Article pubs.acs.org/JPCC
Cooperative Reorganization of Mineral and Template during Directed Nucleation of Calcium Carbonate Jonathan R. I. Lee,*,† T. Yong-Jin Han,† Trevor M. Willey,† Michael H. Nielsen,‡ Liana M. Klivansky,§ Yi Liu,§ Sungwook Chung,∥ Louis J. Terminello,⊥ Tony van Buuren,† and James J. De Yoreo*,§ †
Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94550, United States Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, California 94720, United States § The Molecular Foundry, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States ∥ Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ⊥ Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, Washington 99352, United States ‡
S Supporting Information *
ABSTRACT: Self-assembled monolayers (SAMs) prepared from organic thiol molecules on metal substrates are known to exert substantial influence over mineralization and, as such, provide model systems for investigating the mechanisms of templated crystallization by organic matrices. Characterizing the structural evolution at the organic/inorganic interface in SAM/crystal systems is of paramount importance in understanding these mechanisms. In this study, X-ray absorption spectroscopy is used to characterize the structural evolution of SAMs prepared from purpose-synthesized organic thiols, with similar yet subtly different structures and compositions, during the course of mineralization at their surfaces. The studies reveal that the structure of the thiol molecules strongly affects their ability to reorient within the SAM. Complementary scanning electron microscopy measurements demonstrate that this feature of the SAMs is strongly correlated with the capability of the monolayers to induce preferential ordering among the organic crystals. Consistent with recent modeling studies of SAM/crystal systems, these findings provide experimental evidence that structural flexibility within the SAMs is crucial for achieving templated crystallization and that templating is inherently a cooperative process that selects the most favorable combination of SAM and crystal orientations.
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INTRODUCTION The organic matrix plays a crucial role in the organization of biomineralized structures ranging from the carbonate shells of marine organisms1 to the phosphate-based bones of humans and other mammals.2 The structural complexity and mechanical properties made possible through matrix-mediated mineralization are unparalleled in current synthetic processes. As a consequence, gaining insight into matrix-directed crystallization is both of fundamental interest for piecing together the complex mechanisms of biomineralization and of potential value in the development of new approaches to materials synthesis. Recent work on matrix-directed mineralization of calcium phosphate has demonstrated that the organic matrix can strongly decrease the interfacial energy,3 which is the most important factor controlling the thermodynamic barrier to mineral nucleation.4 At the heart of this control is the structural relationship that governs the molecular interactions between matrix, or “template”, and the forming mineral phase. However, the structural evolution that comes about during matrix mineralization is largely unknown due to the paucity of tools that can address this aspect of the process and the difficulty of applying what tools do exist to natural three-dimensional matrices. Here we successfully investigate the structural dynamics that enable mineral templating by applying © XXXX American Chemical Society
synchrotron-based X-ray absorption spectroscopy (XAS) to 2D organic matrices composed of self-assembled monolayers (SAMs) grown from a suite of tailor-made organothiol molecules. The results demonstrate that, irrespective of whether SAMs exhibit well-defined molecular orientation, it only acts as a template if it can undergo reorientation during mineralization. In other words, SAM flexibility is a necessary condition for successful templating. These findings imply that templating is inherently a cooperative process that selects the most favorable combination of SAM and crystal orientations. SAMs prepared from organothiol molecules on noble metal substrates are known to induce nucleation and growth, most notably of CaCO3, on distinct crystallographic planes with a high degree of specificity.5−9 Even small modifications in SAM composition or structure, such as changing the end group functionality5 or hydrocarbon chain length,6 or altering the noble metal substrate5 can alter the preferred orientation of the final suite of crystals. The ability to control mineral habit, coupled with simplicity of structure and facile preparation, has made the SAM-mineral Received: January 9, 2013 Revised: April 16, 2013
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Figure 1. (a) Structural formulas for the three MDBA molecules and (b) schematic to illustrate the tilt (colatitudal, θ) and twist (dihedral, φ) angles of the aryl ring of p-MDBA. Individual atoms are denoted by filled spheres with the following colors: sulfur, yellow; carbon, gray; oxygen, red; and hydrogen, white. The purple arrow is coincident with the Au(111) surface normal, the dark red arrow corresponds to the TDMV for the π*-orbital of the aryl ring (and the coplanar carboxyl group) and the blue arrow is coincident with the C−C bond between the aliphatic hydrocarbon chain and the aryl ring.
crystals nucleated almost exclusively on the (104) plane. In contrast, in the absence of the SAM or on MP SAMs for which the OH group pointed down into the SAM, making it more hydrophobic, no orientation control was observed. Thus, we concluded that, while the correct headgroup chemistry was required for orientation control, the SAM was not a rigid template, rather it underwent major structural rearrangements during nucleation. Although these conclusions were significant, our previous XAS study had two limitations. First, the SAMs were prepared from short MP molecules, which inherently have more static disorder and lack the orientational stability of the long-chain ω-substituted alkyl thiol SAMs used almost universally in calcite templating experiments. Second, the OH-terminated MP monolayers stabilized the natural (104) face of calcite and, as such, the findings could not be extended to the more common cases of COOH- or SO4-terminated SAMs that have been the focus of most templating studies, because they give rise to otherwise unexpressed nucleation planes, such as (012) and (013).5−7 Extensive investigations of mineral nucleation at Langmuir monolayers,13−30 primarily conducted using grazing incidence X-ray diffraction (GIXD),13−27 have encompassed organic films composed of molecules that contain both the long alkyl chain and the terminal carboxyl functionality missing from the MPs. While these studies yield valuable insight into the mechanisms of crystallization at the surface of Langmuir monolayers, great caution must be taken in extrapolating the results to explain the behavior of carboxyl-terminated SAMs. In fact, direct comparison between the carboxyl-terminated Langmuir monolayers and SAMs is inappropriate without further study for two important reasons: (1) although they are thought to induce the nucleation of [1-10] calcite,13−15 Langmuir monolayers composed of carboxyl-terminated molecules ultimately yield crystallites with a random distribution of orientations,15−20 which contrasts with the high degree of specificity and control observed for carboxyl-terminated SAMs and (2) neither the alkyl tails nor the carboxyl headgroups of the molecules that compose the Langmuir monolayers are tethered to a substrate in the manner of the Au−S bond present in the alkanethiol SAMs, which gives the Langmuir monolayers a far greater degree of flexibility and freedom of movement and could have an important bearing upon the relative ability of the monolayers to influence CaCO3 nucleation and growth. It is also noteworthy that, in contrast to XAS, structural analysis of the Langmuir monolayers with GIXD has not provided a direct
system an excellent model for studying modes and mechanisms of directed mineralization. Nonetheless, due to experimental challenges associated with probing the configuration and bonding of the SAM at the buried organic/inorganic interface, the majority of prior structural studies have relied on characterization of the monolayer before crystal growth with the aim of deriving structural relationships via comparison with the polymorph and orientation of the final crystals. While this approach seems to reveal stereochemical5,6 or epitaxial7,8 arrangements between SAM and crystal, the majority of interpretations to date assume the monolayer structure at the buried interface is just that of the crystal-free SAM. To derive structural information about the interfacial region during the nucleation phase, one must further assume that the initial monolayer structure and orientation and polymorph of the final crystals represent those at the time of nucleation, which may not be appropriate. In fact, recent theoretical treatments concluded that the observed orientations of the calcite nucleation plane on various carboxyl-terminated SAMs would not be expected in the case of rigid SAMs.10 Instead, either specific types of defects in the SAM or flexibility in the molecular orientation must be included for experiment to match theory.10,11 One excellent experimental study12 did address the possibility that the SAM is not a rigid template for growth and the authors derived a model for the strain induced at the organic/inorganic interface and within the organic layer during crystallization. Nevertheless, the model is primarily derived from structural parameters at the extremes of the crystallization process, that is, the initial SAM structure and final orientation and shape of the calcite crystals, and is limited to qualitative agreement with the experimental data. This underscores the need for further characterization of the structure and structural evolution of the monolayer and crystal to build on prior studies and develop new insight into the SAM/crystal systems. We previously showed that synchrotron-based X-ray absorption spectroscopy (XAS) can be used to explore the structural relationship between SAM and crystal at various stages of the crystallization process by enabling determination of SAM orientation even at buried interfaces, especially if the organothiol molecules contain an aromatic ring.9 Starting with an initially well-ordered OH-terminated mercaptophenol (MP) SAM, we found that, upon exposure to CaCO3 solutions, the SAM was covered by a film of amorphous calcium carbonate (ACC) nanoparticles and became completely disordered. Nonetheless, once the ACC film transformed into calcite, the B
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X-ray beam and the surface of the experimental sample within the limits of normal (90°) and approaching grazing (20°) incidence. All contact angle measurements were conducted in a static sessile drop mode using a Krüss G10 analysis system with DSA100S software. XRD spectra were collected over a 10 ≤ 2θ ≤ 60° range using a Scintag PADX diffractometer and Cu Kα radiation. The XRD spectrum collected from a flame-annealed Au(111) on Si(100) reference was used to subtract substrate reflections from samples supporting crystalline CaCO3. SEM images were collected along the Au(111) surface normal using a Hitachi S-800 scanning electron microscope. XAS Analysis. Thorough discussions regarding the application of XAS to structural studies of organothiol SAMs are abundant in the literature9,31−33 (also see SI) and so the processes of data analysis are mentioned only briefly here. XAS enables the quantitative assignment of bond orientation via analysis of the angular dependencies displayed by various resonances. The intensity of a particular XAS resonance is proportional to the dot product between the transition dipole moment vector (TDMV) of the specific unoccupied orbital into which the electron is promoted and the electric vector of the incident X-ray beam. Therefore, linear regression analysis of resonance intensities recorded at multiple angles of incidence between the experimental sample and the incident X-ray photons can yield an assignment of functional group orientation with a predicted accuracy of ±4°. For the π*resonance of the aryl group in the MDBA molecules, the orientation is described by a TDMV that is modeled to lie perpendicular to the plane of the ring. The same approach is taken for the planar carboxyl groups and, therefore, the TDMVs must be identical for the π*-resonances of the aryl ring and carboxyl group(s) because these functionalities are part of an extended, coplanar, π-system. While the TDMV for the aryl π*-resonance is descriptive of the end group arrangement, it does not represent a unique result for the specific orientation adopted by the aryl ring due to the fact that it remains invariant with any in-plane rotation of the ring (i.e., rotation about an axis coincident with the TDMV). As a consequence, additional analysis can be extremely valuable, for which it is beneficial to describe the end group orientation in terms of two angles: (1) tilt (colatitudal, θ) angle and (2) the twist (dihedral, φ) angle (Figure 1b). Comparison of the experimental TDMV with theoretical TDMVs generated using the “building block” approach and a subsequent consideration of steric restrictions yields a manifold of physically viable tilt and twist angles for the aromatic end group that are consistent with the XAS data.9,31,32,34 Restrictions of the experimental geometry preclude efforts to further deconvolve the tilt and twist angles, since one obtains only a single degree of freedom from analysis of the XAS resonance intensities. Nonetheless, identifying the physically viable combinations of tilt and twist angles can provide significant insight for CaCO3 crystallization on MDBA SAMs for two important reasons: (1) each calculated TDMV corresponds to a unique subset of tilt and twist angles, which provides distinct boundaries for the orientation adopted by the MDBA molecules and enables any changes in orientation of the aromatic ring to be identified and (2) the subset of tilt/twist combinations is limited for each TDMV (see SI).
measurement of the end group orientation, which is instead inferred from the tilt angle of the molecule as a whole. In the investigation reported here, we overcome the limitations of our XAS studies of crystallization on MP SAMs and the prior GIXD measurements of Langmuir monolayers by conducting XAS measurements at various stages of CaCO3 crystallization on SAMs prepared from a triad of purposesynthesized organothiol molecules composed of a long alkyl chain terminated by a mercaptan functionality at one end and a carboxyl group (or groups) coupled to an aryl ring at the other (Figure 1a). Each of the three is a mercaptodecyl benzoic acid (MDBA) that differs subtly in structure from the others by the number of carboxyl functional groups and/or their substitutional position(s) on the aryl ring (Figure 1a). Thus, XAS studies of SAMs prepared from p-, m-, and bm-MDBA molecules each contain a single carboxyl group bonded to the aryl ring in the para or meta position with respect to the alkyl group, while the bi-meta- (bm-)MDBA molecules contain two carboxyl groups bonded to the aryl ring in both meta positions. Thus, XAS studies of SAMs prepared from para- (p-), meta(m-), and bi-meta- (bm-)MDBA isomers enables investigation of the effects of SAM structure and its dynamics on matrixdirected nucleation without the limitations of the MP-calcite or Langmuir monolayer-calcite systems. Furthermore, the use of XAS provides the opportunity to explicitly evaluate changes in the orientation of the carboxyl end group functionality that directly interacts with the nascent CaCO3 phase and to deconvolve these changes from any structural rearrangement of the alkyl chain.
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MATERIALS AND EXPERIMENTAL METHODS Sample Preparation. Protocols for the preparation of the MDBA SAMs on Au(111) substrates are described in detail in the literature9,31,32 and the SI. These “as prepared” SAMs were then immediately transferred to ultrahigh vacuum (UHV) for XAS measurements or, alternatively, immersed in 10 mM aqueous (deionized water, 18.2 MΩ, Milli-Q) solutions of CaCl2 for exposure to Ca2+-bearing solution or in preparation for mineral growth. Stabilization of an amorphous mineral phase on the surface of the SAM required use of a 10 mM aqueous solution of CaCl2/MgCl at a ratio of 2:1. Samples that were exposed to Ca2+-bearing solution, but not mineral formation, were removed from the calcium chloride solution after 20 min, dried in a diffuse N2(g) stream without additional rinsing and transferred into the UHV chamber for X-ray spectroscopy studies. The precipitation of Mg-incorporated/ stabilized ACC formation or growth of crystalline CaCO3 on the MDBA SAMs was carried out using the CO2 diffusion method described in the literature.6 Upon formation of the desired phase and surface coverage of the mineral, the samples were removed from the desiccator, rinsed with acetone and dried in N2(g). The Mg-stabilized ACC precipitate formed on the surface of the MDBA SAMs was stable on the time scale of the experiments described in this paper. Even so, all samples supporting a mineral phase were transferred to the vacuum environments of a scanning electron microscope or the UHV X-ray spectroscopy chamber within 90 min of preparation to minimize any degradation of the organic monolayer. Instrumentation. XAS spectra were recorded on beamline 8.2 of the Stanford Synchrotron Radiation Laboratory (SSRL) at the Stanford Linear Accelerator (SLAC) according to welldefined protocols described in the literature9 and the SI. The XAS spectra were recorded for a series of six angles between the
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RESULTS X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) Measurements. XRD and SEM were C
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Figure 2. Characteristic (a) SEM micrograph and (b) XRD pattern for calcite prepared on a p-MDBA SAM on Au(111) and characteristic SEM micrographs for (c) calcite prepared on a m-MDBDA SAM on Au(111), (d) crystalline CaCO3 formed on a bm-MDBA SAM, and (e) Mg-stabilized ACC on a p-MDBA SAM on Au(111). The inset to (a) provides an expanded image of a calcite crystal on a p-MBDA SAM and the inset to (c) provides a second example of calcite crystals formed on an m-MDBA SAM.
demonstrates that, in contrast to the p-MDBA monolayers, the m-MDBA monolayers do not exhibit a preferred orientation for calcite nucleation; instead, a random distribution of calcite rhombohedra orientations was observed at the surface of the SAM. Crystallization on the bm-SAMs also results in a random distribution of calcite crystallites but, in contrast to the behavior of the p- and m-MDBA monolayers, the predominant phase of CaCO3 formed at the surface of the bm-SAMs is vaterite (>75%), which is demonstrated by the representative SEM micrograph displayed in Figure 2d. In all of these experiments, as with our previous work on MP SAMs,9 the first phase to form was ACC. Precipitation of this amorphous phase resulted in the formation of comparable thin ACC layers on each of the three MDBA monolayers. We note that, because ACC on carboxyl-terminated SAMs rapidly transforms into calcite we added Mg2+ to the growth solution
used for identifying the coverage, orientation and polymorph of minerals grown on the surface of the MDBA SAMs. Figure 2a and b display a representative SEM micrograph and XRD pattern, respectively, for CaCO3 crystallites formed on pMDBA SAMs. The SEM data illustrate that calcite rhombohedra predominate on the surface of p-MDBA monolayers and exhibit a strong preference for nucleation on the (01l) crystallographic faces, particularly the (018) plane (>90%). This preference is supported by the corresponding XRD data, which contains an intense peak at 47.6° that corresponds to the (018) plane. We note that the XRD data also reflects the presence of vaterite but, significantly, the proportion of this phase of CaCO3 is far lower than for calcite and only a limited number of vaterite crystallites were observed via SEM. Figure 2c displays a characteristic SEM micrograph recorded for CaCO3 crystallites formed on an m-MDBA SAM, which D
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contrast, the m-MDBA SAMs display far less hydrophilic character via their averaged contact angle of 58°. X-ray Absorption Spectroscopy (XAS) Measurements. XAS spectra were recorded at the carbon K-edge with the aim of determining the statistically averaged molecular orientation within the MDBA SAMs. The upper panel of Figure 3a displays characteristic normalized carbon K-edge XAS spectra collected for the “as prepared” p-MDBA SAMs on Au(111). The corresponding XAS spectra for p-MDBA SAMs following exposure to Ca2+-bearing solution and the formation of Mgstabilized ACC are shown in the upper panels of Figure 3b and c, respectively. The lower panel of Figure 3a−c display difference spectra derived from the normalized XAS data via subtraction of the 20° scan from spectra recorded at each of the other angles of incidence. Pronounced features in the difference spectra typically correspond to angular dependence in the associated molecular orbital (unoccupied electronic state) and, as a consequence, are indicative of the average orientation of a specific bond/functional group. Each of the XAS spectra is composed of a series of resonances that are assigned as follows: the lowest energy signal at ∼285.3 eV, peak I, is attributed to the π* transitions for the six carbon atoms in the aryl ring of p-MDBA.33 An additional π*-resonance, peak III, arises at ∼288.6 eV from the carboxyl carbon.31 Significantly, the carboxyl π*-feature is convolved with resonances from the C−S σ* (∼287 eV) and C−H σ*/R* transitions (∼288 eV), which are responsible for the shoulder (II) observed on peak III. The two broad spectral features (IV) observed at ∼293.8 and 302.6 eV are attributed to the C(1s) → C−C σ* transitions.33 Peak V is present only in the spectrum for a p-MDBA SAM supporting the Mg-stabilized ACC due to the fact that it arises from the π* transition for the carbonate ion of the mineral phase.9 The π*-peak for the carbon atoms in the aryl ring (I) is completely resolved in energy from all other resonances in Figure 1a−c, even after formation of the Mg-stabilized mineral at the SAM surface. As such, analysis of this peak provides the optimal means of determining the end group orientation from the XAS data because it removes any uncertainty introduced by
in order to stabilize the ACC for a sufficient amount of time to allow for extraction of the sample with the ACC film intact. An SEM micrograph of a characteristic amorphous layer, prepared on a p-MDBA SAM, is displayed in Figure 2e. The SEM data illustrate that the layer has a high surface coverage (70−80%) and is primarily composed of rounded particles approximately 30−50 nm in diameter. The high surface coverage is suitable for study with XAS because it ensures that the spectroscopic signal is representative of the interfacial region between the SAM and the amorphous precipitate. In contrast, the surface density of calcite rhombohedra following crystallization is too low for XAS measurements because the majority of the signal would arise from uncovered regions of the SAM. Contact Angle Measurements. Static contact angle measurements with water were carried out to evaluate the hydrophilicity of each MDBA SAM surface and, by extension, determine the accessibility of the carboxyl functional groups within each monolayer. Table 1 contains the measured static Table 1. Static Contact Angles with H2O and TDMV Tilt Angles with Respect to the Au(111) Surface Normal for SAMs Prepared from Each Type of MDBA Moleculea C(1s) XAS aryl π*-resonance TDMV tilt angles
pMDBA mMDBA bmMDBA
contact angle w/H20 (°)
as prepared (°)
after exposure to Ca2+ solution (°)
after Mg-stabilized ACC formation (°)
19
55
67
67
58
64
63
62
25
64
68
63
The static contact angles have an error of ±2−3° and the TDMV tilt angles have an error of ±4°. a
contact angle for each MDBA SAM. The p- and bm-MDBA SAMs exhibit contact angles of 19 and 25°, respectively, which indicates that both have strongly hydrophilic surfaces. In
Figure 3. Normalized carbon K-edge XAS spectra recorded in the TEY mode for p-MDBA SAMs on Au(111): (a) “as prepared”, (b) following immersion in 10 mM aqueous solution of Ca2+ for 20 min, and (c) after precipitation of an ACC layer. The horizontal dashed lines running between the plots serve to aid in direct comparison between XAS spectra collected for the three p-MDBA samples. E
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Figure 4. Normalized carbon K-edge XAS spectra recorded in the TEY mode for bm-MDBA SAMs on Au(111): (a) “as prepared”, (b) following immersion in 10 mM aqueous solution of Ca2+ for 20 min, and (c) after precipitation of an Mg-stabilized ACC layer. The horizontal dashed lines running between the plots serve to aid in direct comparison between XAS spectra collected for the three bm-MDBA samples. These spectra are also representative of the behavior of m-MDBA SAMs under the same conditions.
deconvolving the carboxyl π* signal from other resonances in the same energy range. Inspection of Figure 3a−c reveals that exposure of the “as prepared” p-MDBA SAMs to the Ca2+bearing solution and the precipitation of Mg-stabilized ACC brings about considerable changes in the orientation of the aromatic headgroups within the organothiol molecules. Strong, and similar, angular dependence is observed in the π*resonances of the aryl ring within Figure 3b,c, indicating that the aromatic end groups of the p-MDBA molecules adopt welldefined orientations following immersion in the Ca2+-bearing solution and formation of the amorphous mineral. In contrast, the aryl π*-difference peaks for the “as prepared” monolayer (Figure 3a) are far smaller, as illustrated by the dashed horizontal lines between the lower panels of Figure 3a−c. Linear regression analysis of the XAS peak intensities provides a more quantitative measure of the molecular orientation and was used to derive the angle of the aryl π* TDMV with respect to the Au(111) surface normal for each of the p-MDBA monolayers. The results are displayed in table 1. The TDMV for the “as prepared” p-MDBA SAMs exhibits an angle of 55° that, significantly, resides within experimental error of the magic angle (arcsin[(2/3)1/2], ∼54.74°)33 for XAS, at which the resonance intensities become independent of incident angle of the X-ray photons. Meanwhile, exposure to either Ca2+-bearing solution or Mg-stabilized mineral growth results in an aryl π* TDMV angle of 67° with respect to the Au(111) surface normal. An angle of 67° resides beyond experimental error of the 55° angle exhibited by the “as prepared” SAMs and, therefore, reflects a reorientation of the aromatic end groups. Although inspection of the XAS data appears to reveal angular dependence in the carbonate π*, linear regression analysis indicates that the TDMV angle is within experimental error of the magic angle and, therefore, is consistent with an amorphous phase. This feature of the data is discussed extensively in the SI. Carbon K-edge XAS measurements conducted for the mMDBA and bm-MDBA SAMs reveal a clear contrast to the pMDBA monolayers. Figure 4a−c displays characteristic XAS data and the corresponding difference spectra recorded for an
“as prepared” bm-MDBA SAM and equivalent monolayers that were exposed to Ca2+-bearing solution or precipitation of the Mg-stabilized mineral phase. These spectra are also representative of the m-MDBA SAMs, which exhibit identical behavior to within experimental error under equivalent experimental conditions (see SI). Inspection of the difference spectra for the bm-MDBA (and m-MDBA) monolayers reveals strong angular dependence in the aryl π*-features, indicating welldefined orientation in the aromatic end groups. Moreover, the intensities of the XAS resonances at each specific angle of incidence are closely comparable for the bm-MDBA SAM under all three conditions. This is illustrated for the spectra collected at 90° by the horizontal dashed line running across Figure 4a−c. This feature of the experimental spectra reflects little or no change in the orientation of the aryl π* TDMV during treatment of the bm- and m-MDBA SAMs with Ca2+bearing solution or precipitation of the Mg-stabilized mineral. This behavior contrasts with that of the p-MDBA SAMs for which significant reorientation was observed. Linear regression analysis of the aryl π* resonance intensities (Table 1) provides support for this assessment. The “as prepared” bm-MBDA SAMs exhibit angles for the aryl π* TDMV of 64°, while treatment with Ca2+-bearing solution and amorphous mineral formation result in angles of 68 and 63°, respectively. The corresponding angles for the aryl π* TDMV of the m-MDBA SAMs are 64, 63 and 62° respectively. All of these angles are within experimental error of one another and the angle (67°) adopted by the p-MDBA after exposure to Ca2+-bearing solution and amorphous mineral precipitation. Analysis of the XAS signal intensity following the method of Crain et al.35 provides a quantitative means of comparing the relative coverage/areal density of molecules within each of the MDBA monolayers. Comparison of the post-edge intensity (at 325 eV) following normalization of the pre-edge (at 280 eV) intensity to 1 reveals that the areal density of carbon varied by
