Langmuir films of normal-alkanes on the surface of liquid mercury

Molecular Dynamics Simulation of Alkylthiol Self-Assembled Monolayers on Liquid Mercury

We report computer simulation of the self-assembly of alkylthiols on the surface of liquid mercury. Here we focus mainly on the alkylthiol behavior on mercury as a function of the surfactant surface coverage, which we study by means of large-scale molecular dynamics simulations of the equilibrium structure at room temperature. The majority of the presented results are obtained for octa- and dodecanethiol surfactants. This topic is particularly interesting because the properties of the alkylthiol self-assembled monolayers on liquid mercury are relevant for practical applications (e.g., in organic electronics) and can be controlled by mechanically manipulating the monolayer, i.e., by changing its structure. Our computer simulation results shed additional light on the alkylthiol self-assembly on liquid mercury by revealing the coexistence of a dense agglomerated laying-down alkylthiols with a very dilute 2D vapor on mercury surface rather than a single vapor phase in the low surface coverage regime. In the regimes of the high surface coverage we observe the coexistence of the laying-down liquid phase and crystalline phases with alkylthiols standing tilted at a sharp angle to the surface normal, which agrees with the phase behavior previously seen in X-ray and tensiometry experiments. We also discuss the influence of finite-size effects, which one inevitably encounters in molecular simulations. Our findings agree well with the general predictions of the condensation/evaporation theory for finite systems. The temperature dependence of the stability of the crystalline alkylthiol phases and details of the surfactant chemical binding to the surface are discussed. The equilibrium structure of the crystalline phase is investigated in detail for the alkylthiols of various tail lengths.

Alkyl-Based Surfactants at a Liquid Mercury Surface: Computer Simulation of Structure, Self-Assembly, and Phase Behavior

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.

Liquid-Mercury-Supported Langmuir Films of Ionic Liquids: Isotherms, Structure, and Time Evolution

Ionic liquids have been intensively developed for the last few decades and are now used in a wide range of applications, from electrochemistry to catalysis and nanotechnology. Many of these applications involve ionic liquid interfaces with other liquids and solids, the subnanometric experimental study of which is highly demanding, and has been little studied to date. We present here a study of mercury-supported Langmuir films of imidazolium-based ionic liquids by surface tensiometry and X-ray reflectivity. The charge-delocalized ionic liquids studied here exhibit no 2D lateral order but show diffuse surface-normal electron density profiles exhibiting gradual mercury penetration into the ionic liquid film, and surface-normal structure evolution over a period of hours. The effect of increasing the nonpolar alkyl chain length was also investigated. The results obtained provide insights into the interactions between these ionic liquids and liquid mercury and about the time evolution of the structure and composition of their interface.

Two-dimensional order in mercury-supported langmuir films of fatty diacids

The structure of mercury-supported Langmuir films of dicarboxylic acid molecules with 13 ≤ n ≤ 22 carbons is studied by X-ray methods and surface tensiometry. The molecules lie surface-parallel, forming mono-, bi-, or trilayers, depending on coverage. All films exhibit a full 2D order of the same single-molecule oblique unit cell. In particular, the distinct odd-even structure difference of 3D crystals of the same molecules is not observed. The unit cell's width and angle show a small systematic decrease with n, while the length increases commensurately with the molecular length. These results show the films to consist of closely packed, extended, polymer-like chains of diacid molecules, bound by their carboxyl end groups. Evidence is presented for the inclusion of a single mercury atom in the carboxyl-carboxyl bond. The possible conformation of this bond and implications of the parity-independent structure are discussed.

Langmuir films of chiral molecules on mercury

Homo- and heterochiral Langmuir films of a chiral derivative of stearic acid are studied in situ on the surface of liquid mercury as a function of surface coverage by surface tensiometry and surface-specific synchrotron X-ray diffraction and reflectivity. A transition from a phase of surface-parallel molecules to a phase of standing-up molecules is found. The former shows no surface-parallel long-range order. The standing-up phase of both homochiral and heterochiral compositions exhibit long-range order. However, the former has an oblique unit cell with parallel molecular planes, and the later has a centered rectangular unit cell with a herringbone molecular packing. For both cases, the standing-up molecules are tilted by 44 degrees from the surface normal and pack at a density of 19.5 A(2)/molecule in the plane normal to the molecular long axis. Important differences are found, and discussed, between this behavior and that of a Langmuir film of the nonchiral stearic acid on mercury.

Alkyl-thiol Langmuir films on the surface of liquid mercury

The coverage dependent phase behavior of monolayers of alkyl thiols (CH3(CH2)(n-1)SH, denoted as CnSH) on mercury was studied for chain lengths 9 <or= n <or= 22, using surface tensiometry and surface-specific X-ray scattering methods. At low coverage, a disordered single layer of surface-parallel molecules is found for all n. At high coverage, a monolayer of standing-up molecules is formed, exhibiting well-ordered phases, the structure of which is n- and coverage-dependent. The molecular chains pack in a centered rectangular unit cell, with an approximately 27 degrees tilt from the surface normal toward nearest neighbors. The strong sulfur-mercury bond induces a noncentered unit cell for the headgroups, incorporating one mercury atom per two thiol molecules. The small but significant differences in structure of these films on gold and on mercury are discussed and assigned to the different structure of the subphase: long-range-ordered crystal for gold and short-range-ordered liquid for mercury.

Studying electron transfer through alkanethiol self-assembled monolayers on a hanging mercury drop electrode using potentiometric measurements

A new approach based on measuring the change of the open-circuit potential (OCP) of a hanging mercury drop electrode (HMDE), modified with alkanethiols of different chain length conducted in a solution containing a mixture of Ru(NH3)6(2+) and Ru(NH3)6(3+) is used for studying electron transfer across the monolayer. Following the time dependence of the OCP allowed the extraction of the kinetic parameters, such as the charge transfer resistance (R(ct)) and the electron transfer rate constant (k(et)), for different alkanethiol monolayers. An electron tunneling coefficient, beta, of 0.9 A(-1) was calculated for the monolayers on Hg.

Structure of Mercaptobiphenyl Monolayers on Mercury

The molecular-scale structure and phase behavior of single-component Langmuir films of 4'-methyl-4-mercaptobiphenyl (MMB) and 4'-perfluoromethyl-4-mercaptobiphenyl (FMMB) on mercury were studied using surface tensiometry, grazing incidence X-ray diffraction, and X-ray reflectivity. At low coverages, a condensed but in-plane disordered single layer of surface-parallel molecules is found for both compounds. At high coverages, both compounds exhibit in-plane-ordered phases of standing-up molecules. For MMB, the biphenyl core dominates the structure, yielding a centered-rectangular unit cell with an area A(x) of 21.8 A(2)/molecule, with molecules tilted by approximately 14 degrees from the surface normal in the nearest-neighbor direction, and a coherence length xi of >1000 A for the crystalline domains. For FMMB, the perfluoromethyl group dominates the structure, yielding a hexagonal unit cell with untilted molecules, an area A(x) of 24.2 A(2)/molecule, and a much smaller xi of approximately 110 A. The structure is discussed in comparison with self-assembled monolayers of MMB on crystalline Au(111) and similar-length alkanethiolate SAMs on Au(111) and on mercury. The differences in the structure are discussed and traced to the differences in the substrate's surface structure, and in the molecular cross section and rigidity.

Temperature dependence of the structure of Langmuir films of normal-alkanes on liquid mercury

The temperature dependent phase behavior of Langmuir films of n-alkanes [CH3(CH2)(n-2)CH3, denote Cn] on mercury was studied for chain lengths 19< or =n< or =22 and temperatures 15< or =T< or =44 degrees C, using surface tensiometry and surface x-ray diffraction methods. In contrast with Langmuir films on water, where molecules invariably orient roughly surface normal, alkanes on mercury are always oriented surface parallel and show no long-range in-plane order at any surface pressure. A gas and several condensed phases of single, double, and triple layers of lying-down molecules are found, depending on n and T. At high coverages, the alkanes studied here show transitions from a triple to a double to a single layer with increasing temperature. The transition temperature from a double to a single layer is found to be approximately 5 degrees C, lower than the bulk rotator-to-liquid melting temperature, while the transition from a triple to a double layer is about as much below the double-to-single layer transition. Both monolayer and bulk transition temperatures show a linear increase with n with identical slopes of approximately 4.5 degrees C/CH2 within the range of n values addressed here. It is suggested that the film and bulk transitions are both driven by a common cause: the proliferation of gauche defects in the chain with increasing temperature.

The structure and phase diagram of Langmuir films of alcohols on mercury

The coverage-dependent phase behavior of molecular films of alcohols (CH3(CH2)n-2CH2OH, denoted as CnOH) on mercury was studied for chain lengths 8 < or = n < or = 28, using surface tensiometry and surface specific X-ray methods. Phases with surface-normal-oriented molecules are found at high coverage, showing the CS, S, and LS phases found also on water. Phases comprising surface parallel molecules, which do not exist on water, are found here at low coverage. For the lowest coverage a two-dimensional gas phase is found, followed, upon increasing the coverage, by an n-dependent sequence of condensed phases of up to four layers of surface-parallel molecules before converting to the surface-normal phases. In contrast with the surface-normal phases, all of the surface-parallel phases are found to lack long-range order in the surface-parallel direction. Adsorption energies are derived from the phase diagram for the alkyl chain and the alcohol headgroup.

Fatty acid Langmuir films on liquid mercury: X-ray and surface tension studies

The structure and phase behavior of liquid-mercury-supported molecular films of fatty acids (CH3(CH2)n-2COOH, denoted CnOOH) were studied for molecular lengths 7 < or = n < or = 24, by surface tensiometry and X-ray methods. Two qualitatively different film structures were found, depending on coverage. For high coverage, the film consists of a monolayer of roughly surface-normal molecules, showing a pressure-dependent sequence of structures similar, though not identical, to that of the corresponding water-supported Langmuir films. At low coverage, phases consisting of surface-parallel molecules are found, not observed on the aqueous subphases employed to date. In this range, a two-dimensional (2D) gas followed by a single and, for 14 < or = n < or = 24, also by a double layer of surface-parallel molecules is found as coverage is increased. Depending on chain length, the flat-lying phases have a crystalline 2D-ordered, a smectic-like 1D-ordered, or a disordered in-plane structure consisting of molecular dimers. The structure and thermodynamics of the films are discussed.