Molecular Explanation for the Abnormal Flux of ... - ACS Publications

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

A Molecular Explanation for the Abnormal Flux of Material Into a Hot Spot in Ester Monolayers Vanesa Viviana Galassi, Mario G. Del Popolo, Thomas Martin Fischer, and Natalia Wilke J. Phys. Chem. B, Just Accepted Manuscript • Publication Date (Web): 11 May 2017 Downloaded from http://pubs.acs.org on May 14, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry B is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26

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

The Journal of Physical Chemistry

A Molecular Explanation for the Abnormal Flux of Material into a Hot Spot in Ester Monolayers Vanesa V. Galassi1, Mario G. Del Popolo1, Thomas M. Fischer2 and Natalia Wilke2,3* 1 CONICET, Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Cuyo, Mendoza M5502JMA, Argentina 2 Institut für Experimentalphysik, Universität Bayreuth, 95440 Bayreuth, Germany 3 Centro de Investigaciones en Química Biológica de Córdoba (CIQUIBIC-CONICET), Departamento de Química Biológica, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Córdoba X5000HUA, Argentina. * To whom correspondence should be addressed. E-mail: [email protected] ABSTRACT Langmuir monolayers of certain surfactants show a negative derivative of the surface pressure with respect to temperature. In these monolayers, a local temperature gradient leads to local yielding of the solid phase to a kinetically flowing liquid, so that the material flows towards the hotter region that act as sinks. The accumulation of material leads to the formation of non-equilibrium multilamellar bubbles of different sizes. Here we investigate the molecular factors leading to such a peculiar behavior. First, we identify the required structural molecular moieties, and second we vary the composition of the subphase in order to analyze its influence. We conclude that esters appear to be unique in two key aspects: they form monolayers whose compression isotherms shift to lower areas as the temperature increases, and thus collapse into a hot spot; and they bind weakly to the aqueous subphase, i.e. water does not attach to the monolayer at the molecular level, but only supports it. Molecular simulations for a selected system confirm and help explaining the observed behavior: surfactant molecules form a weak hydrogen bonding network, which is disrupted upon heating, and also the molecular tilting changes with temperature, leading to changes in the film density.

Abbreviations: MP: Methyl Palmitate, MS: Methyl Stearate, MA: Methyl Arachidate, EP: Ethyl Palmitate, ES: Ethyl Stearate, EA: Ethyl Arachidate, BS: Butyl Stearate, HD: hexadecanol, PDA: pentadecanoic acid, DPPC: dipalmitoyl phosphatidyl choline, FM: Fluorescent Microscopy, DIC: Differential Interference Contrast microscopy, DLS: Dynamic Light Scattering, CAC: Critical Aggregate Concentration, pbc: pyrobaric coefficient.

1 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

INTRODUCTION Solids convert into liquids (and vice versa) via thermodynamic phase transitions, i.e. when the external intensive parameters, such as temperature and pressure, cross a phase-coexistence line. Conversely, a solid can also be converted into a fluid by applying shear stress. In this case, the solid undergoes elastic shear deformations up to a critical threshold, the yield stress. Beyond the yield stress the material flows and yields into a liquid. The shear yielding of a solid is not an equilibrium phase transition but a process in which the solid is directly converted into a kinetically flowing liquid. In three dimensions, and under normal circumstances, mass conservation prevents a constant dilatational flow of material towards the center of a three-dimensional solid. On the other hand, for quasi two-dimensional systems, at the interface between 3D phases, molecules can escape into the third dimension when conditions at the surface become too crowded. This is the case for surfactant monolayers at the air-water interface.1–4 These monolayers can be compressed using the barriers of a Langmuir trough, increasing surface pressure and, beyond collapse, pushing surfactants out from the interface into the third dimension When solid monolayers are exposed to a temperature gradient, a surface pressure gradient is generated, that the monolayer bears if it remains at the interface. In a previous work we investigated the effect of a local thermal gradient, generated with a focused IR laser, on monolayers composed of different compounds. Most of the amphiphilic systems investigated melted in the hot region at sufficiently high temperature gradients. However, for some materials, a temperature gradient promoted a flux of material to the hot spot, where a 3D aggregate was formed.5 This phenomenon was explained in our previous work in terms of a thermally induced pressure gradient that led to the conversion of the solid into a fluid monolayer, when it was higher than a threshold value.5 Such transition is different from local melting, and corresponds to the local yielding of the solid into a fluid that moves towards the hot spot. According to the arguments provided by Aliaskarisohi et al.,5 only monolayers where the thermocapillary dilatational stress pushes the material into the laser focus with a strength exceeding the yield pressure will flow to the hot spot. For this to happen, the thermally induced pressure gradient has to be directed towards the focus. This occurs whenever the tension in the focus is higher than in the colder periphery, i.e. when the derivative of the monolayer surface pressure with respect to temperature, at constant surfactant density, (pyrobaric coefficient, pbc = ∂π⁄∂T ) is negative. Correspondingly, amphiphiles that show this abnormal behavior upon local heating, also show compression isotherms with mean molecular areas shifting to lower values as the temperature increases (they show negative pbc values).5 Furthermore, from the pyrobaric coefficient and the minimal temperature gradient needed to observe the yielding of the solid phase, ΔT , it is possible to estimate the dilatational yield pressure, = pbc × ΔT . This estimation neglects the change of surface tension of the bare interface with temperature, which is small compared with the pbc values (-0.17 mN m-1K-1).6


Page 2 of 26

Page 3 of 26

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


In our previous investigation, it was not clear which are the molecular reasons that lead to some materials to exhibiting negative pbc values. The aim of this work is to shed light into that conundrum. Most of the commonly studied surfactants show positive pbc values in the solid state, and the behavior of some of them upon local heating has been checked by Aliaskarisohi et al. 5 Therefore, in the present work we focus on esters, which show negative pbc values. We investigated monolayers of different esters, and their mixtures with fatty alcohols or fatty acids. Our results indicate that solid monolayers composed of esters made from fatty acids and short alcohols yield under a temperature gradient, while the fatty acids or the fatty alcohols with similar acyl chain do not. Furthermore, when mixed with fatty acids or fatty alcohols, esters keep their anomalous thermal behavior only within certain composition range. For ester monolayers, water in contact with the alchohol’s alkyl chains and the carboxylic moiety may be involved in the observed phenomena. Therefore, in order to explore the role of water, we changed the composition of the subphase, adding chaotropic agents or changing the pH. Surprisingly, no measurable effects were detected, suggesting that changes in the level of hydration of the surfactant molecules do not determine negative pbc values, or that water molecules do not interact specifically with ester monolayers, as previously found by Sum-Frequency Spectroscopy experiments.7 The previous considerations led us to explore the phase behavior of a methyl stearate monolayer by means of molecular simulations. We found the presence of weak intermolecular hydrogen bonds between the surfactant molecules, which were gradually lost as temperature increased. Moreover, the tilt of the molecules changed upon heating, as already observed experimentally.8–13 These changes in the monolayer structure led to lattices with decreasing mean molecular areas as the temperature increased. It is important to remark that changes in the tilt angle of the molecules upon heating has also been reported for fatty acids and fatty alcohols,10–14 but not so the concomitant decrease in the area per molecule observed in esters.15,16 The absence of hydration water seems to be the main difference between esters and fatty alcohol/acid monolayers, we propose this as the main cause for the different thermodynamic response to heating. It seems that in the case of alcohols and acids, the reorganization of hydration water upon heating makes extra contributions that need to be considered in the balance of energies leading to the observed structures.17 Regarding the fate of the material accumulated in the hotter regions, 3D aggregates form, which for ethyl stearate were described as bubbles.18 In the present work, we describe the structures formed in all the systems investigated, as well as the process of return of the material to the monolayer upon cooling. In general, slightly different structures were formed randomly, with no relationship with the equilibrium shapes that the molecules adopt in suspension. All the evidences point to the formation of out-ofequilibrium multilamellar bubbles. EXPERIMENTAL SECTION Materials: Ethyl Arachidate (EA), Methyl Arachidate (MA), Ethyl Stearate (ES), Butyl Stearate (BS) Hexadecanol (HD), Pentadecanoic Acid (PDA) and Methyl Stearate (MS) ≥99% purity were purchased from SIGMA. Methyl Palmitate (MP) and Ethyl Palmitate (EP), ≥99% purity were purchased from Fluka. Urea (P.A.) was from GE-Healthcare Life 3 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

Science and D-(+)-Trehalose dehydrate (≥99% purity) was from SIGMA. The fluorescent probe Texas Red 1,2-Dihexadecanoyl-sn-Glycero-3-Phosphoethanolamine, Triethylammonium Salt (Texas Red-DHPE) was from Invitrogen Molecular Probes.

Methods: Isotherms and local heating. A Nima and a KSV film balance were used for the monolayer investigations. Monolayer isotherms were obtained at a compression rate of 0.06 Å2 molecule-1 s-1. Reproducibility was within a maximum of 1mN m-1 for the surface pressure and below 3 Å2 molecule-1 for the molecular areas. The setup for studying local collapse phenomena has been described in detail elsewhere.5 Briefly, it consists of a home-built Langmuir trough placed on the stage of an inverted immersive objective. The temperature of the trough can be controlled precisely by means of a thermocirculator (Fischer Scientific). An IR laser beam (λ = 1064 nm, P = 2 mW to 10 W) was used to locally heat the monolayer in the focus of the objective. The light is partially absorbed by the subphase and heats the monolayer locally around the focus. The surfactants were doped with 0.5-1 mole% of the fluorescent probe and, as in Muruganthan and Fischer,6 the flow was measured quantitatively by following the characteristic texture of the monolayer as a function of time (see Figure S1 as an example). Surface potential measurements were performed with the Kelvin method using a KSV apparatus. Equilibrium structures of the surfactant in aqueous suspension For ethyl stearate (ES), we determined the critical aggregation concentration (CAC), which is the minimal amount of monomer needed for the formation of aggregates. This was performed determining the decrease in surface tension generated by the addition of increasing amounts of ES. ES was added from a concentrated methanolic solution and the surface tension was determined with a KSV apparatus using the Whilhelmy method. Solutions with a concentration of ES higher than the determined CAC were observed by Dynamic Light Scattering using a Submicron Particle Sizer (NicompTM 380). The results could be fitted with at most three population of particle sizes. Molecular models and simulation details Previous works based on mean-field theories,17,19–25 as well as simulations with coarse grained models26–28 have shown that the phase behaviour of Langmuir monolayers can be reliably described by models in which the liquid sub-phase is replaced by an effective tethering potential. From the computational point of view such models offer a great advantage, as they allow for the long simulation times required for investigating solid or rigid phases under tension, where structural relaxation is sluggish. More importantly, these simplified models conform with key experimental evidence (vide infra) that indicates that ester monolayers bind weakly to the aqueous sub-phase, and that their pyrobaric and 4 ACS Paragon Plus Environment

Page 4 of 26

Page 5 of 26

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


thermal expansion coefficients are almost unaffected by salt concentration, the presence of dissolved chaotropic agents, or different pHs in the sub-phase. In view of the previous considerations, a methyl-stearate (MS) monolayer made of 64 molecules, was built forming a hexagonal lattice with an area per molecule of 21Å2. The effective interaction of the polar head groups with the subphase was modelled as a harmonic potential, = − , that tethers the carboxylic carbons to the X-Y plane of the simulation box, located at . Note that the tethering potential acts only along the z direction, so molecules can move freely on the X-Y plane. Different force constants (Kfc) were used: 10, 60, 100 and 10000 kJ/nm2. The lowest values of Kfc were within the range of the ones obtained from relatively short Molecular Dynamics simulations of a fully hydrated MS monolayer at different temperatures. From these simulations, the zfluctuations of the carbonyl groups were fitted to Gaussian distributions conforming to harmonic potentials with force constants between 40 and 60 kJ/nm2 (See examples in Figure S2). The highest value of Kfc (10000 kJ/nm2) was included in our simulation set with the aim of examining the effects of a more stringent confining potential. An Amber compatible atomistic force-field for MS was obtained through generalized Amber force-field (GAFF) parametrization protocol, using the AmberTools package,29 AM1-BCC charges were used, which have been validated by comparison to ab initio HF/6-31G* charges. 30 Lennard-Jones and bonding parameters were obtained by chemical analogy, resorting to the Amber ff99SB force-field database.31 The MS monolayer was initially relaxed by thermal annealing from 177°C to 7°C during a 2 ns simulation in the NVT ensemble. NPT simulations were then performed increasing the temperature from 7°C up to 87°C, at a rate of 6.5x10-4 °C/fs, using Berendsen’s thermostat. Pressure coupling was implemented through Berendsen’s semiisotropic barostat, with a reference pressure of 1 atm on both the monolayer plane and the monolayer normal. The barostat time constant was set to 1 ps. The compressibility on the plane of the monolayer was set to 4.5x10-5 bar-1, and zero along the normal with the aim of preserving the z-dimension of the simulation box (Lz = 122 Å). Electrostatic interactions were computed with the Particle-Mesh-Ewald method, and short range forces were cut-off at 13 Å. For each heating cycle the total simulation time was 130 ns. All simulations were run with the GROMACS 5.0 package.32 During the heating process we investigated the thermal behaviour of four molecular properties: area per molecule, hydrogen bonds, molecular dipole vectors, and tilting or molecular inclination. This last property was determined as the angle formed between the molecular axis (the axis connecting the carboxylic carbon and the alkylterminal carbon) and the z-coordinate axis. Hydrogen bonds were defined between oxygens (acceptors, A) and α and β carbons (donors, D) satisfying the following geometrical criterion: D-A distances of 3.5 Å or shorter, and D-H-A angles between 120° and 180°.



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

RESULTS AND DISCUSSION Flux of material to the hot spot in monolayers composed of different esters on water Figure 1A shows the compression isotherms of the investigated esters at 22ºC. Compounds were chosen with different acyl groups, i.e. esters formed from hexadecanoic acid (Palmitic acid, P), octadecanoic acid (Stearic acid, S), and icosanoic acid (Arachidic acid, A). Moreover, alkyl groups were also varied, including in the analysis esters from Methanol (M), Ethanol (E) and Butanol (B). All the ester surfactants formed stable monolayers that collapsed from a stiff phase. By comparing their compression isotherms in the region of the solid phase, we observed that surface pressures shifted to lower values, at constant mean molecular areas, when the temperature increased, i.e. they showed negative pyrobaric coefficients.5,33–36 Compression isotherms for EA and BS at different temperatures, which were not previously reported, are shown in Figure S3. Despite that negative pyrobaric coefficients have been reported before for ester compounds,5,33–36 to the best of our knowledge, this is the first time they are analyzed in depth, and that a structure-based explanation is proposed for its negative sign. Interestingly, the structurally related compounds, fatty alcohols and fatty acids form monolayers with positive pyrobaric coefficients.15,16 In a different set of experiments, ester molecules were spread on water at 18ºC, compressed up to pressures close to collapse (in the solid state), and imposed a local temperature gradient by means of a focused IR laser beam. At low pressures, local yielding was not observed, but the monolayer melted when close to an LE-LC phase transition.5 Other molecules, such as phospholipids, fatty acids and fatty alcohols, that exhibit positive pyrobaric coefficients, showed no flow to the hot spot.5 At this stage, it is important to clarify that in our previous paper, we made a typographic mistake. We stated that MA did not show inward flux into the hot spot, while hexadecanol (HD) did. We checked the old experiments and performed new ones, and they agreed in that MA does show an abnormal flux (movie 01), while HD melts in the hot spot at high laser power (Figure S4). See also the corresponding shift in the isotherms with temperature in Figure S4. The behavior of the ester monolayers at increasing laser power was subsequently investigated, and the Critical Laser Power (CLP) was determined as the minimum power needed to observe flux to the focus. At laser powers equal or higher than CLP, the material moves to the hot spot with a flux = , being the film density and the radial component of the flux velocity. Outside the region of aggregation, ! must be constant due to mass conservation, and thus, in this region ! = "#$%&'$&, which implies that the flux velocity increases as the material approaches the laser focus as ~ 1⁄!. An individually

tracked particle + will thus have a radial velocity & = ! ,1 − 2 ⁄! & , with ! and

the initial position and velocity of particle +, respectively (see Figure S1). However, a


Page 6 of 26

Page 7 of 26

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


constant rate can be considered in the small interval of distances that we have analyzed, which is within 50 to 80 µm from the laser focus. In our experimental setup, the temperature at the focus was higher than that far away from it by an amount Δ., which can be calculated as Δ. = /31⁄4 3, where / = 0.1 "678 is the adsorption coefficient of water at the wavelength of the IR laser (1064 nm), and 3 = 0.6: 678 78 is the heat conductivity of water.14 Thus, the temperature in the focus varied from 20ºC (1 = 0.5:) to 38ºC (1 = 5:).

Figure 1. (A) Compression isotherms for ester monolayers on water at 22ºC. (B) and (C) Velocity of the flux of material to the hot spot at 50-80 µm from the focus as a function of the power of the IR laser. Each symbol represents average ± standard deviation of the data considering at least 5 determinations such as that showed in Fig. S1. (D) Z- and E-configurations proposed for ester molecules at interfaces. (E), (F) and (G) Critical Laser Powers determined for each system.

Figures 1 B and C show the flux velocity of different ester monolayers when subjected to a temperature gradient. In Figure 1B, the data for octadecyl esters with different alkyl chains are depicted. As the length of this moiety increases, the film flux velocity decreases. This could be due to different driving forces or to different viscosities or both. The viscosity of the kinetically excited liquid will increase in the order butyl>ethyl>methyl if the molecules are in the Z-configuration, which is in fact the proposed structure for these esters at high film density.34,37 In this configuration, the alcoholic moiety points towards water, see Figure 1 D.34,37 This result suggest a different arrangement of the molecules in this excited liquid compared to the liquid expanded phase that is present at low surface pressures, in which the esters are in an E-configuration.34,37 7 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

The Z-configuration is also expected for the solid phase formed by compressing monolayers at high temperatures. Figure S5 provides evidence on that sense. In this set of experiments, the surface potential was determined for ES monolayers at a constant surface pressure of 20 mN m-1 while it was heated or cooled. This led to a low, but reproducible, decrease in the surface potential at high temperatures (0.06±0.02 V). Since E-configurations are expected to lead to higher surface potentials than Zconfigurations,12,37 we conclude that the ES molecules in the solid state were also in the Z-configuration at high temperatures. The small change in the surface potential upon heating may be related to tilting changes of the hydrocarbon chains, or of the ester moiety, relative to the interface. Figure 1 C shows that the flux velocity for molecules with a methyl moiety decreased with increasing length of the acyl chain. This result is expected, since the longer the hydrocarbon chain, the stronger the van der Waals interactions among the molecules, and the higher the film viscosity, with a concomitant decrease in the rate of molecular motion. On the contrary, when the polar region corresponds to an ethyl moiety no appreciable influence of the length of the acyl chain was observed (see Figure S6A). It has to be remarked, however, that the flux velocity in these systems was very low, and thus, any difference may be hidden within the measurement error. The variation of the flux velocity with the excess laser power (Laser power minus CLP) was linear in a double logarithm plot, indicating that ∝ ∆. − ∆. , see Figure S6B. In summary, the temperature gradient promoted the formation of a flowing fluid phase with molecules in the Z-configuration, whose motion depended linearly on the gradient, and with a decreasing rate as the total number of carbon atoms in the molecule increased. Additionally, a global increase in temperature generated a denser solid phase, also with molecules in Z-configuration, and with a slightly lower value of the surface potential compared to colder monolayers. Critical laser power and pyrobaric coefficient for the different monolayers on water We further analyzed the effect of the molecular structure on the critical temperature ∆. . Fig. 1 E-G show the CLP values for all the esters investigated. Panels 1 E and F depict an increase of CLP with the (acyl) hydrocarbon chain length. Fig. 1 G indicates that the larger the size of the alkyl chain (related to the alcohol), the lower the CLP value. It is worth mentioning that Aliaskarisohi et al.,5 reported similar trends but slightly lower values of CLP. In those measurements a different laser was used, so one could expect slightly different light intensities reaching the monolayer in the new setup. Fig. 1 E-G indicate that the sensitivity of the monolayers to a thermal gradient increases as the length of the acyl chains decreases, and as the length of the alkyl chain increases. In principle, such trends can be attributed to differences in the pyrobaric coefficients, or in the yield pressures. In order to discriminate between these two options, compression isotherms of some esters at different temperatures were analyzed. Pyrobaric coefficients were determined as the slope of the surface pressure vs. temperature plot at a fixed (high) density.5 Figure S1 shows examples for BS and EA.


Page 8 of 26

Page 9 of 26

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


Fig. 2 A and B show the absolute values of pbc determined for ethyl esters (A) and for esters from stearic acid (B). The results indicate that the values for pbc are independent of the length of the hydrophobic (acyl) tail, while they decrease as the alkyl chain increases. Fig. 2 C and D show the minimal surface pressure increase needed for the monolayer to yield, i.e. the yield pressure calculated as = pbc × ΔT . There is a slight dependence of the yield pressure on the hydrophobic tail length (Fig. 2C), whilst an increase in the length of the alkyl chain clearly correlates with a decrease in yield pressure (Fig. 2 D). Since the yield pressure is expected to increase as the film stiffness increases, we calculated the compressibility modulus for monolayers close to collapse (maximum value before collapse) from the isotherms at 22°C, as > 78 = −? @ ⁄@?.38 These compressibility moduli are plotted as a function of the yield pressure in the inset of Fig. 2D, where a correlation between both parameters can be observed. Monolayers formed by esters with increasing alkyl chains are less stiff (with lower compressibility modulus), and they are also more viscous (the flow due to convection is impeded). Both features (low compressibility modulus and high shear viscosity) are commonly observed in polymer-like monolayers,39,40 which may explore many different configurations as the film is compressed. With this in mind, we hypothesize that in BS monolayers, the conformational entropy of the alkyl chains probably suppresses the changes in surface pressure with temperature and thereby decreases the absolute value of pbc.

Figure 2 . Absolute values for the Pyrobaric coefficients (A, B) and yield pressures (C,D) for esters with different acyl and alkyl chains.



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

Monolayers formed on subphases containing a chaotropic agent. The chemical composition of the subphase was varied in order to explore the role of water molecules and of the ions of the subphase on the emergence of negative pbc values. As is well known, liquid and solid water exhibit regions of abnormal pressure behavior upon heating due to changes in its hydrogen-bonds network. There are some other examples where a thermally induced change in the H-bonding leads to an anomalous density response. Brovchenko et al.41 determined a negative thermal expansion coefficient for the peptide Abeta42 in solution, which correlates with a decrease in intramolecular H-bonding upon heating. They concluded that intrapeptide H-bonds prevent close packing of the peptide chain, whereas a decrease in the number of such bonds should help packing the peptide more tightly.41 A similar behavior has also been described for other peptides, such as the human islet amyloid polypeptide and elastin-like peptides.42 Upon heating of amide monolayers, Brezesinski et al. observed a phase transition between a dense and a more expanded solid phase.43 This was attributed to changes in intermolecular hydrogen-bonding. They showed that, at high pressure, the shortening of hydrogen bonds between head-groups drives an increase in separation between the alkyl chains, and leads to a more open supramolecular structure. In summary, an abnormal behavior upon heating/compressing is consistent with the observation that highly directional interactions (usually H-bonds) make molecular packing less efficient. Ester molecules can form two types of hydrogen-bond networks: one involving carbonyl oxygen atoms connected through water molecules, and a weaker one involving C-H donors and the oxygen atoms from the ester group.44 We will first consider whether hydrogen bonds via water molecules may affect the structure and determine the phase behavior of ester monolayers. For the formation of an interfacial H-bond network the carbonyl moieties must be exposed to water, which is unlikely for the Z-configuration that these molecules seem to adopt (vide supra). Furthermore, it has been shown using Sum-Frequency Spectroscopy, that water underneath MS monolayers at 30-35 mN/m is disordered, i.e. the presence of this particular surfactant does not induce structuring of the interfacial water layer, as in the case of stearic acid.7 Therefore, we expected that in the absence of a temperature gradient, a hydrogen-bond network via water molecules was not present in the esters. This was corroborated by performing compression isotherms on subphases containing chaotropic agents or high/low pHs. The chaotropic molecules are known to interact with water and biological molecules causing specific effects, i.e. effects not accommodated by classical theories of electrolytes.45 Figure S7 shows, as an example, that ES monolayers kept the property of shifting to lower mean molecular areas at high temperatures both at pH 13 and 2, on a subphase of glycerol/water 1:1, and on trehalose 50 mM. In order to explore the possibility that the interfacial water structure influences the local collapse process, we performed experiments similar to those reported in Figure 1, but using subphases containing different chaotropic agents. If water structure, hydrophobic effects, and/or ion-surfactant interactions were involved in one or more of the stages of the material flux and 3D aggregation, changes in the subphase composition would lead to a 10 ACS Paragon Plus Environment

Page 10 of 26

Page 11 of 26

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


perturbation of the observed phenomena. We determined the effect of the focused IR laser on monolayers of the various esters on solutions of Urea 5M and Trehalose 50 mM. The presence of neither of both solutes modified the CLP value (Figure 3A), or the flow velocity into the focus (Figure 3B). The same results were found for monolayers of ES on acidic (pH2) and basic (pH13) subphases, 5M NaCl and 5M guanidinium chloride (data not shown). All these results indicate that oxygen atoms of the ester moiety were hidden form the aqueous phase during local collapse and 3D aggregation, and also when the monolayer was globally heated since the pyrobaric coefficient was always negative, independently of the composition of the sub-phase.

Figure 3 . Critical Laser Power for the different ester monolayers (A) and velocity of the flux of material to the hot spot at 50-80 µm from the focus in monolayers of MP (B). Subphases: pure water (black), Urea 5M (red) and Trehalose 50 mM (blue).

Mixtures of MP with other lipid On basis of the results shown so far, we propose that esters at the air-water interface form a supramolecular arrangement that is completely dehydrated, and where the alkyl chains act as an “umbrella” that prevents the interaction of the ester moiety with the subphase. Under such conditions, i.e. without the competition of water molecules, surfactant-surfactant hydrogen bonds may form between the C=O and the C-O-C acceptors (strong hydrogen bond acceptors) and the methylene hydrogens (weak donors). This kind of inter-molecular H-bond network is not unique to ester monolayers. Fatty acids are also able to form inter-molecular hydrogen bonds at high packing, even stronger than those in esters, since in the acids the hydrogen bond donor and acceptor groups are both strong. However, fatty acid monolayers do not show negative pbc and do not flow into a hot spot.15,16 Therefore, the presence of the proposed hydrogen bonding network is not sufficient to explain the local collapse phenomena. 11 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

The main difference found so far between esters and fatty acids/alcohols is that in esters, water molecules stay away from the putative hydrogen bonding region, and no structured interfacial water is present.7 Moreover, the carbonyl groups in esters are protected from the aqueous subphase by the alkyl moieties. This shielding effect was proposed previously from determinations of the kinetic of ester hydrolysis on alkaline subphases.37 If water could access the oxygen’s region, as in fatty acids/alcohols, the inter-molecular hydrogen bonds would compete with the stronger hydrogen bonds trough water molecules, and the monolayer structure would be affected by both, the surfactantsurfactant interactions and the water-surfactant interactions. However, since water is precluded to enter in the Z-configuration of ester films, a hydrophobic region is exposed to water, and the subphase simply acts as a support for the monolayer, without participating in the energetic balance that leads to the observed supramolecular structure. In order to test the importance of the proposed shielding, films of MP were mixed with molecules with similar hydrocarbon chain and different polar head group. The added molecules, when pure, did not flow into the hot spot when exposed to a temperature gradient. Three different molecules were tested: PDA, HD and DPPC. The phospholipid bears a highly hydrated polar head group and two hydrocarbon chains, thus it is expected to completely disrupt the supramolecular structure of the ester monolayer. Indeed, the addition of a very small amount of DPPC (5 mole%) to the MP film prevented the flux of material into the focus, and the monolayer melted in the hot spot. On the contrary, the acid and the alcohol may allow the emergence of a well packed H-bonded structure, while the effect of the shielding of the ester moiety decreases as the amount of the non-ester molecules increases (see the chemical structures in Figure 4). Furthermore, HD is expected to open the film structure, and expose the C=O and the C-O-C regions to a higher degree than PDA. In agreement with the previous propositions, in the presence of small amounts of PDA or HD, the mixed films showed inward flux of material at a slightly higher CLPs than the pure MP monolayer. This is shown in Fig. 4, for additions of PDA (red) or HD (blue). Local collapse was observed with PDA and HD proportions up to 71% and 18%, respectively; while at 82% (PDA) and 40% (HD) respectively, the monolayer simply melted in the hot spot. It is also interesting to mention that in the region where the mixed monolayer was still pushed to the hot spot, the sub-phase composition had no measurable effect on the phenomenon, as proved by experiments on 5M Urea (see the orange symbol for 71% PDA in Figure 4).


Page 12 of 26

Page 13 of 26

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


Figure 4. Plot: Critical Laser Power values for monolayers of pure MP (black) and with increasing amounts of HD (blue) or PDA (red) on water. The lines indicate the lower value of HD (blue) or PDA (red) at which local collapse was no more observed. The orange symbol corresponds to monolayers composed of MP/PDA 3:7 on 5M Urea. Left: chemical structures of a group of 3 molecules, highlighting the differences in the chemical moieties exposed to water in MP, PDA and HD.

Molecular simulations The experimental evidence provided in the previous sections strongly suggests that water molecules play a secondary role in determining the thermal behaviour of ester monolayers. The subphase simply provides a supporting interface where amphiphiles get confined. At high packing fractions, no structured water seems to surround the polar headgroups, as indeed occurs for fatty acids.7 Therefore, as detailed in the Experimental Section, we simulated an MS monolayer replacing the aqueous subphase by an effective tethering potential. This model granted the possibility of performing the long simulations required to simulate 2D solid phases. It is important to make clear that our model was formulated with the sole purpose of testing the experimentally motivated hypothesis that a weak binding of the monolayer to the subphase correlates with a negative pyrobaric coefficient. In other words, the model was formulated a posteriori, and aimed at capturing the most basic ingredients of the intermolecular interactions in these systems: molecular shapes, electrostatic and Van der Waals interactions between amphiphiles, and a tunable parameter that regulates the binding of the monolayer to the subphase. In this sense, our model should be considered as a simple step forward in complexity with respect to the rigid rods model proposed by Kaganer et. al.17 Figure 5 presents the evolution of several molecular properties as the temperature of the system was increased from 15°C up to 65°C at constant lateral 13 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

pressure. Curves with different colours and symbols correspond to different tethering coupling constants, Kfc. The thermal evolution of the area per molecule (Figure 5A) shows that for values of Kfc between 10 and 100 kJ/nm2 (i.e. values bracketing the forceconstants obtained from MD simulations of fully hydrated MS monolayers at different temperatures, see Experimental Section and Figure S9), the monolayer exhibited a positive thermal expansion coefficient up to a critical temperature. This threshold value occurred within the 27-37°C interval, but it has to be remarked that given the approximations made in the formulation of our molecular model, it is not surprising that the temperature window where area changes are observed, does not exactly match the experimental values. Above 27-37°C, there is a temperature window where the area per molecule decreased, resulting in a negative thermal expansion coefficient. By standard thermodynamic relationships, a negative thermal expansion coefficient is equivalent to a negative pyrobaric coefficient. Note that the drop in molecular area was quite abrupt for the softest tethering potentials, and disappeared when the amphiphiles were virtually soldered to the X-Y plane (Kfc= and 10000 kJ/nm2). This behaviour is strongly reminiscent of the experimental observations discussed above, negative pbc occurs when the amphiphiles bind weakly and softly to the water surface.


Page 14 of 26

Page 15 of 26

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


Figure 5. Thermal dependence of molecular properties of MS grafted monolayers in heating simulations of monolayers composed of 64 metilestearate molecules. Tethering harmonic potentials applied on the carboxylic carbon, at different harmonic force constants Kfc (10, 60, 100 and 10000 kJ/nm2). (A) Molecular area. (B) Molecular tilting, defined as the angle established between the vector determined by the carboxylic carbon and the terminal carbon, and monolayer normal direction, see scheme. (C) Number of hydrogen bonds between carboxylic oxygens as acceptors, A, and Cα and Cβ as donors, D.

According to our model, the reduction in molecular area is concomitant with, and arguably could be attributed to, the drop in molecular tilting (Figure 5B) and the decrease in the number of MS-MS hydrogen bonds (Figure 5C). At temperatures below the critical value, the monolayer was in a tilted state (Figures 5B and 6A) with the hydrocarbon tails 15 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

collectively oriented in the direction of the next-nearest neighbours (NNN). Upon heating, the tilt angle decreased, and molecules lifted up and reoriented towards the nearest neighbour (NN) (see Figure 6B). In turn, the average number of hydrogen bonds formed between methylenic hydrogens and the esters’ oxygen atoms (see simulation methods), decreased upon heating (Figure 6C). In fact, when plotted against the area per molecule (Figure S9), the average number of hydrogen bonds shows a maximum near 21 or 21.5 Å2/molecule. When the area increased due to thermal expansion, the rising distance between the surfactants hindered hydrogen bonding. On the other hand, when molecules got too close together, the hydrogen-bond network was perturbed and fewer hydrogen bonds were formed. According to Kaganer et. al,17,19 a tilting transition of the kind observed in our simulations results from two competing effects. Internal energy favours the low temperature tilted state, while entropy favours the high-temperature non-tilted arrangement. In Kaganer’s mean-field theory the tilting transition can either be a weak first-order or a continuous transition, depending on the details of the interaction potential between the amphiphiles and the supporting substrate. In this regard, as our model aims at capturing the qualitative phase behaviour of an ester monolayer on water, we have made no attempts to characterise the putative phase transition in detail. Nonetheless, as shown in Figure S10 of the supplementary material, reversing the thermal scans reveals some important findings: As the system is cooled down, starting from a well equilibrated high-temperature state, the monolayer returned to its original low-temperature tilted phase. At the same time, the internal energy, the average area per molecule, and the tilt angle depicted certain degree of hysteresis. Such behaviour is particularly evident for Kfc = 10 kJ/nm2.


Page 16 of 26

Page 17 of 26

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


2

Figure 6. Snapshots from the heating simulation of MS grafted monolayer (Kfc = 10 kJ/nm ). Stick representation of metilestearate molecules, with carbon atoms in cyan and oxygen atoms in red. Only hydrogen potentially establishing hydrogen bonds are represented with white sticks. Carboxyl carbons are represented with orange balls. Hydrogen bonds are indicated with black discontinuous lines. Lateral views of the simulation box are shown in the upper panels and bottom views in the lower panels, with a hexagonal cell schematized in continuous black line. (A and C) Configuration at 20 °C and (B and D) at 60 °C.

Finally, the changes in the inclination of the molecules when the monolayer is heated or cooled, can help explaining the changes in interfacial potential observed experimentally (see Figure S5). Figure 7 shows the z-component of the monolayer dipole vector (panel A), and its angle with the X-Y plane (panel B), as depicted in the inset of panel A. Clearly, the dipole vector became more parallel to the plane of the substrate as the temperature increased and the molecules adopted a more upright position. The main contribution to the molecular dipole is due to the polar bonds between the carboxylic carbon and the oxygen atoms (C=O and C-O). Figure S11 shows the mean angle formed between these bonds and the z-axis. The C=O bond kept nearly normal to z-axis as temperature increased, leading to no significant contribution to the z-component of the 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

monolayer dipole vector. At the same time the angle between the C-O bond and the z-axis increased, leading to a major contribution to the interfacial dipolar potential decrease upon heating.

Figure 7. Thermal dependence of the molecular dipole moment in heating simulations of grafted monolayers (Kfc of 10, 60, 100 and 10000 kJ/nm2). (A) Projection of the total dipolar moment in the monolayer normal direction. (B) Angle formed between the total dipole moment and the z-axis. The measured quantities are schematized in panel A insert.

Shape of the aggregates formed in the hot spot. As a consequence of the inward flux of material, there is always an accumulation of molecules in the hot spot, leading to the formation of 3D structures, whose features were not always the same.


Page 18 of 26

Page 19 of 26

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


In most of the experiments, bright and rounded aggregates, in the range of 10 to 100 µm diameter, were formed (Figure 8 A and C). Usually, there was a central structure that grew due to the addition of molecules, or to the merging with other aggregates (see movie 02.avi). These aggregates were very often observed to rearrange into large bubbles (see Figure 8 A and C), as previously reported.18. When the laser was turned off, the material from the aggregates returned fast to the monolayer, forming a bright cell around them and leaving a central bright spot with flower shape (see the temporal sequence in Figure 8 B). This kind of aggregates were observed for MP, MA, MS, EA and EP on different subphases and for different velocities of the inward flux. In some cases, the large aggregate structure appeared darker than the monolayer. These structures are probably the same than the brighter ones, but with lower amounts of probe or with a quenched probe, since we also observed bright aggregates to become dark (see the temporal sequence in Figure 8C and in movie 01.avi). In some experiments, instead of large bubbles, small aggregates were formed. These structures were below resolution, and the accumulated material always returned to the monolayer once the laser was switched off, forming cells with a bright spot at the center (see a temporal sequence in Figure 8D and movie 03.avi). MP, MS, MA, EP, ES and EA formed these kind of aggregates at the same conditions than the larger structures, i.e. sometimes large structures were formed, and sometimes these smaller structures, randomly. For monolayers of BS, none of these aggregates were observed, but small bubbles, with the appearance of concentric circles, formed very fast and then merged (Figure 8E). Similar structures were also observed rarely in the other systems. In order to inquire whether there was a correlation between the shape of the 3D structure generated at the hot spot and the equilibrium shape of the aggregates formed by the molecules in aqueous suspensions, the hydrodynamic diameter for the latest aggregates was determined by Dynamic Light Scattering (Figure S8A). The critical aggregate concentration of ES, determined as indicated in the Experimental Section, were 13, 10 and 20 µM for suspensions in water, trehalose 50 mM and 5M urea, respectively (see Figure S8B). The most probable hydrodynamic diameters were 114 and 10 nm for suspensions in water and 50mM trehalose, respectively. In urea 5M, two populations were observed with most probable sizes of 43 and 178 nm, i.e. small and large aggregates. Therefore, we conclude that ES formed larger aggregated in water and urea than in trehalose, where micelles are formed. Consequently, we can conclude that the structure of the bubbles formed by local heating is not related to the equilibrium supramolecular structures formed spontaneously by these molecules in aqueous suspensions. Neither were they related to the solubility of these molecules in water or to the stability of the monolayer, since the collapse pressures were in the range of 40-50mN/m for the Methyl esters and about 35 mN/m for the ethyl esters (see Figure 1A).



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

Figure 8. Shape of the aggregates observed in the hot regions. (A) ES on Urea 5 M, laser power: 3W. (B) MP on water, laser off. (C) MA on water, laser power: 4 W. (D) ES on water, laser off. (E) BS on water, laser power: 1W. (F) Aggregates of ES on Glycerol/water 1:1 observed by DIC (left) or by fluorescence microscopy (middle), and a temporal sequence of two bubbles merging. Laser power: 2W.(G) Temporal sequence of the process of return of material to the monolayer after the laser was turned off. System: ES on Glycerol/water 1:1. The scale bars in all images correspond to 50 µm.

The behavior of monolayers of ES prepared on Glycerol/water 1:1 (v/v) showed some interesting features that deserve special attention. On this subphase, the CLP was lower than for the other subphases (0.9±0.2 W on Glycerol/water and 1.5±0.5 W on the other subphases). This can be explained considering that the heat conductivity of pure glycerol is lower than that of pure water (κw=0.6 W/mK and κgly=0.3 W/mK), i.e. for a similar temperature gradient, a lower laser power is necessary when Glycerol/water 1:1 (v/v) is the subphase instead of pure water. Other very interesting difference is that the Glycerol/water 1:1 (v/v) solution is more viscous than pure water, and this translates to a slower kinetics of the material flux, thus indicating that the monolayer and the subphase were mechanically coupled, though not interacting at the molecular level. The slower rates enabled us to better describe the aggregation process, and the return of the material to the monolayer when the laser was switched off. We were able to observe the growth of big bubbles on this subphase, which were always visible by DIC (see Figure 8F, left), indicating that they had different refractive index, as expected for air 20 ACS Paragon Plus Environment

Page 20 of 26

Page 21 of 26

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


bubbles. The bubbles were formed by concentric circles observable by fluorescence microscopy and not by DIC (see Figure 8F, middle); thus these patterns were caused by uneven distribution of the fluorescent probe (or of its quantum yield). This was probably related to the multilayered nature of the bubbles previously reported.18 When the bubbles merged, they rapidly adopted the concentric circles pattern again (see Figure 8F right and movie 04.avi). When the laser was turned off, the material went back to the interface slower than in the other subphases (it took 4-5 times longer). This allowed us to detect the pattern formation during the return of the material, as shown in Figure 8G. First, bright regions of material returned to the monolayer, afterwards, the darker regions, and a bright core remained for longer times (see movie 05.avi). These bright cores were detected by DIC (see movie 06.avi), thus they were not a monolayer but a 3D aggregate indicating that the process was not completely reversed. In the final stage, each core was delimited by a cell, as previously shown for the other systems (Figure 8D). This mechanism for the return of the material is consistent with the concentric fluorescent circles of the bubbles, regions with and without fluorescence were arranged sequentially. The reason for this surprising behavior is still not understood. Movie 06.avi shows an example of the process. The aggregates are formed only at high surface pressures, while at lower pressures the monolayer melts. Therefore, the aggregates can be formed by an increase in the temperature gradient at constant density, as described up to now and shown in Figure 8, or by an increase in surface pressure maintaining a constant gradient of temperature, i.e. maintaining the laser on. Movie 07.avi shows an example of a monolayer compressed with the laser on, at low pressures the monolayer melts at the hot spot while at high pressures, local collapse occurs. Regarding the disassembly of the aggregates, it occurred when the laser was turned off (as shown in Figure 8), and also by decreasing the surface pressure with the laser on (see movie 08.avi).

CONCLUSIONS Solid-state monolayers of a series of ester surfactants were locally heated with an IR-laser, while the texture of the film was followed under a microscope. All the compounds investigated showed a peculiar behavior: in the presence of an appropriate thermal gradient, all monolayers yielded to a kinetically excited liquid that flowed to the hot laser’s spot. These observations tally with the fact that compression isotherms of the same compounds, measured at different temperatures, reveal negative pyrobaric coefficients. Structurally related fatty acids and alcohols, behave in the opposite way: their monolayers show a positive pbc and simply melt under the laser’s hot spot. We conclude that the difference between esters and the other molecules is due to the lack of influence, in the former case, of hydration water at high molecular packing. Ester monolayers seem to be decoupled from water, i.e. there is no hydration water around the polar moieties as is the case for alcohols and fatty acids. Water only provides the interface where the molecules are confined.



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

Molecular simulations of an ester monolayer showed that the change in the tilt angle of the molecules occuring upon heating at constant lateral pressure, correlates with a decrease in the mean molecular area and in the average number of hydrogen bonds between surfactants. In our experiments, a film with negative thermal expansion coefficient leads to a pressure gradient when the monolayer is exposed to a thermal gradient. This induced local stress is borne until the monolayer yields. The yield pressure depends on the monolayer stiffness, so that less compressible monolayers bear higher pressures. Esters sensitivity to temperature gradients depend on the length of the alkyl chain (derived from the alcohol). This alkyl moiety is necessary to decouple the film from water, but the longer they are the lower the thermal sensitivity.

The Supporting Information is available free of charge on the ACS Publications website Figure S1. Determination of the flux velocity. Figure S2. Fluctuations of the carbonyl groups in the direction normal to the monolayer, obtained from Molecular Dynamics simulations of a fully hydrated MS monolayer. Figure S3. Compression isotherms at different temperatures, and curves of surface pressure vs temperature at constant and low mean molecular area for EA and BS. Figure S4. Images of an HD monolayer submitted to different laser powers and isotherms for HD and MA at different temperatures. Figure S5. Surface potential for ES monolayers on water at a constant surface pressure of 20 mN/m and at increasing temperatures (from 15 to 35 ºC). Figure S6. (A) Velocity of the flux of material to the hot spot for EP, ES and EA. (B) Doubly logarithmic plot of the velocity of MP on water versus the laser power minus the critical laser power. Figure S7. Compression isotherms for ES monolayers on different subphases at 24 and 30°C. Figure S8. Equilibrium structures formed by ES in aqueous suspensions Figure S9. Mean number of hydrogen bonds vs molecular area obtained from the simulated experiments. Figure S10. Molecular area (A), molecular tilting (B) and number of hydrogen bonds (C) as a function of temperature obtained from the simulated experiments. Figure S11. Angle formed between C-O bonds and the monolayer plane as a function of temperature. Movie 01.avi: Formation of a bright bubble that turns dark. System: MA on water. Total size: 100 × 100 µm2. Total time: 2.32 s. Movie 02.avi: Fusion of two large bubbles. System: ES on urea 5M. Total size: 240 × 180 µm2. Total time: 0.56s Movie 03.avi: Small aggregates (below resolution) that returned to the monolayer when the laser is off. System: ES on water. Total size: 240 × 180 µm2. Total time: 2.48 s. Movie 04.avi: Formation and fusion of bubbles with concentric circles. System: ES on Glycine/water 1:1 (v/v). Total size: 70 × 70 µm2. Total time: 6.35s.


Page 22 of 26

Page 23 of 26

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


Movie 05.avi: Return of the material to the monolayer after the laser is off observed by fluorescence microscopy. System: ES on Glycine/water 1:1 (v/v). Total size: 115 × 115 µm2. Total time: 5.55s. Movie 06.avi: Return of the material to the monolayer after the laser is off observed by DIC. System: ES on Glycine/water 1:1 (v/v). Total size: 115 × 115 µm2. Total time: 4.03s. Movie 07.avi: Monolayer compressed with the laser on observed by fluorescence microscopy. System: ES on Glycine/water 1:1 (v/v). Total size: 115 × 115 µm2. Total time: 16.44s. Movie 08.avi: Monolayer expanded with the laser on observed by DIC. System: ES on Glycine/water 1:1 (v/v). Total size: 115 × 115 µm2. Total time: 6.25s.

ACKNOWLEDGMENTS NW thanks the Alexander von Humboldt foundation for an AvH fellowship. V. G., M.G. D. P. and N.W. are career investigators of CONICET. M.G.D.P. thanks EU Commission Marie Curie RISE “ENACT” Programme (grant number 643998), SECTyPUNCUYO, and FONCyT (PICT-2011-2128 and PICT-2012-2759) for financial support.

Bibliography (1) (2) (3) (4) (5) (6) (7) (8) (9)

(10) (11)

Gopal, A.; Lee, K. Y. C. Morphology and Collapse Transitions in Binary Phospholipid Monolayers. J. Phys. Chem. B 2001, 105 (42), 10348–10354. Ybert, C.; Lu, W.; Möller, G.; Knobler, C. M. Kinetics of Phase Transitions in Monolayers: Collapse. J. Phys. Condens. Matter 2002, 14, 4753–4762. Baoukina, S.; Monticelli, L.; Risselada, H. J.; Marrink, S. J.; Tieleman, D. P. The Molecular Mechanism of Lipid Monolayer Collapse. Proc. Natl. Acad. Sci. U. S. A. 2008, 105 (31), 10803–10808. Phan, M. D.; Lee, J.; Shin, K. Collapsed States of Langmuir Monolayers. 2016, 2015. Aliaskarisohi, S.; Fischer, T. M.; Wilke, N. Dilatational Yielding of Solid Langmuir Monolayers. J. Phys. Chem. B 2011, 115 (40), 11631–11637. Muruganathan, R. M.; Fischer, T. M. Laser-Induced Local Collapse in a Langmuir Monolayer. J. Phys. Chem. B 2006, 110 (44), 22160–22165. Nickolov, Z. S.; Britt, D. W.; Miller, J. D. Sum-Frequency Spectroscopy Analysis of Two-Component Langmuir Monolayers and the Associated Interfacial Water Structure. J. Phys. Chem. B 2006, 110 (31), 15506–15513. Fischer, B.; Tsao, M.; Ruiz-garcia, J.; Fischer, T. M.; Schwartz, D. K.; Knobler, C. M. Observation of a Change from Splay to Bend Orientation at a Phase Transition in a Langmuir Monolayer. J. Phys. Chem. 1994, 98 (31), 7430–7435. Qiu, X.; Ruiz-Garcia, J.; Stine, K. J.; Knobler, C. M.; Selinger, J. V. Direct Observation of Domain Structure in Condensed Monolayer Phases. Phys. Rev. Lett. 1991, 67 (6), 703–706. Bibo, a M.; Knobler, C. M.; Peterson, I. R. A Monolayer Phase Misclblllty Comparison of Long-Chain Fatty Acids and Their Ethyl Esters. J. Phys. Chem. 1991, 95 (10), 5591–5599. Knobler, C. Phase Transitions in Monolayers. Annu. Rev. Phys. Chem. 1992, 43 (1), 207–236. 23 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

(12)

(13) (14) (15) (16)

(17) (18) (19) (20) (21) (22) (23)

(24) (25) (26) (27) (28) (29) (30)

Teer, E.; Knobler, C. M.; Lautz, C.; Wurlitzer, S.; Kildae, J.; Fischer, T. M.; Teer, E.; Knobler, C. M. Optical Measurements of the Phase Diagrams of Langmuir Monolayers of Fatty Acid , Ester , and Alcohol Mixtures by Brewster-Angle Microscopy Optical Measurements of the Phase Diagrams of Langmuir Monolayers of Fatty Acid , Ester , and Alcohol Mixtures by B. J. Chem. Phys. 2003, 1913 (1997), 1913–1920. Foster, W. J.; Shih, M. C.; Pershan, P. S. The Structure of a Langmuir Monolayer of Methyl Eicosanoate as Determined by X-Ray Diffraction and Brewster Angle Microscopy. J. Chem. Phys. 1996, 105 (8), 3307. Khattari, Z.; Fischer, T. M. Growth of Tilted Domains in an Octadecanol Langmuir Monolayer Using Radial Temperature Gradients. J Phys Chem B 2004, 108 (1), 13696–13699. Gaines, G. L. In Insoluble Monolayers at Liquid-Gas Interfaces; Prigogine, I., Ed.; Wiley-Interscience: New York, 1966. Valdes-Covarrubias, M. a; Cadena-Nava, R. D.; Vásquez-Martínez, E.; ValdezPérez, D.; Ruiz-García, J. Crystallite Structure Formation at the Collapse Pressure of Fatty Acid Langmuir Films. J. Phys. Condens. Matter 2004, 16 (May), S2097– S2107. Kaganer, V. M.; Osipov, M. A.; Peterson, I. R. A Molecular Model for Tilting Phase Transitions between Condensed Phases of Langmuir Monolayers. J. Chem. Phys. 1993, 98 (4), 3512. Gewinner, J.; Fischer, T. M. Heterogeneous Nucleation of Giant Bubbles from a Langmuir Monolayer in a Laser Focus. J. Phys. Chem. B 2013, 117 (47), 14749– 14753. Kaganer, V. M. Structure and Phase Transitions in Langmuir Monolayers. 1999, 71 (3), 779–819. Haas, F. M.; Hilfer, R.; Binder, K. Phase Transitions in Dense Lipid Monolayers Grafted to a Surface: Monte Carlo Investigation of a Coarse-Grained Off-Lattice Model. 1996, 3654 (96), 15290–15300. Selinger, J. V; Wang, Z.-G.; Brusima, R. F.; Knobler, C. M. Chiral Symmetry Breaking in Langmuir Monolayers and Smectic Films. 1993, 1139–1142. Gupta, R. K.; Suresh, K. A.; Kumar, S.; Lopatina, L. M.; Selinger, R. L. B.; Selinger, J. V. Spatiotemporal Patterns in a Langmuir Monolayer due to Driven Molecular Precession. 2008, 1–7. Karaborni, S.; Toxvaerd, S. Molecular Dynamics Simulations of Langmuir Monolayers: A Study of Structure and Thermodynamics. J. Chem. Phys. 1992, 96 (7), 5505. Karaborni, S.; Toxvaerd, S.; Olsen, O. H. Phase Transitions in Langmuir Monolayers: A Molecular Dynamics Study. J. Phys. Chem. 1999, 53 (1), 93–106. Duchs, D.; Schmid, F. Phase Behaviour of Amphiphilic Monolayers : Theory and Simulati. Physics. Condens. matter 2001, 13, 4853–4862. Stadler, C.; Lange, H.; Schmid, F. Short Grafted Chains: Monte Carlo Simulations of a Model for Monolayers of Amphiphiles. 1999, 59 (4), 4248–4257. Brown, F. L. H. Simple Models for Biomembrane Structure and Dynamics. 2007, 177, 172–175. Schmid, F.; Düchs, D.; Lenz, O.; West, B. A Generic Model for Lipid Monolayers , Bilayers , and Membranes. 2007, 177, 168–171. Wang, J.; Wolf, R. M.; Caldwell, J. W.; Kollman, P. A.; Case, D. A. Development and Testing of a General Amber Force Field. 2004, 56531. Jakalian, A.; Jack, D. B.; Bayly, C. I. Fast , Efficient Generation of High-Quality Atomic Charges . AM1-BCC Model : II . Parameterization and Validation. 2002. 24 ACS Paragon Plus Environment

Page 24 of 26

Page 25 of 26

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


(31)

(32) (33) (34) (35) (36) (37) (38) (39) (40) (41) (42) (43)

(44) (45)

Hornak, V.; Abel, R.; Okur, A.; Strockbine, B.; Roitberg, A.; Simmerling, C. Comparison of Multiple Amber Force Fields and Development of Improved Protein Backbone Parameters. 2006, 725 (September), 712–725. Abraham, M. J.; Murtola, T.; Schulz, R.; Pall, S.; Smith, J. C.; Hess, B.; Lindahl, E. GROMACS : High Performance Molecular Simulations through Multi-Level Parallelism from Laptops to Supercomputers. 2015, 1–2, 19–25. Niccolai, A.; Baglioni, P.; Dei, L.; Gabrielli, G. Monolayers of 1,2,3-Propanetriol Esters at the Air-Water Interface: Two-Dimensional Phases and Miscibility. Colloid Polym. Sci. 1989, 267 (3), 262–268. Gericke, A.; Hühnerfuss, H. The Conformational Order and Headgroup Structure of Long-Chain Alkanoic Acid Ester Monolayers at the Air/Water Interface. Berichte der Bunsengesellschaft für Phys. Chemie 1995, 99 (4), 641–650. Gutberlet, T.; Vollhardt, D. Thermally Induced Domain Growth in Fatty Acid Ester Monolayers. J Colloid Int Sci 1995, 173, 429–435. Vila-Romeu, N.; Nieto-Suárez, M.; Dynarowicz-La̧tka, P.; Prieto, I. Mixed Langmuir Monolayers of Gramicidin A and Ethyl Palmitate: Pressure-Area Isotherms and Brewster Angle Microscopy. J. Phys. Chem. B 2002, 106 (38), 9820–9824. Alexander, A. E.; Schulman, J. H. Orientation in Films of Long-Chain Esters. Proc. RS London. Ser. A, Math. Phys. Sci. 1937, 161, 115–127. Wilke, N. Lipid Monolayers at the Air–Water Interface, 1st ed.; Elsevier Ltd., 2014; Vol. 20. Peñalva, D. A.; Wilke, N.; Maggio, B.; Aveldaño, M. I.; Fanani, M. L. Surface Behavior of Sphingomyelins with Very Long Chain Polyunsaturated Fatty Acids and E Ff Ects of Their Conversion to Ceramides. 2014. Davies, J. T.; Rideal, E. K. Interfacial Phenomena; Academic Press: New York, 1963. Brovchenko, I.; Burri, R. R.; Krukau, A.; Oleinikova, A.; Winter, R. Intrinsic Thermal Expansivity and Hydrational Properties of Amyloid Peptide A Β42 in Liquid Water. J. Chem. Phys. 2008, 129 (19), 1–11. Brovchenko, I.; Andrews, M. N.; Oleinikova, a. Volumetric Properties of Human Islet Amyloid Polypeptide in Liquid Water. Phys. Chem. Chem. Phys. 2010, 12 (16), 4233–4238. Brezesinski, G.; Stefaniu, C.; Nandy, D.; Dutta Banik, S.; Nandi, N.; Vollhardt, D. Cross-Sectional Area Increase at Phase Transition on Compression: An Unexpected Phenomenon Observed in an Amide Monolayer. J. Phys. Chem. C 2010, 114 (37), 15695–15702. Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond: In Structural Chemistry and Biology; Oxford Science Publications, 1999. Lo Nostro, P.; Ninham, B. W. Hofmeister Phenomena: An Update on Ion Specificity in Biology. Chem. Rev. 2012, 112 (4), 2286–2322.



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

TOC Graphic


Page 26 of 26

Molecular Explanation for the Abnormal Flux of ... - ACS Publications

Recommend Documents