Article pubs.acs.org/Langmuir
Temperature- and pH-Dependent Shattering: Insoluble Fatty Ammonium Phosphate Films at Water−Oil Interfaces Joe Forth,† David J. French,† Andrei V. Gromov,‡ Stephen King,§ Simon Titmuss,† Kathryn M. Lord,∥ Mike J. Ridout,⊥ Pete J. Wilde,⊥ and Paul S. Clegg*,† †
School of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3FD, U.K. EaStChem, School of Chemistry, University of Edinburgh, Edinburgh EH9 3FJ, U.K. § STFC ISIS Facility, Rutherford Appleton Laboratory, Harwell Oxford Campus, Didcot OX11 0QX, U.K. ∥ School of Biological Sciences, University of Edinburgh, Edinburgh EH9 3BF, U.K. ⊥ Institute of Food Research, Norwich Research Park, Colney, Norwich NR4 7UA, U.K.
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‡
S Supporting Information *
ABSTRACT: We study the films formed by tetradecylamine (TDA) at the water−dodecane interface in the presence of hydrogen phosphate ions. Using Fourier transform infrared spectroscopy (FTIR), interfacial shear rheology, confocal fluorescence microscopy, cryo-scanning electron microscopy (cryo-SEM), and small-angle neutron scattering (SANS), we find that between pH 5 and 8 tetradecylammonium cations bind to hydrogen phosphate anions to form needle-shaped crystallites of tetradecylammonium hydrogen phosphate (TAHP). These crystallites self-assemble into films with a range of morphologies; below pH 7, they form brittle, continuous sheets, and at pH 8, they form lace-like networks that deform plastically under shear. They are also temperature-responsive: when the system is heated, the film thins and its rheological moduli drop. We find that the temperature response is caused by dissolution of the film in to the bulk fluid phases. Finally, we show that these films can be used to stabilize temperature-responsive water-in-oil emulsions with potential applications in controlled release of active molecules.
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INTRODUCTION Interactions between multiple components, mediated by an oil−water interface, can be used to create complex interfacial films with novel properties.1,2 The mechanisms by which such interfacial layers are formed include polyelectrolyte complexation,3,4 interfacial polymerization,5,6 and particle−surfactant complexation.7,8 These mechanisms typically lead to the formation of an insoluble, rather elastic layer that is confined to the interface and can sustain nonequilibrium droplet shapes.9,10 Such elastic multilayer films have received recent attention for their potential applications in encapsulation and controlled release of active molecules for uses in the food and pharmaceuticals industry.11,12 These films are often stimulusresponsive and provide a high stability against aggregation and coalescence but can require rather intensive multistep processing such as layer-by-layer film formation.3,13 One method of avoiding layer-by-layer film fabrication is by dispersing two components in separate, immiscible phases, such that they interact only at the interface.14,15 However, encapsulation and film formation by this method has been explored using only a narrow range of materials. Given the potential range of applications of such systems, a cheap material that creates temperature-responsive films with tunable inter© 2015 American Chemical Society
facial rheological properties, and that is suitable for large-scale production, is therefore rather desirable. In this paper we study the temperature-responsive films formed by the binding of tetradecylamine (TDA, CH3(CH2)13NH2), a water-insoluble fatty amine, to hydrogen phosphate counterions, at the dodecane−water interface. This is illustrated in Figure 1. Fatty amines with a range of aliphatic chain lengths have already been extensively studied in the context of insoluble monolayers formed at the air−water interface, where it has been shown that these molecules bind to a variety of divalent counterions.16,17 Fourier transform infrared (FTIR) spectroscopy and neutron activation analysis studies showed that within a pH range of 4.5 and 7.5 these monolayers have a well-defined stoichiometry and consist mostly of two fatty ammonium cations bonded to a single HPO 4 2− anion.18−20 This paper studies the TAHP films formed at the oil−water interface, which leads to markedly different behavior. Rather than a monolayer, we find that the films are thick and extremely Received: June 2, 2015 Revised: July 13, 2015 Published: August 11, 2015 9312
DOI: 10.1021/acs.langmuir.5b01981 Langmuir 2015, 31, 9312−9324
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fluorescence microscopy, suggesting a number of potential applications in controlled release and delivery.
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MATERIALS AND METHODS
Sample Preparation. Tetradecylamine (≥95%) and dodecane (ReagentPlus, ≥99%) were purchased from Sigma-Aldrich, UK, and used as obtained. Water was deionized and filtered through a Milli-Q reverse osmosis unit (resistivity >1018 Ω m). Sodium hydrogen phosphate salts (monobasic monohydrate and dibasic dihydrate, ≥99%, Sigma-Aldrich, UK) were used as the source of phosphate ions. Minor adjustments to pH were made using 1 M NaOH. All experiments presented here were performed using an aqueous solution of 100 mM HnPO4(3−n)−, with the ratio [HPO42−]/[H2PO4−] in the aqueous phase controlled by varying the initial pH. The volumetric oil:water ratio for all experiments was 4:1. Emulsions were formed by shear using a Polytron PT-3100 rotor stator (Kinematica, Switzerland) operated for 1 min at a shear rate of 30 000 s−1. Additionally, water-in-oil capsules were made by injecting aqueous hydrogen phosphate solution into a coflowing stream of 5 mM TDA dispersed in dodecane, as described elsewhere.21 The equipment used in this experiment differs slightly from that described in Umbanhowar et al. in that rather than using a compressed nitrogen cylinder to drive fluid flow, a syringe pump with an electric motor was used to give greater control over the injection rate of the aqueous phase. 0.5 mL of aqueous solution was injected in to the dodecane bath at a constant rate of 0.1 mL/min through a glass needle with a tip width of 150 μm, which was placed 2 cm from the cup center. The cup was rotated at approximately 1 Hz. Fourier Transform Infrared Spectroscopy. FTIR measurements were performed in reflection mode using a Smiths IlluminatIR spectrometer mounted on top of a Renishaw inVia Raman microscope equipped with a noncontact all-reflection objective (ARO, 10× magnification, Smiths Detection). TAHP synthesis for these experiments was carried out over 60 h in the crystallization dishes as shown in Figure 2. After synthesis, the materials from the dishes were isolated by filtration, washed consecutively with ethanol and hexane, and dried. Samples were prepared by drop-casting thick dispersions of TAHP or TDA powders in pentane on to IR-reflective low-E glass slides (Smiths Detection). A linear baseline subtraction procedure was applied to the spectra after the measurements were performed. Interfacial Rheology. Interfacial rheology results were taken using a TA DHR-2 rheometer (TA Instruments, US) equipped with a Pt−Ir DWR (double wall ring) interfacial geometry described elsewhere.22 The Pt−Ir ring was flame-treated between experiments using a butane burner. To take measurements, the aqueous phase was added in to a Delrin trough and the ring was lowered such that the sharp edges of the ring were level with both the air−water interface and the lip of
Figure 1. (top) Chemical structure of tetradecylamine. (bottom) Schematic illustrating the interfacially mediated binding of the (waterinsoluble) tetradecylamine and (dodecane-insoluble) hydrogen phosphate. This interaction rapidly forms thick, multilayer films, as shown in Figure 2, which consist of micron-sized crystallites.
strong, and that they have a hierarchical structure. Scattering experiments show the system contains lamellar order, and we find that these lamellae assemble into crystallites which are predominantly needle-shaped, with the crystallite size dependent on the conditions of film formation. These needles are then found to assemble in to macroscopic films with a tunable structure and surface rheological properties that can be directly related to the structure. The presence of the second liquid (oil) phase, in which the TAHP is sparingly soluble, also leads to the films being temperature-responsive. We use FTIR spectroscopy to study powder samples of tetradecylammonium hydrogen phosphate (TAHP) to give information about the chemical constituents of the film. We use small-angle neutron scattering on TAHP-stabilized emulsions to show that these films assemble in to structures with lamellar order. We use interfacial shear rheology to show that the fatty amines form a film with an elastic modulus that is both highly tunable and unusually large. The tunable mesoscopic structure of the film allows us to vary the film rheology from plastic to brittle. Finally, we study the temperature-responsive emulsion droplets stabilized by this film using cryo-SEM and confocal
Figure 2. TAHP films formed at a dodecane−water interface at various pH values, photographed after 60 h of formation, along with light micrographs of the films formed at pH 7 and 8 (inset, scale bar is 125 μm). The pH of the aqueous phase is given in the images. Films were also formed at pH values 5 and 6 for chemical analysis and were identical in yield and appearance to the film formed at pH 7. The diameter of the crystallization dishes is approximately 7 cm. 9313
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maximize surface area and scattering signal, 6.5 mM h-tetradecylamine was used, and emulsions were formed at a higher shear rate of 60 000 s−1 for 1 min using an UltraTurrax T10 Basic rotor stator with S10N5G stainless steel attachment (IKA, Germany). This yielded droplets qualitatively similar in appearance from those used elsewhere in this work, albeit with a smaller droplet diameter (on the order of 20 μm). The emulsions were then pipetted into 0.5 mL volume, 2 mm path length quartz cuvettes held in a temperature-controlled rack. Incoherent background scattering from the solvents was corrected for by measuring the background scattering from an empty quartz cuvette, as well as from cuvettes containing the appropriate ratio of dand h-dodecane or D2O. Scattering from the solvents was measured at 25, 40, 55, and 65 °C with scattering for intermediate temperatures calculated by interpolation. Background subtractions were calculated using relative volume fractions in each phase, which was measured after the experiment by weighing the mass of the two fluid phases after the emulsions had coalesced in response to heating to 80 °C. In the case of the measurements performed above 45 °C, coalescence and sedimentation of the droplets in the samples meant that this procedure resulted in an oversubtraction of the incoherent background, and a small (∼5%) reduction to the absolute value of the used background subtraction was made. The SANS2D instrument used in this work, described elsewhere,25 uses a white beam containing neutrons with wavelengths in the range 1.75−16.5 Å, where q is determined by time-of-flight. Δq/q is taken to be 5%, in line with estimates used elsewhere for the same instrument.25,26 Scattering data were reduced using Mantid,27 with measured intensity being placed on an absolute scale using the scattering from a standard sample (a solid blend of hydrogenous and perdeuterated polystyrene) in accordance with established procedure.28 A simultaneous q-range of 0.005−1.78 Å was achieved by employing both front and rear detectors at sample-to-detector distances of 2.4 and 4 m, respectively. Least-squares fits and data analysis for scattering experiments were performed in Igor Pro using the NIST SANS data analysis macros. Data were modeled using the modified Caillé theory described by Nallet et al.,29,30 which models the powder-averaged scattering from a system of lamellar sheets with a separation, d, and bilayer thickness, δ, undergoing thermal fluctuations in the structure factor. We also include a Lorentzian term to describe background scattering at high q. Full details of the model are given in the Supporting Information. When fitting the data, the low-q region of the data was used to fit the scale factor and number of sheets. A further two parameters, the Caillé parameter η and the length scale of the nonpolar region δ, are then found from fits to the data in the region of the Bragg peaks. Scattering length densities for the polar region were estimated using the density of diammonium phosphate, assuming full deuteration of the salt, and for the nonpolar region using the volume of an aliphatic chain, v, using the empirical formula obtained by Tanford, v = 27.4 + 26.9nc,31 with nc = 14, yielding ρpolar = 3.85 × 10−6 Å2 and ρnonpolar = −4.73 × 10−7 Å2.
trough. 10 mL of dodecane containing tetradecylamine was then gently added on top. The rheometer was operated in controlled strain mode during film formation, with an oscillating 0.5 Hz strain, γs, chosen to be in the linear deformation region of the film (γs = 0.01%). The large interfacial storage modulus of the films meant that the signal-to-noise ratio of the measurements was excellent even at small strains. Measurements on the stress response of the film were performed in controlled stress mode as controlled strain measurements on the brittle films yielded poor results in the region of nonlinear response. For temperature measurements, the trough was covered with an aluminum lid and samples were equilibrated for 2 h before the measurement was performed. Further measurements were taken using a TA AR-G2 rheometer with a stainless steel 22 mm diameter bicone geometry to rule out geometry- and instrumentspecific effects. Sample preparation and experimental procedure were identical for experiments on both devices. Confocal Microscopy. Confocal microscopy measurements were performed on a Zeiss LSM T-PMT/LSM700 confocal laser scanning inverted microscope with Zeiss ZEN software (Carl Zeiss AG, Germany). Images were taken using custom-made imaging chambers that consisted of a 7 mL glass vial with the base removed and attached to a ground glass coverslip using Norland 61 UV-setting optical adhesive. Sealed imaging chambers were heat-hardened in an oven at 50 °C for 24 h and then aged for a week at room temperature prior to use to prevent dissolution of the optical adhesive. Fluorescent doping of the fatty amine film was performed using NBD-Cl (4-chloro-7nitrobenzofurazan, 98%, Sigma-Aldrich, UK), which is known to form fluorescent compounds upon reacting with primary amines.23 The NBD-Cl was used as obtained, and a small quantity of it (100 mN/m) more rapidly than can be detected by the apparatus, explaining the significant deformation of the droplets in Figure 3. At pH 7, the rate of film formation is slowed somewhat, but the final values of the rheological moduli are similar for both pH 5 and 7, in agreement with the observation that the quantity of TAHP formed at these pH values is similar. The final values of the interfacial rheological moduli depend on the initial quantity of tetradecylamine used. Varying the initial TDA concentration between 2.5 and 6.5 mM allows the film strength to be varied by a factor of 3. At TDA concentrations below 1 mM, the films typically reach a peak in their rheological moduli after 2 h before decreasing by up to an order of magnitude over the course of up to 60 h. Below a fatty amine concentration of 200 μM no film is observed to form, suggesting there is insufficient TAHP at the oil−water interface to form a percolating network at this concentration. Above 1.5 mM TDA, Gs′ reaches a constant value on the order of 10 N/m after approximately 2 h. These values are over an order of magnitude larger than typical values for interfacial films, which range from 10−6 N/m for particle monolayers55 up to 0.1 N/m for films consisting of proteins or polysaccharides.56,57 We attribute the strength of the TAHP film to its thick, multilayer structure: estimating molecular area as 20 Å2 (in line with previous work on TAHP monolayers16) yields a film thickness of 1000 layers (approximately 500 nm) when using 10 mL of 2.5 mM TDA. By contrast, the majority of interfacial shear measurements have been performed on 9318
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Figure 9. (top) Photos of a 10 μL TAHP-coated aqueous hydrogen phosphate solution (pH 5) droplet in dodecane, heated to (left to right) 38, 48, 57, and 66 °C. The droplet was suspended from a needle in a bath of dodecane containing 0.5 mM TDA. The diameter of the needle is 1.83 mm. Photos were taken after the temperature of the system had equilibrated (approximately 30 min). (bottom) Interfacial shear rheology measurements of a TAHP film during heating [TDA] = 2.5 mM (left) and 6.5 (right), pH 5. Results show averages over three measurements; error bars correspond to standard error in the mean.
film breaks and the inertia of the instrument dominates the measurement. At pH 8, the rheological behavior is markedly different. Rather than abruptly yielding, the film deforms plastically at increasing strain, exhibiting a small peak in the loss modulus and a crossover of Gs′ and Gs″ at γs ∼ 0.25%. Experiments on the pH 5 film, repeated using a stainless steel bicone geometry, yielded qualitatively identical behavior (shown in Supporting Information, Figure S3). Similar variation in interfacial rheological properties has been observed in experiments performed on two-component interfaces consisting of DNA complexed with varying concentrations of a cationic surfactant (DTAB).62,63 In these studies, a bicone geometry was used to show that the interfacial rheology of the DNA−surfactant complexes varied from linear, to plastic, to brittle as surfactant concentration was increased and that this change could be directly linked to a change in the relaxation dynamics of the monolayers. Brewster angle microscopy images also showed that in the brittle regime the film could be fractured using a glass pipet, resulting in film ruptures similar in appearance to those shown in Figure 8a. Rather than being due to a change in the dynamics of the system, the switchable interfacial rheology of the TAHP films studied here is a direct consequence of the mesocopic structure of the films, which we can directly visualize, as in Figure 2. The interfacial rheological moduli of the TAHP films is independent of the frequency of the oscillatory stress, as shown in Figure 7, inset. Given the chemical similarity of the pH 5 and pH 8 films, the change in yielding behavior seems to be a direct consequence of the lower interfacial coverage at higher pH values. Complete interfacial coverage appears to inhibit structural rearrangements that would give rise to plastic deformation, and thus brittle behavior is observed. Response of Film to Temperature Increase. Interfacial shear rheology and pendant drops have been used to study the origin of the temperature response of the TAHP films and how
it can be tuned. The temperature response of the TAHP film is qualitatively studied in Figure 8 using a pendant drop. The pendant drop was suspended from the needle at 25 °C, and the film was left to form for 1 h, shown in Figure 8a. In the case of the measurement performed at 65 °C the system was then heated, as shown in Figure 8b. The images show the droplet as the aqueous phase is withdrawn by the syringe. At 25 °C the film is observed to rupture and detach from the droplet. At high (i.e., >60 °C) temperatures, withdrawal of the water results in a self-affine reduction in the droplet volume, showing the absence of significant interfacial elasticity. Images of the pendant drop at various temperatures during heating, with photos taken after the temperature of the system had equilibrated, are shown in Figure 9. Rather than there being a rapid change in the appearance of the pendant drop over a narrow temperature range, the film is seen to thin at 48 °C, and at 57 °C the droplet has only a partial coating of TAHP. The remaining TAHP visible at the drop interface at 57 °C does not detach from the interface at this temperature, nor did it disperse over the course of 15 min of observation. As the system is heated further, to 66 °C, the remaining TAHP at the drop interface is no longer visible. The steady decrease in the quantity of TAHP present at the interface suggests that the change in the shape of the pendant drop, rather than being driven by a melting transition, is driven by an increase in the saturation concentration of TAHP in the fluid phases, in particular the dodecane. Similar observations were made in the interfacial rheology experiments. In the temperature range in which the rheological moduli of the film are within detection limits of the equipment, the film is an elastic solid. Increasing the temperature of the system leads to a steady, rather than a discontinuous, decrease in the interfacial storage modulus of the film. There is a similar decrease in the interfacial loss modulus of the system. However, the values of Gs′ and Gs″ are never observed to cross over until 9319
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form. At pH 7 and below, highly stable water-in-oil emulsions drops are formed. TAHP-stabilized water-in-oil emulsions formed between pH 5 and 7 have been found to be stable against coalescence for over two years. The majority of the tetradecylamine present in the system is present in the final interfacial films. Testing of the nonaqueous supernatant using a fluorometric assay, in which an aliquot of supernatant was tested in a 10 mM solution of fluorescamine in acetone,64 found a residual primary amine of approximately 150 μM. This implies a similar, or slightly higher, uptake of primary amine to the case of the films formed at a planar interface. Adding NBD-Cl to the aqueous phase leads to the formation of fluorescently doped films on the droplet surfaces that can be directly imaged using fluorescence microscopy. The excitation and emission spectrum of NBD-based fluorophores depends strongly on the dielectric constant of the surrounding phase,65 meaning that the fluorescence signals from the dodecane and the interface can be readily separated. A weak signal from the water droplets could also be detected, suggesting at least a sparing solubility of TDA-NBD in both fluid phases. For clarity, the fluorescence signal from the oil phase is not shown here. Confocal fluorescence micrographs of TAHP-stabilized droplets and capsules are shown in Figure 11. The water droplets are seen to be coated by TAHP crystallites of a range of shapes. The capsules (Figure 11b) are coated with large plates of TAHP, with some cracks in the film seen, due to either an inhomogeneous coating of the interface or droplet collisions during emulsion formation. Emulsions formed using high-shear show TAHP films consisting of both large plates (small droplet, Figure 11c) and needle-shaped crystallites (large droplet, Figure 11c). The image shown here took 7 min to obtain during which time no motion of the crystallites was observed. Further observations lasting an hour have also detected no movement in the interfacial films of stable emulsion droplets. A weak fluorescence signal can also be seen in the darker regions of the interface, suggesting continuous or near-continuous interfacial coverage. Cryo-SEM images of emulsion drops, prepared under similar conditions to those in Figure 11c, are shown in Figure 12. Needle- and ribbon-shaped TAHP crystallites, similar in appearance to those seen in the confocal micrographs, can be clearly seen in Figures 12b and 12c, showing that these are not artifacts of the freezing process. The interfacial structure of the TAHP is most reminiscent of the crystallites formed by acid soaps.66 Extensive work has been performed on these systems to understand the rather complex relationship between the stoichiometry, phase behavior, and concentration dependence of the crystallite morphology.67−69
the raw phase between the force and displacement of the oscillation reaches ≥170°, at which point instrument inertia dominates the measurement, making extraction of the rheological moduli challenging.58 Changing the fatty amine concentration does not change the form of the temperature dependence of the rheological moduli but instead only varies the breakdown temperature of the film. At 2.5 mM TDA (left, in Figure 9), the film cannot be detected by the apparatus above 55 °C. When TDA concentration is increased (right, same figure), the temperature at which the film cannot be detected is shifted above 65 °C, which is above the accessible temperature range of our apparatus. The concentration dependence of the “breaking” temperature of the film further suggests that the temperature dependence of the film strength is driven by the saturation concentration of TAHP in the fluid phases at a given temperature, rather than a melting transition of the film. This was also suggested by the SANS experiments in the previous section, in which the prominence of the Bragg peaks detected from scattering by TAHP-stabilized emulsions was found to decrease as the system was heated. Light scattering experiments on the dodecane supernatant performed at high temperature (65 °C) showed no significant increase in scattering intensity that could be attributed to micelle formation, suggesting the TAHP is present in the dodecane as monomers. Water-in-Oil Emulsions. Temperature-responsive waterin-oil emulsions stabilized by a TAHP film, formed by shearing using a rotor stator, are shown in Figure 10. These have
Figure 10. TAHP-stabilized water-in-dodecane emulsions formed at increasing (left to right) pH. The dodecane contains 5 mM tetradecylamine. The pH of the aqueous phase is (left to right) 5, 6, 7, 8, and 10. Shear rate = 30 000 s−1, [TDA] = 5 mM, pH = 5.
potential applications for controlled release of active ingredients. At pH 8 and above, insufficient interfacial material is formed on the time scale of coalescence for stable emulsions to
Figure 11. (a) Annotated cross section of TAHP-stabilized capsules in dodecane, made using the needle-in-coflowing-stream method. Only the fluorescence signal from the interface is shown. (b) Confocal micrographs consisting of summed slices of the capsules shown in (a). (c) Confocal micrograph of TAHP stabilized water-in-oil emulsion drops (shear rate = 30 000 s−1, [TDA] = 5 mM, pH = 5); slices summed in the z-direction. 9320
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Figure 12. Cryo-SEM images of a TAHP-stabilized water-in-dodecane emulsion. The region magnified is denoted in each image. Shear rate = 30 000 s−1, [TDA] = 5 mM, pH = 5.
Figure 13. (top) TAHP-stabilized emulsion droplets heated to (left to right) 40, 50, and 60 °C. Low concentrations of TDA-NBD are present in the water and fluoresce under the same frequency of light as the TDA-NBD present at the drop interface. The right two micrographs have had the intensity doubled to make features in the image clearer. (bottom) TAHP-stabilized emulsions in quartz glass cuvettes imaged after the cuvettes have been immersed for 30 min at the stated temperature. [TDA] = 5 mM; aqueous phase is at pH 5. Image contrast has been enhanced for clarity.
The marked similarity in crystallite morphology variation suggests that the work presented here is complementary to the extensive body of work already performed on acid soaps. Direct imaging of the system during heating shows similar behavior to that previously observed on macroscopic systems, as shown in Figure 13. The interface can be seen to thin inhomogeneously, and the fluorescence signal from the interface, which overwhelms the signal from the droplets at room temperature, becomes significantly weaker. Typically, smaller droplets retain at least a partial covering of the interfacial film at higher temperatures. Needle-like crystals of TAHP and TDA-NBD can be clearly seen projecting from the droplet interface in the bottom left of Figure 13, center. The droplet concentration in the fluorescence micrographs in Figure 13, top, is significantly lower than in cuvettes in the same figure, and so the droplets in the confocal micrographs are unstable at lower temperatures. Also shown in Figure 13 is the effect of temperature upon emulsion stability. The emulsions were formed at [TDA] = 5 mM, with the aqueous hydrogen phosphate solution at pH 5, and pipetted in to the glass cuvettes. The volume fraction of emulsion in the samples shown here is lower than that used in the scattering experiments, and so coarsening and sedimenta-
tion have taken place to a slightly greater extent. The coarsening process takes place gradually as temperature is increased, with the majority of the coarsening happening between 50 and 60 °C. The pH and temperature dependence of the stability of the emulsions along with the observation that the TAHP films surrounding the droplets are static on a time scale of approximately 1 h shows that the mechanism preventing coalescence of these emulsions is the steric barrier of the elastic layer of TAHP. The point at which the rheological moduli of the film fall below the detection sensitivity of the interfacial rheology apparatus, combined with the direct fluorescence imaging of the droplet interfaces during heating, shows that it is the thinning of the film that causes the droplets to coalesce. Other workers have seen similar, temperature-responsive behavior in foams stabilized by fatty acid needles, albeit arising from a slightly different cause.70 The temperature response in the fatty acidstabilized systems is caused by the increase in temperature driving a phase transition from tubule to micelle conformation.70 The temperature-responsive behavior in the TAHP films occurs due to the dispersion of the TAHP in to the fluid phases and is hence driven by the temperature dependence of the 9321
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Langmuir equilibrium concentration of TAHP in the fluid phases. While the acid-soap-stabilized foams exhibit a marked drop in stability at a single temperature (∼60 °C) due to a change in the conformation of the fatty acids, the coalescence temperature of TAHP-stabilized emulsions in the temperature range probed is dependent on TAHP film thickness and the volume fraction of emulsion in the dodecane. Indeed, we have found that simply dispersing an aliquot of TAHP-stabilized droplets in a large volume of dodecane at room temperature induces coalescence of TAHP-stabilized droplets.
wealth of structures with potential applications in the controlled release of active molecules.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b01981. Analysis of scattering data; Figures S1−S3 (PDF)
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CONCLUSION Confocal fluorescence microscopy, small-angle neutron scattering, cryo-SEM, Fourier transform infrared spectroscopy, and interfacial shear rheology have been used to study the chemical and physical properties of interfacial films formed by tetradecylamine and hydrogen phosphate at the dodecane− water interface. We find that these films are formed by interfacially mediated counterion binding and that between pH 5 and 8 they consist predominantly of two tetradecylammonium tails stoichiometrically bound to HPO42−. Our results show that the binding of the tetradecylamine to the hydrogen phosphate counterion leads to a self-assembled, tunable, hierarchical structure forming at the oil−water interface. Small-angle neutron scattering data show that the film consists of lamellae with a repeat distance of 37.9 Å. Confocal fluorescence microscopy and cryo-SEM show that these lamellae assemble in to needle-shaped crystallites, which then aggregate to form interfacial films. The interfacial rheology of the TAHP films is elasticdominated, and the film is notable for its large interfacial storage modulus (Gs′ ∼ 10 N/m at 2.5 mM TDA), which is attributed to the large mass of material formed at the interface. The strength of these films can be tuned by varying both the pH of the aqueous phase, which determines the extent of protonation and the surface activity of the amine head groups, and the fatty amine concentration in the oil. The macroscopic structure of the films can be varied from lace-like to a continuous sheet. The continuous sheets are highly brittle, while the lace-like films deform plastically under shear. The film is also temperature-responsive. Rather than this temperature response occurring abruptly at a single temperature, it occurs due to dissolution of the TAHP in to the fluid phases. The breaking temperature of the film can thus be varied by controlling the initial thickness of the film. We also find that the films can be used to form highly stable water-in-oil emulsions using two different methods. Like the films that stabilize them, these capsules and emulsions have been shown to be temperature-responsive and coalesce upon being heated. Images obtained, combined with interfacial rheology measurements, show that coalescence occurs due to the thinning and dissolution of the interfacial film. Inclusion of NBD in the aqueous phase leads to the formation of fluorescent interfaces that can be directly imaged. Future work must focus on directly studying the switchable interfacial rheology of the film by using rheo-optic methods and tuning surface coverage via pH. Insight into the mechanism by which the interfacially mediated reaction yields micron-sized crystallites, rather than a monolayer, is also of particular relevance given importance of the thick TAHP film in emulsion stabilization. Finally, work performed on similar systems suggests that varying fatty amine concentration, along with varying the counterion used in the aqueous phase, could yield a
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (P.S.C.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Andrew Schofield, Job Thijssen, and Michiel Hermes for useful and stimulating conversations. We are grateful to Wilson Poon for suggesting the use of NBD in the system, to Jan Vermant for advice regarding interfacial rheology techniques, to Stephen Mitchell and the Electron Microscope Laboratory in the School of Biological Sciences for use of facilities, and to the STFC and ISIS for beam-time and consumables support. J.F. was supported by a DRINC Scheme Studentship (BB/J50094/1).
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REFERENCES
(1) Maas, M.; Ooi, C. C.; Fuller, G. G. Thin film formation of silica nanoparticle/lipid composite films at the fluid-fluid interface. Langmuir 2010, 26, 17867−73. (2) Cui, M.; Emrick, T.; Russell, T. P. Stabilizing liquid drops in nonequilibrium shapes by the interfacial jamming of nanoparticles. Science (Washington, DC, U. S.) 2013, 342, 460−464. (3) Dickinson, E. Interfacial structure and stability of food emulsions as affected by protein-polysaccharide interactions. Soft Matter 2008, 4, 932−942. (4) Ogawa, S.; Decker, A.; McClements, D. J. Production and characterization of O/W emulsions containing droplets stabilized by lecithin-chitosan-pectin mutilayered membranes. J. Agric. Food Chem. 2004, 52, 3595−3600. (5) Zoldesi, C. I.; Van Walree, C. A.; Imhof, A. Deformable hollow hybrid silica/siloxane colloids by emulsion templating. Langmuir 2006, 22, 4343−4352. (6) O’Sullivan, M.; Vincent, B. Aqueous dispersions of silica shell/ water-core microcapsules. J. Colloid Interface Sci. 2010, 343, 31−35. (7) Whitby, C. P.; Fornasiero, D.; Ralston, J. Effect of oil soluble surfactant in emulsions stabilised by clay particles. J. Colloid Interface Sci. 2008, 323, 410−9. (8) Santini, E.; Guzmán, E.; Ferrari, M.; Liggieri, L. Emulsions stabilized by the interaction of silica nanoparticles and palmitic acid at the waterhexane interface. Colloids Surf., A 2014, 460, 333−341. (9) Erni, P. Deformation modes of complex fluid interfaces. Soft Matter 2011, 7, 7586−7600. (10) Ganzevles, R. A.; Zinoviadou, K.; Van Vliet, T.; Cohen Stuart, M. A.; De Jongh, H. H. J. Modulating surface rheology by electrostatic protein/polysaccharide interactions. Langmuir 2006, 22, 10089− 10096. (11) Andersson Trojer, M.; Nordstierna, L.; Nordin, M.; Nydén, M.; Holmberg, K. Encapsulation of actives for sustained release. Phys. Chem. Chem. Phys. 2013, 15, 17727−41. (12) McClements, D. J.; Li, Y. Structured emulsion-based delivery systems: controlling the digestion and release of lipophilic food components. Adv. Colloid Interface Sci. 2010, 159, 213−28. 9322
DOI: 10.1021/acs.langmuir.5b01981 Langmuir 2015, 31, 9312−9324
Article
Langmuir (13) Decher, G. Fuzzy Nanoassemblies: Toward Layered Polymeric Multicomposites. Science 1997, 277, 1232−1237. (14) Kaufman, G.; Boltyanskiy, R.; Nejati, S.; Thiam, A. R.; Loewenberg, M.; Dufresne, E. R.; Osuji, C. O. Single-step microfluidic fabrication of soft monodisperse polyelectrolyte microcapsules by interfacial complexation. Lab Chip 2014, 14, 3494−3497. (15) Whitby, C. P.; Fornasiero, D.; Ralston, J.; Liggieri, L.; Ravera, F. Properties of Fatty AmineSilica Nanoparticle Interfacial Layers at the HexaneWater Interface. J. Phys. Chem. C 2012, 116, 3050−3058. (16) Gaines, G. LangmuirBlodgett films of long-chain amines. Nature 1982, 298, 544−545. (17) Vollhardt, D.; Wittig, M.; Petrov, J.; Malewski, G. Infrared spectroscopic study of phosphate counter ions bonded in skimmed monolayers and transferred multilayers of octadecylamine. J. Colloid Interface Sci. 1985, 106, 28−32. (18) Petrov, J. G.; Kuleff, I.; Platikanov, D. Interaction of octadecylamine monolayers with phosphate counterions. J. Colloid Interface Sci. 1986, 109, 455−460. (19) Angelova, A.; Petrov, I.; Kuleff, I. Stoichiometry of LangmuirBlodgett multilayers of docosylammonium arsenate. Langmuir 1992, 8, 213−216. (20) Angelova, A.; Petrov, J. G.; Dudev, T.; Galabov, B. Infrared spectra of Langmuir-Blodgett docosylammonium phosphate multilayers of. Colloids Surf. 1991, 60, 351−368. (21) Umbanhowar, P. B.; Prasad, V.; Weitz, D. A. Monodisperse Emulsion Generation via Drop Break Off in a Coflowing Stream. Langmuir 2000, 16, 347−351. (22) Vandebril, S.; Franck, A.; Fuller, G. G.; Moldenaers, P.; Vermant, J. A double wall-ring geometry for interfacial shear rheometry. Rheol. Acta 2010, 49, 131−144. (23) Ghosh, P. B.; Whitehouse, M. W. 7-Chloro-4-nitrobenzo-2-oxa1,3-diazole: a New Fluorigenic Reagest for Amino Acids and other Amines. Biochem. J. 1968, 108, 155−156. (24) Lichius, A.; Lord, K. M.; Jeffree, C. E.; Oborny, R.; Boonyarungsrit, P.; Read, N. D. Importance of MAP kinases during protoperithecial morphogenesis in Neurospora crassa. PLoS One 2012, 7, e42565. (25) Heenan, R. K.; Rogers, S. E.; Turner, D.; Terry, A. E.; Treadgold, J.; King, S. M. Small Angle Neutron Scattering Using Sans2d. Neutron News 2011, 22, 19−21. (26) Rennie, A. R.; Hellsing, M. S.; Wood, K.; Gilbert, E. P.; Porcar, L.; Schweins, R.; Dewhurst, C. D.; Lindner, P.; Heenan, R. K.; Rogers, S. E.; Butler, P. D.; Krzywon, J. R.; Ghosh, R. E.; Jackson, A. J.; Malfois, M. Learning about SANS instruments and data reduction from round robin measurements on samples of polystyrene latex. J. Appl. Crystallogr. 2013, 46, 1289−1297. (27) Arnold, O.; Bilheux, J.-C.; Borreguero, J.; Buts, A.; Campbell, S. I.; Chapon, L.; Doucet, M.; Draper, N.; Leal, R. F.; Gigg, M. Mantid Data analysis and visualization package for neutron scattering and μ SR experiments. Nucl. Instrum. Methods Phys. Res., Sect. A 2014, 764, 156− 166. (28) Wignall, G. D.; Bates, F. S. Absolute Calibration of Small-Angle Neutron Scattering Data. J. Appl. Crystallogr. 1987, 20, 28−40. (29) Caillé, A. Remarques sur la diffusion des rayons X dans les smectiques A. C. R. Acad. Sci. Paris, Ser. B 1972, 274, 891−893. (30) Nallet, F.; Laversanne, R.; Roux, D. Modelling X-ray of neutron scattering spectra of lyotropic lamellar phases: interplay between form and structure factors. J. Phys. II 1993, 3, 487−502. (31) Tanford, C. Micelle shape and size. J. Phys. Chem. 1972, 76, 3020−3024. (32) Binks, B. P.; Horozov, T. S. Colloidal Particles at Liquid Interfaces; Cambridge University Press: New York, 2006. (33) Davis, J. T.; Rideal, E. K. Interfacial Phenomena; Academic Press: New York, 1963. (34) Peters, R. A. Interfacial Tension and Hydrogen-Ion Concentration. Proc. R. Soc. London, Ser. B 1931, 109, 88−90. (35) Perrin, D. D. Dissociation Constants of Organic Bases in Aqueous Solution; Butterworths: London, 1972.
(36) Adam, N. K. The Structure of Surface Films. Part XV. Amines. Proc. R. Soc. London, Ser. A 1930, 126, 526−541. (37) Betts, J. J.; Pethica, B. A. The ionization characteristics of monolayers of weak acids and bases. Trans. Faraday Soc. 1956, 52, 1581. (38) Goddard, E. Ionizing monolayers and pH effects. Adv. Colloid Interface Sci. 1974, 4, 45−78. (39) Siegel, S.; Honig, D.; Vollhardt, D.; Mobius, D. Direct Observation of Three-Dimensional Transformation of Insoluble Monolayers. J. Phys. Chem. 1992, 96, 8157−8160. (40) Vollhardt, D.; Retter, U. Nucleation In Insoluble Monolayers. 1. Nucleation and Growth Model for Relaxation of Metastable Monolayers. J. Phys. Chem. 1991, 95, 3723−3727. (41) Vollhardt, D.; Retter, U. Nucleation in Insoluble Monolayers. 3. Overlapping Effect of the Growing Centers. Langmuir 1992, 8, 309− 312. (42) Pezron, E.; Claesson, P. M.; Berg, J. M.; Vollhardt, D. Stability of arachidic acid monolayers on aqueous salt solutions. J. Colloid Interface Sci. 1990, 138, 245−254. (43) Angelova, A.; Vollhardt, D.; Ionov, R. 2D-3D Transformations of Amphiphilic Monolayers Influenced by Intermolecular Interactions: A Brewster Angle Microscopy Study. J. Phys. Chem. 1996, 100, 10710− 10720. (44) Barnes, G. T.; Gentle, I. R. Interfacial Science: An Introduction, 2nd ed.; Oxford University Press: New York, 2011. (45) Linstrom, P. J., Mallard, W. G., Eds.; Infrared Spectra; NIST Chemistry WebBook. (46) Miller, F. A.; Wilkins, C. H. Infrared Spectra and Characteristic Frequencies of Inorganic Ions. Anal. Chem. 1952, 24, 1253−1294. (47) Polak, M.; Gruebele, M.; DeKock, B. W.; Saykally, R. J. Velocity modulation infrared laser spectroscopy of molecular ions. Mol. Phys. 1989, 66, 1193−1202. (48) Fripiat, J. J. Influence of the Van Der Waals Force on the Infrared Spectra of Short Aliphatic Alkylammonium Cations Held on Montmorillonite. Clay Miner. 1969, 8, 119−134. (49) Douliez, J. P.; Navailles, L.; Nallet, F.; Gaillard, C. Self-assembly of unprecedented swollen multilamellar twisted ribbons from a racemic hydroxy fatty acid. ChemPhysChem 2008, 9, 74−77. (50) Fameau, A. L.; Houinsou-Houssou, B.; Ventureira, J. L.; Navailles, L.; Nallet, F.; Novales, B.; Douliez, J. P. Self-assembly, foaming, and emulsifying properties of sodium alkyl carboxylate/ guanidine hydrochloride aqueous mixtures. Langmuir 2011, 27, 4505− 4513. (51) Verruto, V. J.; Kilpatrick, P. K. Water-in-model oil emulsions studied by small-angle neutron scattering: interfacial film thickness and composition. Langmuir 2008, 24, 12807−22. (52) Reynolds, P. A.; Gilbert, E. P.; White, J. W. High Internal Phase Water-in-Oil Emulsions Studied by Small-Angle Neutron Scattering. J. Phys. Chem. B 2000, 104, 7012−7022. (53) Fameau, A. L.; Houinsou-Houssou, B.; Novales, B.; Navailles, L.; Nallet, F.; Douliez, J. P. 12-Hydroxystearic acid lipid tubes under various experimental conditions. J. Colloid Interface Sci. 2010, 341, 38− 47. (54) Fameau, A. L.; Cousin, F.; Navailles, L.; Nallet, F.; Boué, F.; Douliez, J. P. Multiscale structural characterizations of fatty acid multilayered tubes with a temperature-tunable diameter. J. Phys. Chem. B 2011, 115, 9033−9039. (55) Keim, N. C.; Arratia, P. E. Mechanical and Microscopic Properties of the Reversible Plastic Regime in a 2D Jammed Material. Phys. Rev. Lett. 2014, 112, 028302. (56) Ridout, M. J.; Mackie, A. R.; Wilde, P. J. Rheology of Mixed beta-Casein/beta-Lactoglobulin Films at the Air Water Interface. J. Agric. Food Chem. 2004, 52, 3930−3937. (57) Erni, P.; Windhab, E. J.; Fischer, P. Emulsion Drops with Complex Interfaces: Globular Versus Flexible Proteins. Macromol. Mater. Eng. 2011, 296, 249−262. (58) Jaishankar, A.; Sharma, V.; McKinley, G. H. Interfacial viscoelasticity, yielding and creep ringing of globular proteinsurfactant mixtures. Soft Matter 2011, 7, 7623−7634. 9323
DOI: 10.1021/acs.langmuir.5b01981 Langmuir 2015, 31, 9312−9324
Article
Langmuir (59) Wilde, P.; Mackie, A.; Husband, F.; Gunning, P.; Morris, V. Proteins and emulsifiers at liquid interfaces. Adv. Colloid Interface Sci. 2004, 108−109, 63−71. (60) Hermans, E.; Vermant, J. Interfacial shear rheology of DPPC under physiologically relevant conditions. Soft Matter 2014, 10, 175− 86. (61) Graužinyte, M.; Forth, J.; Rumble, K. A.; Clegg, P. S. ParticleStabilized Water Droplets that Sprout Millimeter-Scale Tubes. Angew. Chem., Int. Ed. 2015, 54, 1456−1460. (62) McLoughlin, D.; Langevin, D. Surface complexation of DNA with a cationic surfactant. Colloids Surf., A 2004, 250, 79−87. (63) Espinosa, G.; Langevin, D. Interfacial shear rheology of mixed polyelectrolyte-surfactant layers. Langmuir 2009, 25, 12201−12207. (64) Imai, K.; Toyo’oka, T.; Miyano, H. Fluorigenic reagents for primary and secondary amines and thiols in high-performance liquid chromatography. A review. Analyst 1984, 109, 1365−1373. (65) Tsukanova, V.; Grainger, D.; Salesse, C. Monolayer behavior of NBD-labeled phospholipids at the air/water interface. Langmuir 2002, 18, 5539−5550. (66) Douliez, J.-P.; Gaillard, C. Self-assembly of fatty acids: from foams to protocell vesicles. New J. Chem. 2014, 38, 5142−5148. (67) Stuart, M. C. A.; van Esch, J.; van De Pas, C.; Engberts, J. B. F. N. Chain-length and solvent dependent morphological changes in sodium soap fibers. Langmuir 2007, 23, 6494−6497. (68) Kralchevsky, P. A.; Danov, K. D.; Pishmanova, C. I.; Kralchevska, S. D.; Christov, N. C.; Ananthapadmanabhan, K. P.; Lips, A. Effect of the precipitation of neutral-soap, acid-soap, and alkanoic acid crystallites on the bulk pH and surface tension of soap solutions. Langmuir 2007, 23, 3538−3553. (69) Boneva, M. P.; Danov, K. D.; Kralchevsky, P. A.; Kralchevska, S. D.; Ananthapadmanabhan, K. P.; Lips, A. Coexistence of micelles and crystallites in solutions of potassium myristate: Soft matter vs. solid matter. Colloids Surf., A 2010, 354, 172−187. (70) Fameau, A.-L.; Saint-Jalmes, A.; Cousin, F.; Houinsou Houssou, B.; Novales, B.; Navailles, L.; Emile, J.; Nallet, F.; Gaillard, C.; Boué, F.; Douliez, J.-P. Smart foams: switching reversibly between ultrastable and unstable foams. Angew. Chem., Int. Ed. 2011, 50, 8264−8269.
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