Temperature- and pH-Dependent Shattering: Insoluble Fatty

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Temperature- and pH-dependent shattering: Insoluble fatty ammonium phosphate films at water-oil interfaces Joe Forth, David J. French, Andrei V Gromov, Stephen M. King, Simon Titmuss, Kathryn M. Lord, Michael Ridout, Peter James Wilde, and Paul. S. Clegg Langmuir, Just Accepted Manuscript • Publication Date (Web): 11 Aug 2015 Downloaded from http://pubs.acs.org on August 11, 2015

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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,k Pete J. Wilde,k and Paul S. Clegg∗,† School of Physics and Astronomy, University of Edinburgh, Edinburgh, EH9 3FD, UK, EaStChem, School of Chemistry, University of Edinburgh, Edinburgh, EH9 3FJ, UK, STFC ISIS Facility, Rutherford Appleton Laboratory, Harwell Oxford Campus, Didcot, OX11 0QX, UK, School of Biological Sciences, University of Edinburgh, Edinburgh, EH9 3BF, and Institute of Food Research, Norwich Research Park, Colney, Norwich, NR4 7UA, UK E-mail: [email protected]

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, ∗

To whom correspondence should be addressed School of Physics and Astronomy, University of Edinburgh, Edinburgh, EH9 3FD, UK ‡ EaStChem, School of Chemistry, University of Edinburgh, Edinburgh, EH9 3FJ, UK ¶ STFC ISIS Facility, Rutherford Appleton Laboratory, Harwell Oxford Campus, Didcot, OX11 0QX, UK § School of Biological Sciences, University of Edinburgh, Edinburgh, EH9 3BF k Institute of Food Research, Norwich Research Park, Colney, Norwich, NR4 7UA, UK †

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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, 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 temperatureresponsive water-in-oil emulsions with potential applications in controlled release of active molecules.

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 polymerisation, 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 non-equilibrium droplet shapes. 9,10 Such elastic multi-layer 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 stimulus-responsive and provide a high stability against aggregation and coalescence, but can require rather intensive multi-step 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 2


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that creates temperature-responsive films with tunable interfacial rheological properties, and that is suitable for large-scale production, is therefore rather desirable.

Figure 1: (Top) Chemical structure of tetradecylamine. (Bottom) Schematic illustrating the interfacially-mediated binding of the (water-insoluble) tetradecylamine and (dodecaneinsoluble) hydrogen phosphate. This interaction rapidly forms thick, multi-layer films, as shown in Fig. 2, which consist of micron-sized crystallites. .

In this paper we study the temperature-responsive films formed by the binding of tetradecylamine (TDA, CH3 (CH2 )13 NH2 ), a water-insoluble fatty amine, to hydrogen phosphate counter-ions, at the dodecane-water interface. This is illustrated in Fig. 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 counter-ions. 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 18–20 ammonium cations bonded to a single HPO2− 4 anion.

This paper studies the films TAHP films formed at the oil-water interface, which leads to markedly different behaviour. Rather than a monolayer, we find that the films are thick and extremely 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

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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 fluorescence microscopy, suggesting a number of potential applications in controlled release and delivery.

Materials and Methods Sample Preparation Tetradecylamine (≥ 95%) and dodecane (ReagentPlus, ≥ 99%) were purchased from SigmaAldrich UK, and used as obtained. Water was de-ionized and filtered through a Milli-Q reverse osmosis unit (resistivity > 1018 Ωm). 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 1M NaOH. All experiments (3−n)−

presented here were performed using an aqueous solution of 100mM Hn PO4 ratio

[HPO2− 4 ] [H2 PO− 4 ]

, with the

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, 4


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Switzerland) operated for one minute at a shear rate of 30000s−1 . Additionally, water-in-oil capsules were made by injecting aqueous hydrogen phosphate solution in to a co-flowing stream of 5mM 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. 0.5ml of aqueous solution was injected in to the dodecane bath at a constant rate of 0.1ml/minute through a glass needle with a tip width of 150µm, which was placed 2cm from the cup centre. The cup was rotated at approximately 1Hz.

Fourier Transform Infra-Red 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 non-contact all-reflection objective (ARO, 10x magnification, Smiths Detection). TAHP synthesis for these experiments was carried out over 60 hours in the crystallization dishes as shown in Fig. 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 5


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of trough. 10ml of dodecane containing tetradecylamine was then gently added on top. The rheometer was operated in controlled strain mode during film formation, with an oscillating 0.5Hz 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 non-linear response. For temperature measurements, the trough was covered with an aluminium lid and samples were equilibrated for 2 hours before the measurement was performed. Further measurements were taken using a TA AR-G2 rheometer with a stainless steel 22mm diameter bicone geometry to rule out geometry- and instrument-specific effects. Sample preparation and experimental procedure was 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 7ml 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 hours 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-7-nitrobenzofurazan, 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 (< 1mg/20ml) was dispersed in phosphate solutions using an ultrasonic bath for several hours. Samples were then filtered using a Millex syringe-driven polyethersulfone filter unit with a pore size of 200nm. Addition of this aqueous solution to a dispersion of tetradecylamine in dodecane led to the formation of fluorescent interfacial 6


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films with rheological and solubility characteristics very similar those of films formed with no NBD present. Fluorescence microscopy images were taken 24 hours after emulsification to allow the fluorophore to react.

Pendant Drop Tensiometry Pendant drop images were obtained using the Kr¨ uss Easy Drop apparatus and accompanying software (Kr¨ uss GmbH, Germany), which allows for controlled rates and volumes of droplet injection. Measurements were performed with the sample inside a temperature-controlled chamber and samples cooled at a constant rate of 0.5◦ C or, where temperatures are quoted, after the sample had been left for 30 minutes for the temperature to equilibrate. The dodecane was held in a 6ml cubic borosilicate glass cuvette. The needle was lowered in to the dodecane bath and the pre-determined volume of water was injected at 0.5ml min−1 . The injection and ageing of the droplet was recorded at 31fps.

Cryo-Scanning Electron Microscopy Cryo-scanning electron microscopy was performed using a Hitachi S-4700 Cold Field-Emission Scanning Electron Microscope (Hitachi High-Tech. Europe Gmbh, Germany) fitted with a Gatan Alto 2500 cryo-specimen system (Gatan UK, UK), using a method adapted from imaging biological specimens. 24 The samples were mounted on brass specimen stubs. The specimen stub was plunged into slushed nitrogen at -210◦ C, and then rapidly transferred into the microscope to achieve best sample preservation. The specimen was fractured in the cryo-preparation chamber using a mounted scalpel, and then sputter-coated with ∼ 10nm of 60%:40% gold:palladium alloy. Specimens were manually rotated for part of the coating period, to allow for even sputter-coating of rough surfaces, which were almost at right angles to the plane of the stub. An accelerating voltage of ∼ 5kV was used to image the sample in the microscope, and both upper and lower detectors were used for imaging.

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Small-Angle Neutron Scattering Small-angle neutron scattering experiments were performed at ISIS on the SANS2D instrument using TAHP-stabilized water-in-dodecane emulsions. Dodecane with a scattering length density matched (5% v/v h-dodecane, 95% v/v d-dodecane) and off-matched (100% d-dodecane) to D2 O (98-atom %, d-dodecane, QMX Laboratories; D2 O, Sigma Aldrich) was used. To maximize surface area and scattering signal, 6.5mM h-tetradecylamine was used and emulsions were formed at a higher shear rate of 60000s−1 for 1 min using an UltraTurrax T10 Basic rotor stator with S10N-5G 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 20µm). The emulsions were then pipetted in to 0.5ml volume, 2mm path length quartz cuvettes held in a temperaturecontrolled 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 d- and h-dodecane or D2 O. 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 over-subtraction 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˚ A, where q is determined by time-of-flight. ∆q/q is taken to be 5%, in line with estimates used elsewhere for the same instrument. 25,26 Scattering data was reduced using Mantid, 27 with measured intensity being placed on an absolute scale using the scattering from a standard sample (a solid blend of 8


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hydrogenous and perdeuterated polystyrene) in accordance with established procedure. 28 A simultaneous q-range of 0.005 - 1.78˚ A was achieved by employing both front and rear detectors at sample-to-detector distances of 2.4m and 4m respectively. Least-squares fits and data analysis for scattering experiments was performed in Igor Pro using the NIST SANS data analysis macros. Data was modelled 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 Supplementary 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 non-polar 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 non-polar 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 ˚ A2 and ρnon-polar = −4.73 · 10−7 ˚ A2 .

Results and Discussion Film Formation Photographs of the TAHP films formed after 60 hours at a flat, macroscopic, water-dodecane interface are shown in Fig. 2. The structure of the film is strongly dependent on the pH of the aqueous phase, and varies from a thick sheet that strongly scatters light at pH 7, to a self-affine, lace-like structure at pH 8. No film can be seen by eye to form at pH 9. Mass measurements of the TAHP isolated 60 hours after the system was formed suggest that approximately 90% of the tetradecylamine added to the system is present at 9


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the interfacial film at pH 7 and below, but approximately 30% at pH 8 (assuming a perfect 2:1 amine:phosphate stoichiometry). Micrographs of the films formed at pH 7 and 8 (Fig. 2, inset) show that at pH 7 the oil-water interface is fully covered with TAHP crystallites, whilst at pH 8 the film only partially covers the interface, and consists predominantly of aggregated needle-shaped TAHP crystallites. At pH 8 these crystallites can be directly visualised, and can be as large 150µm in length. Breaking the continuous sheets formed at pH 7 using a glass pipette reveals a similar, self-assembled structure (shown in Supplementary Info, Fig. S1) but with markedly smaller crystallites. Qualitatively, no long-range orientational ordering of the crystallites has been observed. The crystallites can be seen to significantly deform the interface, most clearly at pH 8 where the TAHP crystallites are largest (shown in both Fig. 2 and Supplementary Info Fig. S2). This suggests that attractive interactions due to capillarity are likely to drive the self-assembly of these films. 32

Figure 2: TAHP films formed at a dodecane-water interface at various pH values, photographed after 60 hours of formation, along with light micrographs of the films formed at pH 7 and 8 (inset, scale bar is 125µm). 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 7cm. Images of the change in appearance of the TAHP film with time during the first 30 seconds of film formation, and how this is affected by the pH of the aqueous phase, are shown in Fig. 3. Here, a drop of aqueous hydrogen phosphate solution at pH 5 or 7 is injected into a dodecane bath containing tetradecylamine. In this experiment, measurements were begun as 10


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charged interface. 33,34 The pH at a charged air-water interface, pHs , is related to the pH in the bulk aqueous solution, pHb , by the relation pHs = pHb +

eψs , 2.3kB T

where kB and T are

the Boltzmann constant and absolute temperature respectively, e is the fundamental unit of charge, and ψs is the surface potential. 33 This explains both the apparent contradiction of the high mass TAHP formed even though at pH 5 the HPO2− 4 concentration is extremely low, and the decrease in TAHP yield and the slowing of the formation kinetics as the pH increased above pH 7. The acid dissociation constant of tetradecylamine is 10.6, 35 and the increased pH near the (positively charged) interface leads to a reduced concentration of tetradecylammonium cations. This trend was observed as long ago as the 1920s, 34,36 and more recently in work studying the stoichiometry of fatty ammonium phosphate and fatty ammonium arsenate monolayers. 19,20 Our results for TAHP films formed at the oil-water interface show an interfacial pH shift of approximately 3 towards more basic values, in good agreement with the other work mentioned here. This implies an effective interfacial pKa of the tetradecylamine at the interface of approximately pH 7.5, and a surface potential on the order of 200mV, in line with values obtained for monolayers of primary amines. 37,38 Rather than forming an insoluble monolayer, as observed for fatty amines spread at the air-water interface, 16 the TAHP formed at the oil-water interface forms a thick film. The presence of a monolayer in some regions of the film cannot be ruled out, however the majority of the mass of the TAHP is clearly present as crystallites. The 2D-3D transformation of amphiphilic monolayers has been extensively studied, both experimentally and theoretically, for insoluble monolayers at the air-water interface. 20,39–41 Monolayer collapse has been studied in similar, fatty acid-based systems at the air-water interface, 42 which suggests that monolayer collapse may play an important role in the nucleation of the TAHP crystallites at the oil-water interface. However, the importance of the sparing solubility of the TAHP in the dodecane, along with the speciation of the phosphate ions, are also likely to be important factors, especially given the importance of ionic strength, ion speciation, and head group ionization in governing monolayer collapse behaviour. 43 Whilst monolayer collapse is often

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undesirable in certain processing applications involving monolayers, 44 in this work we find that the formation of thick TAHP films at the oil-water interface leads to both highly stable water-in-oil emulsions and novel interfacial rheological properties.

Infrared Spectroscopy FTIR spectra for powder TAHP and TDA samples are compared with FTIR data for dibasic ((NH4 )2 HPO4 ) and monobasic (NH4 H2 PO4 ) ammonium phosphate dispersions in Nujol −1 (obtained from the NIST Webbook 45 ) in Fig. 4. Peaks characteristic of HPO2− 4 at 1077cm

(νas,HP O42− ), 965cm−1 (νs,HP O42− ), and 900cm−1 (νs,HP O42− ) can be clearly identified in samples formed at pH 5 and pH 8, as seen elsewhere in the literature. 17,20 The bands in the region between 3000 and 2800cm−1 originate from C-H stretching vibrations in the aliphatic chains that are present in both the TDA and the synthesized TAHP. The sharp band in the synthesized TAHP at 1470cm−1 , present in both the ammonium phosphate salts as a broad peak, and in the TDA as a sharp peak, arises from the collective ν4 stretching vibration of 46–48 the NH+ The sharpening of this band for the TAHP and the organic cations 3 groups.

appears to be due to the change in the symmetry of the ammonium moiety, in which only three hydrogens participate in the vibration, as opposed to four in the case of the inorganic salt. The labelled peak at 3340cm−1 , present in the TDA and characteristic of free amino groups, is absent from the TAHP powder samples. This does not entirely exclude the possibility that free tetradecylamine adsorbs on to the interfacial film during formation, and is then removed when the film is rinsed (tetradecylamine’s solubility in ethanol and hexane being far higher than TAHP’s). However, as noted, the TAHP yield at pH 7 and below is 90% of all the TDA added to the system, meaning the measurements performed here correspond to the majority constituents of the film. Measurements were also performed at pH 7, but are not shown due to their similarity with those taken of films formed at pH 5. Our results suggest that the interfacial film 13


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Figure 4: ARO-FTIR spectra obtained for TAHP formed at pH 5 and 8, tetradecylamine, and (NH4 )2 HPO4 and NH4 HPO4 dispersions in Nujol. Bands discussed in the text have been denoted. Spectra presented have been shifted vertically relative to one another for clarity. consists largely of two tetradecylammonium molecules bonded to a single HPO2− 4 anion, in good agreement with results obtained for monolayers formed at the air-water interface in other work. 18,19

Small Angle Neutron Scattering Scattering data for both 95% v/v d-dodecane 5% v/v h-dodecane (D2 O-matched) and 100% d-dodecane (off-matched) h-TAHP-stabilized emulsions, along with fits to the off-matched emulsion data, are shown in Fig. 5. Bragg peaks can be seen at q = 0.166, 0.332, and 0.504˚ A−1 (i.e., a ratio of 1:2:3), showing that the TAHP film is multi-lamellar in structure, along with a fainter, rather broad peak at q = 0.224˚ A−1 , which most likely corresponds to short-range order in the plane of the lamellae. The troughs in the scattering intensity due to minima in the form factor that, considering the tail and repeat lengths of the system, might have been expected to be seen near the Bragg peaks, are not observed due to both the 14


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contribution to the scattering from the diffusive thermal fluctuations of the non-deuterated amine tails and smearing due to finite instrumental q-resolution. The first order Bragg peak corresponds to a lamellar repeat distance of 37.9˚ A, which is somewhat smaller than the estimated length of two tetradecylamine molecules (∼ 41˚ A). 31 Further, this distance does not account for the polar diammonium phosphate regions that the FTIR results in Fig. 4 show are present in the film. At 35◦ C and below, good fits to the data are obtained for a tail group thickness of 33.7 ± 0.4˚ A (corresponding to a polar region thickness of 4.2˚ A). The TAHP crystallites, which give rise to this scattering, must therefore contain little-to-no inter-lamellar water, and with the lipid tails either interdigitated or tilted with respect to the surface normal of the bilayer. Similar lamellar structures have been found to be formed by fatty acid soaps, though the TAHP crystallites incorporate significantly less solvent, and therefore have a much smaller inter-lamellar spacing. 49 Both interdigitation of the tails and tilting has been observed in similar, fatty acid-based systems, in which the tail conformation was dependent upon the structure of the aliphatic chain. 50 Matching the scattering length densities of the solvents allows inferences to made about the structure of the interfacial film by studying the scaling of the scattered intensity at low q values. The power-law scaling in the low-q region (q ≤ 0.37˚ A−1 ) was determined by fitting aq −b to q 4 I(q). The intensity decreases as q −3.87±0.05 , and is independent of both sample temperature and, more importantly, h-dodecane concentration. Matching the scattering length densities of the solvents leads to a slight decrease in the scattering intensity at low q (from 360 to 280cm−1 ), along with an increase in incoherent background due to the larger incoherent scattering cross-section of the hydrogenous solvent, but has no effect upon the scaling at low-q. A thin film surrounding the droplets, with the scattering length densities of the solvents being matched, would be expected to yield q −2 scaling of the intensity. 51,52 From the q −4 scaling measured for the TAHP-stabilized emulsions studied here, we can infer that the interfacial TAHP film is rather thick. We have also confirmed this observation by directly imaging the film using confocal fluorescence microscopy and scanning electron

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microscopy, as shown later in Fig. 11, using samples prepared using similar conditions. At 35◦ C and below, fitting the data to the first and second order Bragg peaks gives values of the Caillé parameter of 0.001 or less. Decreasing η below this value does not significantly alter the shape of the curve, meaning this should be considered an upper bound for η. In this temperature range, η is found to be independent of temperature, suggesting that the elastic properties of the bilayers are not strongly affected by heating at these temperatures. This low value for η, along with the small repeat distance of the film and the predicted η ∼ d−2 behaviour of the Caillé parameter, suggest that the film has rather large mechanical moduli that can be attributed, at least in part, to the rather strong electrostatic interactions and hydrogen bonding likely present in the polar part of the TAHP film. The effect that heating the emulsions above 40◦ C has on the Bragg peaks is shown in the lower graphic in Fig. 5. As temperature increases, the Bragg peaks become less prominent due to the reduced quantity of TAHP present at the interface. The third order peak is only visible below 40◦ C. As the system is heated above 45◦ C a second length scale is observed in the system. This is detected at 40◦ C as a broadening of the principle peak at q = 0.166˚ A−1 , at which point Caillé theory can no longer be used to fit the peaks. This second length scale is then present as a shoulder at 45◦ C and can be resolved as a separate peak at 50◦ C. The A−1 at q-value of this peak decreases further as temperature increases, reaching q = 0.114˚ 65◦ C. The emergence of the second, lower-q peak, shows that two co-existing structures are present in the TAHP at these temperatures, corresponding to lamellae with repeat distances of 37.9 and 55.1˚ A (at 65◦ C). The experiments were performed at sufficiently high emulsion volume fraction and fatty amine concentration ([TDA] = 6.5mM in the emulsions used in the scattering experiment) that the emulsions were stable against coalescence at these elevated temperatures; thus, the appearance of this second, larger length scale can not be directly related to the coalescence of the emulsions. Similar temperature-driven changes in bilayer separation have been observed in multi-layer assemblies of fatty acids, with the change in

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Figure 5: (Top) Measured scattering intensity at 25-40◦ C and fit to the 30◦ C data for the full q-range for TAHP-stabilized emulsions containing D2 O-matched and off-match dodecane, and 30◦ C data with fit in the region of the primary Bragg peak, inset. The dashed line in the low-q region scales as q −4 . (Bottom) Measured scattering for the h-TAHP-stabilized D2 Oin-d-dodecane (off-match) emulsions in the region of the primary Bragg peak, measured at 45 (green), 50 (orange), and 55◦ C (purple).

separation between lamellae being due to incorporation of water in to the region between the bilayers. 53,54 Whilst chemically and structurally somewhat similar to these acid soap systems, the TAHP system studied here exhibits some significant differences. The acid soap needles studied by Fameau et al had a rather large separation between bilayers (often several hundred nm), with the crystallites containing typically only three or four lamellae. Contrastingly, in the case of TAHP crystallites, the bilayer separation remains rather small (i.e., never greater 17


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than the expected length of the fully extended aliphatic chains plus the size of the ammonium phosphate head group), with good fits to the data obtained when the number of lamellae is between 40 and 50. In the case of the TAHP, the emergence of the second length scale at higher temperatures ostensibly corresponds to a change in the conformation of the aliphatic tails, in which the temperature increase drives a transition to either a lower tilt angle or a non-interdigitated phase.

Interfacial Rheology Film Formation Interfacial shear rheology is used to study the TAHP films during formation, as well as their strain-reponse. Droplet shape analysis allows the first few seconds of film formation to be qualitatively studied; however, the elastic TAHP film makes drop shape analysis an unsuitable method for studying the system at larger timescales. The time-dependence of the interfacial shear moduli of the TAHP films during 10 hours of film formation are shown in Fig. 6. For all systems, independent of pH, the film is highly elastic, with Gs ’ exceeding Gs ” by over an order of magnitude. The pH of the aqueous phase affects both the final rheological moduli of the film and the rate at which the film forms. At pH 5, Gs ’ reaches large (>100mN/m) more rapidly than can be detected by the apparatus, explaining the significant deformation of the droplets in Fig. 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 6.5mM allows the film strength to be varied by a factor of three. At TDA concentrations below 1mM, the films typically reach a peak in their rheological moduli after two hours before decreasing 18


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time after which a film can be detected has been observed to range from the order of minutes up to several hours, with some films requiring 60 hours for the interfacial rheological moduli to reach stable values. The poor repeatability of the measurements at all TDA concentrations at pH 8, and at low TDA concentrations at pH 5 and 7, can be qualitatively explained in terms quantity of interfacial material formed and the retarded kinetics of TAHP formation. At pH 8, the quantity of TAHP formed is much smaller, and the kinetics of film formation much slower. Film detection requires the TAHP to precipitate at the oil-water interface, and for the needleshaped crystallites shown in Fig. 2 to form a percolating network on the length scale of the shear gap of the rheology geometry (about 4mm in the case of the DWR). In the case of low TDA concentrations at pH 5, whilst the kinetics of TAHP formation are very rapid, the quantity of material formed is still insufficient to completely cover the amount of interface present. Incomplete coverage of the interface may lead to long time-scale rearrangements of the film structure due to convective motion at the interface, explaining the variation of the film rheological moduli during formation under these conditions. Strain Response The response of TAHP films formed at different pH values to increasing interfacial shear stresses is shown in Fig. 7. At pH 7 and below the film formed is extremely brittle. At low strains (γs < 0.1%), the film behaves like a Hookean solid. As shear stress is increased, the strain then diverges over a very narrow range of stresses, at which point the film breaks and the inertia of the instrument dominates the measurement. At pH 8, the rheological behaviour is markedly different. Rather than abruptly yielding, the film deforms plastically at increasing strain, exhibiting a small peak in the loss modulus, and a cross-over of Gs ’ and Gs ” at γs ∼ 0.25%. Experiments on the pH 5 film, repeated using a stainless steel bicone geometry, yielded qualitatively identical behaviour (shown in Supplementary Information, Fig. S3).

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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 Fig. 8 using a pendant drop. The pendant drop was suspended from the needle at 25◦ C and the film was left to form for one hour, shown in Fig. 8a. In the case of the measurement performed at 65◦ C the system was then heated, as shown in Fig. 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.

Figure 8: Images of a TAHP-coated water droplet suspended from a needle in a dodecane/fatty amine bath at (left) 25◦ C and (right) 65◦ C. Photos were taken of the droplet were taken as the water was withdrawn out via the needle. [TDA]=1mM, pH 5, needle diameter = 1.83mm. Images of the pendant drop at various temperatures during heating, with photos taken after the temperature of the system had equilibrated, are shown in Fig. 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 minutes 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 22


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2.5mM TDA (left, in Fig. 9), the film can not be detected by the apparatus above 55◦ C. When TDA concentration is increased (right, same figure), the temperature at which the film can not 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 water-in-oil emulsions stabilized by a TAHP film, formed by shearing using a rotor stator, are shown in Fig. 10. These have 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 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 non-aqueous supernatant using a fluorimetric assay, in which an aliquot of supernatant was tested in a 10mM solution of fluorescamine in acetone, 64 found a residual primary amine of approximately 150µM. 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. 24


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formation. Emulsions formed using high-shear show TAHP films consisting of both large plates (small droplet, Fig. 11c) and needle-shaped crystallites (large droplet, Fig. 11c). The image shown here took seven minutes 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

Figure 12: Cryo-SEM images of a TAHP-stabilized water-in-dodecane emulsion. The region magnified is denoted in each image. Shear rate = 30000s−1 , [TDA] = 5mM, pH = 5 in Fig. 11c, are shown in Fig. 12. Needle- and ribbon-shaped TAHP crystallites, similar in appearance to those seen in the confocal micrographs, can be clearly seen in Figs. 12b) and c), showing that these are not artefacts 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 behaviour, and concentration-dependence of the crystallite morphology. 67–69 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 behaviour to that previously observed on macroscopic systems, as shown in Fig. 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 26


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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 Fig. 13, center. The droplet concentration in the fluorescence micrographs in Fig. 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.

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 minutes at the stated temperature. [TDA] = 5mM, aqueous phase is at pH 5. Image contrast has been enhanced for clarity. Also shown in Fig. 13 is the effect of temperature upon emulsion stability. The emulsions were formed at [TDA]=5mM, 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 sedimentation 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. 27


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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 timescale of approximately one hour show 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 behaviour in foams stabilized by fatty acid needles, albeit arising from a slightly different cause. 70 The temperature-response in the fatty acid-stabilized systems is caused by the increase in temperature driving a phase transition from tubule to micelle conformation. 70 The temperatureresponsive behaviour 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 equilibrium concentration of TAHP in the fluid phases. Whilst 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 TAHPstabilized droplets.

Conclusion Confocal fluorescence microscopy, small-angle neutron scattering, Cryo-SEM, fourier transform infra-red 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

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by interfacially-mediated counter-ion binding, and that between pH 5 and 8 they consist predominantly of two tetradecylammonium tails stoichiometrically bound to HPO2− 4 . Our results show that the binding of the tetradecylamine to the hydrogen phosphate counter-ion 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˚ A. 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 elastic-dominated, and the film is notable for its large interfacial storage modulus (Gs ’∼ 10N/m at 2.5mM 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, whilst 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

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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 counter-ion used in the aqueous phase, could yield a wealth of structures with potential applications in the controlled release of active molecules.

Acknowledgement 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. JF was supported by a DRINC Scheme Studentship (BB/J50094/1).

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 (80-. ). 2013, 342, 460–464. (3) Dickinson, E. Interfacial structure and stability of food emulsions as affected by proteinpolysaccharide 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. 30


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(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 Surfaces A Physicochem. Eng. Asp. 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.; Stuart, M. A. C.; 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. (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.

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(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 Langmuir-Blodgett multilayers of docosylammonium arsenate. Langmuir 1992, 8, 213–216. (20) Angelova, A.; Petrov, J. G.; Dudev, T.; Galabov, B. Infrared spectra of LangmuirBlodgett docosylammonium phosphate multilayers of. Colloids and Surfaces 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 2009, 49, 131–144. (23) Ghosh, P. B.; Whitehouse, M. W. 7-Chloro-4-nitrobenzo-2-oxa-1,3-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 .

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(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 Cryst 1987, 20, 28–40. (29) Caillé, A. Remarques sur la diffusion des rayons X dans les smectiques A. C.R. Acad. Sci. Paris Série 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 France 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, 2006. (33) Davis, J. T.; Rideal, E. K. Interfacial Phenomena; Academic Press, 1963. (34) Peters, R. A. Interfacial Tension and Hydrogen-Ion Concentration. Proc. R. Soc. B Biol. Sci. 1931, 109, 88–90. 33


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(35) Perrin, D. D. Dissociation constants of organic bases in aqueous solution; Butterworths, 1972. (36) Adam, N. K. The Structure of Surface Films. Part XV. Amines. Proc. R. Soc. A Math. Phys. Eng. Sci. 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 ThreeDimensional 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 Edition; Oxford University Press, 2011. (45) Linstrom, W. G., P. J.; Mallard, Ed. Infrared Spectra; NIST Chemistry WebBook. 34


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(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.

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(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 / betaLactoglobulin Films at the Air Water Interface. J. Agicultural 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. (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) Grauˇzinyte, M.; Forth, J.; Rumble, K. A.; Clegg, P. S. Particle-Stabilized Water Droplets that Sprout Millimeter-Scale Tubes. Angew. Chemie 2014, 53, 1–6. (62) McLoughlin, D.; Langevin, D. Surface complexation of DNA with a cationic surfactant. Colloids Surfaces A Physicochem. Eng. Asp. 2004, 250, 79–87. (63) Espinosa, G.; Langevin, D. Interfacial shear rheology of mixed polyelectrolytesurfactant layers. Langmuir 2009, 25, 12201–12207.

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Langmuir

(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, 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 Jan 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 neutralsoap, 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 Surfaces A Physicochem. Eng. Asp. 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. Engl. 2011, 50, 8264–9.

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TOC Graphic

Figure 14: Table of Contents Graphic

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Temperature- and pH-Dependent Shattering: Insoluble Fatty

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