Alkyl-Based Surfactants at a Liquid Mercury Surface: Computer

Letter pubs.acs.org/JPCL

Alkyl-Based Surfactants at a Liquid Mercury Surface: Computer Simulation of Structure, Self-Assembly, and Phase Behavior Anton Iakovlev,† Dmitry Bedrov,*,‡ and Marcus Müller† †

Institut für Theoretische Physik, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany Department of Materials Science & Engineering, University of Utah, 122 South Central Campus Drive, Salt Lake City, Utah 84112, United States

‡

S Supporting Information *

ABSTRACT: Self-assembled organic films on liquid metals feature a very rich phase behavior, which qualitatively differs from the one on crystalline metals. In contrast to conventional crystalline supports, self-assembled alkylthiol monolayers on liquid metals possess a considerably higher degree of molecular order, thus enabling much more robust metal− molecule−semiconductor couplings for organic electronics applications. Yet, compared to crystalline substrates, the self-assembly of organic surfactants on liquid metals has been studied to a much lesser extent. In this Letter we report the first of its kind molecular simulation investigation of alkyl-based surfactants on a liquid mercury surface. The focus of our investigation is the surfactant conformations as a function of surface coverage and surfactant type. First, we consider normal alkanes because these systems set the basis for simulations of all other organic surfactants on liquid mercury. Subsequently, we proceed with the discussion of alkylthiols that are the most frequently used surfactants in the surface science of hybrid organometallic interfaces. Our results indicate a layering transition of normal alkanes as well as alkylthiols from an essentially bare substrate to a completely filled monolayer of laying molecules. As the surface coverage increases further, we observe a partial wetting of the laying monolayer by the bulk phase of alkanes. In the case of alkylthiols, we clearly see the coexistence of molecules in laying-down and standing-up conformations, in which the sulfur headgroups of the thiols are chemically bound to mercury. In the standing-up phase, the headgroups form an oblique lattice. For the first time we were able to explicitly characterize the molecular-scale structure and transitions between phases of alkyl-based surfactants and to demonstrate how the presence of a thiol headgroup qualitatively changes the phase equilibrium and structure in these systems. The observed phenomena are consistent with available direct and indirect experimental evidence.

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the opposite electrode.27 All these features promote a wide usage of self-assembled alkyl-based films in the rapidly developing field of organic electronics.25−30 The profound understanding of and the ability to predict the properties of organic monolayers on Hg surfaces and what drives their structural changes would greatly facilitate progress in this promising field.25 From this perspective, the development of computer simulations of organic self-assembled layers on liquid Hg appears to be crucial for further advancement; however, no such attempt has previously been undertaken. Recent Fourier-transformed infrared spectroscopy results clearly showed an increasing ordering in alkylthiol (thiol) monolayers on liquid mercury as the surface coverage of the surfactants grows.27 Tensiometry and X-ray reflectivity measurements on similar systems indicated coexistence regions of single-molecular layers laying parallel to the mercury surface and of standing molecules tilted with respect to the surface normal.31,32 In addition, these experiments suggested that the patches of standing molecules were highly ordered and featured

hile the structure and self-assembly properties of alkylbased films on crystalline nanoparticles and flat surfaces of gold, silver, graphite, mica, etc. have been extensively investigated by both experiments1−6 and simulations,7−24 the behavior and structure of alkyl-based surfactants on liquid metal surfaces, such as liquid mercury (Hg), are studied to a much lesser extent. Nevertheless, the usage of liquid mercury surface as a supporting substrate for organic self-assembled monolayers (SAMs) and multilayers offers a number of advantages over conventional materials with crystalline structure. The seamless, liquid, yet very dense Hg surface of high tension enables creation of organic self-assembled layers that are not influenced by the features of underlying crystal lattice and concomitant defects. These unique properties of liquid Hg make it exceptionally apt for the studies of charge transfer through organic self-assembled layers, where a liquid Hg droplet is used as one of the electrodes simply by being gently set on top of the organic film, which is adsorbed on another, typically crystalline, electrode.25 Such setup of a hybrid metal−molecule−metal (or −semiconductor) junction leaves the structure of the organic layer intact.26 Furthermore, Hg droplet electrodes covered with an organic layer opposing the one on the crystalline electrode are applied to inhibit amalgamation of Hg by the material from © 2016 American Chemical Society

Received: March 2, 2016 Accepted: April 5, 2016 Published: April 5, 2016 1546

DOI: 10.1021/acs.jpclett.6b00494 J. Phys. Chem. Lett. 2016, 7, 1546−1553

Letter

The Journal of Physical Chemistry Letters rectangular packing with two thiols per unit cell and respective base vector lengths of 5.52 and 8.42 Å at room temperature.31,32 Such a unit cell corresponds to the thiol surface coverage of 4.3 molecules/nm2. The possibility of oblique packing was also mentioned, though no quantitative estimates were given.32 In both cases these packings appear to be qualitatively different from the ones reported on gold (Au), where the headgroups as well as tails are arranged in a hexagonal manner that is commensurate with the gold’s underlying order.3 More generally, phase coexistence regions were also found for alcohols,33 fatty acids,34 and even for normal alkanes (n-alkanes) on liquid Hg.35 The case of alcohols probably exhibits the most rich phase behavior, featuring such phases as single, double, triple, and quadruple layers of surfaceparallel molecules as well as a monolayer of molecules standing normally to the surface.33 Fatty acids exhibit a phase diagram that is similar to the one of alcohols with the primary difference that no triple or quadruple multilayers of laying molecules were detected.34 Moreover, n-alkanes were shown to have phases of single, double, and triple layers of laying flat molecules on Hg surface.35 In this case, no “standing” phases were found in the experiments. Such behavior of n-alkanes on liquid Hg completely opposes to the one on water, where the alkanes, due to the strong hydrophobic interaction with water, stand titled at a sharp angle to the surface normal and exhibit hexagonal ordering.36 Additionally, it was inferred that respective bulk phases of n-alkanes on liquid Hg begin to emerge after a certain number (depending on the alkane length) of fully completed alkane layers parallel to the Hg surface are reached.37 Low-density two-dimensional (2D) gas phases were deduced from the fits by the Volmer isotherm of the available tensiometry data in all of the above cases (i.e., thiols, fatty acid, alcohols, and n-alkanes on liquid Hg) for surfactant surface coverages lower than the one required to completely fill a layer of laying molecules.31,33−35 Finally, the latest optical tensiometry results revealed multiscale temporal adsorption of thiols on liquid mercury, thereby giving additional indirect evidence of the intricate structural transformations within the monolayer of thiols as the surfactant surface coverage increases.38 While these experimental studies provide a great deal of valuable information, the molecular-scale understanding of the self-assembly of organic surfactants on the liquid Hg interface is incomplete and is difficult to gain from experimental measurements alone (e.g., whether or not thiols are already chemisorbed in the “laying-down” conformations, arrangement of S−Hg−S bonds, structure and mechanisms of growth of alkane bulk phases, etc.). Hence, here we report the first of its kind molecular simulation investigation of alkyl-based surfactants on a liquid mercury interface. Motivated by the above experimental findings, we focus on the surfactant conformations as a function of surface coverage and surfactant type at room temperatures. First, we start with the case of n-alkanes as these systems set the basis for the simulation of other surfactants on liquid Hg discussed above. Our simulations not only reproduce a number of experimentally observed phases of n-alkanes as well as of thiols on liquid Hg but also provide direct insights into the structure and intermolecular interactions of these systems. Then we proceed with the discussion of alkylthiols on liquid Hg, because these surfactant systems are one of the most frequently used in the surface science of hybrid organometallic interfaces.38

We utilize large-scale molecular dynamics (MD) simulation techniques using a united-atom (UA) representation for nalkanes39,40 and thiols,20 while employing our optimized density-independent (ODI) atomistic force field for liquid Hg.41 The ODI model enables a computationally efficient and physically robust simulation of the liquid mercury surface.41 The main challenge for the modeling of liquid Hg surface is the ability to simultaneously capture its unusual (as for liquid) properties, namely, its strongly stratified surface density profile and extremely high surface tension.41 The importance of these two concepts (density and surface tension) for the force field development for the case of organic and biological surfactants on crystalline supports has also been reported previously.23,24 Figure 1 demonstrates a schematic view of the systems under

Figure 1. Schematic view of the investigated systems. (a) Sketch of a simulation cell. (b) Studied alkanes physisorbed on Hg in the UA representation: top, dodecane (C12); bottom, docosane (C22). (c) Studied alkylthiols chemisorbed on Hg in the UA representation: top, dodecanethiol (SC12); bottom, octadecanethiol (SC18). (d) Typical density profile perpendicular to mercury surface of a hybrid liquid Hg−alkyl-based surfactant interface. Color code: bulk Hg, red; bound Hg, purple; CH2,3 group, green; S, yellow. For brevity, hydrogens are omitted in the labels and chemical designation.

investigation. We used a slab configuration in which the Hg film was placed in the center of the simulation cell (Figure 1a). Normal alkanes (Figure 1b) or alkylthiols (Figure 1c) were symmetrically preadsorbed from both sides on the surface of liquid Hg. An empty space was added from both sides of the film ensuring that the molecules from one interface do not interact with molecules from another interface through periodic boundary conditions, which were applied in all directions. Figure 1d depicts typical density profiles developed by each type of the studied surfactants on the stratified surface of liquid mercury. Detailed descriptions of our simulation protocol and force fields are included in the Supporting Information. Figure 2a,b shows top and side views of dodecane (C12) systems at liquid Hg interface as obtained from the MD simulations at different alkane surface coverages σ. The respective carbon density profiles parallel to the Hg interface are given in Figure 2c. At low molecular surface coverages (σ < 1.11 nm−2), all dodecane molecules are laying-down on the Hg surface with their molecular axes parallel to the surface and primarily confined to a single (incomplete) layer. The distribution of molecules is not homogeneous at these coverages, and we observe a phase coexistence between a 1547


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Figure 2. Behavior of dodecanes on liquid Hg as a function of surface coverage σ: (a) dodecane conformations for selected values of σ and systems with lateral dimensions of 10 nm; (b) dodecane conformations for σ = 2.0 nm−2 and system with lateral dimensions of 30 nm; and (c) evolution of density profiles and distributions (inset) of carbon atoms in the dodecane self-assembled film on Hg as a function of the increasing surface coverage. The thin solid black line is the Hg interface and is shown as a reference.

observations,37 in which the dewetting of alkanes from physisorbed laying-down phases has also been seen. Such behavior of n-alkanes is an indicator of the strong short-ranged Hg−alkane interaction. The low-coverage phase behavior of longer alkanes is similar to their shorter counterparts. At molecular coverages smaller than that of the completely filled monolayer of molecules laying parallel to the Hg surface, docosanes also stepwise fill the first layer until it is completed at σ = 0.64 nm−2. At higher molecular surface coverages, the formation of the bulk phase of docosanes (C22) sets in as one can see from the systems snapshots (Figure 3a,b). Because the bulk of docosane at room temperatures is crystallized, we observe here the formation of docosane thin crystallites of four layers for σ = 1.5 nm−2 and the system with the cross section of 10 × 10 nm2 (Figure 3a). The crystalline structure of the three-dimensional (3D) docosane bulk on liquid Hg is also clearly manifested in the development of sharp peaks in the interface density profiles as the docosane surface coverage rises (Figure 3c). No further increase in the number of layers in the docosane crystallites was observed as the surface coverage was subsequently increased. Instead, the number of docosane crystallites grew, forming multiple stripe patterns on the surface, which could cluster. The lamellar structures that we obtained have reached the local equilibrium but are still kinetically trapped. A further merging of crystalline lamellae such as in Figure 3b is a very protracted process and it is unlikely to see any further perfection of the crystalline order on the time scales of our simulations. The snapshots for σ = 1.5 nm−2 (Figure 3a) also reveal regions of premelted liquid docosanes at the interface between the crystalline and layingdown monolayer phases of docosane. Such findings qualitatively agree with the theory of polymer crystallization42 and experiments.43,44 We expect that for a particular temperature, molecular coverage, and system sizes, the extent of these premetled regions, and consequently the height of the corresponding 3D bulk phase, is governed by the contact

bare Hg surface and 2D aggregates of dodecane molecules. This can be clearly seen on the inset of Figure 2c where the probability of finding a surface element with cross-section of 1 × 1 nm2 and certain amount of carbon atoms is plotted. For σ = 0.67 nm−2, the inset shows that the probability of finding such a volume almost empty (i.e., bare Hg surface) is the highest, and there is a well-pronounced peak around 10 carbons. Such coexistence is accompanied by the growth of a single peak at the Hg interface position in the carbon density profiles up to a point when the complete layer of laying-down molecules is formed at about σ = 1.11 nm−2. At this stage, the first peak stops growing and the density profiles indicate the development of a second layer and finally start to exhibit the second peak as the surface coverage reaches the value of 2 dodecane molecules per 1 nm2. In contrast to the layering transition from an “empty” surface to the first layer, we observe here partial wetting by the bulk alkane liquid of the layer of laying-down molecules. (At room temperature, the bulk of dodecanes is liquid.) This can be clearly seen for the system with σ = 2 nm−2, the density profile of which substantially extends away from the interface. The probability distribution of finding the surface element with cross-section of 1 × 1 nm2 and some carbons inside it also demonstrates a greatly extended shoulder tending to form another peak at the regions with higher carbon surface densities. These regions can be clearly seen on the snapshots featuring a dodecane droplet, which is in equilibrium with the completely filled first layer of laying-down dodecanes. The droplet height and curvature are dictated by its contact angle and thus depend on the overall system size. We anticipate that it would continue to change until macroscopic droplet dimensions are reached. In the largest system investigated (1800 dodecanes on the liquid mercury surface of 30 × 30 nm2), the droplet height is about 2 nm, which corresponds to more than four van der Waals diameters of an alkyl group (Figure 2b). The above predicted phases of dodecane on liquid Hg are in excellent agreement with the available experimental 1548


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alkanes on liquid mercury, but clearly no long-range order in the laying-down phase is present (see for example Figures 2b and 3b). Let us now explore the effect of adding a headgroup with a strong affinity to the mercury surface. Figure 4a shows the top and side views of octadecanethiol (SC18) conformations on the surface of liquid Hg for various values of σ. The evolution of the monolayer density profiles with increasing σ are presented in Figure 4b. We see that the low-coverage behavior of octadecanethiols is similar to that of n-alkanes. Namely, there is a layering transition to the first monolayer of molecules laying with their alkyl tails flat on the surface of liquid Hg. At the surface coverage of about 0.84 molecules/nm2, octadecanethiols complete the formation of this layer. As is evident from the corresponding snapshots (Figure 4a), no long-range order is observed for the low-coverage phases of thiols on Hg, which contrasts to the respective thiol phases on gold.3 Similar to the case of n-alkanes, we also see a reorganization of alkylthiols upon increasing σ. Here, for σ > 0.84 nm−2, however, we observe the formation of crystalline islands of the thiol molecules standing tilted at a sharp angle to the surface normal, as is clearly seen in the snapshots (Figure 4a). One can also trace the onset and growth of the islands from the respective density profiles (Figure 4b). The single peaks of sulfur and alkyl density profiles are localized in the same region of the interface at molecular coverages less than or equal to the one of the complete first layer of the laying-down molecules. The onset of the reorientation of alkyl tails is signaled by the delocalization of these two peaks and by the growth of the range of the alkyl density profile as σ grows. The two-phase coexistence also becomes evident from the two-peak probability distributions of finding a particular number of carbons in the 1 × 1 nm2 interface regions (see the inset in Figure 4b). At the highest simulated octadecanethiol surface coverage, σ = 1.61 nm−2, the second peak of the carbon density distribution attains its maximum at ∼75 carbon units per 1 nm2. This corresponds to 4.17 octadecanethiols per 1 nm2 inside the crystalline island of standing thiols. A schematic view of the structure in the island of standing thiols is given in Figure 4c. For σ = 1.61 nm−2, both the distribution of relative angles between neighboring S−Hg−S bonds as well as the distribution of angles between xy-projections of alkyl tails of the same R−S− Hg−S−R complex peak at 0° (Figure S2). This indicates that the S−Hg−S bonds are aligned and that the alkyl tails are tilted in the same direction within the island of standing thiols. As one can seen from Figure 5a−c, the structure of shorter thiols (dodecanethiols) is very similar to that of the longer ones. However, in this case, one can conclude from the lower peak values and broader distributions of the relative angles between neighboring S−Hg−S bonds and between the xy-projections of alkyl tails (Figure S3) that the islands of shorter thiols possess a larger degree of translational and orientational disorder. Figure 5a shows the distributions of tilt angles for short and long thiol molecules. The average tilt angle, θ, reduces from 39° for octadecanethiols to 35° for dodecanethiols in the islands of standing thiols. The broad peaks around 90° are due to the laying-down phase and Hg surface corrugations that serve for the wide scatter of the molecular axes orientations of laying thiols. [For such system sizes (29.808 × 30.312 nm2), corrugations due to capillary waves may reach several angstroms.] Furthermore, we treated a R−S−Hg−S−R molecule as a standing one if both of its alkyl tails were not tilted by more than 45° to the surface normal. This criterion

Figure 3. Behavior of docosanes on liquid Hg: (a) docosane conformations for selected values of σ and systems with lateral dimensions of 10 nm; (b) docosane conformations for σ = 1.5 nm−2 and system with lateral dimensions of 30 nm; and (c) evolution of density profiles of carbon atoms in the docosane self-assembled film on Hg as a function of the increasing surface coverage for the system with lateral dimensions of 10 nm. The thin solid black line is the Hg interface and is shown as a reference.

angle between the premelted docosanes at the crystalline interface and the respective monolayer of laying-down molecules. This suggestion is confirmed by the simulation of a docosane self-assembled film with the same surface coverage (σ = 1.5 nm2) but larger cross-section area of 30 × 30 nm−2 (Figure 3b). For this system, we detect the formation of the seven-layer docosane crystallites as is also evident from the side views of the system (Figure S1). We expect a further growth in height of the 3D dodecane crystals after the complete coverage of the surface with the crystallites of seven layers thickness will be attained. These observations of the bulk-like crystalline phases of docosanes on liquid Hg surface are also confirmed experimentally.37 However, experiments suggest the formation of two additional layers of laying-down molecules prior to seeing the onset of the crystalline bulk phase, which emphasizes the intricate structure of longer alkanes on liquid Hg and the need for further theoretical as well as experimental studies of these films. Similar to n-alkanes, the nucleation of bulk phases on physisorbed laying-down layers of short-chain n-alkanes was also detected on silver at lower temperatures.6 These experiments demonstrated that the laying-down as well as the bulk phases of n-alkanes had a long-range order epitaxially imposed by the silver substrate. Highly ordered lamellar monolayers of n-alkanes were also found on gold.4,5 In comparison, while we do see short-range (around five alkanes) correlations in the molecular orientations of laying-down n1549


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Figure 4. Behavior of octadecanethiols on liquid Hg: (a) octadecanethiol conformations for selected values of σ; (b) evolution of density profiles and surface density distributions (inset) of carbon units in the octadecanethiol self-assembled film on Hg as a function of the increasing surface coverage; and (c) schematic depiction of the alkylthiol structure in the standing phase on liquid Hg. The x-axis is aligned along the S−Hg−S bond of a selected molecule. Rnn1, Rnn2, and Rnn3 are the 2D vectors connecting the selected bound Hg atom with its first, second, and third nearest neighbors, respectively. α1, α2, and α3 (not shown) are the angles between the x-axis and Rnn1, Rnn2, and Rnn3 vectors, respectively. Tilt angle, θ, and azimuthal tilt direction angle, φ, are defined as an angle between the vector connecting S and CH3 groups of the same thiol and its projection onto the z-axis, and an angle between its projection onto the xy-plane and x-axis, respectively.

Figure 5. Characteristics of alkylthiol packing on liquid mercury. (a) Distribution of tilt angles for octadecanethiols (black squares) and dodecanethiol (blue circles). (b) 2D radial distribution functions for bound Hg atoms in the islands of standing thiols. (c) Angular distributions of bound Hg atoms in the first, second, and third coordination spheres and tilt directions in islands of standing thiols. Distributions are symmetric around 90°. (d) Packing of S−Hg−S bonds in the island of standing octadecanethiols for the overall molecular surface coverage of 1.61 nm−2. The inset demonstrates the corresponding unit cell of the headgroups.

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The Journal of Physical Chemistry Letters yielded around 51% and 27% of the standing alkylthiols at σ = 1.61 nm−2 for octadecanethiols and dodecanethiols, respectively. The distributions of the azimuthal angle, φ, are shown in Figure 5c. The lower peak and slightly broader distribution of dodecanethiols compared to octadecanethiols confirm the previous indication of a larger degree of disorder in the dodecanes islands of the “standing” phase. This observation might also depend on the size of the islands. The xy-projection of an alkyl tail creates an angle of 75.1° and 81.2° with the S− Hg−S bond in the case of octadecanethiols and dodecanethiols, respectively. From the distributions we also see that alkyl tails tilt very close in the direction of the first nearest-neighbors. In both cases, the packing of bound Hg atoms in the standing phase, and thus the packing of the sulfur headgroups as well, is virtually identical, as is clearly seen from the 2D radial distribution functions (Figure 5b) and angular distributions of the first, second, and third nearest neighbors (Figure 5c). Figure 5d shows a typical arrangement of the S−Hg−S bonds in an island of “standing molecules” obtained from MD simulations in the case of octadecanethiols and σ = 1.61 nm−2. The bound Hg atoms, and thus the sulfurs, in the islands of standing thiols form an oblique 2D lattice with the base vectors of 8.2 and 5.82 Å and an angle of 97.22° between the base vectors. This results in the unit cell area of 47.34 Å2 and two alkylthiols per unit cell, as is clearly seen from the inset in Figure 5d. Hence, one obtains the octadecanethiol surface coverage of 4.22 molecules/nm2 in the densely packed “standing” island. This agrees very well with the abovementioned experimental value of 4.3 nm−2 of a completely assembled monolayer of standing thiols. The fact that we have recovered experimentally observed coexistence of standing and laying-down phases of alkythiols with our simulation model, where thiols were chemisorbed in both phases, may suggest that the first stage of the adsorption of thiols on liquid Hg, the physisorption, does not last very long (or is absent) and the chemisorption of the sulfur headgroups onto mercury sets in much faster compared to the adsorption on gold, where the transition from laying-down to standing thiolate phase is associated with the formation of chemical bonds between sulfur headgroups and gold atoms.3 This can be due to high mobility of Hg atoms compared to gold, and due to which the right mutual orientations of the electronic orbitals of mercury and sulfur atoms responsible for the chemical bonding are more likely to be achieved than those of gold and sulfur atoms. This supposition is strongly supported by recent experimetal results, in which we have shown that the twostage adsoption model involving physi- and chemisorption of thiols on Hg (as two main rate-limiting steps) failed to explain the experimental observations, whereas a simple diffusionlimited Langmuir adsorption appeared to be superior in interpreting the results.38 In summary, our simulations reproduce well experimentally observed coexistences of laying-down and standing tilted alkylthiols as well as the number of phases of normal alkanes on liquid Hg surface and provide insights into the structure of those phases, such as unit cell characteristics and molecular orientations. Our model enabled us to directly access molecular level features of these self-assembled phases. We found the condensated 2D phase of thiols and n-alkanes in the lowcoverage regime rather than the 2D gas phase conjectured from simple fits in the experiments. We have also captured the coexistence of laying-down normal alkanes with their bulk-like phases at the interface with liquid Hg, as well as the difference

between bulk phases (liquid droplets vs nanocrystallites) as function of alkyl length. At the same time we observed that the phases of alkylthiols of various alkyl chain lengths on liquid Hg are qualitatively similar. While these observations agree very well with experiments, our simulations also indicate that a better understanding of the fine structure of longer alkanes on liquid Hg requires further investigation. When a Hg droplet with a thiol SAM on it is pressed against an opposing electrode, the Hg droplet increases its surface that is accessible to the SAM molecules. As is evident from our simulations, in an extreme case, the thiol monolayer might drastically change its phase behavior and transform into a condensated phase of laying-down molecules, which, in turn, would expose the bare surface of mercury and therefore increase the danger of a short circuit. The present work sets the foundation for future theoretical studies of the self-assembly of organic films on liquid metals, which provide a very rich and intricate phase behavior, as we discussed above. Qualitatively different packings of standing thiols on mercury and gold surfaces may result in emerging defects at the interfaces, where these monolayers are brought into contact, which would deteriorate the performance of such junctions. Alternatively, one could expect that a thiol monolayer on liquid mercury surface is laterally much more mobile compared to the one on a crystalline gold substrate. Consequently, such a monolayer could adopt the structure of the opposing monolayer on gold, which would promote a better quality of such molecular junctions. Such an effect might for example depend on the length difference of thiols in the opposing monolayers of the molecular junction. Another interesting aspect for future studies is what happens with the Hg−thiol−Au (or any other crystalline second electrode) junction under external stress conditions, because in experiments the junction properties are controlled via mechanically manipulating a Hg drop electrode. Additionally, the extension of the current work to bioorganic surfactants, which were intensively studied on crystalline substrates,24 also represents an interesting topic.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b00494. Molecular force fields, simulation protocols, the details of systems set up, and additional figures on the structural properties of alkylthiols and n-alkanes (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank B. Pokroy for stimulating discussions. A.I. and M.M. acknowledge the financial support from the Volkswagen Foundation within the joint German-Israeli program under Grant VW-ZN2726 and the GWDG Göttingen, the HLRN Hannover/Berlin and the von Neumann Institute for the computational resources. D.B. acknowledges the financial support from the Alexander von Humboldt Foundation through an Experienced Researcher Fellowship and the 1551


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(20) Hautman, J.; Klein, M. L. Simulation of a Monolayer of Alkyl Thiol Chains. J. Chem. Phys. 1989, 91, 4994−5001. (21) Heinz, H.; Castelijns, H. J.; Suter, U. W. Structure and Phase Transitions of Alkyl Chains on Mica. J. Am. Chem. Soc. 2003, 125, 9500−9510. (22) Heinz, H.; Vaia, R. A.; Farmer, B. L. Relation between Packing Density and Thermal Transitions of Alkyl Chains on Layered Silicate and Metal Surfaces. Langmuir 2008, 24, 3727−3733. (23) Heinz, H.; Vaia, R. A.; Farmer, B. L.; Naik, R. R. Accurate Simulation of Surfaces and Interfaces of Face-Centered Cubic Metals Using 12−6 and 9−6 Lennard-Jones Potentials. J. Phys. Chem. C 2008, 112, 17281−17290. (24) Heinz, H.; Ramezani-Dakhel, H. Simulations of inorganicbioorganic interfaces to discover new materials: insights, comparisons to experiment, challenges, and opportunities. Chem. Soc. Rev. 2016, 45, 412−448. (25) Weiss, E. A.; Kriebel, J. K.; Rampi, M.-A.; Whitesides, G. M. The Study of Charge Transport through Organic Thin Films: Mechanism, Tools and Applications. Philos. Trans. R. Soc., A 2007, 365, 1509− 1537. (26) Zhu, L.; Popoff, R. T. W.; Yu, H.-Z. Metastable Molecular Metal-Semiconductor Junctions. J. Phys. Chem. C 2015, 119, 1826− 1831. (27) Babayco, C. B.; Chang, P.; Land, D. P.; Kiehl, R. A.; Parikh, A. N. Evolution of Conformational Order during Self-Assembly of nAlkanethiols on Hg Droplets: An Infrared Spectromicroscopy Study. Langmuir 2013, 29, 8203−8207. (28) Seitz, O.; Dai, M.; Aguirre-Tostado, F. S.; Wallace, R. M.; Chabal, Y. J. Copper-Metal Deposition on Self Assembled Monolayer for Making Top Contacts in Molecular Electronic Devices. J. Am. Chem. Soc. 2009, 131, 18159−18167. (29) Kiehl, R. A.; Le, J. D.; Candra, P.; Hoye, R. C.; Hoye, T. R. Charge Storage Model for Hysteretic Negative-Differential Resistace in Metal-Molecule-Metal Junctions. Appl. Phys. Lett. 2006, 88, 172102− 172104. (30) Nitzan, A.; Ratner, M. A. Electron Transport in Molecular Wire Junctions. Science 2003, 300, 1384−1389. (31) Kraack, H.; Tamam, L.; Sloutskin, E.; Deutsch, M.; Ocko, B. M. Alkyl-Thiol Langmuir Films on the Surface of Liquid Mercury. Langmuir 2007, 23, 7571−7582. (32) Ocko, B. M.; Kraack, H.; Pershan, P. S.; Sloutskin, E.; Tamam, L.; Deutsch, M. Crystalline Phases of Alkyl-Thiol Monolayers on Liquid Mercury. Phys. Rev. Lett. 2005, 94, 017802. (33) Kraack, H.; Ocko, B. M.; Pershan, P. S.; Sloutskin, E.; Tamam, L.; Deutsch, M. The Structure and Phase Diagram of Langmuir Films of Alcohols on Mercury. Langmuir 2004, 20, 5386−5395. (34) Kraack, H.; Ocko, B. M.; Pershan, P. S.; Sloutskin, E.; Tamam, L.; Deutsch, M. Fatty Acid Langmuir Films on Liquid Mercury: X-Ray and Surface Tension Studies. Langmuir 2004, 20, 5375−5385. (35) Kraack, H.; Ocko, B. M.; Pershan, P. S.; Tamam, L.; Deutsch, M. Temperature Dependence of the Structure of Langmuir Films of Normal-Alkanes on Liquid Mercury. J. Chem. Phys. 2004, 121, 8003− 8009. (36) Kaganer, V. M.; Möhwald, H.; Dutta, P. Structure and phase transitions in Langmuir monolayers. Rev. Mod. Phys. 1999, 71, 779− 819. (37) Kraack, H.; Ocko, B. M.; Pershan, P. S.; Sloutskin, E.; Deutsch, M. Langmuir Films of Normal-Alkanes on the Surface of Liquid Mercury. J. Chem. Phys. 2003, 119, 10339−10349. (38) Haimov, B.; Iakovlev, A.; Glick-Carmi, R.; Bloch, L.; Rich, B.; Müller, M.; Pokroy, B. Kinetics of nano-scale self-assembly measured on liquid drops by macroscopic optical tensiometry: from mercury to water and fluorocarbons. J. Am. Chem. Soc. 2016, 138, 2585−2591. (39) Escobedo, F. A.; Chen, Z. Liquid crystalline behavior of a semifluorinated oligomer. J. Chem. Phys. 2004, 121, 11463−11473. (40) Amat, M. A.; Rutledge, G. C. Liquid-vapor equilibria and interfacial properties of n-alkanes and perfluoroalkanes by molecular simulation. J. Chem. Phys. 2010, 132, 114704.

University of Utah Center for High Performance Computing for computational resources.

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Alkyl-Based Surfactants at a Liquid Mercury Surface: Computer

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