DOI: 10.1021/cg900398j
Calcite Nucleation on Mixed Acid/Alcohol Monolayers Conrad D. Lendrum†,‡ and Kathryn M. McGrath*,†
2009, Vol. 9 4391–4400
†
MacDiarmid Institute for Advanced Materials and Nanotechnology, School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington, New Zealand, and ‡ Industrial Research Limited, Wellington, New Zealand Received April 9, 2009; Revised Manuscript Received June 9, 2009
ABSTRACT: Nucleation of calcium carbonate was achieved on mixed octadecanoic acid/octadecanol Langmuir monolayers. Two different nucleation regimes are distinguished based on monolayer alcohol content corresponding to the zone axes [01.0] (high alcohol content, weak or no interaction between the critical calcium carbonate nucleus and the monolayer) and [11.0] (low alcohol content, stronger face selective nucleation). The cation-mediated hydrogen-bonded soap network is disrupted by the presence of the alcohol, resulting in a reduced interaction between the monolayer and the subphase. This leads to little or no control over the nucleating crystal. The alcohol concentration at which the transition between the two preferential zone axes occurs is weakly dependent on growth pressure. No other variation with growth pressure was observed.
*To whom correspondence should be addressed. Address: 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. Office: 503 Laby building, Kelburn Campus. E-mail:
[email protected]. DDI: þ64 4 463 5963. Fax: þ64 4 463 5237.
been well studied individually,18 with both being flagged as being important in calcium carbonate biomineralization.19 When placed on a calcium carbonate crystallizing subphase, these mixed monolayer systems display a significant reduction in the area per molecule (Am), the loss of a tilting transition, and rule of mixtures behavior of the surface potential as compared to their behavior on other subphases (water, calcium chloride, and sodium bicarbonate). These effects were attributed to the interfacial interaction, or more specifically the formation of a cation-mediated hydrogen-bonded soap network (Figure 1). At high charge densities, binding and network formation result in a degree of layering of charge parallel to the interface. Along with an altered subphase dielectric constant, this ordering of charge leads to a large decrease in the measured surface potential. The extent of this effect is reduced with increasing alcohol content, as the reduced charge density leads to a reorientation of the charged layers so as to present a more neutral arrangement of ions. Although comparatively disordered and hydrated, we believe that such a network forms as the precursor to crystal nucleation. The nature of the cation-mediated hydrogenbonded network provides a mechanism for achieving face selective nucleation. Moreover, since the formation of the network is adversely affected upon increasing monolayer alcohol content we would anticipate that face selective nucleation should be reduced as monolayer alcohol content is increased in accordance with the observed weakening of the cation-mediated hydrogen-bonded network. Here we present the results for calcium carbonate crystallization (specifically calcite) under a range of octadecanoic acid/octadecanol mixed monolayers. In particular, the aims of this paper were to (i) investigate the influence mixed monolayer composition has on calcite crystallization; (ii) elucidate the mechanism of the monolayer/crystal interaction; and (iii) assess if there is any correlation between this mechanism and the hypothesis that the formation of a cation-mediated hydrogen-bonded soap network functions as a nuclei precursor and defines face selective nucleation.
r 2009 American Chemical Society
Published on Web 08/20/2009
Introduction Synthetic investigations of biomineralization utilize many approaches aimed at exploring different aspects of the proposed generalized mechanism of biomineralization.1-3 A key component in the formation of biominerals, and indeed any solid substance, is the nucleation event. In the case of heterogeneous nucleation, nucleation on the cellular membrane, on a biopolymer scaffold, or induced by a macromolecule (either singular or as an aggregate) are the most important in the context of biomineralization. Confining ourselves to substratebased or templated nucleation, that is, mimics of the cellular membrane or biopolymer scaffold, we may loosely define three domains of investigation. The first explores the purely physical confinement afforded by a static substrate.4 The second also utilizes a largely static substrate but one that has specific chemical functionality in a defined symmetry, setting among other things, charge density and specificity.5 Finally, the third is similar to the second in that control of the nucleation event is of chemical and physical origin, but these aspects are now incorporated into a responsive substrate, the most common such example is growth on Langmuir monolayers.6-12 The first example of using a Langmuir monolayer as a substrate for nucleation was by Landau et al. in 1985.6 Since that time there have been many subsequent uses of such monolayers to explore the effects of, among other things, chemical functionality, charge density, spatial geometry, symmetry and dipolar interactions on crystal nucleation and in particular the ability of the monolayer to define face selective nucleation.13 Substantial evidence now exists that monolayer chemistry and symmetry exert an influence on crystal nucleation and growth.14-16 Precisely how this comes about however remains an enigma. In a previous study,17 we explored the intrinsic monolayer properties of mixed octadecanoic acid/octadecanol monolayers, the carboxylic acid and alcohol functionalities having
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Figure 1. An idealized schematic of the cation-mediated hydrogen-bonded network hypothesized to form under ODA/ODOH monolayers. The two views: (a) pure ODA, and (b) mixed ODA/ODOH monolayers indicate possible rearrangements necessary to accommodate different surface charge, lattice spacing, etc. and thus resulting in different crystal nucleation faces. It is important to note that the actual structure will be much less ordered with considerable water of hydration and structural defects. (Key: black is carbon, gray is hydrogen, red is oxygen, green is calcium, and the blue dashed bonds represent a O-H-O hydrogen bond).
These aims were explored by correlating the monolayer behavior with two key aspects of the resulting calcium carbonate crystal properties: face selective nucleation (or preferential orientation, qualitatively determined for each system using the interedge angle method9) and morphology. The results are also discussed from the perspectives of the roles played by charge density, lattice matching, and symmetry in controlling face selective nucleation. Experimental Section Octadecanoic acid (ODA, >99%, Merck) and octadecanol (ODOH, g 99%, Fluka) were used without further purification. Analytical grade chloroform (Labscan AR) was used as the spreading solvent and ultrapure deionized water (double-distillation fed Millipore purification unit, resistivity of 18.2 MΩ cm) was used to prepare all other solutions. ODA/ODOH ratios were established on a weight percent basis, with these values subsequently converted to mole percentages. Crystal growth experiments were performed in a NIMA 102 PTFE trough. Aqueous solutions of 20 mM CaCl2 (calcium chloride dihydrate, >99%, Sigma Aldrich) and 20 mM NaHCO3 (>99%, Romil) were bubbled with carbon dioxide at a rate of ∼10 L/min for at least 30 min. Equi-volumes of the two solutions were combined and added to the trough (pH ∼ 5.6). Fifteen microliters of ∼1.2 mg/mL surfactant solution was applied to the air/water interface. Pressures of either 10 or 25 mN m-1 were applied. These two fixed pressures were chosen as they lie in the tilted and untilted phases, respectively, for the majority of systems investigated.17 The trough was temperature controlled at 20 ( 1.0 °C. Additionally, humidity and air temperature were maintained constant. Crystals were harvested after ∼16 h of growth using 12 mm diameter hexamethyldisilazane treated glass coverslips via the horizontal Langmuir-Schaefer method.18 Crystals were imaged using a JEOL 5300 LVM SEM using backscattered mode, after being sputter coated with 4 nm of gold. Measurement of interedge angles, allowing the nucleation face to be ascertained, as described by Archibald et al.,9 was used to analyze crystal nucleation. From the digitized image the interedge angles were
measured using SemAfore software (Version 5.0, JEOL). 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).
Results Mineralization at the air/water interface in the absence of a monolayer is nonspecific. The product is randomly oriented rhombohedral calcite crystals with very little elongation and regions of intergrowth. Nucleation occurs preferentially at the interface where the surface energy is high; however, the induction times are typically longer than those in the presence of a monolayer. This is consistent with nucleation observed by Loste et al. under similar conditions.11 Calcium carbonate nucleation under monolayers has been well characterized,7,20-23 particularly for the more common surfactant systems of pure octadecanol and octadecanoic acid, though no studies have involved the mixed acid/alcohol systems. As such, in evaluating the growth form, under the mixed monolayers employed here we utilize the following crystal descriptors for calcite: type I (large rhombohedral plate-like crystals with {10.4} side faces), II (elongated irregular truncated rhombohedra), and III (truncated rhombohedra), in addition to the classic rhombohedral growth (see Figure 2 for examples of each of these morphological manifestations).8,11,18,24,25 Isotherm Pressure. Low Pressure (Π = 10 mN m-1). A comparison of the crystals grown under mixed monolayers at a fixed surface pressure of 10 mN m-1 is shown in Figure 3. The pure alcohol system (100 mol % ODOH) displays classic rhombohedral calcite with nucleation on the {10.4} faces (Figure 3a). There are very few defects and minimal elongation of the crystals. Upon initial substitution of ODOH with ODA (up to ∼40% substitution) little alteration in the form of
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the crystals is observed (see for example Figure 3b, 75 mol % ODOH system) with the exceptions that there is a greater number of nucleation events on increased substitution, and type III crystals begin to be sparsely evident. By 50 mol %
Figure 2. Exemplars of the four calcite morphologies (a) classic rhombohedral, (b) type III (truncated rhombohedral), (c) type II (elongated irregular truncated rhombohedral), and (d) type I (large rhombohedral plate-like crystals with {10.4} side faces). Scale bar = 10 μm.
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ODOH (Figure 3c), there is a significant increase in the propensity for truncation with very few {10.4} oriented rhombohedral crystals now apparent. The truncated rhombohedra are defect free (type III) though interdispersed is a small amount of type II crystals. Upon decreasing the alcohol content further (e.g., 25 mol. % ODOH, Figure 3d), a dominance of the irregular type II crystals arises and there is a concomitant increase in the average crystal size. Possibly as a result of the increased elongation and irregularity of the crystals, a substantial number of crystals are inverted during the harvesting procedure. This inversion permits a view of the crystal surface in contact with the monolayer, which shows considerable topography. A small proportion of the type II
Figure 3. Overview of the changes in crystal morphology as the octadecanol content of the monolayer is decreased at a constant pressure of 10 mN m-1. (a) 100 mol % ODOH, (b) 75 mol % ODOH, (c) 50 mol % ODOH, (d) 25 mol % ODOH, (e) 10 mol % ODOH, and (f) 0 mol.% ODOH. The insets show enlarged images of a typical crystal (the scale bar = 10 μm).
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crystals display the beginnings of a central depression as described by Rajam et al.,18 attributed to diffusion-limited growth. Decreasing alcohol content further (20 mol % ODOH) leads to the complete domination of type II crystals, the first occurrence of type I crystals, and almost no type III crystals. A further reduction of alcohol to 10 mol % (Figure 3e) shifts the numbers toward type I crystals with reduced type II and no type III. Crystal density remains high. In contrast, in the pure acid system (0 mol % ODOH, Figure 3f) there is a significant reduction in the number of crystals successfully harvested with a predominance for type II crystals with a small amount of defect-ridden type III. At low pressure, on decreasing the alcohol content in the mixed monolayers, a transition occurs from the classical rhombohedral morphology to an elongated irregular truncated calcite. This transition reflects the variation in binding capacity and double layer potential of the different monolayer compositions. Considering crystal morphology in isolation, two transitions are evident on decreasing alcohol substitution (the starting point being 100% classical rhombohedral calcite for 100 mol % ODOH): (1) the classical rhombohedral calcite to truncated rhombohedral calcite (type III) associated with nucleation on a face other than {10.4} occurring at ∼60 mol % ODOH, and (2) a switch from oriented but truncated rhombohedral calcite (type III) to elongated irregular calcite of types I and II occurring at ∼40 mol % ODOH, with type II initially dominating and finally type I dominating at 10 mol % ODOH. High Pressure (Π =25 mN m-1). Increasing the pressure does not alter the morphology realized under pure alcohol monolayers (Figure 4a), the crystals exhibit defect-free rhombohedral morphology with some elongation. Such crystals persist down to ∼40 mol % ODOH or lower, at high pressure (see for example 50 mol % ODOH, Figure 4b). As such, a considerably greater extent of acid substitution can be achieved at this higher pressure before a significant effect on crystal nucleation and growth is observed. Although delayed, type III crystals are in evidence on increasing acid content of the monolayers. By 25 mol % ODOH (Figure 4c), the crystal morphology realized at high pressure differs significantly from that observed at low pressure (Figure 3d). Rather than the general increase in crystal size and elongation moving toward type II crystals seen at 10 mN m-1, at 25 mN m-1 the morphology remains very similar to that obtained under 50 mol % ODOH. The morphology is dominated by type III crystals that exhibit few defects and little elongation. Further, a significant number of rhombohedral crystals are present, which are effectively absent at low pressures. As such, the transition from classical rhombohedral crystals to type III crystals is now a gradual one when the surface pressure under which the crystals are grown is increased. No distinct concentration corresponding to an abrupt change in morphological abundance is observed. By 20 mol % ODOH type I and II crystals are finally evident (these were first observed at ∼50 mol % ODOH at low pressure); however, type III crystals remain dominate with little elongation and only a small increase in the frequency of defects. At 10 mol % ODOH (Figure 4d) the degree of elongation, irregularity, and frequency of defects is increased from the 20 mol % ODOH system. However, in comparison to the low pressure system, morphology
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continues to be dominated by types II and III truncated rhombohedra. Crystallization under pure acid (0 mol % ODOH, Figure 4e) monolayers at high pressure is characterized by a decrease in nucleation density, increased elongation, size, and truncation. However, in comparison to the low pressure system, the increase in elongation, size, and truncation is lessened at high pressure. A transition on decreasing alcohol content in mixed monolayers from the classical rhombohedral morphology to an elongated irregular truncated rhombohedral calcite is evident. However, in contrast to the low pressure crystal morphology, this transition is significantly delayed. Taken from the perspective of the degree of truncation, elongation, and irregularity, the 0 mol % ODOH system at high pressure is equivalent to somewhere in the range of 50-25 mol % ODOH system at low pressure. Monolayer Stability. Investigating monolayer stability allows us to probe the interfacial interaction via changes to the equilibrium spreading pressure brought about by the nucleation and growth processes. The inherent equilibrium spreading pressure of a monolayer arises from the inter- and intramolecular interactions at the interface including surface pressure, hydrophilicity, hydrophobicity, solvation, hydrogen bonding, and ion-ion interactions among others. Consequently, the monolayer stability was monitored over a period of 16 h corresponding to the crystallization period. Under isobaric conditions, measurement of the area between the barrier arms and therefore the nominal area per molecule (Am) is a direct measure of monolayer stability. Monolayers were found to generally have reduced stability over this time period on a calcium carbonate crystallizing subphase as compared to pure water, 20 mM CaCl2 or 20 mM NaHCO3 subphases (see Suppporting Information ESD Figure 1). On water, pure ODOH has an equilibrium spreading pressure of ∼35 mN m-1 as compared to that for pure ODA, 2-7.3 mN m-1.26-28 As such, the ability of ODOH-rich monolayers to maintain surface pressures of 10 and 25 mN m-1 over the 16 h will be greater than monolayers rich in ODA. The observation of increased monolayer stability at low pressures, independent of alcohol content (see Suppporting Information ESD Figure 2), and with increased substitution amounts is consistent with the difference in the equilibrium spreading pressure. However, the extent of the monolayer stability variation on the calcium carbonate crystallizing subphase, on increasing ODA content, is incommensurate with the differences in the surfactant equilibrium spreading pressures alone (Figure 5). Two nearly constant regimes of stability are evident: 100-50 mol % ODOH where the monolayer is very stable and greater than 50 mol % ODA where the monolayer is very unstable (Figure 5). The data implies a stronger interaction between the monolayer and the subphase with the monolayer directly participating in the nucleation event for monolayers comprising greater than 50 mol % ODA. Crystal Orientation. The interedge angle method popularized by Archibald et al. was employed to probe the nucleation face/orientation of the nucleated crystals.9 While this technique does not result in a categorical assignment of the nucleation face, it is a good method for giving qualitative support for a preferred orientation. Results for crystals grown at low and high pressure are summarized in Figure 6. The relatively low percentage returns (∼20% for the most dominant faces) for any one
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Figure 4. Overview of the changes in crystal morphology as the octadecanol content of the monolayer is decreased at a constant pressure of 25 mN m-1. (a) 100 mol % ODOH, (b) 50 mol % ODOH, (c) 25 mol % ODOH, (d) 10 mol % ODOH, and (3) 0 mol % ODOH. The insets show enlarged images of a typical crystal (the scale bar = 10 μm).
face is consistent with the literature9 and is a reflection of errors in the technique and the dynamic nature of Langmuir monolayers. However, the consistent return of the {11.15} and {10.4} faces as dominant nucleation faces at both pressures supports the qualitative value of the technique. The {11.15} face is consistently the most common nucleation face at low alcohol contents for both pressures investigated. At high alcohol contents, a switch to the classic
rhombohedral {10.4} face occurs. Interestingly, the transition between these two faces differs for the two pressures investigated. At high pressure, an abrupt change from crystals nucleating on {10.4} faces to {11.15} faces occurs on increasing acid content at ∼60 mol % ODOH. In contrast, at low pressure this transition is more gradual, involving an intermediate orientation, {10.16}. The transitions occurring are, on increasing acid content: {10.4} to
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{10.16} at ∼60 mol % ODOH and {10.16} to {11.15} at ∼40 mol % ODOH. Upon reviewing the secondary faces (the second and third most common faces), the theme of a common zone axis becomes evident. At low alcohol contents, the faces (11.12) and (11.9) have the greatest representation after {11.15}, with all three faces belonging to the [11.0] zone axis
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(accounting for ∼30% of all crystals). This is in contrast to the [01.0] zone axis for the {10.4} and (10.16) and (10.10) faces (a cumulative total of ∼35%). Two transition events occur with decreasing alcohol content in crystals grown under mixed octadecanoic acid/octadecanol monolayers: (1) A nucleation face or orientation change from [01.0] to [11.0] (Figure 6), and (2) A morphological change from truncated rhombohedra (type III) to type II and I calcite. The specific monolayer composition for these observed transitions is pressure dependent. The zone axis transition is significantly delayed (i.e., the transition occurs at higher acid substitution) at 10 mN m-1 as compared to at 25 mN m-1. In contrast, the morphological change is shifted to higher acid contents at 25 mN m-1 as compared to at 10 mN m-1. Discussion
Figure 5. A comparison of the loss in area after 4 h of crystal growth for the two pressures (LP = 10 mN m-1 and HP = 25 mN m-1). Horizontal lines are to guide the eye only, highlighting the two almost constant domains.
For the systems investigated here it can be summarized that upon increasing surface pressure and/or decreasing ODOH monolayer content monolayer stability consequently decreases (Figure 5). Hence the most unstable monolayers are those with high ODA content at high pressures. Increasing alcohol content decreases crystal irregularity, irrespective of surface pressure (Figures 3 and 4). Consequently, for a given surface pressure increasing crystal irregularity corresponds to a decrease in monolayer stability (correlation of Figure 5 with Figures 3 and 4), with the crystal-centered transitions loosely
Figure 6. Synopsis of the low and high pressure nucleation face assignment data, showing the most prominent nucleation faces for the different mixed monolayer systems. Charge density is based upon a full ionized monolayer (given a surface pH of 7 at the point of crystallization) and an average Am of 20 A˚2/molecule. Where two crystal types are listed the predominant type is given first. A switch in zone axis is shown by a color change, blue [11.0] and green [01.0].
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correlating to a step change in the monolayer stability. Paradoxically, crystal irregularity is also decreased by increasing surface pressure. As such, an additional effect beyond the hypothesized cation-mediated hydrogen-bonded network must be influencing crystal nucleation and growth at high surface pressure and high ODA content. Generically, nucleation is driven by the metastability of the solution and the desire to minimize energy. It is for these reasons that heterogeneous nucleation is ubiquitous, with nuclei forming at interfaces. In the absence of a monolayer, crystal orientation is random but almost exclusively involves one of the faces of the form {10.4}. This is due to the {10.4} faces having the lowest surface energy (especially when hydrated),29 which is, in part, due to the high atomic density. The final crystal habit is a rhombohedron under equilibrium conditions. In nonequilibrium cases or when a third body is present, crystal morphology is also a function of crystal orientation and growth kinetics. Here the introduction of a monolayer sets up the propensity for enhanced matching at the interface leading to the appearance of higher energy crystal faces. Consequently, the expression of {10.4} faces under the high alcohol monolayers is indicative of little or no templation. This lack of templation for high ODOH content monolayers correlates directly with a weak interaction between the monolayer and the subphase, which is reflected in the stability of the monolayer, and points to the absence of a cationmediated hydrogen-bonded network for monolayers with high ODOH contents.17 While the headgroup of ODOH participates in H-bonding, the extent of H-bonding alone (without ion-ion interactions) between the monolayer and the subphase is insufficient to induce network formation. Further, there is evidence that the majority of the alcoholbased H-bonding occurs within the monolayer between the surfactants themselves.17 Hence, a weak interaction at high ODOH content irrespective of surface pressure leads to expression of the thermodynamically favored regular smooth {10.4} faces with little or no elongation. Effectively the monolayer acts purely to reduce the cost of nucleation but little or nothing else. Conversely expression of {11.15} faces under low alcohol content monolayers implies a stronger interaction across the interface, and a monolayer structure that has properties commensurate with that particular crystal face; the cationmediated hydrogen-bonded network now actively and directly participates in the nucleation event. The domination of the {11.15} faces over the compositional ranges of 0-40 and 0-60 mol % ODOH for the low and high pressure systems, respectively, reflects a considerable versatility and ability to accommodate defects by both the monolayer and the crystal nuclei, facilitated by the presence of the hydrogenbonded network. As such, the delineation of the orientation according to alcohol content is strongly indicative of face selective nucleation, as exemplified by the assignment of two zone axes. The instigation of face selective nucleation on decreasing alcohol content coincides with the formation of the cationmediated hydrogen-bonded network that exists at the interface.17 Nucleation begins with the agglomeration of ions, a process facilitated by the monolayer. Interactions (such as charge neutralization, dissociation, like-charge repulsion, hydrogen bonding, dipole interactions, dispersion forces among others) between the monolayer head groups, the head groups and hydrated ions, and between the ions themselves leads to some distribution of ions at the interface. The primary role for
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the arrangement of these ions is energy minimization and may involve bond formation. The concept of a linear chain-like structure, loosely styled on the cation-mediated hydrogenbonded network of crystalline NaHCO3,30,31 as a potential structure at the interface, accounts for this range of interactions. Further, such a structure conceivably directs face selective nucleation (Figure 1). Separation of similarly charged faces is not possible using charge density alone but can be achieved via network formation. The inclusion of the surfactant head groups in the network facilitates complementarity between the monolayer carboxylates and the calcite crystal carbonates leading to a particular orientation. The heterogeneity and dynamic nature of the monolayer and the variable subphase chemistry, on a molecular length scale, means that the interfacial interaction is more a case of synergy rather than rigid templation. The observed crystal morphologies/irregularities and reduced monolayer stability can also be attributed to the formation of the hydrogen-bonded network. Exploring monolayer stability first we find no reference to its role in defining crystal morphology in the literature. This is quite surprising given the strong correlation observed here. Monolayer stability is dependent on many factors including temperature, compression speed, equilibrium spreading pressure (surfactant and subphase chemistry), experimental pressure, and defects/perturbations. In combination these different effects result in very unpredictable behavior. For the systems investigated here the most important of these is the equilibrium spreading pressure of the surfactant itself and its modification through its interaction with the subphase. The step change seen with respect to monolayer stability on altering the ratio of alcohol to acid (Figure 5) correlates directly with an increase in crystal elongation and irregularity. Hence, monolayer stability is intimately linked to the crystal morphology, though at higher pressures it is modified through electrostatic effects and a reduction in steric freedom. As discussed above this transition at ∼50 mol % ODOH also correlates with the first evidence for formation of a hydrogenbonded network. How then can monolayer stability and the presence of a hydrogen-bonded network account for crystal elongation and irregularity? Elongation reflects anisotropic growth of the crystal, ultimately due to differing growth rates for the exposed facets. Given that classic or truncated rhombohedra have {10.4} faces exposed to the subphase it is unlikely that elongation is caused by face-specific surface energies. A more plausible explanation was put forward by Pokroy and Aizenberg for nucleation on a self-assembled monolayer (SAM).32 They correlated the direction of asymmetric lateral growth of the crystals to the direction(s) of greatest lattice match between the SAM and the crystal. In our case, this is facilitated through the chain-like hydrogen-bonded network that extends laterally inducing crystal elongation. For high pressure systems (and therefore high charge density), the influence of the network to extend laterally is moderated resulting in less irregularity and elongation. The reason for this will be elucidated below. The formation of the hydrogen-bonded network therefore participates directly and actively in the crystal nucleation. This interaction is found to be modified via the surface pressure. At low pressures, the relative rotational freedom of the individual surfactant molecules is likely to result in larger domains and generally greater monolayer stability. The suggestion of pressure-dependent domain size is supported
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Table 1. Selected Properties of the Dominant Nucleation Faces As Determined by Interedge Angle Measurements zone axis
angle to (00.1)
Ca2þ lattice spacing/A˚
Ca2þ lattice area/nm
(10.4)
[01.0]
44.63
a = 4.0320 b = 4.9688
0.20
(10.16)
[01.0]
13.86
a = 4.9688 b = 11.8195
0.59
(10.10)
[01.0]
21.54
a = 4.9687 b = 8.1015
0.40
(11.15)
[11.0]
24.51
a = 8.606 b = 13.7285
1.18
(11.12)
[11.0]
29.68
a = 6.3989 b = 8.6061
0.55
(11.9)
[11.0]
37.23
a = 8.6061 b = 9.4719
0.82
face
by increased nucleation density (see Supporting Information ESD Figure 3), most evident in the early stages of growth before ripening processes begin to dominate, where the high energy domain boundary sites readily promote nucleation. This relative rotational flexibility facilitates an improved interaction with the hydrogen-bonded soap network thus stabilizing the monolayer further. However, the resulting lateral growth (thus greater elongation and irregularity) associated with a strong interfacial interaction leads to a gravityinduced overall decrease in monolayer stability. Conversely, at higher pressures steric limitations restrict the monolayers ability to facilitate network formation through spacing and symmetry rearrangements, the network domain size is smaller and therefore more soluble. Coupled with the effect of the increased pressure and nucleation-related buckling, the monolayer is commensurately more unstable. The smaller network domains also result in less lateral growth in subsequent crystallization. While causality is difficult to determine at this stage the data allude to the direction being one of enhanced monolayer/ subphase interaction via the formation of the cation-mediated hydrogen-bonded network, which leads to enhanced crystal elongation and irregularity and a more unstable monolayer. The most important driver being the strong interaction between the monolayer and the subphase reflected in the formation of a hydrogen-bonded network. Such a network is not formed for monolayers with high ODOH content. That is, monolayer instability is not the cause of crystal morphology modifications; rather, at low pressures, it is an expression of a strong interfacial interaction associated with an extended cation-mediated hydrogen-bonded network. At high pressures, monolayer instability is more directly related to the low equilibrium spreading pressure of the monolayer and the associated network. Finally, closer examination of the properties of the faces assigned to the two zones ([01.0] and [11.0]) leads to the following correlations (Table 1). • All faces in the [01.0] zone have one Ca2þ lattice dimension of length 4.97 A˚, whereas the [11.0] zone faces have one dimension of 8.61 A˚. • Based on the two-dimensional Ca2þ lattice area, on average the [01.0] zone faces have a higher density than the [11.0] zone faces. Both are consistent with the low alcohol templated nucleation occurring on a high energy low density face.
Historically, face selective nucleation has been attributed to lattice matching,24 spatial geometry matching (often incorrectly termed stereochemistry)15 and more recently nonspecific electrostatics.12 In the following discussion, for each of these in turn, we will illustrate how the idea of a single dominant effect is overly simplistic. Lattice Matching. Lattice matching is related to the identification of equivalent lattice parameters and symmetry between the monolayer and specific crystal faces. The pure ODA monolayer at a pressure of ca. 25 mN m-1 has an Am of 19.2 A˚2 and a hexagonal lattice cell (a = 4.71 A˚).10 Therefore, given the slightly larger Am of 19.8-20.5 A˚2 at low pressure and 19.3-19.7 A˚2 at high pressure for the mixed systems on the calcium carbonate crystallizing subphase, and assuming a similar symmetry, nucleation on the hexagonal (00.1) face of calcite (a = 4.97 A˚) might be expected. This approach however ignores the capacity for the monolayer to change with both time and the nucleation event.33 The likelihood of monolayer rearrangement is not inconsequential, with the extent of monolayer restructuring depending on the strength of the overall interfacial interaction and steric considerations. Although monolayer restructuring complicates the design process, it does offer improved matching facilitated by the ability (even if limited) for the monolayer and nuclei to find the most energetically favorable conformation. Assessing the level of lattice matching in this study, we find a significant mismatch between the monolayer (Am ∼ 20 A˚2) and the {11.15} faces (Ca2þ lattice area ∼ 118 A˚2). At room temperature on a calcium carbonate crystallizing subphase, the monolayer has a hexagonal lattice10 which is inconsistent with the rectangular symmetry of the {11.15} faces. The large crystal lattice may however originate from the necessity of finding a common multiple for the two sets of lattice dimensions as described by Kewalramani et al.34 Alternatively, this disparity could reflect a low surface charge in the mixed monolayer systems, brought about by the presence of alcohol and/or significant protonation of the acid molecules.17 Therefore, considering simultaneously the initial interfacial pH, the dynamics of the monolayer, the ability of the crystal nuclei to accommodate defects, and the hydrated and likely amorphous nuclei then there is reasonable scope for a good interfacial fit. However, there is insufficient evidence to state that lattice matching is the controlling interaction. Spatial Geometry Matching. A comparison of the interplane angles for the different nucleation faces enables an
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assessment of the spatial geometry matching between carbonate anions and the carboxylate head groups. In calcite the carbonate ions are aligned perpendicular to the c axis, and therefore the angle of the nucleation face to the (00.1) face provides a measure of the angle of the planar carbonates to the nucleation plane. These angles for the various predominant nucleation faces are given in Table 1. Given that the monolayer is untilted35 and assuming that the chain is in an all-trans configuration with herringbone packing (likely due to the reduced area) then the theoretical carboxylate orientation would be at an angle of 45-90° to the interface and consequently perpendicular to the nucleation plane. However, the combination of low surface pressure, reduced likecharge repulsion, and the smaller alcohol headgroup allows greater freedom for the carboxylate to reorientate. Molecular modeling performed by Duffy and Harding shows that there may be a considerable range of orientations present.36 The system modeled was a SAM of 25 16-mercaptohexadecanoic acid molecules in vacuo at 2 K. The angle of the final H2C-COOH bond is ∼45° to the substrate. In contrast, the addition of water, Ca2þ and HCO3- ions at 300 K leads to a very broad bimodal distribution spanning a of 0.5-75° for an even H2C-COOH bond angle √ range √ length chain with a 3 3 R30° structure. The most intense peak (frequency ∼3.0 molecules) is located at ∼25° and a second weaker broad peak (frequency ∼ 1.5 molecules) is centered at ∼54°. The two peaks correlating to the {01.2} (27°) and {01.5} (52°) faces rather than the {00.1} (90°) hexagonal face, which based on symmetry provides a better match to the hexagonal SAM. While this investigation is based on a SAM model, involving tilted molecules, which are therefore well spaced providing the carboxylate head groups significant rotational freedom, the results do suggest that the angles for all the faces given in Table 1 are possible. As for the case of lattice matching, there is insufficient evidence to suggest that spatial geometry matching of the carboxylate and carbonate groups directs face selective nucleation. However, the importance of spatial geometry matching in face selective nucleation is dependent on steric implications, and therefore its role will vary with surface pressure and surfactant chemistry. Electrostatics. Electrostatics is the most commonly employed explanation for face selective nucleation. Typically, discussion of electrostatics is based around charge density with a focus on ion-ion interactions, with the potentially important dipole interactions, which can impart directionality, being neglected. If we consider the case where monolayer charge density is moderate then there exists some reasonably large number of calcite faces that can match the monolayer charge density. Therefore, any experimental realization of face selective nucleation from this subset of possible crystal faces must be the result of other factors. For medium to low charge densities, dipole, symmetry and lattice interactions play an increasingly important role. This is apparent in a number of contradictory and anomalous trends in the results including: • two faces of relatively fixed surface termination, (11.15) and (10.4), dominate comparatively large ranges of monolayer surface charge; • the transition between these two faces differs for low and high pressures, yet the mean monolayer surface charge varies negligibly; and
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• the transition at high pressure occurs relatively abruptly (at ∼40 mol % ODOH), which is inconsistent with the rule of mixtures behavior of the maximum surface potential.17 In contrast if monolayer charge density is high then there are relatively fewer crystal faces that will match this high charge density. In the case of calcite, these are the {00.1}, {01.2}, and {01.5} faces. Hence, one can stipulate in this case that electrostatics alone, or indeed ion-ion interactions, induce face selective nucleation. However, because there are now so few crystal faces that are feasible face selective nucleation results effectively by default, assuming a strong interfacial interaction exists. Moreover, high charge densities also influence nucleation kinetics, potentially limiting the options for face selective nucleation. The evidence supporting the importance of electrostatics in face selective nucleation is strong, but the inconsistencies highlighted above suggest that they are neither the sole nor necessarily the dominant effect. Individually, lattice, spatial geometry, or electrostatic matching is unable to explain the observed results. The concept of a cation-mediated hydrogen-bonded network incorporates all the above aspects and adequately accounts for the results. In the pure alcohol system, interaction between the monolayer and subphase is weak with little or no control over crystal nucleation. As alcohol is substituted by acid, the interaction begins to be enhanced and the cation-mediated hydrogen-bonded network begins to be evident. With substitution comes increasing monolayer charge. When this is sufficient, face selective nucleation is observed which occurs at a lower level of substitution for higher monolayer pressure. The enhanced flexibility of the monolayer at low surface pressure allows for increased lateral interaction between the monolayer and the growing crystal resulting in greater crystal elongation. Conclusions In this study, we investigated the properties of calcite nucleation and growth under a range of mixed octadecanoic acid/octadecanol monolayers at two pressures. The results provide evidence of face selective nucleation of calcite. An orientation transition with respect to nucleation zone axis occurs at ∼50 mol % ODOH (the actual value is surface pressure dependent), corresponding to a switch from the [01.0] axis to [11.0] the axis. Additionally, there is an associated transition in the morphology of the crystals from classic rhombohedra to irregular plate-like (type I) crystals with decreasing ODOH content. These transitions are associated with a change in the monolayer stability indicative of a strong interfacial interaction. Evaluation of the roles played by electrostatics, lattice, and spatial geometry matching in the templation of calcite crystals shows that individually they cannot explain the observed face-selective nucleation. A cation-mediated hydrogen-bonded network that includes surfactant, counter- and co-ions accounts for the observations. Such a network encompasses the above effects while providing a mechanism for the directionality required to differentiate between different calcite crystal faces. Network formation explains crystal morphological effects and combined with the implications of the equilibrium spreading pressure explains the monolayer behavior.
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Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. (19)
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