.; Malac, M. Radiation damage in the TEM and SEM. Micron 2004, 35, 399. (23) Machesky, M. L.; et al. Potentiometric titrations of rutile suspensions to 250°C. J. Colloid Interface Sci. 1998, 200, 298. (24) Koch, C. A. Determination of core structure periodicity and point defect density along dislocations. Ph.D. Thesis, Arizona State University, 2002.

simultaneous adsorption of Ca and glutamate were successfully predicted.34,35 In all crystals, steps present special bonding environments for adsorbing molecules, and knowledge of the proportions of steps present on solid surfaces at the atomic scale is therefore essential to an understanding of how organic molecules attach to inorganic mineral surfaces, as already demonstrated for amino acids/peptides on calcite crystals. From our observations, it is clear that the external morphology of rutile nanocrystals imaged at the coarse scale of scanning electron microscope images is not an accurate indicator of the atomic smoothness or of the proportions of steps present. We have demonstrated that, for the case of these rutile crystals, the step proportions are, in fact, independent of surface roughness and contain proportions of steps that reflect their relative surface energies. This observation was made possible only by counting individual Ti atoms on the entire {110} edge. Although this will need to be verified on other materials synthesized under other conditions, our data suggest the potential for predicting the proportion of steps in cases where equilibrium crystal shapes may have been obtained. By bridging the atomic and whole crystal length scales, STEM HAADF imaging has opened a new window to investigating the topography of nanocrystal mineral surfaces and identifying the sites where organic molecules attach.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (K.J.T.L.). Present Address &

B.S.: Gatan GmbH, Ingolstädter Str. 12, 80807 München, Germany. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Chuong Huynh and Carl Zeiss, NTS, for He ion imaging; NSF grants 1021023889/1023865, DOE grant DE-FG02-96ER-14616, and EPSRC grant (EP/D040205/1) for financial support and for catalyzing collaboration through the Leeds-EPSRC Nanoscience Research Equipment Access Facility (LENNF); K. Sader for initial STEM imaging; and Budhika Mendes and M. Shannon for their helpful discussions. We gratefully acknowledge EPSRC for access to the UK National Facility for Aberration-corrected STEM (SuperSTEM) at Daresbury.

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REFERENCES

(1) Brown, G. E. J.; et al. Metal oxide surfaces and their interactions with aqueous solution and microbial organisms. Chem. Rev. 1999, 99, 77. (2) Zhang, Z.; et al. Ion adsorption at the rutile-water interface: Linking molecular and macroscopic properties. Langmuir 2004, 20, 4954. (3) Stumm, W.; Huang, C. P.; Jenkins, S. R. Specific chemical interaction affecting the stability of dispersed systems. Croat. Chem. Acta 1970, 42, 223. (4) Machesky, M. L.; Wesolowski, D. J.; Palmer, D. A.; Ridley, M. K. On the temperature dependence of intrinsic surface protonation equilibrium constants: an extension of the revised MUSIC model. J. Colloid Interface Sci. 2001, 239, 314. 6882

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(25) Kirkland, E. J. Advanced Computing in Electron Microscopy; Plenum: New York, 1998. (26) Cromer, D. T.; Herrington, K. The Structures of anatase and rutile. J. Am. Chem. Soc. 1955, 18, 4708. (27) Onishi, H.; Iwasawa, Y. STM-imaging of formate intermediates adsorbed on a TiO2 (110) surface. Chem. Phys. Lett. 1994, 226, 111. (28) Onishi, H.; Fukui, K.-I.; Iwasawa, Y. Atomic-scale structures of TiO2 (110) surface determined by scanning tunneling microscopy: A new surface-limited phase of titanium oxide. Bull. Chem. Soc. Jpn. 1995, 68, 2447. (29) Diebold, U.; et al. Intrinsic defects on a TiO2 (110)(1 × 1) surface and their reaction with oxygen: a scanning tunneling microscopy study. Surf. Sci. 1998, 411, 137. (30) Martinez, U.; et al. Reduced step edges on rutile as competing defects to oxygen vacancies on the terraces and reactive sites for ethanol dissociation. Phys. Rev. Lett. 2012, 109, 155501. (31) Sverjensky, D. A. Prediction of the speciation of alkaline earths adsorbed on mineral surfaces in salt solutions. Geochim. Cosmochim. Acta 2006, 70, 2427. (32) Hardcastle, T. P.; Brydson, R. M. D.; Livi, K. J. T.; Seabourne, C. R.; Scott, A. J. Ab-initio modelling, polarity and energies of clean rutile surfaces in vacuum and comparison with water environment. J. Phys.: Conf. Ser. 2012, 371, 012059. (33) Fukushi, K.; Sverjensky, D. A. A surface complexation model for sulfate and selenate on iron oxides consistent with spectroscopic and theoretical molecular evidence. Geochim. Cosmochim. Acta 2007, 71, 1−24. (34) Lee, N. Amino acid stability and interaction with mineral surfaces. Ph.D. Thesis, Johns Hopkins University, 2012. (35) Lee, N.; Sverjensky, D. A.; Hazen, R. M. Cooperative and competitive adsorption of amino acids with Ca2+ on rutile. Goldschmidt Geochemistry Conference Abstract, Florence, Italy, in press.

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