Article pubs.acs.org/Langmuir
Morphology and Composition of Structured, Phase-Separated Behenic Acid−Perfluorotetradecanoic Acid Monolayer Films Jeveria Rehman,†,§ Hessamaddin Younesi Araghi,†,§ Anqiang He,‡ and Matthew F. Paige*,† †
Department of Chemistry, University of Saskatchewan, 110 Science Place, Saskatoon, Saskatchewan S7N 5C9, Canada University of Alberta-nanoFAB, W1-060 ECERF Building, 9107−116 Street, Edmonton, Alberta T6G 2V4, Canada
‡
S Supporting Information *
ABSTRACT: The phase separation of immiscible surfactants in mixed monolayer films provides an approach to physically manipulate important properties of thin films, including surface morphology, microscale composition, and mechanical properties. In this work, we predict, based upon existing miscibility studies and their thermodynamic underpinnings described in the literature, the miscibility and film morphology of mixed monolayers comprised of behenic acid (C21H43COOH) and perfluorotetradecanoic acid (C13F27COOH) in various molar ratios. Predictions are tested using a combination of experimental surface characterization methods for probing miscibility and film morphology at the solid/air and air/water interfaces. Film components were immiscible and phase-separated into chemically welldefined domains under a variety of experimental conditions, with monolayer morphology consistent with initial predictions. The extensibility of these basic predictions to other systems is discussed in the context of using these works for different perfluorinated surfactant molecules.
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tant monolayers,8−10 with a particular emphasis on using miscibility and phase separation to control and pattern film morphology at micrometer to nanometer length scales. A rich diversity of monolayer film morphologies have been reported in systems ranging from single-component surfactants11 to simple mixtures of hydrogenated and perfluorinated fatty acids,3,9,10 to mixtures involving novel hybrid diblock (CnF2+1CmH2m+1) amphiphiles,12 and a host of others.13 The physical chemical properties of the constituent surfactants that contribute to phase morphology in monolayer systems are complex, but some important generalities have been elucidated in the literature; for single-component phospholipid films, McConnell11 has modeled domain morphology as being broadly controlled by a combination of dipole−dipole headgroup repulsions, leading to extended domains, in combination with line tension, which leads to contraction of domains. In fluorinated systems, more exotic crystalline structures in singlecomponent diblock surfactant systems have been described by Krafft et al.12 and others, in which novel self-assembled hemimicelles formed spontaneously under a variety of film preparation conditions. In these systems, a two-phase, liquid− liquid thermodynamic model, in which dipole−dipole interactions played a key role, was used to successfully describe the
INTRODUCTION Fluorinated surfactants are of enormous technological importance in applied surface science, with uses ranging from wetting agents and detergents in enhanced oil-recovery operations, to spreading agents in fire-fighting foams, as well as for additives in biomedical applications such as lung surfactant formulations.1−3 Fluorosurfactants exhibit remarkable wetting, spreading, and surface tension-lowering properties, while simultaneously exhibiting excellent chemical and thermal inertness. For many technological applications, fluorosurfactants are used in carefully formulated mixtures in combination with hydrogenated surfactants, as mixtures of the two often perform synergistically, in comparison with singlecomponent surfactants alone. Factors that regulate miscibility (or in extreme cases, phase separation) in fluorosurfactanthydrogenated surfactant mixtures are complex, and significant efforts have been aimed at understanding and controlling these effects, because of their potential impact in improving the technological performance of surfactant mixtures. Early research efforts aimed at understanding and controlling miscibility and phase separation in mixed fluorosurfactanthydrogenated systems have focused on surfactant monolayer films,4−7 as tractable, minimal model systems for complex surfactant mixtures. A primary motivation of these studies has been unravelling the various molecular-level interactions that control system miscibility. Our group and others have been systematically exploring conditions that regulate miscibility in binary or ternary mixed fluorosurfactant-hydrogenated surfac© 2016 American Chemical Society
Received: March 21, 2016 Revised: May 5, 2016 Published: May 10, 2016 5341
DOI: 10.1021/acs.langmuir.6b01104 Langmuir 2016, 32, 5341−5349
Article
Langmuir morphology of these films at the air/water interface, as well as their response to applied surface pressures.14 The morphologies for binary, two-component mixed surfactant films with a fluorinated component are also often complex because of the various homomolecular and heteromolecular interactions that can occur, but similar underlying driving forces as described by McConnell remain important. Research from Matsumoto’s group8,15 has demonstrated control over surface morphology of phase-separated diblock fatty acids (CkH2k+1COOH and CmF2m+1CnH2nCOOH), and correlated phase-separated film morphology in these systems to an interplay of dipole−dipole repulsion between carboxylic acid headgroups with line tension between phases. This allowed the film morphology of the mixed monolayer systems to be adjusted from extended “nanowires” (phase morphology dominated by dipole−dipole repulsions) to circular, compact domains (phase morphology dominated by line tension) through careful selection of the chemical structure of the surfactant. Similar effects have been reported in affiliated systems by others (e.g., Kataoka et al.16). Research in our group has focused on systematically controlling and manipulating monolayer film morphology through phase separation in binary hydrogenated-perfluorinated fatty acid systems at both the solid/air and air/water interfaces using Langmuir and Langmuir−Blodgett (LB) approaches.9,10,17−19 Based on this previous work, a simple (but incomplete; vide inf ra) measurable that correlates with film morphology in these systems is the difference in chain length of the constituent surfactants, which we will describe here using the simple parameter ΔH−F:
While we currently lack a detailed model that will fully allow predictions with regard to which hydrogenated-perfluorinated surfactant combinations will undergo phase separation, and what types of film morphologies will form, the body of empirical data accumulated on these binary mixed fatty acid systems is now sufficient to enable rudimentary predictions of this properties. For binary systems with fixed carboxylic acid headgroups, differences in surfactant chain length in excess of ΔH−F = +4, one can predict that the system will undergo phase separation to yield compact, hexagonal domains that are enriched in the hydrogenated fatty acid. We note that the factors that control domain structure and composition are, in some cases, more complex than can be described by one simple parameter; for example, in the AA−PF (ΔH−F = +6) system, domain growth kinetics as well as surface pressure effects strongly influenced overall film morphology. Nonetheless, the approach does provide a simple, testable metric for predicting phase separation in these systems. In this work, we test this simple hypothesis on the binary behenic acid (BA, C21H43COOH)−PF (ΔH−F = +8) system, by measuring monolayer film morphology and composition at both the solid/air and air/water interfaces, and explore the underlying miscibility of the two components using classical thermodynamic (additivity rule and Gibbs excess energies of mixing) analyses of Langmuir monolayer compression isotherm data. Data are compared with affiliated monolayer systems described previously in the literature, and further refinements to the simple predictions for monolayer morphologies for these systems are discussed.
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ΔH−F = number of methylene groups in hydrocarbon chain
MATERIALS AND METHODS
Chemicals and Substrates. Behenic acid (BA) and perfluorotetradecanoic acid (PF) were purchased from Sigma−Aldrich Corporation and used as received. Solvents were purchased from Merck EM Science or EMD Millipore. Stock solutions of surfactant were prepared by dissolving solid BA and PF in 9:1 volume ratio of hexane:THF to attain a final total surfactant concentration of 2 × 10−3 mol L−1. When a mixture of BA and PF was required, the stock solutions were mixed in appropriate volumes to give a solution with the final desired mole ratio of surfactants. Microscope cover glass slides (VWR International) were rinsed with absolute ethanol, dried in nitrogen, and cleaned in a plasma cleaner (Harrick Plasma) for 20 min before use in deposition experiments. Surface Pressure Isotherms and Langmuir−Blodgett (LB) Film Preparation. Surface pressure−area (π−A) compression isotherm measurements and LB film depositions were performed on a KSV 2000 Langmuir trough system (KSV Instruments), using symmetric compression at 22 °C with ultrapure water (resistivity = 18.2 MΩ cm−1) as a subphase. The trough was cleaned thoroughly by suction before each measurement, and no appreciable change in surface pressure was observed during blank runs (compression of the clean water surface). Isotherms were collected for pure BA, pure PF and for various mixtures of the two using a platinum plate Wilhelmy plate for surface pressure measurements. The composition of mixed films is described hereafter using the nomenclature X BA:Y PF to indicate a mixed film with a relative BA:PF mole ratio of X:Y. For each measurement, a 100 μL aliquot of surfactant solution was spread on the subphase surface and the solvent was allowed to evaporate for at least 10 min prior to compression. A compression rate of 4.8 Å2 molecule min−1 was used for isotherm measurements and a compression rate of 2.4 Å2 molecule min−1 for deposition measurements was used. For deposition measurements, films were compressed to the desired surface pressure and the film was allowed to stabilize for ∼20 min prior to withdrawing the substrate at a rate of 5 mm min−1 through the air/water interface. The film was allowed to dry for a
− number of difluoromethylene groups in perfluorocarbon chain
In previous studies, we have reported that simple, binary mixed films of arachidic acid (AA, C19H39COOH) and perfluorotetradecanoic acid (PF, C13F27COOH), ΔH−F = +6, undergo phase separation at the air/water interface to form hexagonal domains enriched in the hydrogenated surfactant;9 after an initial nucleation event, domain growth proceeded via Ostwald ripening, and monolayer films could be transferred to solid substrates via Langmuir−Blodgett (LB) deposition with negligible perturbation to film structure. Brewster angle microscopy (BAM) imaging anisotropy suggested the hexagonal domains were crystalline in nature.17 Decreasing the chain length of the hydrogenated surfactant by two methylene units (stearic acid; C17H35COOH) while maintaining the same chain length in the perfluorocarbon (ΔH−F = +4) resulted in films with extended linear domains of hydrogenated surfactant, reflecting the relative increase in the influence of dipolar repulsion on film morphology.10 An intriguing alteration in film morphology was observed with a binary combination of palmitic acid (C15H31COOH) and perfluoroctadecanoic acid (C17F35COOH), with ΔH−F = −2.19 For this system, discrete hexagonal domains were again observed, but with a complete reversal of chemical composition; the domains were enriched in the perfluorinated surfactant instead of the hydrocarbon, although the underlying mechanism that gave rise to this effect was complicated by the slow dissolution of the slightly soluble palmitic acid into the underlying subphase in the Langmuir trough. 5342
DOI: 10.1021/acs.langmuir.6b01104 Langmuir 2016, 32, 5341−5349
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Figure 1. (A) Surface pressure−area compression isotherms for BA, PF, and their mixed monolayers at T = 22 °C on a pure water subphase. (B) Compressibility plots for BA, PF, and a 1BA:1PF mixture under the same conditions as those for panel (A). minimum of ∼20 min at room temperature before imaging in the atomic force microscopy (AFM) system. Surface Microscopy. AFM images were collected on a Dimension Hybrid Nanoscope system (Veeco Metrology Group) in either contact or tapping mode. For contact mode, silicon nitride AFM probes (Bruker Instruments) with a nominal spring constant of 0.12 N m−1 were used while for tapping mode, silicon AFM probes (Bruker Instruments) with a resonance frequency of 276−349 kHz and a spring constant of 12−103 N m−1 were used. All images were collected in air, and no appreciable difference between tapping-mode and contact-mode images was observed. A scan rate of 0.50 Hz and resolution of 512 pixels per line were used. Under these imaging conditions, monolayer films could be imaged repeatedly without any observable tip-induced damage. For some samples, the film could be forcibly removed (“scratch test”) by selecting a smaller area of the sample and repeatedly scanning this region at the maximum possible operating force in the microscope. BAM imaging of monolayer films was carried out at the air/water interface using a Langmuir trough equipped with a commercial BAM system (UltraBAM, KSV-NIMA). The BAM was equipped with 658 nm illumination laser and a CCD camera detector, and was capable of imaging diffraction-limited (∼2 μm for the optical system used here) images at a rate of 20 frames per second. The secondary ion mass images were obtained using Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), with a ToF-SIMS IV instrument (ION-ToF GmbH). Bi+ ions were used as the analytical source, operated at 25 kV in static mode. The selected sample areas were imaged over 128 × 128 collecting pixels. Charge neutralization was applied because the sample was deposited on an insulating glass substrate.
temperature, and P the pressure), as a function of mean molecular area for the pure films and a 1BA:1PF mixture are shown in Figure 1B. For phase assignment, we will use the terminology recommended by Kaganer et al.20 for simple fatty acid systems. For pure BA, three peaks were distinguishable in the compression modulus plots (at ∼16, ∼17, and ∼19 Å2 molecule−1), which we interpret as corresponding to transitions from the collapse pressure plateau to the untilted condensed (UC) phase, from the UC to the tilted condensed (TC) phase, and the UC to liquid expanded (LE) phase transition, respectively. For pure PF, only two peaks were observed. While the phases of the PF monolayer systems are not as wellcharacterized as the simple fatty acids, the results here are consistent with those reported elsewhere,3 and can be attributed to a transition from the film collapse plateau to an ordered phase and a transition from the ordered to a disordered (similar to LE) phase. For the mixed films, three inflection points in the isotherms were still detectable, but as the mole fraction of PF increased, the ability to distinguish the UC-TC transition became increasingly difficult. Limiting areas (A0) of the condensed films were determined by fitting a straight line to the UC phase for pure BA and the ordered phase for pure PF, and extending the fit to the abscissa. This yielded values of ∼25 Å2 and 18 Å2 for PF and BA, respectively. The monolayer films exhibited collapse pressures of ∼60 mN m−1 for pure PF and ∼68 mN m−1 for BA, and the film collapse pressures for the mixtures decreased as a function of increasing PF content, indicating that PF destabilizes monolayers of the pure fatty acid film in approximate proportion to the amount of perfluorocarbon. Isotherms were consistent in phase behavior and collapse pressure values with isotherms found in the literature for comparable systems.9,10 For the mixed films, the π−A isotherms fell between two extrema defined by individual pure components, with mixed films being more expanded than pure BA, but more compacted than pure PF. Isotherms for all of the monolayer films, including both the pure single components, as well as the mixtures, exhibited significant hysteresis upon multiple compression−expansion cycles. While hysteresis in the mixed monolayer systems might be reasonably postulated to the formation of poorly redispersible (e.g., crystalline) domains, this is complicated by the hysteresis observed for all systems,
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RESULTS AND DISCUSSION Compression isotherms for monolayer films of PF, BA, and their mixtures on a water subphase were collected and are shown in Figure 1A. Isotherms for the films consisted of a slowly increasing region at low film compression, followed by a steep increase in surface pressure at higher compressions until reaching the collapse plateau. The steep increase in pressure typically occurred between 18 and 28 Å2 molecule−1, with the area of steepest increase depending on film composition. In the case of the pure BA isotherm, an additional inflection point was observed, indicative of an extra phase transition; to further aid visualization of phase transitions in the isotherm data, compression modulus plots (C = A(dA/dπ)T,P, where A is the mean molecular area, π the surface pressure, T the 5343
DOI: 10.1021/acs.langmuir.6b01104 Langmuir 2016, 32, 5341−5349
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deviations (below the line predicted from eq 1 for attractive interactions and above the line for repulsive interactions) from additivity. For reference purposes, Figure 2 includes the behavior predicted by eq 1 with values for A1 and A2 taken from the film measurements for the pure components. All of the mixtures showed small (typically 0 ⎝2⎠
(2)
where N is the total number of molecules in the system, z is the coordination number, χi are mole fractions, and ΔkεAA is the change in homointeraction energies between molecules produced by increasing fatty chain length “k” by one. While this model accounts only for dispersive interactions in mixed diblock−fatty acid systems, and should be viewed as only
where A12 is the mean molecular area for the mixed film, Ai is the mean molecular area for the ith component, and χi is the mole fraction of the ith component. Net attractive or repulsive interactions between different film components result in
Figure 3. AFM height mode image (15 μm × 15 μm) and corresponding cross-sectional analysis of a 1:1 BA:PF mixed LB film deposited at π = 30 mN m−1 and T = 22 °C on cover glass. The difference in heights between the smooth continuous layer (dark) and the hexagonal domains (light yellow) was typically ∼1.0 nm. 5344
DOI: 10.1021/acs.langmuir.6b01104 Langmuir 2016, 32, 5341−5349
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‘08, V1.2.0) for PF is 1.8 and 2.8 nm for the fully extended conformation of BA. The height difference between the hexagonal domains and the continuous layer measured by AFM is consistent with the difference in length between vertically adsorbed PF and BA, which suggests the film consists of phase-separated regions of PF (the smooth continuous film) and BA (the elevated hexagonal domains). This is the same phase-separated film morphology and domain composition observed for the AA−PF system, and in agreement with the original hypothesis of this work. Furthermore, the morphology agrees with the necessary morphology for eq 2 to hold. However, a series of additional tests were required in order to definitively assign chemical identities to the different phases. To verify monolayer formation, “scratch tests”, in which the film was removed from the underlying glass substrate by repeated imaging at high operating force, followed by imaging of the height difference, were performed on films of mixed 1:1 BA:PF and pure BA. Results (see Figures S3 and S4 in the Supporting Information) indicated that the films were indeed monolayers. For the mixed films, the height difference between the bottom of the scratched film region and the different phases was consistent with the continuous substrate and the hexagonal domains consisting of PF and BA adsorbed normal to the substrate, respectively. The pure BA film height difference was also consistent with vertically adsorbed and fully extended BA. For additional confirmation of the chemical identity of the different phases, a systematic survey of film structure as a function of film composition was carried out. A series of LB films with different BA:PF molar ratios were prepared and measured in the AFM, and for each sample, the fractional area occupied by the continuous layer (determined by the total number of image pixels in the continuous layer divided by the total number of pixels per image) was calculated. Typical images for these measurements are shown in Figure 4 and the fractional areas for the different film compositions are summarized in Table S1. It was found that, as the molar ratio
semiquantitative, Kimura’s model predicts an increase in length of hydrocarbon chain will result in a less miscible system, and our results agree well with this. This, in combination with the minimal change in dipole moment with chain length, suggests that the film expansion that is observed in the BA−PA system is the result of increased interactions between the hydrogenated fatty acid chains. Further, this suggests a possible film structure consisting of vertically aligned surfactants at the air−water interface, though direct imaging measurements are required for validation of this supposition. AFM measurements that confirm this structure are described later in this manuscript. The mixing thermodynamics and molecular-level interactions in the mixed films was further assessed by calculating the excess Gibbs free energy of mixing (ΔGE) for the mixtures at different surface pressures.21 The extent of film stabilization (or destabilization) due to interactions between film components can be evaluated from the π−A isotherm data by calculating excess Gibbs free energies of mixing using eq 3: ΔGE =
∫0
π
[σ12 − (χ1 σ1 + χ2 σ2)] dπ
(3)
where σi is the molar area of the pure film and σij is the molar area of the mixed film. Positive or negative excess values of the Gibbs excess function indicate either repulsive or attractive interactions, respectively. As shown in Figure S1 in the Supporting Information, the mixed monolayer systems showed positive (ΔGE) values for all surface pressures measured, with values ranging from
