Article pubs.acs.org/JPCB
Molecular Interactions behind the Synergistic Effect in Mixed Monolayers of 1‑Octadecanol and Ethylene Glycol Monooctadecyl Ether Diana N. H. Tran,† Emma L. Prime,† Michael Plazzer,‡ Andy H. M. Leung,† George Yiapanis,‡ Andrew J. Christofferson,‡ Irene Yarovsky,*,‡ Greg G. Qiao,*,† and David H. Solomon*,† †
Department of Chemical and Biomolecular Engineering, University of Melbourne, Parkville, VIC, 3010, Australia School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, VIC, 3001, Australia
‡
ABSTRACT: Mixed monolayers of 1-octadecanol (C18OH) and ethylene glycol monooctadecyl ether (C18E1) were studied to assess their evaporation suppressing performance. An unexpected increase in performance and stability was found around the 0.5:0.5 bicomponent mixture and has been ascribed to a synergistic effect of the monolayers. Molecular dynamics simulations have attributed this to an additional hydrogen bonding interaction between the monolayer and water, due to the exposed ether oxygen of C18E1 in the mixed system compared to the same ether oxygen in the pure C18E1 system. This interaction is maximized around the 0.5:0.5 ratio due to the particular interfacial geometry associated with this mixture.
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INTRODUCTION The study of Langmuir monolayers is important because it allows the fabrication of ultrathin, highly ordered organic films. Insoluble monolayers are formed on the surface of a liquid which has a high surface tension (e.g., water) by the spreading of insoluble, nonvolatile substances over the surface. Spreading occurs when the molecules of the substance possess a hydrophilic head, which is attracted to water, and a hydrophobic hydrocarbon tail that repels water. A one molecule thick film is formed at the air−water interface, which can form multilayer films provided that the surface area (e.g., a solid substrate) is sufficient to accommodate all the molecules spread.1,2 A number of potential applications with commercial interest have been developed for monolayers, such as modeling biological cell membranes using phospholipids,3,4 investigating molecular recognition processes,5 and building molecular electronic devices.6,7 Other applications can include the lubrication of thin film magnetic disks to enhance the shelf life of high density hard disks,8 ultrafiltration membranes,9 stabilizing and controlling emulsions and foams,10,11 and water conservation effects.12 Monolayers have been used to cover standing water sources, such as dams and lakes as a means to reduce the loss of water by reducing the evaporation rate.13 Numerous laboratory studies have identified the characteristics of monolayers in terms of evaporation suppression. Among the monolayer compounds tested, long chain fatty alcohols, especially 1hexadecanol (cetyl alcohol) and 1-octadecanol (stearyl alcohol), have demonstrated various evaporation savings when used on large water reservoirs.14−16 However, results © 2013 American Chemical Society
are dependent on a number of factors, such as air and water temperatures, water surface conditions, film maintenance, wind, dust, and rain.13 The influence of these factors will reduce the effectiveness of the monolayer film on the water surface, and in the case of 1-hexadecanol and 1-octadecanol evaporation, suppression can be severely reduced. It is therefore necessary to formulate new and/or improved compounds to overcome these mechanisms that deteriorate the films’ efficiency in reducing water evaporation.12 Previous studies have investigated different compounds with 12 or more carbon atoms, as well as unsaturation and branching of the hydrocarbon chain, but this has had adverse effects on the monolayer’s performance.17 We recently developed a strategy which involved increasing the molecular weight and/or polarity of the hydrophilic headgroup of the amphiphilic molecules, in order to improve its “anchoring” ability to the water surface. We identified the best performing monolayer to be ethylene glycol monooctadecyl ether (C18E1) compared to 1-octadecanol (C18OH).18 Although this strategy was successful in reducing the degradation of the film, the enhanced monolayers still have room for further improvement. If the pure compounds form monolayers by themselves and behave ideally in the mixture, the overall evaporation resistance of the mixed film is equal to the sum of the evaporation resistance times the mole fractions of the pure components in the mixture at a given surface pressure. This equation has been Received: January 29, 2013 Revised: March 3, 2013 Published: March 8, 2013 3603
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grade, Chem-Supply). The compositions comprising a mixture of C18OH and C18E1 in desired mole ratios were then prepared by combining aliquots of the individual solutions in the amounts described in Table 1. All solutions were then mixed on a Vortex Mixer (Ratek Instruments Pty Ltd., Model VM1, power 20 W) at maximum speed for 30 s to ensure homogenization.
validated on several mixtures of long chain alcohols, and their relationships have been shown to be linear and are of equal ratio to the corresponding components. Nonideal behavior of the compounds results in a positive or negative deviation from linearity.19−21 It has been suggested for some mixed monolayers that these effects may be due to van der Waals interactions between the hydrocarbon chains, hydrogen bonding between neighboring polar head groups, and between monolayers and water molecules.22 Previous work focused on mixed monolayer films has shown that the evaporation resistance of films of pure fatty alcohols can be decreased or increased by adding small amounts of other compounds (e.g., surfactants), the latter effect being attributed to the less defective structure of the mixed films studied.23−27 The majority of this work has studied blends of 1-hexadecanol and 1-octadecanol, and the increase in evaporation resistance reported by Barnes and La Mer14 may be due to the synergistic effect observed in the mixtures, which leads to the formation of a more condensed film than those of the pure alcohols. The greater the cohesion of monolayers, the better their performance in retarding water evaporation. However, their effect on monolayer durability and function has been shown to be minimal.19,27,28 Further studies have also investigated the difference in volume and polarity of the molecule headgroups in mixed monolayers. Stosch and Cammenga29 studied the phase behavior and evaporation resistance of binary mixed monolayers of octadecanoic acid with octadecylamine, octadecanamide, and octadecylurea. Their findings showed enhanced van der Waals attraction between the hydrocarbon chains and cross hydrogen bonding between two types of headgroups induced maximum lateral interaction in the mixed monolayers, thus lowering the evaporation resistance of water. Rao and Shah20 demonstrated that cholesterol molecules can occupy the molecular cavities present in oleic acid monolayers and thereby reduce the evaporation of water. In all these systems, nonideality is caused by the different shapes and structures of the molecules that either expand or condense the monolayer. The objective of this paper is to investigate the mechanism of the synergistic effect observed by films of binary mixtures of C18OH and C18E1 at the air−water interface. Surface techniques are employed to characterize the mixed monolayers, and their water mitigation performances under extended wind stresses are investigated. Brewster angle microscopy (BAM) is used to visualize the films at different stages during compression. Computer simulation using classical molecular dynamics algorithm is then used to understand the molecular interactions of the mixed monolayers at the water surface relevant to their evaporation suppressing and wind resistance properties, and to provide an explanation for the observed synergistic effect in comparison with previous simulation studies of pure C18OH and C18E1 monolayer systems.18,30
Table 1. Compositions of the Various Mixtures of C18E1 and C18OH mole ratio (C18E1:C18OH)
volume of C18E1 (mL)
volume of C18OH (mL)
0:1 0.3:0.7 0.4:0.6 0.5:0.5 0.8:0.2 1:0
0 1.00 1.00 1.00 1.00 1.00
1.00 2.00 1.29 0.86 0.22 0
Apparatus. A Teflon Langmuir trough (76 cm × 10 cm, Nima Technology Ltd.) model 711D with a single Delrin barrier (11.2 cm × 1.6 cm) was used to characterize the properties of the monolayer film. Before each experiment, both the trough and barrier were thoroughly cleaned with chloroform (AR grade, Chem-Supply) and a Wilhelmy plate (2.35 cm × 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 25 ± 1 °C. The water surface was swept clean until the surface pressure reading was zero. Generally, 50 μL of the solution containing the monolayer was applied to the water surface (700 cm2 area) using a microsyringe, unless otherwise stated. A period of 30 min was allowed for the solvent to evaporate,32 after which the following characterization studies of the deposited film were conducted. Surface Pressure/Area Isotherms. The Delrin barrier was used to slowly compress the monolayer film at a rate of 50 cm2/ min while measuring the surface pressure as a function of the area per molecule (Å2/mol) until the monolayer reached its collapse pressure, as indicated by a constant or falling pressure. The barrier was then opened by changing the barrier speed to −50 cm2/min, and the surface pressure was also recorded. This isotherm cycle was repeated three times to confirm the stability of the monolayer film and to ensure the reproducibility of the data. Equilibrium Surface Pressure (ESP). In order to measure the ESP, the barrier was used to close 3/4 of the trough (approximately 175 cm2, barrier is closer to the pressure sensor) and then a solution of the monolayer forming material was added on the water surface. The quantity of solution added should contain 3 times the amount required to cover the water surface in the designated area with a close-packed monolayer. This amount was calculated using the area required to occupy one monolayer molecule (Å2) and the area of the Langmuir trough used. The surface pressure was then allowed to equilibrate for 30 min before the surface pressure was recorded. The barrier was then opened slightly and closed again to a different position on the trough and allowed to equilibrate for another 30 min and the pressure reported. This step was repeated a third time and an average of the three measurements taken.
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EXPERIMENTAL SECTION Materials. 1-Octadecanol (C18OH) was purchased from Orica Pty. Ltd. (99% pure) and was used as received. Ethylene glycol monooctadecyl ether (C18E1) was synthesized according to the general method31 and was characterized by 1H NMR using a Varian Unity 400 (400 MHz) spectrometer (>95% pure). Details have already been reported elsewhere.18 Individual solutions of the monolayer compounds were prepared at concentrations of 1 mg/mL in chloroform (AR 3604
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Loss of Monolayer Material. To determine the half-life of the monolayer on the water surface, approximately 65 μL of the monolayer forming material was applied with the barrier in the open position. A period of 30 min was allowed for the solvent to evaporate. The barrier speed was set to 50 cm2/min and the monolayer was compressed to a surface pressure of 35 mN/m, after which the barrier was instructed to hold the monolayer constant at that pressure for 24 h. During this time, as the monolayer material disappeared, the barrier moved to compress the monolayer to maintain this set pressure. The change in surface area can be correlated to the rate of loss of monolayer material, and the results were presented as the percentage of monolayer material remaining on the water surface as a function of time. The water level in the trough was maintained by placing the tip of a separating funnel (held by a retort stand) filled with Milli-Q water on the opposite side of the barrier without monolayer. The change in height allowed water in the separating funnel to flow, thus maintaining the water level in the trough relatively constant. An average of three runs was taken for reproducibility. Surface Canal Viscometry. A specially designed Delrin barrier containing a central 2 mm wide canal (6 mm high) at the bottom was attached to the standard trough barrier. 50 μL of the monolayer solution was applied to the water surface and was left undisturbed for 30 min to let the solvent evaporate, before being compressed to 35 mN/m using both barriers. After a holding time of 30 min, the two barriers were separated. The standard barrier was moved to an open position at a rate of 50 cm2/min, leaving the canal barrier in place. During this time, the monolayer molecules will diffuse through the canal into the clean water surface behind the standard barrier. The flow of monolayer material through the canal was detected by a decrease in the surface pressure as a function of time. An average of two measurements was taken. Wind Studies. A gravimetric method was used to measure the evaporation under exposure to wind. The wind was generated by a centrifugal fan (RS Components Pty Ltd.) connected to a wind tunnel made in-house.18 The wind tunnel ensured that the air flow out of the tunnel was laminar, and the wind speed was set at 25 km/h (7.0 m/s, measured by a hot wire anemometer (Control Company) placed at the mouth of the wind tunnel). The mouth of the wind tunnel was positioned at the end of a digital balance (Mettler-Toledo Limited) where a plastic rectangular container (10.5 cm × 16.3 cm) filled with 800 mL of Milli-Q water was placed on top. The digital balance was connected to a computer installed with the BalanceLink program, which was set to record the mass of the container and water every minute. Three times the amount of material required to form one monolayer was applied on the water surface and the solvent left to evaporate for 30 min before turning on the fan. The change in weight of the container and water was monitored for 12 h. A container of water with no monolayer was used as the control. An average of three measurements was taken. Brewster Angle Microscopy (BAM). Images of the films at the air−water interface were obtained using a Brewster angle microscope (BAM, KSV NIMA), mounted on a Langmuir trough (KN 2003, KSV NIMA). The BAM was equipped with a 50 mW laser emitting p-polarized light 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. Molecular Dynamics (MD) Simulations. MD simulations on systems composed of C18OH and C18E1 in the ratios 1:0, 0.75:0.25, 0.66:0.33, 0.5:0.5, 0.33:0.66, 0.25:0.75, and 0:1 were performed. Each system was constructed by placing 320 monolayer molecules aligned parallel to the z-direction in periodic units with dimensions 80 × 80 × 140 Å3. The positions of the molecules in the mixed systems were chosen so as to mimic a uniformly mixed system. The cell size in the zdirection was chosen to accommodate a water layer ≥20 Å thick situated directly beneath the headgroup and vacuum space approximately 100 Å in length above the aliphatic chains to mimic a quasi 2D interface. The OPLS33 force field was employed, as it was previously found to efficiently and accurately model the interatomic interactions in pure monolayer/water systems.18,34 The TIP4P35 model was used to represent the water molecules. The particle mesh Ewald method36 was employed to calculate the electrostatic interactions with the real space part of the Ewald sum and the Lennard-Jones van der Waals interaction cutoff at 13 Å. To eliminate any inherent strain in the initial system, the steepest descent method was used to minimize the potential energy with the convergence criterion set to 1 kJ/mol. Following this, MD was performed in the NPT (constant number of particles (N), constant pressure (P), constant temperature (T)) ensemble for 10 ns with an integration time step of 1 fs. The temperature was maintained at 298 K using the thermostat of Bussi et al.,37 while the surface pressure was maintained using the Berendsen coupling regime. The systems were considered equilibrated when fluctuation in the x−y box vectors was below approximately 0.2 Å, at which point the thermodynamic properties, volume, and total energy reached equilibrium. The data acquisition was performed over the final 5 ns with surface pressure coupling applied in the x−y directions. The simulated systems were evaluated over a range of surface pressures in order to observe any conformational changes in the molecules and structural changes in the monolayer that may affect their evaporation mitigation performance. The surface pressure range was between 10 and 50 mN/m, with 5 mN/m intervals. Above 50 mN/m, the monolayer system begins to collapse,18 while below 10 mN/m areas of phase coexistence occur, which cannot be modeled in a simulated NPT regime. Surface pressure−area isotherms have been employed to compare packing properties of different systems across a range of surface pressures. Importantly, a good agreement with experimentally measured isotherms is indicative of the efficacy and accuracy of the MD technique for these types of systems.18 Specific interactions between the monolayer and water were characterized as a function of surface pressure. The average number of hydrogen bonds occurring between donor−acceptor pairs was calculated on the basis of the cutoff criteria for the hydrogen donor−acceptor angle (≤30°) and the donor− acceptor distance (≤3.5 Å).38 We note that we have earlier modeled systems of similar amphiphiles on water using smaller system sizes,18 which compared to their larger-sized counterparts exhibit similar behavior. However, the increased size systems employed here enabled us to observe larger scale molecular features at the water/monolayer interface. 3605
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Figure 1. (a) Mass of water lost over time for a 0.5:0.5 mixture of C18OH and C18E1 compared to C18OH, C18E1 (alone), and a control with no monolayer and (b) percentage of water savings for different proportions of C18E1 in the mixture calculated after 12 h. The dashed line represents the calculated line of water savings for the mixtures if they behaved ideally. All surface films were applied at 3 monolayers18 under exposure to wind at 25 km/h.
Figure 2. Change in surface pressure over time for (a) a 0.5:0.5 mixture of C18OH and C18E1 compared to its pure components and (b) the time taken for different mixtures of C18OH and C18E1 to reach zero pressure in the canal experiment. The dashed line represents the calculated line for the mixtures if they behaved ideally.
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RESULTS AND DISCUSSION Wind Resistance of Monolayers at the Air−Water Interface. Using the in-house wind tunnel apparatus, the compositions prepared in accordance with Table 1 were tested and compared with a control sample (water only) and with water samples having films formed with C18E1 and C18OH alone. Figure 1a shows the performance of the two components alone and that of the 0.5:0.5 blend. C18OH by itself did not save any water from evaporation under the conditions used in this test; however, by creating a blend with C18E1, a significant improvement in the performance of the surface film was observed. The water savings observed after 12 h are summarized for all blends in Figure 1b. If the mixed monolayers were to behave ideally, then their performance would represent the calculated line. However, all compositions of C18OH and C18E1 tested gave water savings above the calculated line, which indicates that the mixtures did not behave ideally; there is a synergy effect. Of the mixtures of C18OH and C18E1 tested, it was found that the 0.5:0.5 mixture demonstrated the maximum
evaporation reduction (80%, similar performance to C18E1 alone). Increasing the proportion of C18E1 in the blends beyond 50% did not show further improvement in the monolayer’s performance. On the other hand, high proportions of C18OH (>50% by mole) only achieved 40−50% water savings. This could be due to disruption of the surface layer formed by the C18OH composition. Similar results were also obtained from a previous study for wind tests carried out in Petri dishes and at low wind speeds for 1 h.39 However, there are many contradictory results within the works of these authors, even though the same compounds were used.39−41 Canal Viscometry. The surface viscosity of the monolayer can provide important information about the intermolecular interactions and phase transformations in monolayers by flow. It is expected that the higher the surface viscosity of the film, the slower the material will flow through a canal, and hence the slower the decrease in surface pressure over time will be.42 The results from the canal viscometry experiments using the pure components and the 0.5:0.5 mixture are shown in Figure 2a. It is clearly evident that there is a strong interaction between the C18OH and C18E1 molecules at the air−water interface as the 3606
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Figure 3. Percentage of monolayer remaining on the water surface over time for (a) a 0.5:0.5 mixture of C18OH and C18E1 compared to C18OH and C18E1 (alone) and (b) the different mixtures of C18OH and C18E1 calculated after 24 h. The dashed line represents the calculated line for the mixtures if they behaved ideally.
Figure 4. (a) Experimental data and (b−d) simulated (dotted) surface pressure−area isotherms for a 0.5:0.5 mixture of C18OH and C18E1 compared to C18OH and C18E1 (alone).
mixed film takes longer to flow through the canal. This indicates a more viscous film as it reaches zero pressure at 120 s, compared to C18OH (60 s) and C18E1 (90 s) alone. Interestingly, when compared to other compositions of C18OH and C18E1, the 0.5:0.5 mixture took the longest time to flow through the canal, as seen in Figure 2b. This suggests that the distributions of C18E1 and C18OH molecules (at this particular molar ratio) are packed in a unique way at the air− water interface in order to provide this synergistic effect.
Stability of Monolayers. From our previous work,18 we have shown that the stability of C18E1 on the water surface is superior to C18OH due to the additional ethylene oxy group, which is capable of forming more hydrogen bonds to the water subphase, thus achieving greater anchoring. The film stability results for films containing C18OH and C18E1 as well as a 0.5:0.5 mixture of C18OH and C18E1 are shown in Figure 3a. Similar to the wind resistance results, the influence of C18E1 in the mixture is evident as the stability of the monolayer is more comparable to C18E1 alone, rather than falling along the 3607
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interspersed throughout the film (indicated by the white regions). As the film is compressed even further to 50 mN/m, near the point of collapse, the film becomes homogeneous with minimal thick white regions, and a near uniform gray coverage. Films of pure C18OH, on the other hand, form a close-packed film even at low surface pressures, without the distinct microdomains observed in C18E1. As the film is compressed, small islands of thicker regions begin to appear (white spots in Figure 5) and the number of these increase as the film is compressed further, until at a surface pressure of 50 mN/m the film becomes homogeneous, resembling that of pure C18E1. The 0.5:0.5 bicomponent film displays features observed in both pure component films. At low surface pressures, the mixed film displays distinct domains, separated by regions of pure water; however, these domains are bigger than in the pure C18E1 film. As the film is compressed, these domains merge into a film with distinct regions and the presence of ribbon-like thicker regions, similar to C18E1. The film at 50 mN/m is homogeneous, similar to that of both pure components. The bicomponent film clearly undergoes a different formation mechanism to either of the pure components; however, it does show more characteristics similar to the pure C18E1 film than the C18OH film. This agrees with other observed properties where the mixed film shows a performance more similar to that of pure C18E1 than expected if the film was behaving ideally. Molecular Simulations. We have previously reported theoretical simulation studies of the interactions contributing to the monolayer stability and water evaporation mitigation properties,30,34,43 including the analysis of the effect of headgroups on the monolayer structure and performance.18 The simulations demonstrated that the headgroup chemistry plays a profound role in the monolayer structure and dynamics, and we have been able to relate the microscopic effects to the evaporation resistance of the monolayers measured experimentally. In this section, we present molecular details of the behavior of the mixed monolayers at the water surface obtained using all-atom molecular dynamics simulations and relate them to experimental observation whenever possible. Surface Pressure−Area Isotherms. The surface pressure−area isotherms for C18OH, C18E1, and the 0.5:0.5 blend were calculated and are presented in Figure 4b−d. Similarly to previous calculations,18 good agreement between the experimental and simulated data, especially at the higher surface pressure region (>30 mN/m), was found. The simulated C18E1 system shows slightly less packing at higher surface pressures, and a greater degree of packing at lower surface pressures compared to its experimental counterpart. Hydrogen Bonding. Insights into how a particular monolayer system anchors to the aqueous surface can be obtained by comparing the hydrogen bond formation between the monolayer and water molecules. The average number of instantaneous hydrogen bonds (H-bonds) was normalized with respect to the surface area per monolayer molecule in order to account for any additional hydrogen bonding (H-bonding) associated with an increased contact area with water. Strong contrast between a pure C18E1 and a C18OH system can be seen in their respective water−monolayer H-bonding profiles, as shown in Figure 6. In particular, C18E1 maintains a higher and more consistent bond count across the entire pressure range simulated. The mixed systems display behavior similar to pure C18E1 at higher surface pressures (>30 mN/m); however, the predominantly C18OH mixed systems show an abrupt
midpoint line between the two pure components, which would be expected if the mixture was behaving ideally. Figure 3b shows the percentage of monolayer remaining on the water surface for the other mixtures of C18OH and C18E1, and likewise, the 0.5:0.5 mixture was found to be the best performing ratio. Surface Pressure−Area Isotherms. The synergistic performance of mixed monolayers of C18OH and C18E1 at the air−water interface was further investigated by examining the physical properties of these films to determine whether there were differences in the structures. Measurements on the surface pressure versus area per molecule (Å2) of C18OH, C18E1, and the 0.5:0.5 mixture of C18OH and C18E1 spread from chloroform are presented in Figure 4. The monolayer film properties obtained from these experiments are listed in Table 2. Table 2. Monolayer Properties of Investigated Compounds Spread from Chloroform Obtained from Experimental Data mole ratio (C18E1:C18OH) 0:1 0.3:0.7 0.4:0.6 0.5:0.5 0.8:0.2 1:0
area/moleculea (Å2)
ESP ± 1 (mN/m)
± ± ± ± ± ±
39 43 45 45 46 48
22 23 23 24 23 21
0.2 0.2 0.3 0.2 0.1 0.2
a
Extrapolated from the solid phase of the isotherm from the experimental curves.
As can be seen from the isotherms of the pure components, the solid-like phase of C18E1 is more closely packed than C18OH at high surface pressures. Interestingly, the contour of the isotherm for the 0.5:0.5 mixture of C18OH and C18E1 is similar to that for C18E1 at low pressures and C18OH at high pressures. This indicates that C18E1 is the dominating component in the mixed film, which is in agreement with the results observed for the wind and stability experiments. However, isotherms for all mixtures studied reached a similar collapse pressure of 50 mN/m, a high ESP, and packed to 23− 24 Å2/molecule, as shown in Table 2. This indicated there was no distinct difference between the packing densities of the mixed films, thus signifying the importance of hydrogen bonding of C18OH and C18E1 occurring at the air−water interface. Brewster Angle Microscopy (BAM). BAM is a probe-free technique for visualizing films at the air−water interface. pPolarized light incident at the Brewster angle (ca. 53°) has a reflectivity of zero from a pure water surface. The presence of a surface film alters the reflectivity of the interface, and hence, a fraction of the light is reflected and an image can be obtained. The light intensity at each point in the image is dependent on the local thickness and optical properties. BAM images of the two pure films (C18E1 and C18OH) and the 0.5:0.5 bicomponent film at a range of surface pressures as the films have undergone compression are shown in Figure 5. The pure component films show different formation mechanisms as the films are compressed. C18E1 at low surface pressures has distinct microdomains, with the domains separated by pure water. As the films are compressed to 10 mN/m, the microdomains merge into a film with distinct domains of different thicknesses (indicated by the different gray scale regions in Figure 5), with ribbons of thicker regions 3608
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Figure 5. BAM images of films formed at the air−water interface by the two pure components (C18E1 and C18OH) and the 0.5:0.5 bicomponent film. The scale-bar is 100 μm.
Figure 6. Time averaged hydrogen bond count between water and monolayer normalized with respect to monolayer packing area, as a function of surface pressure for the predominant (a) C18OH systems 0.66:0.33 (green) and 0.75:0.25 (blue) and (b) C18E1 systems 0.33:0.66 (orange) and 0.25:0.75 (pink). The pure C18OH (red), C18E1 (black), and 0.5:0.5 blend (gray) are included in both parts a and b for comparison.
water−EO H-bonding despite having half the number of sites. The remaining mixed systems also have noticeably higher water−EO H-bonding compared to the pure system of C18E1. To account for these results, the typical equilibrium snapshots of interface cross sections from the 0.5:0.5 mixture and the pure C18E1 system were compared, as shown in parts a and b of Figure 8, respectively. It can be seen that the pure system presents a comparatively “planar” interface to the water which prevents hydrogen bonding between water and the ether oxygen (illustrated in Figure 8d). However, the mixed systems present a considerably more uneven interface to the water, exposing a greater part of the headgroups (Figure 8c). This
decrease in bonding at lower surface pressures (
