Water Interface: Improved Lifetime through Ionic

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Duolayers at the Air/Water Interface: Improved Lifetime Through Ionic Interactions Emma Louise Prime, David H. Solomon, Ian J Dagley, and Greg G. Qiao J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b04273 • Publication Date (Web): 15 Jul 2016 Downloaded from http://pubs.acs.org on July 19, 2016

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The Journal of Physical Chemistry

Duolayers at the Air/Water Interface: Improved Lifetime Through Ionic Interactions Emma L. Prime†,‡, David H. Solomon†, Ian J. Dagley§, Greg G. Qiao*,† †

Department of Chemical & Biomolecular Engineering, University of Melbourne, Parkville, VIC, 3010, Australia §

Cooperative Research Centre for Polymers, 8 Redwood Drive, Notting Hill, VIC, 3168, Australia

ABSTRACT

Ionic interactions to stabilize Langmuir films at the air/water interface have been used to develop improved duolayer films. Two component mixtures of octadecanoic (stearic) acid and poly(diallyldimethylammonium chloride) (polyDADMAC) with different ratios were prepared and applied to the water surface. Surface pressure isotherm cycles demonstrated a significant improvement in film stability with the inclusion of the polymer. Viscoelastic properties were measured using canal viscometry and oscillating barriers, with both methods showing that the optimum ratio for improved properties was four octadecanoic acid molecules to one DADMAC unit (1:0.25). At this ratio it is expected multiple strong ionic interactions are formed along each polymer chain. Brewster Angle Microscopy showed decreased domain size with increased ratios of polyDADMAC, indicating that the polymer is interspersed across the surface. This new method to stabilize and increase the viscoelastic properties of charged monolayer films, using a pre-mixed composition, will have application in areas such as water evaporation mitigation, optical devices and foaming. 1 ACS Paragon Plus Environment


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INTRODUCTION The formation of Langmuir monolayers at the air/water interface can lead to exciting applications in a range of fields such as biomimetics,1-2 water evaporation mitigation,3-4 optical and piezoelectric devices,5-6 and solar cells.7 All these applications require stable films to be formed at the interface, however not all molecules of potential interest can form stable films by themselves. For example, ionizable molecules such as octadecanoic acid or octadecylamine often form unstable films as they are lost by dissolution into the subphase over time, a process which can speed up if the molecules are ionized.8-11 The introduction of an additional component into the film composition could assist in overcoming this limitation. Ionic interactions have previously been reported between charged surfactants such as sodium dodecyl sulfate (SDS), dodecyltrimethylammonium bromide, and oppositely charged polymers such as poly(acrylamide sulfonate).12-13 These films have shown greater surface tension lowering,14 and increased surface viscoelasticity.13, 15-16 Similarly, the addition of various salts to the water sub-phase has been shown to stabilize oppositely charged monolayers, through suppressing dissolution of the molecules.17-18 More recently, Regen et al. have reported systems which use water insoluble components such as calix[6]arene-based amphiphiles, or polymers containing quaternary amines, in combination with water soluble polymers such as poly(styrenesulfonate) or poly(acrylic acid).19-20 The resultant films showed increased viscosity, reduced defects when transferred to solid substrates as Langmuir Blodgett films,21 and have shown potential as gas separation membranes.22 However, this method requires comparatively large amounts of water soluble polymer to be used as the entire water subphase needs to contain the determined concentration of the polymer. In another method reported by this group, the two components are applied to the surface separately with the insoluble film applied first, and the water soluble polymer ‘glue’ carefully syringed under the established film.19 Neither of these techniques can be feasibly scaled up. This means that they are unsuitable for

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applications such as water evaporation mitigation, or large scale manufacturing of gas separation membranes. An alternative method for applying a multi-component mixture to the water surface is the recently reported duolayers: surface films formed by combining a water insoluble monolayer compound with a water soluble polymer prior to application to the water surface.23 The inclusion of the polymer into the system was demonstrated to provide increased surface viscosity and improved water evaporation capabilities. The novelty of the duolayer system is that the two components are mixed together first, and then applied to the water surface as a single application. However, the previously reported system of ethylene glycol monooctadecyl ether (C18E1) and poly(vinyl pyrrolidone) (PVP) was shown to be dynamic, with the polymer diffusing away from the interface over time, and therefore having only a short-lived impact on surface film characteristics.23 This short lifetime has been ascribed to relatively weak hydrogen bonding being the main mechanism anchoring the water-soluble polymer to the monolayer.24 The use of a stronger interaction between the two components could therefore increase the lifetime of the resultant duolayer system. Combination of the duolayer system with the use of ionic interactions to glue the surface film is proposed to overcome the limitations with both methods. By pre-mixing the two components prior to application to the water surface, substantially less water soluble polymer is required to obtain the gluing effect. Additionally, the use of ionic interactions instead of hydrogen bonding is expected to lead to stronger interactions and therefore improve film properties, including longevity of the improvement. This paper thereby reports on the properties of ionic duolayers formed by octadecanoic (stearic) acid and poly(diallyldimethylammonium chloride), (polyDADMAC).

EXPERIMENTAL SECTION Materials. Octadecanoic (stearic) acid (C17COOH) was purchased from Sigma Aldrich, poly(diallyldimethylammonium chloride) (polyDADMAC, MW = 240,000) was purchased from 3 ACS Paragon Plus Environment


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PolySciences. Ethylene glycol monooctadecyl ether was purchased from Advanced Molecular Technologies Pty. Ltd. Ethanol (AR, ChemSupply), hydrochloric acid and sodium hydroxide were used as received. Preparation of samples. Solutions were prepared by dissolving the water insoluble monolayer forming component and the water soluble polymer separately in ethanol at a known concentration of 1 mg/mL. To make the duolayer solutions, at ratios of 1:1, 1:0.5 and 1:0.25 molar ratio of monolayer component to polymer monomeric unit, the appropriate ratios of the two components were mixed. For example, with 1 mg/mL solutions of C17COOH and polyDADMAC, 0.57 mL of polyDADMAC was added to 1 mL of C17COOH to make the 1:1 composition. Subphase solutions at pH 4 and 10 were prepared by addition of 1 M HCl or 1 M NaOH to distilled water until the desired pH was reached. Surface pressure/area isotherms. A Teflon® Langmuir trough (KN 2003 model, 58 cm x 14.5 cm, KSV Nima) with two Delrin® barriers moving symmetrically to compress to the centre of the trough, was used to measure the surface pressure/area isotherms of the duolayer films. Before each experiment, the trough and barriers were thoroughly cleaned with chloroform and a Wilhelmy plate (2.35 cm x 1 cm, Whatman CHR1 filter paper) was attached to the pressure sensor. The trough was then filled with Milli-Q water (18.2 MΩ.cm Millipore) and allowed to equilibrate with the air at a temperature of 21±1 °C. The water surface was swept clean until the surface pressure reading was zero. The appropriate film composition was applied to the water surface and left for 30 minutes to allow the solvent to evaporate.25 The barriers were used to slowly compress the film at a rate of 10 mm/min while measuring the surface pressure as a function of the area per molecule (Å2/molecule) until the film reached its collapse pressure. The barrier was then opened at the same speed of 10 mm/min and the surface pressure also recorded. This isotherm was repeated three times to confirm the stability of the film and to ensure the reproducibility of the data. Continuous cycles of surface pressure/area isotherms were also undertaken by repeatedly closing and opening the barrier at 10


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mm/min. The barrier was held for 5 minutes in the open position (film relaxed) and closed (film compressed) states between cycles. Canal viscometry. A Teflon® Langmuir trough (model 711D, 76 cm x 10 cm, Nima Technology Ltd) with a single Delrin® barrier (11.2 cm x 1.6 cm) was used to characterize the canal viscometry properties of the surface film. Before each experiment, the trough and barrier were thoroughly cleaned with chloroform and a Wilhelmy plate (2.35 cm x 1 cm, Whatman CHR1 filter paper) was attached to the pressure sensor. The trough was then filled with the desired subphase, either Milli-Q water (18.2 MΩ.cm Millipore) or distilled water adjusted to pH 4 or 10, and allowed to equilibrate with the air at a temperature of 21±1 °C. The water surface was swept clean until the surface pressure reading was zero. A specially designed Delrin® barrier containing a central 2 mm wide canal (6 mm high) was attached to the standard barrier. The appropriate composition was applied to the water surface, left for 30 min for solvent to evaporate, before being compressed to 25 mN/m using both barriers. After a certain waiting time the two barriers were separated and the standard barrier opened at a rate of 50 cm2/min leaving the canal barrier in place. Surface film flows through the canal, reducing the surface pressure measured by the pressure sensor. This decrease in surface pressure with time was recorded. Each measurement was repeated three times to ensure reproducibility. Oscillating barriers. The Teflon® Langmuir trough (KN 2003 model, 58 cm x 14.5 cm, KSV Nima) with two Delrin® barriers moving symmetrically to compress to the centre of the trough (described previously) was used. The surface film was compressed at 10 mm/min, to a surface pressure of 25 mN/m upon which the barriers were set to oscillate at 75 mHz for 300 cycles at an area change of 5% (barrier speed 10 mm/min). The software was used to calculate the viscoelastic parameters. Brewster Angle Microscopy. The Teflon® Langmuir trough (KN 2003 model, 58 cm x 14.5 cm, KSV Nima) with two Delrin® barriers moving symmetrically to compress to the centre of the trough (described previously) was used in conjunction with the Brewster Angle Microscope (BAM, KSV NIMA) to visualize the films. The BAM was equipped with a 50 mW laser emitting p-polarized light 5 ACS Paragon Plus Environment


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with a wavelength of 658 nm which was reflected off the air-water interface at the Brewster angle (ca. 53°). The lateral resolution of the microscope was 2 µm. Images were obtained during the compression process and recorded along the isotherm and were processed using the accompanying software. The light intensity at each point of the BAM image depends on the local thickness and film optical properties.

RESULTS AND DISCUSSION Octadecanoic (stearic) acid (C17COOH) (1 in Figure 1) does not form a stable monolayer by itself on water of pH greater than 3.0,18 due to ionization of an increasing number of acid groups as the pH is increased above 3.0. Octadecanoic acid has a pKa of 4.89,26 meaning on water of pH 4.0 there is essentially no dissociation, while at pH 10.0 there is near complete dissociation of the monolayer.26 At pH 6.0 – 7.0, that of MilliQ water, around 25% of the octadecanoic acid should be ionized.26 These ionized fatty acid molecules have a degree of water solubility, reported as 2.9 mg/L,9, 27 and are therefore more rapidly lost into the subphase than the undissociated acid molecules. Albrecht et al. have also demonstrated that for C14 and C16 fatty acids the primary mechanism of loss is dissolution into the subphase.28


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Figure 1. Chemical structure of octadecanoic (stearic) acid (C17COOH, 1), ethylene glycol monooctadecyl ether (C18E1, 2) and poly(diallyldimethylammonium chloride) (polyDADMAC, 3). Therefore, if a surface film of 100% C17COOH is applied to the surface of Milli-Q water (pH unadjusted) and compressed, some molecules are expected to dissociate and material is expected to be gradually lost into the water subphase. Repeatedly compressing and expanding the film will force more material into the subphase. To demonstrate this isotherm cycles were carried out on a Langmuir trough where the film was compressed, held for 5 minutes, then expanded and held for a further 5 minutes, before repeating the cycle.23 The isotherm cycle for C17COOH alone is shown in Figure 2a.



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Figure 2. Isotherm cycles of (a) C17COOH (1) alone, and duolayers of C17COOH + polyDADMAC (1 + 3) at a (b) 1:1 ratio, (c) 1:0.5 ratio and (d) 1:0.25 ratio. Despite applying sufficient material to form a solid-phase monolayer when compressed, C17COOH alone only reaches this solid state for the first two compressions, subsequent compressions can only reach a maximum surface pressure of 24 mN/m. It can also be seen that when the film is held in the compressed state for 5 minutes, the surface pressure drastically reduces; from 50 mN/m to 18 mN/m for the first two compressions. Both of these observations indicate that a solid state monolayer film of C17COOH is not stable at a surface pressure of 50 mN/m. Heikkila et al. have previously observed that when films of octadecanoic acid are held at a surface pressure above its equilibrium pressure (quoted as 7.5 mN/m) the film is unstable and collapses into the bulk phase.29 This observation supports the isotherm cycles reported here. In an attempt to improve the stability of octadecanoic acid films a duolayer was formed using water soluble poly(diallyldimethylammonium chloride) (polyDADMAC, 3 in Figure 1). The cationic 8 ACS Paragon Plus Environment

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charges on the polymer are expected to interact with the acid groups of dissociated octadecanoic acid, and thereby provide stabilization of the layer. This concept is similar to the quaternary amine ionic gluing process used by Regen et al.,30 and the previous use of poly(ethylenimine) to stabilize acid containing monolayers.31 Three different molar ratios of octadecanoic acid (1) to polymer monomeric unit (DADMAC) were investigated: 1:1, 1:0.5 and 1:0.25. The isotherm cycles for these duolayers are shown as (b), (c) and (d) respectively in Figure 2. The addition of polyDADMAC in a 1:1 ratio (Figure 2b) stabilized the octadecanoic acid monolayer so that the repeated isotherms were stable, all reaching a pressure of 50 mN/m. This is consistent with the expected presence of ionic interactions between the two components. Reducing the amount of polymer in the system still provides stabilization; however, the effect is reduced with a ratio of 1:0.25 collapsing slightly when held in the compressed state. Nevertheless it appears the film recovers to the water surface on expansion as the next cycle is able to again reach 50 mN/m at a similar area per molecule. Film viscosity properties We have previously shown that incorporating a water soluble polymer into the surface film composition to create a duolayer provides increased surface viscosity when compared to the monolayer alone.23 The surface viscosity of these ionic duolayers was therefore investigated using canal viscometry with the results for the various ratios shown in Figure 3a.



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Figure 3. Canal viscometry of the surface film systems: (a) films containing C17COOH (1) alone (1:0 ratio), or duolayers with increasing ratio of polyDADMAC (1 + 3) (1:0.25, 1:0.5, 1:1), (b) corresponding films with C18E1 (2), (c) C17COOH and polyDADMAC (1:0.25) at different subphase pH. Interestingly, the ratios of 1:1 and 1:0.5 show a similar surface viscosity to C17COOH with no polyDADMAC, whereas the ratio 1:0.25 shows a significantly increased surface viscosity. This is in agreement with the previous observation by Li et al. who showed that in the calix[6]arene and poly(4-styrenesulphonate) glued film system, when the ratio of water soluble polymer to surface active molecule was increased, the film flowed through the canal (6 mm) faster.20 The explanation for this is that the presence of more water soluble polymer (polyDADMAC) chains means more competition for binding to the ionized octadecanoic acid and therefore, less likelihood of multiple


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interactions per polymer chain. At lower ratios of polyDADMAC multiple ionized octadecanoic acid molecules can interact with one polymer chain providing enhanced stabilization to the film. In order to demonstrate that this increased viscosity effect was due to ionic interactions between the two components, and not another form of interaction, the C17COOH was replaced with nonionic ethylene glycol monooctadecyl ether (C18E1, 2 in Figure 1) with the results shown in Figure 3b. All films containing C18E1 showed similar canal viscometry results, indicating that the improvement in performance was due to ionic interactions between the two components in the C17COOH system. To further verify this, the effect of changing the subphase pH, and therefore the degree of acid ionization, was investigated (Figure 3c). At pH 4 the duolayer film flows through the canal rapidly, as expected due to the negligible level of C17COOH ionization expected at this acidic pH.26 However, at pH 10 the film shows extremely high viscosity, with very little drop in surface pressure after 800 seconds. At this pH there is expected to be almost 100% ionization of octadecanoic acid,26 resulting in increased availability for ionic interactions with polyDADMAC and therefore significantly increased surface viscosity. The observed dramatic effect that subphase pH was on the surface viscosity of this duolayer system verifies that the effect must be due to ionic interactions between the two components. The previously reported non-ionic duolayer system comprising C18E1 and poly(vinyl pyrrolidone) (PVP) was shown to lose the increased viscosity effect after 3 hours, indicating that the polymer was gradually diffusing away from the interface.23 In this previous system the interactions between polymer and monolayer were proposed to be through hydrogen bonding, a weak bonding force.24 Also, the presence of water provided competition for hydrogen bonding leading to the polymer gradually diffusing from the interface. The system reported in this paper utilizes ionic interactions between the monolayer and polymer, a stronger interaction than hydrogen bonding, and without competition from water. Therefore, it is expected that the longevity of the ionic duolayer system will be increased over the non-ionic system. In order to demonstrate this, canal viscosity of the films



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containing 1:0.25 ratio of C17COOH (1) to polyDADMAC (3) were run after they were maintained at a surface pressure of 25 mN/m for various holding times, with the results in Figure 4.

Figure 4. Canal viscometry of the 1:0.25 ratio film after various holding times. Pure C17COOH film is included as the control. This ionic duolayer film is still stable after 28 hours with the canal viscometry relatively unchanged from the initial curve. This indicates that, unlike the previously reported non-ionic duolayer system of C18E1 and PVP which completely lost its increased viscosity after a 3 hour holding time, polyDADMAC remains at the interface for an extended period providing continued stabilization to the octadecanoic acid monolayer. This indicates that the ionic interactions between the duolayer components are considerably stronger than the proposed hydrogen bonds occurring between the C18E1:PVP duolayer. The increased strength and longevity of this duolayer system could have important implications for areas such as water evaporation mitigation, lipid membranes, drug delivery, optoelectronic devices and the stabilization of monolayer films containing acid groups. Another method for measuring the viscoelastic properties of a surface film is the use of oscillating barriers. For this technique a Langmuir trough with two barriers was used to compress the surface film to the same pressure as the canal viscometry experiments (25 mN/m). The barriers then oscillate, allowing determination of the viscoelastic parameters: compression modulus (|G|), 12 ACS Paragon Plus Environment

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storage (elastic) modulus (G’) and loss modulus (G’’).32 This experiment was conducted for the films containing ionic C17COOH (1), as well as non-ionic C18E1 (2) with the results shown in Table 1.

Table 1. Viscoelastic properties of the surface films measured using the oscillating barriers method.

C17COOH (1) 1 1 1 1 C18E1 (2) 1 1

Ratio PolyDADMAC (3) 0 0.25 0.5 1 PolyDADMAC (3) 0 0.25

|G|

G’

G’’

53 127 23 47

49 121 23 47

20 39 3 1

90 42

84 42

32 5

These results agree with the canal viscometry, with the 1:0.25 C17COOH to polyDADMAC composition showing a significant increase in viscoelastic parameters, while the other C17COOHcontaining compositions are both similar to C17COOH alone. The significant increase in the elastic modulus (G’) at this ratio can be attributed partly to the introduction of polymer into the system, and also to the free dissociated acid groups that would be present at this ratio, with insufficient DADMAC units available to neutralize them. This results in some charge repulsion between neighboring molecules which, when combined with the presence of polymer, leads to high surface elasticity.33 A similar increase in viscoelastic parameters is not observed when the ionic C17COOH is replaced with non-ionic C18E1 in the 1:0.25 composition, providing further evidence that the improved performance is due to the presence of ionic interactions. Brewster angle microscopy The films were looked at using Brewster angle microscopy (BAM) with the results for films formed using the various ratios of octadecanoic acid to polyDADMAC, and compressing the film to different surface pressures, shown in Figure 5.



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Figure 5. BAM images of the monolayer (C17COOH, 1) alone, and the duolayer containing C17COOH (1) and polyDADMAC (3) in the ratios 1:0.25, 1:0.5 and 1:1. The scale bar is 100µm. There is a marked difference between the film formed from C17COOH alone, and those containing polyDADMAC. The polyDADMAC containing films showed decreased domain size compared to C17COOH alone, with the domain size decreasing as the ratio of polyDADMAC in the system increased. This is likely due to polymer chains inhabiting the space between the octadecanoic acid molecules, forcing the octadecanoic acid to aggregate into domains. As the film is compressed some polymer is pushed out from these phase boundaries, resulting in white spots above otherwise homogenous films, with the 1:1 composition showing the most spots as would be expected with the most polymer present.

CONCLUSION The utilization of ionic bonds to increase the strength of interactions in the duolayer system has been shown to significantly increase the stability of an otherwise unstable octadecanoic acid monolayer, with repeated isotherm compressions possible with the presence of polyDADMAC. This 14 ACS Paragon Plus Environment

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technology also overcomes a key limitation of previously reported glued surface film systems as it mixes the two components together prior to application. This allows significantly reduced levels of polymer to be used, and allows for simplified scale-up of this approach. The utilization of ionic bonds to increase the strength of interactions in the duolayer system has also increased the lifetime of the duolayer at the air/water interface compared to previously reported non-ionic duolayer films. An increase in the viscoelastic properties of the film were observed through both canal viscometry and the use of oscillating barriers, with this effect increasing on a subphase of pH 10. The film with a ratio of four octadecanoic acid units to one polyDADMAC monomeric unit to (1:0.25) was shown to have the most significant increase, consistent with more interactions between each polymer chain and multiple ionized acid units. Brewster Angle Microscopy showed the presence of polyDADMAC led to the formation of smaller domain sizes, with some polymer pushed out and above the film upon compression. The incorporation of ionic interactions in the duolayer system through mixing the two components prior to application leads to improved properties and a simplified process which could have application in areas such as water evaporation, foam formation and the production of defect-free Langmuir Blodgett films.

ACKNOWLEDGEMENTS We gratefully acknowledge the financial support of the Cooperative Research Centre for Polymers.

AUTHOR INFORMATION Corresponding Author * Email: [email protected], Telephone: (+613) 8344 8665 Present Addresses ‡

Present address: Institute for Frontier Materials, Deakin University, Geelong, VIC 3216, Australia 15 ACS Paragon Plus Environment


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REFERENCES 1.

Girard-Egrot, A. P.; Marquette, C. A.; Blum, L. l. J. Biomimetic Membranes and Biomolecule

Immobilisation Strategies for Nanobiotechnology Applications. Int. J. Nanotechnol. 2010, 7, 753–780. 2.

Alhakamy, N. A.; Kaviratna, A.; Berkland, C. J.; Dhar, P. Dynamic Measurements of

Membrane Insertion Potential of Synthetic Cell Penetrating Peptides. Langmuir 2013, 29, 15336– 15349. 3.

Rideal, E. K. Influence of Thin Surface Films on the Evaporation of Water. J. Phys. Chem.

1925, 29, 1585–1588. 4.

Prime, E. L.; Tran, D. N. H.; Plazzer, M.; Sunartio, D.; Leung, A. H. M.; Yiapanis, G.; Baoukina,

S.; Yarovsky, I.; Qiao, G. G.; Solomon, D. H. Rational Design of Monolayers for Improved Water Evaporation Mitigation. Colloid Surface A 2012, 415, 47–58. 5.

Ferreira, G. C.; Caseli, L.; Peres, L. O. Block Copolymers of o-PPV Organized at the Molecular

Scale as Langmuir and Langmuir-Blodgett Films. Synthetic Met. 2014, 194, 65–70. 6.

Forrest, S. R. The Path to Ubiquitous and Low-Cost Organic Electronic Appliances on Plastic.

Nature 2004, 428, 911–918. 7.

Kocher-Oberlehner, G.; Bardosova, M.; Pemble, M.; Richards, B. S. Planar Photonic Solar

Concentrators for Building-Integrated Photovoltaics. Sol. Energ. Mat. Sol. C. 2012, 104, 53–57. 8.

Lee, Y.-L.; Liu, K.-L. Relaxation Behaviors of Monolayers of Octadecylamine and Stearic Acid

at the Air/Water Interface. Langmuir 2004, 20, 3180–3187. 9.

McNamee, C. E.; Kappl, M.; Butt, H.-J.; Nguyen, H.; Sato, S.; Graf, K.; Healy, T. W. Effect of

the Degree of Dissociation of Molecules in a Monolayer at an Air/Water Interface on the Force


Page 17 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Between the Monolayer and a Like-Charged Particle in the Subphase. J. Phys. Chem. B 2012, 116, 13731–13738. 10.

Avazbaeva, Z.; Sung, W.; Lee, J.; Phan, M. D.; Shin, K.; Vaknin, D.; Kim, D. Origin of the

Instability of Octadecylamine Langmuir Monolayer at Low pH. Langmuir 2015, 31, 13753–13758. 11.

Aveyard, R.; Binks, B. P.; Carr, N.; Cross, A. W. Stability of Insoluble Monolayers and

Ionization of Langmuir-Blodgett Multilayers of Octadecanoic Acid. Thin Solid Films 1990, 188, 361– 373. 12.

Asnacios, A.; Langevin, D.; Argillier, J.-F. Complexation of Cationic Surfactant and Anionic

Polymer at the Air-Water Interface. Macromolecules 1996, 29, 7412–7417. 13.

Regismond, S. T. A.; Gracie, K. D.; Winnik, F. M.; Goddard, E. D. Polymer/Surfactant

Complexes at the Air/Water Interface Detected by a Simple Measure of Surface Viscoelasticity.

Langmuir 1997, 13, 5558–5562. 14.

Goddard, E. D.; Hannan, R. B. Cationic Polymer/Anionic Surfactant Interactions. J. Colloid

Interf. Sci. 1976, 55(1), 73–9. 15.

Regismond, S. T. A.; Winnik, F. M.; Goddard, E. D. Surface Viscoelasticity in Mixed Polycation

Anionic Surfactant Systems Studied by a Simple Test. Colloid Surface A 1996, 119, 221–228. 16.

Kuznetsov, V. M.; Akentiev, A. V.; Noskov, B. A.; Toikka, A. M. Spread Films of Synthetic

Polyelectrolyte-Surfactant Complexes: Dilational Viscoelasticity and Effect on Water Evaporation.

Colloid J+ 2009, 71, 202–207. 17.

Langmuir, I.; Schaefer, V. J. Effect of Dissolved Salts on Insoluble Monolayers. J. Am. Chem.

Soc. 1937, 59, 2400–2414. 18.

Brzozowska, A. M.; Duits, M. H. G.; Mugele, F. Stability of Stearic Acid Monolayers on

Artificial Sea Water. Colloid Surface A 2012, 407, 38–48. 17 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

19.

Page 18 of 20

Wang, M.; Janout, V.; Regen, S. L. Minimizing Defects in Polymer-Based Langmuir-Blodgett

Monolayers and Bilayers via Gluing. Langmuir 2012, 28, 4614–4617. 20.

Li, J.; Janout, V.; Regen, S. L. Glued Langmuir-Blodgett Film: An Unexpected Dependency of

Gluing on Polyelectrolyte Concentration. Langmuir 2004, 20, 2048–2049. 21.

Li, J.; Janout, V.; Regen, S. L. Sticky Monolayers and Defect-Free Langmuir-Blodgett Bilayers

Using Poly(acrylamide) Glue. Chem. Mater. 2006, 18, 5065–5069. 22.

Li, J.; Janout, V.; McCullough, D. H., III; Hsu, J. T.; Truong, Q.; Wilusz, E.; Regen, S. L.

Exceptional Gas Permeation Selectivity of a Glued Langmuir-Blodgett Bilayer by pH Control.

Langmuir 2004, 20, 8214–8219. 23.

Prime, E. L.; Tran, D. N. H.; Leung, A. H. M.; Sunartio, D.; Qiao, G. G.; Solomon, D. H.

Formation of Dynamic Duolayer Systems at the Air/Water Interface by using Non-ionic Hydrophilic Polymers. Aust. J. Chem. 2013, 66, 807–813. 24.

Christofferson, A. J.; Yiapanis, G.; Leung, A. H. M.; Prime, E. L.; Tran, D. N. H.; Qiao, G. G.;

Solomon, D. H.; Yarovsky, I. Dynamic Performance of Duolayers at the Air/Water Interface. 2. Mechanistic Insights from All-Atom Simulations. J. Phys. Chem. B 2014, 118, 10927–10933. 25.

Wu, Y.; Jia, Q.; Wang, B.; Zheng, W.; Yi, S. Effect of a Thin Spreading Solvent Film on the

Efficiency of the Hexadecan-1-ol Monolayer Deposited for Water Evaporation Retardation. Russ.

Chem. B+ 2010, 59, 1686–1691. 26.

Mercado, F. V.; Maggio, B.; Wilke, N. Phase Diagram of Mixed Monolayers of Stearic Acid

and Dimyristoylphosphatidylcholine. Effect of the Acid Ionization. Chem. Phys. Lipids 2011, 164, 386– 392. 27.

CRC Handbook of Chemistry and Physics, 76th ed.; Boca Raton, FL, 1995.


Page 19 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


28.

Albrecht, O.; Matsuda, H.; Eguchi, K.; Nakagiri, T. The Dissolution of Myristic Acid

Monolayers in Water. Thin Solid Films 1999, 338, 252–264. 29.

Heikkila, R. E.; Kwong, C. N.; Cornwell, D. G. Stability of Fatty Acid Monolayers and the

Relation between Equilibrium Spreading Pressure, Phase Transformations, and Polymorphic Crystal Forms. J. Lipid Res. 1970, 11(3), 190–194. 30.

McCullough, D. H., III; Regen, S. L. Don't Forget Langmuir-Blodgett Films. Chem. Commun.

2004, 24, 2787–2791. 31.

Hwang, M.-J.; Kim, K. Poly(ethylenimine) as a Subphase Stabilizer of Stearic Acid Monolayers

at the Air/Water Interface: Surface Pressure-Area Isotherm and Infrared Spectroscopy Study.

Langmuir 1999, 15, 3563–3569. 32.

Harrison, K. L.; Biedermann, L. B.; Zavadil, K. R. Mechanical Properties of Water-Assembled

Graphene Oxide Langmuir Monolayers: Guiding Controlled Transfer. Langmuir 2015, 31, 9825–9832. 33.

Noskov, B. A.; Loglio, G.; Miller, R. Dilational Viscoelasticity of Polyelectolyte/Surfactant

Adsorption Films at the Air/Water Interface: Dodecyltrimethylammonium Bromide and Sodium Poly(styrenesulfonate). J. Phys. Chem. B 2004, 108, 18615–18622.



TOC Graphic

+ + + +

+

25

air

--------------

+ + + +

water

++ + + +

+

Surface pressure (mN/m)

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Page 20 of 20

Canal viscometry 20 15

-

++ + + +

10 5

-

0 0

100

200

300

Time (s)


Water Interface: Improved Lifetime through Ionic

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