Toward Controlled Nucleation: Balancing Monolayer Chemistry with

Sep 13, 2010 - Synopsis. Six fatty acid Langmuir monolayer systems were investigated from which we deduced the three key attributes of the monolayer ...
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DOI: 10.1021/cg1006764

Toward Controlled Nucleation: Balancing Monolayer Chemistry with Monolayer Fluidity

2010, Vol. 10 4463–4470

Conrad Lendrum†,‡ and Kathryn M. McGrath*,‡ †

Industrial Research Ltd, Wellington, New Zealand, and ‡The MacDiarmid Institute for Advanced Materials and Nanotechnology, School of Chemical and Physical Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6012, New Zealand Received May 20, 2010; Revised Manuscript Received August 9, 2010

ABSTRACT: Langmuir monolayers offer the possibility of a facile method for controlling crystal nucleation. Despite such promise, to date, while many studies have been reported, an understanding of the fundamental aspects of the monolayer in order to readily and reproducibly achieve controlled and defined facial nucleation has not been achieved. Six fatty acid monolayer systems, based on octadecanoic acid, have been investigated that differ by one functional group only, five at the R position, H, OH, Br, CH3, and COOH, and one at the β position (OH). The monolayer behavior of these six systems was investigated on each of pure water, calcium chloride, sodium bicarbonate, and calcium carbonate crystallizing subphases. Isotherm characteristics, modified on addition of subphase ions, are explained based on the strength of interaction of the surfactant headgroup with the calcium counterion: COOH and OH (R position) being strong binders of calcium ions, H and OH (β position) medium binders, and CH3 and Br weak binders. This coupled with the influence of the bicarbonate co-ion inducing the formation of a cationmediate hydrogen bonded network and the monolayer fluidity reveal that the greatest degree of control over calcium carbonate nucleation events is afforded by disordered (predominantly in the hydrocarbon tail region) monolayers composed of weak counterion binding head groups. Collectively, these enable significant participation of the co-ion in generating a network preemptive of calcium carbonate nucleation.

*To whom correspondence should be addressed. Phone: þ64 4 463 5963. Fax: þ64 4 463 5237. E-mail: [email protected].

complementarity (outlined by Mann et al.12) as the dominant interaction that accounts for the observed crystal orientation, morphology, and/or polytype. The relationship between cause and effect is easier to elucidate in the less dynamic often highly structured systems, such as SAMs,13 where the number of degrees of freedom is restricted. In contrast, in the more dynamic simple aliphatic Langmuir monolayers, where the “seed” consists of a weakly bound collection of surfactants, attributing crystal properties to a particular mode(s) of complementarity is difficult, more so when the only means to ascertaining the mechanism is via a before and after structural comparison of the monolayer and the crystal. For small molecule dynamic Langmuir monolayer systems, epitomized by the initial work of Mann et al.14,15 involving fatty acids, alcohols, amines, sulfates, and phosphonates, little was deduced with regard to the mechanism of templation. This was due, at least in part, to the complexity of the system, exacerbated by the differences in the monolayer chemistry, leading to changes in monolayer packing, cation binding, and the co-ion effect. Analysis of the data in these systems has relied heavily on comparing before and after snapshots from which inferences are made. Advanced experimental techniques such as synchrotron liquid surface scattering can provide significant insight into the system, but ultimately they remain limited with regard to both spatial and temporal resolution relative to nucleation events. Ultimately, the biggest advances with respect to the role of the monolayer in the nucleation process will be achieved when it is possible to probe the interfacial domain directly and also simultaneously monitor both the soft dynamic organic template and the mineral crystallization process. While recent studies using in situ cryo-TEM16 or synchrotron-based GIXD17 have allowed one or other of

r 2010 American Chemical Society

Published on Web 09/13/2010

Introduction Over the past two decades, a major focus in materials science and engineering has been the development of technologies that allow control over system fabrication from the nanometer on up. Many of the techniques that have evolved are in one way or another inspired by Nature’s use of selfassembly to construct complex hierarchical structures. While spectacular advances have been made in areas such as lithography and self-assembled monolayers (SAMs) leading to device manufacture, attempts to truly imitate Nature and harness the complexity imparted by self-assembly have been limited. One method that has received considerable attention, with respect to achieving controlled and defined crystal nucleation, is the use of Langmuir monolayers as templates. The Langmuir monolayer is effectively a simplified form of a cell or vesicle, having the capacity to template inorganic crystallization. Despite this simplification, crystallization under Langmuir monolayers is intrinsically complex. Fundamentally, the monolayer functions as a “seed” mediating heterogeneous nucleation. For the case of Langmuir monolayers, the seed is dynamic, with a changing responsive structure. A number of studies have been undertaken in which a range of different organic molecules and structures have been employed to act as seeds for nucleation both within the context of Langmuir monolayers and also other templating surfaces. These include β-pleated sheets,1 SAMs,2-4 hydrogen bonded amine networks,5 macrocyclic polyacids,6,7 and simple aliphatic surfactants8-11 among many others. The result in many of these cases being the assignation of a mode of

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these to be investigated, there are no techniques currently available that pertain to an investigation of all three. Consequently, the comparative analysis approach, outlined above, remains a valid though limited approach to advancing our understanding of templated nucleation. Given these limitations and the accepted understanding that self-assembly is dominated by weak intermolecular interactions, it is paramount, when performing a comparative study of Langmuir monolayers, that only minor modifications are made to the chemical structure or physicochemical environment of the system under study. It has been shown1,8,18 that a Langmuir monolayer is a soft dynamic template undergoing structural change in the presence of subphase calcium ions. Therefore, to understand the influence of chemical changes on the crystallization process, the changes need to be incremental to aid assignation of cause and effect. One such example of the deductive power of subtle systematic changes to the monolayer system on crystallization is the examination of crystallization under mixed octadecanoic acid/octadecanol monolayers.19 Through small stepwise changes in the monolayer and subphase composition, the importance of the bicarbonate co-ion in the templation process was deduced. This study culminated in the proposition that a cation-mediated hydrogen bonded network provides a conduit for crystal templation, the network participating in crystal nucleation, and growth beyond just pure electrostatics or lattice matching. Another recent study explored monolayers formed from surfactants with amino acid-like headgroups. The hydrophobic character of the R position was altered, which concomitantly increased the size of the headgroup. The authors concluded that through an increase in headgroup size the ability of the monolayer to rearrange is enhanced, resulting in greater morphological control over the calcium carbonate crystals.18 Expanding on the above investigations, we report a study of six surfactants, differing from each other by small quantifiable changes in their chemical structure. Here the focus is upon the affect substitution has on the molecular level electrostatics, in particular the binding strength of the headgroup, as opposed to the lattice spacing. The comparison of subtly different substituted acids on the energy landscape of the interface is expected to aid the elucidation of calcium carbonate templation. The six surfactant monolayers investigated were: octadecanoic acid (ODA), 2-hydroxyoctadecanoic acid (2-HSA), 2-methyloctadecanoic acid (MODA), 2-bromooctadecanoic acid (BODA), octadecylmalonic acid (ODMA), and 3-hydroxyoctadecanoic acid (3-HSA). The two main criteria for the selection of these surfactants were (1) the electronegativities of the primary substitution atom, (H 2.2, C 2.5, Br 2.7, and O 3.5) and (2) the proximity of the substituted functional group to the headgroup (2-HSA vs 3-HSA). Collectively, these six surfactants allow us to correlate the presence of electron donating or withdrawing groups, as seen in the change in electrostatic potential around the terminal carboxylic acid group (Figure 1), with cation binding and therefore the subtleties (or lack of) of the interfacial interaction leading to directed nucleation. To decipher the monolayer behavior on a calcium carbonate crystallizing subphase (CCCS), the surfactant systems were characterized on the constituent subphases: pure water, CaCl2(aq), and NaHCO3(aq). This allows the influence of specific subphase ions on monolayer behavior to be ascertained aiding elucidation of the dominant interfacial interactions controlling calcium carbonate crystal nucleation and growth. The ultimate aim of this investigation was to understand how a soft dynamic surface or framework can manipulate

Lendrum and McGrath

Figure 1. Electrostatic potential isosurfaces (A) 2-HSA, (B) ODMA, (C) ODA, (D) 3-HSA, (E) MODA, and (F) BODA (plotted using an electrostatic potential contour value of 0.01). Ab initio calculations were performed on the C6 version of the C18 surfactants used.

inorganic crystallization: its occurrence, shape, size, and orientation and from this to define a set of monolayer attributes, set by the surfactant chemistry, that enables controllable crystal nucleation to be achieved. Materials and Methods DL-2-hydroxyoctadecanoic acid (2-HSA, >99), 2-methyloctadecanoic acid (MODA, 97%), and 2-bromooctadecanoic acid (BODA, 97%) were supplied from Sigma Aldrich, octadecanoic acid, (ODA, >99%) was purchased from Merck, and 3-hydroxyoctadecanoic acid (3-HSA, 97%) was obtained from Indofine Chemicals Inc. All surfactants were used as received. ODMA was synthesized following the method of Shamas and Biswas.20 In brief, 40 mL of sodium ethoxide were freshly prepared from dry ethanol (EtOH) and sodium metal under nitrogen flow. Upon completion of the reaction, 9 mL of diethylmalonate (>99%, Fluka) were added, followed by slow addition of octadecylbromide (98%, Sigma Aldrich). The solution was refluxed to neutral pH. The solution was filtered and washed with ethanol. The ethanol was removed under vacuum. The product was washed with water. The lower aqueous layer was extracted twice with ethylacetate and the product dried with magnesium sulfate (>99%, Pure Science). The ester was recrystallized from ethylacetate, followed by deprotection using 2 M KOH. The parent acid was recovered upon acidification.

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Analytical grade chloroform (Labscan AR) was used as the spreading solvent. Concentrations were approximately 0.5 mg/mL. The aqueous subphase solutions were made from calcium chloride dihydrate (Sigma Aldrich, 20 mM), sodium bicarbonate (Romil, 20 mM), and ultrapure deionized water. The pH of the NaHCO3 subphase was reduced by bubbling carbon dioxide, generating a CO2 supersaturated solution. The fourth subphase “calcium carbonate crystallizing subphase (CCCS)”, was obtained by combining equal concentration volumes of CaCl2 (20 mM) and NaHCO3 (20 mM) solutions, both presupersaturated in CO2(g). The pH of all subphases was in the range 5.5-6, below the pH at which calcium carbonate crystallization is induced. For the CCCS subphase, calcium carbonate nucleation occurs upon effusion of CO2 from the subphase, increasing the pH of the solution. Surface pressure and potential isotherms were measured at room temperature using a NIMA 702BAM PTFE trough. Isotherms were obtained at a compression speed of 100 cm2/min. This speed ensures that nonequilibrium conditions are maintained and calcium carbonate nucleation is minimized. Surface potential measurements were made using a Trek electrostatic voltmeter (320C) and a 3250 highsensitivity vibrating-plate probe from Trek Inc., Medina, NY. In all cases, the null voltage was established on the bare subphase and the measurements were logged in conjunction with the surface pressure and area by the NIMA software (version 5.16). Brewster angle microscopy (BAM) images of the monolayer were collected using a MicroBAM2 from Nanofilm Technologie GmbH, fitted with a class B laser diode, λ = 659 nm and >20 mW, with a maximum optical power of 30 mW at the aperture of the instrument. The beam is collimated with a diameter of approximately 6 mm. Calcium carbonate crystals were grown under isobaric conditions (Π = 10 or 25 mN m-1). Crystals were harvested after approximately 16 h of growth using 12 mm diameter glass coverslips, treated with hydrophobic hexamethyldisilazane, via the horizontal LangmuirSchaefer method.21 A constant humidity and air temperature was maintained during the crystallization period. A JEOL 5300 LVM SEM, operated in backscattered mode, was used to image the crystals, previously sputter coated with a 4 nm layer of gold. Interedge angles were measured as described by Archibald et al.,22 here using SemAfore software (version 5.0, JEOL (Skandinaviska) AB, Hammarbacken 6 A, Sweden). Nucleation face assignment was based on a comparison of these angle measurements with computer-generated idealized rhombohedral models of calcite with a known orientation, using SHAPE for Windows (version 7.2.2, Shape Software). All semiempirical and ab initio computational chemistry calculations were performed using Hyperchem. Calculations were performed on molecules with only six carbon atoms. Both methods returned qualitatively the same results with respect to hydrogen bonding and molecular dipoles for all levels of theory investigated.

Results and Discussion Monolayer Behavior on Water. Figures 2 and 3 show representative surface pressure (Π-A) and potential (ΔV-A) isotherms, respectively, for the six acids on a subphase of pure water at pH 5.6. The data will be briefly discussed with reference to the base ODA system. The surface pressure isotherm of ODA on pure water displays the characteristic fatty acid transition from a gaslike layer to a L2 tilted phase at the point of lift off. Upon increasing the applied pressure further, an untilted hexagonal LS phase forms. The surface potential profile reflects this transitional behavior: a sudden marked rise, associated with the formation of a condensed monolayer, where the surfactant goes from independent, randomly arranged molecules at the air/water interface to a weakly ordered, tilted monolayer with a spatially averaged surface potential of ∼250 mV, consisting primarily of the molecular dipole. As the surface pressure is increased, the L2 phase is continually condensed with a gradual reduction in tilt, which is reflected in a gradual rise in ΔV signifying the increase in the molecular dipole

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Figure 2. Surface pressure isotherms on pure water.

Figure 3. Surface potential isotherms on pure water.

perpendicular to the interface. ΔV reaches a maximum immediately prior to the L2-LS phase transition. Further compression coincides with a rapid increase in surface pressure before collapse of the monolayer. The ΔV-A isotherm displays a gradual decrease throughout the compression of the LS phase consistent with the “slow” collapse23,24 or loss of individual or small numbers of surfactant to the subphase. This surfactant, lost to either the subphase or to multilayer formation, counters the molecular dipole of the “in-tact” monolayer, thus reducing the overall surface potential. Substitution of a proton at the C2 position for a methyl group (MODA) subtly changes the monolayer behavior on pure water. The area per molecule (Am) at onset is increased (∼34 A˚2), consistent with the larger surfactant cross-section of MODA compared with ODA. Furthermore, the onset transition is curvilinear, indicative of a second-order phase transition. The tilted to untilted transition is not observed on increasing pressure. Indeed, this transition is not observed for any of the five substituted acids. BAM shows a lack of contrast, indicating an absence of any tilt ordering. The data are consistent with the presence of the methyl group restricting surfactant packing (larger Ams), allowing the tails greater

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structural tilt and rotational freedom, resulting in a lack of long-range ordering and a less stable monolayer. This reduced structural ordering of the tails is also evident in the ΔV-A data; a lower initial rise and plateau reflect the reduced molecular dipole of the disordered tails. The gradual increase in ΔV at higher surface pressure is steeper than that for the ODA system, consistent with compaction of the film, forcing the tails to become more ordered, resulting in an increase of the molecular dipole perpendicular to the air/ water interface. Finally, a significantly larger decrease in ΔV, at surface pressures beyond ΔVmax, and larger Ams at collapse are the result of the reduced stability of the monolayer. Substitution of a proton with a bromine at the C2 position (BODA), a larger and more electronegative group, results in a curvilinear Π-A isotherm with a single tilted phase similar to that seen for MODA. The ΔV-A isotherm in contrast shows a large depression of the molecular dipole throughout the isotherm. The chaotropic nature of the bromine significantly alters the packing of interfacial waters, reducing the overall ΔV.25 As with MODA, there is a gradual rise associated with a decrease in molecular tilt on increasing pressure before the point of collapse. ODMA, a dicarboxylic acid, presents the largest of the polar substitutions and is therefore the most unstable of the six systems investigated. The decreased monolayer stability arising from the increased molecular solubility associated with the large polar headgroup. The size of the headgroup is reflected in the Am at onset, which is ∼55 A˚2, more than twice that of ODA (∼24.5 A˚2). This is followed by a substantial region of low, but nonzero, pressure before the pressure rises leading to collapse. The ΔV profile is somewhat truncated due to the increased surfactant solubility; twice the amount of surfactant is required to reach collapse. However, the ΔV profile displays a region of gradually increasing potential with decreasing Am, characteristic of a reduction in surfactant tilt. As with the other surfactants, a plateau is reached just prior to collapse, as seen in the Π-A isotherm before ΔV decreases with the formation of multilayers. 2-HSA monolayers on pure water represent the most highly ordered of the monolayers investigated. This is evident in the sharp first-order transition and the low Am at onset (∼25.5 A˚2) in the Π-A isotherm, in conjunction with the absence of either a L2-LS transition, or a rise in the ΔV-A isotherm associated with the loss or reduction in tilt with decreasing Am. These observations are indicative of the formation of fully condensed monolayer domains immediately upon addition of the surfactant. Ab initio modeling studies indicate that the source of this high degree of monolayer condensation is hydrogen bonding between the hydroxyl group of one molecule and the carboxyl group of a neighbor. Such hydrogen bonding is only possible for this surfactant. The influence of this hydrogen bonding was explored further by shifting the hydroxyl group to the C3 position. This has a dramatic affect on the behavior of the monolayer on pure water. The Π-A and ΔV-A isotherms are more akin to those of MODA than the hydrogen bonded 2-HSA system. The Am at onset (∼34 A˚2) is comparable with that of MODA, and the transition is once again curvilinear. The ΔV-A profile indicates a reduction of tilt with decreasing Am before the steep decline indicative of multilayer formation, occurring at Ams larger than the Π-A collapse point. The shift of the hydroxyl group from the C2 to the C3 position removes the capacity for intramonolayer hydrogen bonding.

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Figure 4. Surface pressure isotherms of CCCS.

Figure 5. Representative BAM images at low (left) and high (right) surface pressures, here for a 2-HSA monolayer on CCCS. Scale bar corresponds to 1 mm.

The hydroxyl functionality is therefore a passive branching group. On the basis of ΔVmax, the electron-withdrawing tendencies of the hydroxyl group reduce the headgroup electron density and consequently the overall molecular dipole. Monolayer Behavior on CCCS. Strong Cation Binding: ODMA and 2-HSA. Monolayer behavior is significantly altered upon introducing ions to the subphase, particularly calcium ions, the Π-A isotherms being the most sensitive to this addition (Figure 4). Isotherm profiles are highly irregular, particularly for 2-HSA monolayers. The monolayer presents as a heterogeneous film with incomplete coverage of the surface at all surface pressures. BAM images show rigid nonuniformly shaped monolayer domains that have considerable size polydispersity; polydisperse bare water domains are also evident (Figure 5). For 2-HSA, this behavior is also evident on a CaCl2 subphase but is absent on NaHCO3 and pure water (Figure 2). These data reflect a strong calcium ion effect dominating the response of the system. While ODMA and 2-HSA monolayers are affected similarly for a CCCS, ODMA monolayers exhibit quite different behavior on a CaCl2 subphase. The presence of calcium ions in the subphase increases the order of the ODMA monolayer, but it is only with the addition of the bicarbonate anion that the monolayer exists as polydisperse rigid islands. These islands perturb the Wilhelmy plate leading to premature lift off and therefore inflated onset Am values. Beyond the onset, the behavior of the two surfactants is analogous, including the occurrence of significant slow collapse. The calcium binding in the 2-HSA system results in significant condensation of the monolayer and the absence of any tilt (ΔV-A data, Figure 6). On the basis of the evidence available, this is not the case for ODMA. This propensity to

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Figure 6. Surface potential isotherms of CCCS.

bind calcium reflects the chemistry of the head groups, where the diacid is expected to create a six-membered ring with the subphase calcium ions, resulting in a coupling second only to the five-membered ring of 2-HSA in strength. Medium Cation Binding: ODA and 3-HSA. The introduction of ions into the subphase has a moderate affect on the 3-HSA and ODA monolayers, indicative of a reduced interaction (cf. 2-HSA) between the monolayer head groups and the subphase. For ODA, switching from pure water to CCCS results in a significant reduction in Am at onset (24.1 ( 0.1 to 20.1 ( 0.1 A˚2/molecule, respectively) and the loss of the L2 tilted phase. Condensation of the monolayer brought about by the interaction with the subphase ions results in an untilted monolayer from the outset. Unlike for 2-HSA, the presence of the cation alone is not sufficient to stop the formation of the tilted phase, rather both calcium and bicarbonate ions are required. Similarly, the introduction of subphase ions to the 3-HSA system results in condensation of the monolayer and increased order. On water, the Π-A isotherm shows a curvilinear onset transition followed by a shallow rise with pressure (Figure 2). In contrast, on CCCS (Figure 4), 3-HSA has a clearly defined first-order transition occurring at smaller Ams (23.1 ( 0.1, cf. 27.1 ( 0.8 A˚2/molecule on water). Further, the rise with pressure is steeper, associated with less compressibility, and includes a weak inflection suggestive of a transition to an untilted phase. In terms of the ΔV-A isotherm, both surfactant systems lose the rise in potential associated with reducing tilt upon application of pressure (Figure 6). Combined, the Π-A and ΔV-A observations point to a moderate interaction with the subphase ions; there is no evidence of rigid domains to suggest a strong interaction. Shifting the hydroxyl group one position further away from the carboxylic acid group significantly alters the binding capacity of the surfactant and thereby tempers the interaction between the monolayer and subphase ions. The consequence being that the behavior is consistent with that of ODA, highlighting the importance of the 2-position in binding interactions. Weak Cation Binding: MODA and BODA. The surface pressure isotherm profile of MODA (Figure 4) shows very little change with subphase ions. The Am is shifted to slightly smaller values on changing the subphase from pure water

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(31.9 ( 0.3 A˚2/molecule) to CaCl2 (30.7 ( 0.3 A˚2/molecule), to NaHCO3 (29.8 ( 0.6 A˚2/molecule), and finally to CCCS (29.0 ( 0.1 A˚2/molecule). On the CCCS, the monolayer is more compressible, with a greater loss in Am at Π = 25 mN m-1. Although at a reduced magnitude, the surface potential profile for MODA does display typical features, including a small rise at ∼30 A˚2 associated with the gradual loss of tilt. The suggestion of tilt is a deviation from the behavior exhibited by ODA where the presence of calcium and bicarbonate ions resulted in the loss of tilt at any pressure. The Π-A isotherm of BODA (Figure 4) exhibits an early onset followed by a curvilinear transition to the high pressure regime. The ΔV-A isotherm (Figure 6) is consistent with this; a gradual rise in ΔV with pressure is indicative of the decrease in tilt angle with reduction in Am. However, the ΔV-A profile contains a very significant decrease at large Am and Π = 0 mN m-1; the entire potential isotherm is shifted to negative potentials. Similar behavior is also observed on the NaHCO3 subphase. These data imply that a strong interaction with the bicarbonate anion dominates the surface potential profile for both subphases. This interaction is attributed to formation of a cation-mediated hydrogenbonded network26 that defines the molecular dipole leading to negative potentials. Overall, for both MODA and BODA, the interaction with the subphase ions is muted from that observed for the earlier systems. Crystallization. Two significant differences are apparent when comparing the characteristics of the calcium carbonate crystals grown under the substituted acid monolayers with those grown under an ODA monolayer (Figure 7). First, irregular growth, common for crystals grown under an ODA monolayer, is almost completely absent in the substituted acid-derived crystals, and second, the crystal morphology and orientation tends to be more homogeneous within a particular monolayer system. The lack of crystal irregularity is a kinetic phenomenon, brought about by a reduction in the average monolayer charge density and a more uniform rate of CO2(g) evolution, both factors being associated with the larger Ams observed for the substituted acids. ODMA and 2-HSA. The crystal morphology of calcium carbonate grown under ODMA or 2-HSA monolayers is rhombohedral-like, that is, the crystals tend to present four or five (10.4) faces to the subphase as opposed to the three (10.4) faces of the truncated crystals (Figure 7). Crystal orientation is somewhat random. The greatest number of different nucleation facets (35) of any of the six systems is observed for the 2-HSA system. The (10.4) facet is marginally dominant, with ∼30% of nucleation facets belonging to each of the zone axes Æ1-10æ and Æ010æ. We note that the occurrence of the (10.4) nucleation facet is synonymous with little or no interaction between monolayer and nucleating crystal. Consequently, the 2-HSA system lacks orientational control to any great degree. In contrast, for the ODMA system, >50% of crystals nucleate on the (10.4) face, contributing to a total of >80% of nucleation facets belonging to the Æ010æ zone axis, and therefore the ODMA monolayer does not actively participate in controlling the facial selection of crystal nucleation. This preference for the low energy (10.4) face is consistent with calcium carbonate nucleation under octadecanol monolayers,18 where the monolayer acts largely as a passive surface. The lack of face selective nucleation for the strongest cation binding monolayers is due to the inflexibility of the

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Figure 7. Representative SEM images of calcium carbonate crystals produced under the six fatty acid monolayers (CCCS). (A) 2-HSA, (B) ODMA, (C) ODA, (D) 3-HSA, (E) MODA, and (F) BODA. Π = 25 mN m-1. A summary of the analysis performed using the interedge angle method is given in Supporting Information Figure 1.

monolayer during crystal nucleation. This is consistent with studies by Popescu et al.18 and Cavalli et al.,1 where it was found that fluidity in the monolayer was paramount for crystal templation. The monolayer crystal interaction is a synergistic process requiring both mediums to rearrange and accommodate defects. That is, the monolayer must have the capacity to respond to crystal nucleation and growth, modifying its structure accordingly. For ODMA and 2-HSA, the strong cation binding results in rigid monolayer domain structures with little fluid character. Such rigid domains are unable to respond to the process of calcium carbonate nucleation resulting in nonspecific rhombohedral growth. ODA and 3-HSA. Shifting the hydroxyl group to the C3 position significantly affects monolayer characteristics and therefore its likely, and actual, crystal templation properties.

The inability of the hydroxyl group positioned at C3 to actively participate in cation binding reduces its influence to that of minimally decreasing the electron density of the headgroup while slightly increasing the headgroup spacing. Compared with 2-HSA, there is also a reduced propensity to form intramolecular hydrogen bonds. These differences are manifested in morphological similarity of the crystals grown under 3-HSA with those grown under ODA, optimized by the observation of triangular truncated crystals largely absent for 2-HSA. However, similar to 2-HSA, 3-HSA nucleated calcium carbonate crystals do not exhibit significant preferential orientation, with nucleation facets spread between three zone axes (Æ1-10æ ∼30%, Æ010æ ∼30%, and Æ100æ 25% of crystals) (Figure 7). These data add support to those obtained in the isotherm investigation, showing that 3-HSA behaves similarly to

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ODA in terms of the extent of the interfacial interaction, that is, the intermediate binding strength of 3-HSA and ODA is manifested in an intermediate degree of crystal truncation. However, the directive nature of this interaction appears to continue to be disrupted by the presence of the hydroxyl group, even at the C3 position. MODA and BODA. In dramatic contrast to the previous surfactant systems, calcium carbonate crystallization under MODA and BODA monolayers results in highly oriented, truncated rhombohedra (Figure 7). The weak cation binding, and disorder of the monolayer tails, ensuring the fluidlike nature of the monolayer, favors a strong interfacial interaction. This is evident in the frequency of truncated crystals (∼80% for MODA and ∼95% for BODA, cf. ∼55% for ODA). In terms of orientation, the Æ1-10æ and Æ100æ zone axes account for >70% of the truncated rhombohedra grown under both MODA and BODA monolayers. While these functional group substitutions have not resulted in a large improvement in the extent of preferential orientation, the lack of (10.4) nucleated rhombohedra (∼4% of MODA and ∼0% for BODA) is a significant shift. The absence of (10.4) nucleated crystals is indicative of a strong interfacial interaction, based on the propensity for (10.4) nucleated rhombohedra to form under octadecanol monolayers18 and the size and topology of the truncated nucleation face. Given the range of behaviors observed for the monolayers on the different subphases and the observed crystal properties, the improved interfacial interactions appear to have their foundation in allowing the system to self-assemble. Stronger cation binding leads to inflexibility (reduced fluidity) in the interfacial domain, resulting in template mismatch with the monolayer acting as a passive surface in terms of structural control. In contrast, weak cation binding in conjunction with increased bicarbonate interaction improves the interfacial interaction without dominating it. That is, a more effective soap-like hydrogen-bonded network is formed in these systems. Collectively, these data highlight the importance of cation binding as opposed to charge density per se with regard to the ability of the monolayer to define and control face selective nucleation. This contrasts the conclusions drawn by Fricke and Volkmer,27 who, working with Langmuir monolayers comprised of large rigid macrocyclic polyacids, stated that nondirectional electrostatic parameters, such as average charge density and mean monolayer dipole moment, determined the orientation and polymorph of the calcium carbonate crystals. Following these ideas, a significant change in the extent of acid dissociation of the six surfactants would be required to account for the observed differences in both the monolayer characteristics and the calcium carbonate crystallization data. While it is not possible to directly measure the local interfacial pKa of surfactant monolayers, for the surfactants investigated here, it is unlikely that this is the cause of the observed difference in the facial nucleation. Further as mentioned earlier, if charge density and molecular dipole, as controlled by the chemical functionality of the substituent and therefore the degree of dissociation, are the fundamental drivers for face selective nucleation, the data obtained here represent a trend which is counterintuitive. Calculated molecular dipole moments varied little for the six surfactants and were not correlated directly with the experimental data. The strongly electron-withdrawing substituted groups in

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2-HSA and ODMA, which are expected to have the lower pKas and therefore the higher charge density, nucleate crystals on the neutral (10.4) faces. The system showing the greatest extent of crystal truncation is the BODA system. It is also this system that displays the largest changes in monolayer response upon addition of bicarbonate anions to the subphase. This suggests that the capacity for the co-ion to participate directly in the interfacial interaction is, in addition to the strength of the cation binding, important. Furthermore, the average charge density and the extent of cation binding are moderated to balance the attraction of both the counterions and the co-ions to the interfacial region. This role of the co-ion in establishing a cation-mediated hydrogen bonded network has already been highlighted as a potential mechanism for introducing preferential orientation into the crystallizing system,25 however the idea is extended here with evidence that the strength of the cation binding and the fluidity of the monolayer are also important parameters in setting facial nucleation. Conclusions Three monolayer attributes are seen to be critical in achieving face selective nucleation: (1) the strength of the counterion binding, weak binders being the most favorable; (2) the extent of monolayer disorder, the more fluid-like the monolayer, the greater the level of orientational control, consistent with Popescu et al.;18 and (3) the formation of a cation-mediated hydrogen bonded network as induced by the direct interaction of the co-ion with the monolayer. 2-HSA and ODMA exhibit strong calcium binding which leads to an electrostatics-dominated interaction and subsequently random or unassisted crystal growth. In contrast, MODA and BODA more favorably participate in the bicarbonate inspired network and exhibit crystal truncation and preferential orientation to a greater level than that observed in the mixed systems. In the case of 3-HSA, shifting the hydroxyl group one position further away from the carboxylic acid headgroup removes the possibility of achieving strong bidentate cation binding and consequently the 3-HSA system is more commensurate with the ODA system under all conditions. Previously,1,8,18 it had been shown that surfactant spacing was important in imparting the necessary freedom for the monolayer to restructure in response to nucleation. Here we show that the cation binding strength of the surfactant is also a strong influence over the control of subsequent calcium carbonate crystallization. To attain face selective nucleation of calcium carbonate crystals under a Langmuir monolayer, the average charge density and molecular dipole must be designed to accommodate the attraction of both counterions and co-ions to the interfacial region. Second, the surfactant molecules must have sufficient rotational and conformational freedoms to respond to the nucleation event. Finally, to maximize uniformity of crystal morphology across the monolayer, it is best to achieve a homogeneously disordered system as opposed to a 2D polycrystalline monolayer. Supporting Information Available: Assigned zone axes and nucleation faces for strong, medium, and weak cationic binders, respectively, as determined via the inter-edge angle method.

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This material is available free of charge via the Internet at http:// pubs.acs.org.

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