Nonequilibrium 2-Hydroxyoctadecanoic Acid Monolayers: Effect of

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Nonequilibrium 2-Hydroxyoctadecanoic Acid Monolayers: Effect of Electrolytes Conrad D. Lendrum,†,‡ Bridget Ingham,‡ Binhua Lin,§ Mati Meron,§ Michael F. Toney,|| and Kathryn M. McGrath†,* †

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MacDiarmid Institute for Advanced Materials and Nanotechnology, School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington, New Zealand ‡ Industrial Research Limited, Gracefield, Lower Hutt, New Zealand § Center for Advanced Radiation Sources University of Chicago, Chicago, Illinois, United States Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California, United States

bS Supporting Information ABSTRACT: 2-Hydroxyacids display complex monolayer phase behavior due to the additional hydrogen bonding afforded by the presence of the second hydroxy group. The placement of this group at the position R to the carboxylic acid functionality also introduces the possibility of chelation, a utility important in crystallization including biomineralization. Biomineralization, like many biological processes, is inherently a nonequilibrium process. The nonequilibrium monolayer phase behavior of 2-hydroxyoctadecanoic acid was investigated on each of pure water, calcium chloride, sodium bicarbonate and calcium carbonate crystallizing subphases as a precursor study to a model calcium carbonate biomineralizing system, each at a pH of ∼6. The role of the bicarbonate co-ion in manipulating the monolayer structure was determined by comparison with monolayer phase behavior on a sodium chloride subphase. Monolayer phase behavior was probed using surface pressure/area isotherms, surface potential, Brewster angle microscopy, and synchrotron-based grazing incidence X-ray diffraction and X-ray reflectivity. Complex phase behavior was observed for all but the sodium chloride subphase with hydrogen bonding, electrostatic and steric effects defining the symmetry of the monolayer. On a pure water subphase hydrogen bonding dominates with three phases coexisting at low pressures. Introduction of calcium ions into the aqueous subphase ensures strong cation binding to the surfactant head groups through chelation. The monolayer becomes very unstable in the presence of bicarbonate ions within the subphase due to short-range hydrogen bonding interactions between the monolayer and bicarbonate ions facilitated by the sodium cation enhancing surfactant solubility. The combined effects of electrostatics and hydrogen bonding are observed on the calcium carbonate crystallizing subphase.

’ INTRODUCTION Hydroxy fatty acids have attracted attention as model bipolar amphiphiles for studying the effect of secondary polar groups.1
3 These studies have illustrated the inherent complexity of these systems, brought about by the competition between the two polar groups. Substitution of the hydroxy group at the C2 position results in very different behavior from that of midchain and terminal substitutions.4 These differences arise from the ability for the two polar groups to function as a large single polar entity. In addition to the capacity for the hydroxy group to hydrogen bond with neighboring molecules, hydroxy acids more generally have been linked with strong chelation of calcium ions.5 The relative ubiquity of 2-hydroxy acids in food (including lactic and citric acids6,7), therapeutic and cosmetic (for example glycolic and 2-hydroxyoctadecanoic acid8,9) products, purports to the importance of understanding these systems. The 2-hydroxy acid system investigated here is based on the octadecanoic acid parent. r 2011 American Chemical Society

To date, research into the behavior of long chain hydroxy acids has focused on the fundamental questions surrounding the bipolar nature of the molecules, with the 2-hydroxy moieties being explored as part of comparative investigations.4,10
12 Thus the extent of our current knowledge with regard to monolayer phase behavior is limited to equilibrium conditions on passive subphases (low pH and/or high ionic strength). We have little understanding of the nonequilibrium behavior of the monolayer; additionally, the influence of chelating ions has not been addressed. Fundamental equilibrium studies provide the foundation upon which the response of monolayers to nonequilibrium conditions, associated with a supersaturated calcium carbonate crystallizing subphase can be compared. Vollhardt et al.10 showed that 2-hydroxyoctadecanoic acid (2-HOA) forms a two-phase coexistence region at zero surface pressure. This contrasts with the Received: December 12, 2010 Revised: February 25, 2011 Published: March 18, 2011 4430

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Langmuir bipolar midchain substituted hydroxy acids, which present twophase coexistence regions (represented as a plateau in a Π
A isotherm) at higher pressures.10,12 This difference lies in the separation of the hydrophilic groups within the surfactant and their capacity to hydrogen bond. The monopolar nature of 2-hydroxy acid enables it to form condensed domains stabilized by intermolecular hydrogen bonds without the application of pressure. In contrast, the bipolar midchain molecules require pressure to drive the hydroxy group from the subphase, at which point it becomes available for intermolecular hydrogen bonding leading to the stabilization of condensed domains. Kellner and Cadenhead1 showed that for 2-hydroxyhexadecanoic acid (2-HHA) the monolayer is condensed at all pressures with a very rigid film that displaced the Wilhelmy plate from vertical. The rigidity of the monolayer was attributed to hydrogen bonding between the hydroxy group and the neighboring carbonyl oxygen; also resulting in an increased equilibrium spreading pressure (ESP) and melting point for 2-HHA compared to hexadecanoic acid, (17.8 ( 0.2 mN m
1 and 87 °C, 2-HHA, cf. 10.7 ( 0.8 mN m
1 and 63 °C, hexadecanoic acid). The increased monolayer stability, due to intermolecular hydrogen bonding between neighboring molecules, has also been associated with higher pKa values for 2-HOA.13 In addition to enhanced film rigidity, large areas per molecule, ∼0.25 nm2/molecule cf. ∼0.20 nm2/molecule for a typical fatty acid, result from the presence of the bulky side group.1 The large area per molecule (Am), and associated headgroup/tail size mismatch, decreases the capacity for tail
tail interactions affording greater configurational freedom to the alkyl chains leading to considerable tail disorder. One consequence of this tail disorder is the inability for 2-HOA to achieve any degree of long-range tilt order. Grazing incidence X-ray diffraction (GIXD) studies14 of 2-HHA monolayers highlight the implications of this inability for coherent packing in the long-range order of the monolayer. Weidemann et al.14 reported a distribution of diffraction intensity along a characteristic arc in reciprocal space. The arc-shaped diffuse diffraction intensity was attributed to a superposition of different lattices, ascribed to variations of tilt azimuth, arising from the disordered packing of the alkyl chains. Again the disorder was attributed to a size mismatch between the enlarged headgroup and the hydrocarbon chain.15 With packing limited by the headgroup size, the tails are afforded greater lateral freedom, diminishing the potential for tilt ordering. The propensity of hydroxy acids to chelate calcium, their capacity to hydrogen bond, and the combination of the carboxylate and alcohol groups (previously found to be influential in monolayer behavior16) in a single molecule, makes 2-HOA a good candidate for biomineralization studies. Mineralization under a monolayer involves nonequilibrium conditions, due to the inherently dynamic heterogeneous environments and comparatively complex subphase compositions. Additionally, fast compression speeds are required in order to avoid premature mineralization. This is in contrast to the fundamental studies discussed above. In this study, we have characterized the behavior of 2-HOA monolayers on a calcium carbonate crystallizing subphase (CCCS), along with subphases of the constituent components: water, aqueous calcium chloride, aqueous sodium bicarbonate, and aqueous sodium chloride. In particular, we have probed the influence of the hydroxy group on the formation of the cation-mediated hydrogen-bonded network, proposed as a precursor to calcium carbonate crystallization in the octadecanoic acid system. These investigations form the basis for the subsequent crystallization study. Experimentation involved surface pressure and potential measurements

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complemented by Brewster angle microscopy imaging, grazing incidence X-ray diffraction, and X-ray reflectivity.

’ EXPERIMENTAL SECTION DL-2-hydroxyoctadecanoic acid (2-HOA, 99%, Sigma Aldrich) was used without further purification. Analytical grade chloroform (Labscan AR) was used as the spreading solvent. The 2-HOA in chloroform (∼0.5 mg/mL) was added dropwise to the surface of the aqueous subphase solutions and left for a minimum of 10 min to equilibrate. The aqueous subphase solutions (20 mM unless stated otherwise) were made from calcium chloride dihydrate (Sigma Aldrich), sodium bicarbonate (Romil), sodium chloride (Fisher Scientific) and ultrapure deionized water (resistivity of 18.2 MΩ 3 cm). The pH of the NaHCO3 subphase (pH ∼8.5) was reduced by bubbling carbon dioxide through the solution, generating a CO2 supersaturated solution with a pH ∼6.0. The calcium carbonate crystallizing subphase (pH ∼5.8) was obtained by combining equal volumes of 20 mM CaCl2 and 20 mM NaHCO3 solutions, presupersaturated in CO2(g). Surface pressure and potential isotherms were measured at room temperature (∼20 °C) using a NIMA 702BAM PTFE trough. Isotherms were performed at a compression speed of 100 cm2/min. This relatively high speed was adopted in order to minimize the risk of calcium carbonate nucleation, which would modify the monolayer behavior. Surface potential measurements were made using a Trek Electrostatic Voltmeter (320C) and a 3250 high-sensitivity vibrating-plate probe from Trek INC, Medina NY. In all cases a zero voltage was established on the bare subphase before the surfactant solution was added and equilibrated. The surface potential measurements were collected in conjunction with the surface pressure and area by the NIMA software (Version 5.16). For all systems at least 5 isotherms were performed and the data averaged. Brewster angle microscopy (BAM) images of the monolayer were collected using a MicroBAM2 from Nanofilm Technologie GmbH. With a fixed angle of incidence 53 ( 2° (the Brewster angle for an air/water interface) and parallel polarized light, the reflectivity differences associated with variation in the refractive index created by the monolayer were imaged. A class B laser diode light of 659 nm and >20 mW, with a maximum optical power of 30 mW at the aperture of the instrument was used as the light source. The beam is collimated with a diameter of approximately 6 mm. Grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity (XRR) were performed at the Argonne National Laboratory, Advanced Photon Source (APS) on ChemMatCARS beamline 15ID at an X-ray wavelength of 0.1240 nm (10.0 keV). The beamline was fitted with a Langmuir trough for in situ X-ray scattering from the liquid surface. For a detailed description of the trough setup see Schultz et al.17 Pressures were maintained using the NIMA 5.16 software, and the trough temperature was maintained at ∼20 °C. GIXD data were collected using an area detector, PILATUS 100K, in either a pinhole geometry with a resolution of 5 mrad, or line scans taken in a two-slit geometry with a resolution of 2 mrad. XRR measurements were performed using an Oxford Cyberstar 1000 scintillation photomultiplier and the Pilatus 100K detector. Measurements were made for Qz values up to 0.8 Å
1 (∼37Rc), with a resolution of 3 mrad. Further details of the GIXD and XRR data analyses performed can be found in the Supporting Information.

’ RESULTS AND DISCUSSION Pure Water Subphase. The surface pressure profile of the 2-HOA monolayer on a pure water subphase displays a single phase transition at an area per molecule of ∼0.25 nm2 (Figure 1). This single phase transition is consistent with literature studies performed at pH 34, despite the increased pH (5.6) and compression speed (0.05
0.1 nm2 molecule
1 min
1). 4431

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Figure 1. Representative surface pressure and potential isotherm data for 2-HOA on the four subphases examined: pure water, 20 mM calcium chloride, 20 mM sodium bicarbonate, and a calcium carbonate crystallizing subphase. The latter two subphases were supersaturated with carbon dioxide to reduce the pH.

Immediately following evaporation of the spreading solvent the 2-HOA monolayer is comprised of two coexisting phases (liquid-like and condensed phases), as indicated by Brewster angle microscopy (BAM). The application of pressure results in a comparatively sharp onset transition associated with the loss of the liquid-like phase. Increasing pressure leads directly to collapse (Figure 1) without any apparent secondary transition, such as the tilted to untilted L2
LS transition observed for octadecanoic acid.18 BAM is sensitive to changes in refractive index, or more specifically, changes in the thickness and/or density of the monolayer. Therefore the absence of BAM contrast when probing the condensed 2-HOA monolayer is consistent with the monolayer existing as either an untilted phase or as an universally heterogeneous film over BAM length scales. Since no L2
LS-like transition was observed at higher pressures these data are more consistent with a heterogeneous monolayer. The GIXD data (Figure 2) support this conclusion with the observation of outof-plane diffraction peaks. At all pressures investigated, the presence of variable tilt was observed. The GIXD distribution of scattering intensity is dominated by three distinct peaks along an arc. The distribution of scattering intensity observed in the 2-HOA GIXD data is somewhat similar to that presented by Weidemann et al.14 (note that while data were presented for a number of different surfactants, 2-HOA was not among them). The majority of the data presented by Weidemann et al.14 however show an approximately smoothly decaying intensity along an arc. This contrasts with the data obtained here where the distribution is dominated by three distinct peaks and is therefore more consistent with the behavior observed for mixed hexadecanoic acid/2-HHA monolayers.15

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Figure 2. Top: GIXD of 2-HOA monolayer on a pure water subphase at Π = 10 mN m
1, the circles highlight the peak intensities. Bottom: integrated intensity fits of three peaks, which include a background term. Integration involved a complete summation of data in either the Qxy or Qz directions.

The calculated domain correlation lengths (DCL) and the BAM data indicate that the 2-HOA monolayer exists as a heterogeneous film, with respect to tilt variation, which appears uniform at the mm length scale probed by BAM. The inability of the monolayer to develop long-range tilt order may be a result of the high compression speeds, inhibiting domain coalescence. The observation that the distribution of scattering intensity consists of three peaks is normally associated with the monolayer being comprised of an intermediate or oblique tilted lattice, however for this to be the case the condition Qaz = Qbz þ Qab z must be met with approximately equal integrated peak areas.18 This is not the case for the 2-HOA GIXD data (Figure 2). For example, at Π = 0 mN m
1, the Qz values of 0.844, 0.525, and 0.00 Å
1 do not fulfill the requirements for intermediate tilt. Therefore the occurrence of three peaks in the GIXD data for 2-HOA points to the 2-HOA monolayer existing as a coexistence of three different lattice symmetries. Specifically, a combination of nearest neighbors (NN), next nearest neighbors (NNN) tilted phases and an untilted (U) phase. The result of this postulation is common dimensional lattices between the phases. The a lattice parameters of the NNN and U phases are equal, similarly the b lattice parameters of the NN and U phases are also equal (Supporting Information, Table S1). The latter reflecting the shared in-plane peak for the NN and U phases, though we note that most commonly untilted phases have smaller lattice parameters and therefore diffract at a higher Qxy. The occurrence of two tilted phases may reflect alternative conformational arrangements to suit the energetics of the surfactant/subphase interaction, maximizing hydrogen bonding and/or the propensity of the hydrophilic hydroxy group to interact with the water subphase. The equivalent lattice parameters thereby reflect the untilted phase fulfilling an intermediate role between the two tilted phases. 4432

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Langmuir Peak assignation (see Supporting Information) has the highest Qz peak arising from a combination of the (1, 1) (1,
1) degenerate NN and the (0, 2) nondegenerate NNN reflections, the middle peak representing the (1, 1) (1,
1) degenerate NNN reflections, while the in-plane peak embodies contributions from both the (0, 2) nondegenerate NN and the triply degenerate U reflections. This therefore allows us to estimate the phase composition of the 2-HOA monolayer on the pure water subphase by correlating the integrated peak areas. For example at Π = 0 mN m
1 the ratio of NNN:NN:U phases is estimated to be 51:49:0, based on the expected integrated area ratios associated with the respective degeneracy of the three phases. Hence, approximately half of the monolayer is tilted in the NNN direction. This value immediately increases to approximately 60% at Π = 5 mN m
1, followed by a monotonic decrease of the NNN proportion of the 2-HOA monolayer as surface pressure is increased to 30 mN m
1. In contrast, the initial amount of the NN phase is reduced in favor of the NNN phase and the formation of an U phase. The amount of U phase present increases to ∼8% of the 2-HOA monolayer and then remains constant. Beyond Π = 10 mN m
1, the decreasing trend in the proportion of NN phase is reversed. At Π = 30 mN m
1 the three phases coexist as NNN: NN:U = 41:51:8. There is a suggestion that the NNN phase content is reduced in favor of the NN phase. Consequently, there is evidence for some reordering in response to the application of pressure. A summary of the change in the phase composition of the monolayer as a function of surface pressure is given in the Supporting Information (Figure S2). Calculated lattice parameters (Supporting Information, Table S1) show that the reduction in area per molecule is accommodated through a linear reduction of the aNN (from 0.597 to 0.521 nm) and bNNN (from 0.986 to 0.891) dimensions with pressure (Supporting Information, Figure S2). This reduction in the lattice parameters correlates with the reduction in tilt angle from 38.7 and 33.5° to 24.7 and 21.5° for NN and NNN phases, respectively, such that the direction of shrinkage of the lattice is consistent with the reduction of tilt angle for a specific tilt direction. Thus the application of pressure and associated reduction in area per molecule is accommodated by both a reduction in lattice spacing and tilt angle, and the appearance of an U phase, but there is no improvement in the film order. Heterogeneity in the tilt direction of the monolayer has previously been attributed to a size mismatch between the large monopolar headgroup and the tail.14,15 The molecular spacing in the monolayer is dominated by the large head groups, limiting the tail
tail interactions. A variation of the tilt angle is energetically expensive therefore the tails adopt the observed tilt variation to increase monolayer entropy: the headgroup/tail mismatch leading to an entropically driven disordering of the monolayer tilt angle. However, with the application of pressure the degree of monolayer disorder is reduced with the reduction in tilt (which is observed for both the NN and NNN phases), as indicated by the shift of the peaks to smaller Qz, and larger Qxy or smaller lattice values with increasing pressure. For the 2-HHA system Wediemann et al.15 showed that such a transition culminates as a single, in-plane peak at Π = 45 mN m
1. However this was not observed in the 2-HOA system. This is most likely due to the higher subphase pH used here (5.6 cf. 2) which results in greater monolayer instability, leading to a somewhat premature collapse. Although not directly observed in the pressure/area isotherm, collapse is consistent with brittle-fracture, characterized by a sudden and jerky movement of the monolayer. The brittleness of the monolayer

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reflects the extensive hydrogen bonding that the hydroxy group adds to the system.1 In terms of the surface potential behavior it is helpful to compare the profile with a generalized Demchak Fort model,19 the inverse ‘s’ curve providing some insight into the molecular rearrangements that account for the observed potential profile. From large to small areas per molecule (Figure 1), a typical profile shows a rise from ∼0 mV to a plateau at ∼50 mV at an area per molecule of ∼0.45 nm2. This transition occurs at larger area per molecule than observed for octadecanoic acid (∼0.35 nm2), reflecting the immediate formation of two coexisting phases. Furthermore, the plateau potential is significantly lower for the 2-HOA system compared with that observed for octadecanoic acid (∼50 mV cf. ∼250 mV, respectively, Supporting Information, Figure S3), which is a reflection of chain disorder leading to a reduced chain length perpendicular to the interface, resulting in a reduced molecular dipole. Further differences of the 2-HOA surface potential data from those obtained for the octadecanoic acid system are evident with the lack of a rise in the potential profile associated with the onset transition. For octadecanoic acid, as the surface pressure increases the observed reduction in tilt angle is reflected by a gradual increase in the surface potential as the tail becomes more erect and elongated. This is not observed for 2-HOA monolayers on a pure water subphase. This is due to the small portion of the monolayer that becomes untilted (∼8%). Finally, there is a small drop in the potential profile, which in straight chain fatty acids is associated with the initiation of collapse, where the formation of 3-D structures results in a countering of the original molecular dipole. The nature of this decrease in surface potential is specific to the mechanism of collapse. Overall the average maximum surface potential of 2-HOA on a water subphase is ∼52.6 mV, which is considerably lower than the ∼260 mV and ∼420 mV recorded for pure octadecanoic acid and octadecanol monolayers, respectively (Supporting Information, Figure S3). Such a significant drop in the maximum potential can, at least in part, be attributed to the larger inherent molecular dipole, combined with the capacity for chain disorder, and intermolecular and intramolecular order, the latter also affecting the film thickness and therefore the molecular dipole. Additionally, the increased propensity for intermolecular hydrogen bonding with the hydroxy group could affect the potential in two ways: (1) it may result in significant changes in the water structuring at the interface; (2) it may increase the double layer potential by increasing the degree of dissociation. Ultimately further work is required to elucidate this drop in the measured surface potential. XRR data for 2-HOA on a water subphase were measured at two pressures, Π = 10 and 25 mN m
1 in order to monitor film thickness and to probe subphase/monolayer interactions and potential monolayer collapse (Supporting Information, Figure S4
S7). Given the three-phase coexistence (NN, NNN, and U) and the three hour measurement time, the XRR data are a weighted average. Using the weighted average Am from the GIXD data, the total electron count was calculated. On the basis of the electron density ratio of the box model fits and the number of electrons, molecular groups were assigned to the boxes. At both pressures a two box model provided the best fit despite the enhanced H-bonding, which may manifest itself as three distinct electron density domains (Supporting Information Figure S4 and S6), however the difference in the electron density due to this enhanced H-bonding is most likely below the resolution of the technique. For Π = 10 mN m
1 this resulted in a model consisting of a [CH3(CH2)13] box and a 4433

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Langmuir [(CH2)2CHOHCOOH þ 2  H2O] box. In comparison, at Π = 25 mN m
1, the first box accounts for the 16 tail-carbons, and the second box accounted for the [CHOHCOOH þ H2O] headgroup. The differences in electron density profiles between these two surface pressures are subtle and may reflect conformation differences. The total film thickness for Π = 10 mN m
1 is 2.30 nm, in comparison to 2.21 nm for Π = 25 mN m
1. Both values indicate a thinner monolayer than that typically reported for straight chain fatty acids of 2.5 nm,20 which is consistent with some degree of conformation modification. In summary, the data presented here for 2-HOA monolayers on a water subphase are consistent with literature given the higher subphase pH. The monolayer consists of a three-phase coexistence of NNN, NN, and U phases up to Π = 30 mN m
1. Collectively, surface pressure, surface potential, BAM, GIXD, and XRR data point to some degree of chain disorder brought about by the headgroup/tail size mismatch. Calcium Chloride Subphase. The introduction of calcium ions to the aqueous subphase has a dramatic effect on monolayer phase behavior, consistent with its use as a calcium chelator in therapeutic products.9 A comparison of the Π
A isotherm data with those measured for 2-HOA on the water subphase highlights an increased Am at both onset and Π = 10 mN m
1, a trend reversed at higher pressures (Figure 1). Beyond onset, the rise in pressure is curvilinear in shape compared to the sharp rise observed on the water subphase. This is a reflection of the increased solubility of the surfactant brought about by the capacity for cation binding and the associated disruption to intermolecular hydrogen bonding. Loss of surfactant from the monolayer is evidenced by an Am at collapse of ∼0.17 nm2, which is smaller than the cross-sectional area of the molecule. Collapse is characterized by a smooth inflection of the isotherm correlating with a similarly gradual increase in the nucleation of 3-D discontinuities as directly observed using BAM. The small spherical, highly reflecting nature of these defects is consistent with surfactant solidification. In some instances a solid precipitate was observed macroscopically at the air/water interface subsequent to the experiment. In contrast to the Π-A isotherm data, GIXD data show a single untilted monolayer phase with hexagonal symmetry and an Am of ∼0.20
0.21 nm2 at all pressures (Figure 3). This is considerably lower than the Am of 0.45 nm2 at onset indicated by the Π
A isotherm data. However, the regions between the rigid islands are likely to contain gaseous-like surfactant, which would not diffract. This leads to a large isotherm-derived Am compared to that from GIXD. BAM images help to further elucidate this difference, showing that the monolayer consists of rigid multidomain islands (based on contrast differences). These are approximately 0.3 to 3.0 mm in size, with the majority larger than ∼1 mm (Supporting Information, Figure S10). The application of pressure causes these islands to collide and fracture rather than merge, indicative of strongly bound rigid domains. It is only at higher pressures that there is sufficient force to lead to a merging of the islands. Thus the perturbed isotherm and large isotherm-derived Ams are consequences of macroscopic island packing defects, brought about by the strong Ca2þ binding capacity of the hydroxy acids. On the CaCl2 subphase the surface potential profile (Figure 1) is similar to that observed on the water subphase. Surprisingly, given the contrasting Π
A and GIXD data for the two subphases, the only notable difference in the potential profiles is a small rise at ∼0.25 nm2. However, as the 2-HOA monolayer is untilted at all pressures, this rise in the potential cannot be associated

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Figure 3. GIXD of 2-HOA on the calcium chloride subphase at Π = 10 mN m
1.

with a loss of tilt. Rather, the rise is consistent with the condensation of the monolayer at high pressures, where the higher pressure forces the rigid islands to fracture and pack together. The resulting rise in potential is then followed by a decline, again associated with the nucleation of 3-D phases. The maximum potential is slightly higher than that observed for the water subphase (68.8 mV compared to 52.6 mV for water; N.B., these values are averages obtained over multiple experiments, the data presented in Figure 1 is for a single experiment), which can largely be attributed to the absence of tilt on the CaCl2 subphase. As with the Π
A data, the XRR measurements are also confounded by the heterogeneity of the 2-HOA film. At Π = 10 mN m
1, the coexistence of two phases results in a complex electron density profile (Supporting Information, Figure S11). In contrast, at Π = 25 mN m
1, the profile reflects a more coherent single-phase film, however the XRR data are now complicated by the solubilization of surfactant into the subphase and/or the formation of 3-D structures (Supporting Information, Figure S12). This is evidenced in the low pressure electron density profile by an artificially shortened tail, while an extended subphase region is present in the high pressure profile. Qualitative measurements of the film thickness at both pressures highlight a significant increase in the thickness (>2.5 nm) compared to the 2-HOA monolayer on the water subphase, consistent with the absence of tilt. In summary, the introduction of calcium ions into the subphase has a dramatic effect on the monolayer phase behavior. Strong cation binding results in the formation of rigid domains and increased surfactant solubility, which perturb the Wilhelmy plate (and therefore the surface pressure measurements), and distort the XRR data. However, the combination of techniques enables the determination of the molecular interactions and thus clearly illustrates the different monolayer behavior imparted by the introduction of electrolyte. Sodium Bicarbonate Subphase. 2-HOA monolayer phase behavior was also investigated on a sodium bicarbonate subphase given the integral role that the bicarbonate anion plays in any aqueous-based calcium carbonate mineralization processes. Despite being anionic, a strong interaction is expected between the monolayer and the bicarbonate anion based on our previous investigation of octadecanoic acid/octadecanol monolayers.16 This enhanced interaction is also present when the monolayer is 2-HOA. The strength of the interaction is evidenced by the 4434

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Figure 4. BAM image of 2-HOA on a NaHCO3 subphase, illustrating phase coexistence. The BAM image covers a monolayer area of 4.128 by 3.616 mm, based on a 640  480 pixel image and a x-scale of 5.65 μm/ pixel and a y-scale of 8.60 μm/pixel.

strikingly different 2-HOA isotherm profile on NaHCO3 subphases as compared to that observed on the water or calcium chloride subphases (Figure 1). The monolayer on the sodium bicarbonate subphase is a two phase system at zero pressure (Figure 4). At areas per molecule greater than 0.30 nm2, BAM images show the monolayer to consist of relatively rigid islands, where rigidity was gauged according to how readily the islands merged to form a coherent monolayer, which for NaHCO3-based systems did not occur until after onset. This behavior is similar to, but weaker than, that observed for 2-HOA on the CaCl2 subphase pointing to a reasonably strong interfacial interaction. Onset is marked by a curvilinear transition at an area per molecule of ∼0.26 nm2, similar to the transition observed on the pure water subphase. Beyond onset, the monolayer compressibility, as indicated by the isotherm slope, is similar to that on the CaCl2 subphase. At the relatively low pressure of ∼17 mN m
1 a second curvilinear transition is observed leading to a brittle fracture collapse at ∼22 mN m
1. The overall picture is of a monolayer with very poor stability: the measurement of Am values of 5 mN m
1 (Figure 1). The subsequent dramatic drop into negative potentials at an area of 0.2 nm2 implies significant collapse and/or the presence of an extensive double layer, similar to the behavior observed on the bicarbonate subphase. The ΔV
A data point to a composite behavior with intermediate film stability between that observed for the CaCl2 and NaHCO3 subphases. The incorporation of NaHCO3 reduces, in comparison to on the CaCl2 subphase, the domain rigidity allowing the formation of a coherent film in the early stages of compression. This results in early condensation of the gaseous phase regions, which is indicated by the absence of a rise in the potential profile with compression, as seen on CaCl2 subphases at ∼0.3
0.2 nm2. However, this reduction in domain rigidity reduces film stability, which translates into considerable slow collapse evidenced by the large deviation from the ΔV
A plateau and negative potentials. Instability of the monolayer manifests itself in ambiguous XRR data. The best fit models yielded unusual electron density profiles. As with the bicarbonate data, the long collection times and the monolayer instability make the collection of reliable XRR data difficult. Generally, the behavior of 2-HOA on CCCS is characterized by the dominant electrostatics of the calcium ion. Incorporation of the bicarbonate ion decreases BAM domain rigidity and increases monolayer instability but its influence is less pronounced than that observed for octadecanoic acid,16 where the greater difference between the CaCl2 and CCCS subphases was attributed to the bicarbonate anion. Cation-Mediated Hydrogen Bonding. Comparing the behaviors probed in the pure acid,16 hydroxy acid (this work) and dihydroxy22,23 acid systems on a pure water subphase illustrates the role the hydroxy group plays in defining monolayer phase behavior. The hydrophilic nature of the hydroxy group and its capacity for hydrogen bonding dominates the system, with the formation of a complex multiphase film, the specificities of which are temperature and pressure dependent. The results outlined above highlight how this capacity for hydrogen bonding can be further enhanced or destroyed with the introduction of subphase electrolytes. Such an enhancement in the propensity for hydrogen bonding is illustrated in a comparison between sodium chloride and sodium

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bicarbonate subphase systems. The presence of the bicarbonate anion results in greater monolayer condensation, culminating in ∼80% of the film being in the U phase at Π = 0 mN m
1. The presence of an U phase at low pressures reflects monolayer condensation to an extent that like-charge repulsion is mitigated and the surfactant conformation is consistent with close packing. Sodium is unable to achieve this in the form of a NaCl subphase, and the co-ion nature of the bicarbonate makes a direct interaction unlikely. Hence, the strong monolayer/subphase interaction is attributed to the formation of a cation-mediated hydrogenbonded network.16 That is, hydrogen bonding between bicarbonate anions and the surfactant is facilitated by the presence of sodium cations. However the failure to effectively model the XRR data prevented this hypothesis from being fully validated. In contrast, the introduction of the divalent cation resulted in significant condensation of the monolayer, consistent with literature reports.24 Brought about by strong cation binding the condensation and associated conformational changes led to the formation of an U phase and the disruption of any hydrogen bonding. The CCCS, being a combination of CaCl2 and NaHCO3, is dominated by the electrostatic interaction of the cation. However, the Π
A, ΔV
A, and XRR data hint at the influence of the bicarbonate ion, evident in the decreased monolayer stability. The reduction in monolayer stability is the result of an increase in monolayer solubility upon association with the subphase ions. The dominance of the cation is likely due to the strong binding interaction with the hydroxy acid, which sterically restricts the propensity for intramonolayer hydrogen bonding.

’ CONCLUSIONS The influence of subphase electrolytes on the behavior of 2-HOA monolayers was investigated using surface pressure, potential isotherms, BAM, GIXD, and XRR. Substitution of a hydroxy group at the R position leads to an enhanced interfacial interaction from that observed for octadecanoic acid. The nature of this interfacial interaction is a complex balance of hydrogen bonding, electrostatics, and steric effects. On pure water and sodium bicarbonate subphases the monolayer/subphase interaction is dominated by hydrogen bonding. The interaction is sufficient to condense the monolayer, however there is evidence for a lack of tilt order. This monolayer-scale heterogeneity or disorder is attributed to a size mismatch between the polar headgroup and the hydrocarbon tail. In contrast, on the calcium ion containing subphases the interfacial interaction is heavily influenced by electrostatics. The nondirectionality and strength of the electrostatic interaction results in the formation of an untilted hexagonal phase. The structural packing limitations imparted by the headgroup/ tail size mismatch, at least in the absence of calcium ions, results in a multiphase system depending on the localized packing within the monolayer. This lack of tail order reduces the tail
tail van der Waals interactions and the monolayer is less stable, contributing to the occurrence of slow collapse. Monolayer instability and slow collapse is greatest on bicarbonate ion containing subphases. It is proposed that this instability is due to increased monolayer solubility brought about by short-range, cation-mediated hydrogen-bonding involving the surfactant, the cation, and the bicarbonate co-ion. In general, the interfacial interaction is dominated by the divalent cation however there is some evidence of the bicarbonate ion influencing monolayer behavior. The electrolyte effect is species 4437

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Langmuir specific, with the electrostatic binding of divalent cations having the most significant impact on monolayer phase behavior.

’ ASSOCIATED CONTENT

bS

Supporting Information. Details of the GIXD and XRR data analyses performed, a summary of the variation of phase composition (on a pure water subphase only) and lattice parameters extracted from the GIXD data as a function of surface pressure for all subphases investigated, additionally, pressure/ area isotherm and surface potential data for the pure octadecanoic and octadecanol monolayers on a pure water subphase, and BAM images showing island formation on the calcium chloride subphase. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

ARTICLE

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*E-mail: [email protected].

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