Transferable Gaussian Attractive Potentials for Organic/oxide Interfaces

Organic/oxide interfaces play an important role in many areas of chemistry, and in particular for lubrication and corrosion. Molecular dynamics simulations are the method of choice for providing complementary insight to experiments. However, the force fields used to simulate the interaction between molecules and oxide surfaces tend to capture only weak physisorption interactions, discarding the stabilizing Lewis acid/base interactions. We here propose an improvement of the usual molecular mechanics description (based on Lennard-Jones and electrostatic interactions) by addition of an attractive Gaussian potential between reactive sites of the surface and heteroatoms of adsorbed organic molecules, leading to the GLJ potential. The interactions of four oxygenated and four amine molecules with the typical and widespread hematite and γ-alumina surfaces are investigated. The total RMSD for all probed molecules decreases from 29.2 to 5.7 kcal/mol, and the corresponding percentage from 107.4 to 22.6% over hematite, while on γ-alumina the RMSD decreases from 21.5 to 7.6 kcal/mol, despite using a single parameter for all five chemically inequivalent surface aluminum atoms. Applying GLJ to the simulation of n-octadecanamine and N-tetradecyldiethanolamine adsorbed films on hematite and alumina respectively demonstrates that mobility of the surfactants is overestimated by the common LJ potential, while GLJ shows a strong structuration and slow dynamics of the surface films, as could be expected from the first-principles adsorption energies for model head-groups.

Transferable Gaussian Attractive Potentials for Organic/oxide Interfaces

Organic/oxide interfaces play an important role in many areas of chemistry, and in particular for lubrication and corrosion. Molecular dynamics simulations are the method of choice for providing complementary insight to experiments. However, the force fields used to simulate the interaction between molecules and oxide surfaces tend to capture only weak physisorption interactions, discarding the stabilizing Lewis acid/base interactions. We here propose an improvement of the usual molecular mechanics description (based on Lennard-Jones and electrostatic interactions) by addition of an attractive Gaussian potential between reactive sites of the surface and heteroatoms of adsorbed organic molecules, leading to the GLJ potential. The interactions of four oxygenated and four amine molecules with the typical and widespread hematite and γ-alumina surfaces are investigated. The total RMSD for all probed molecules decreases from 29.2 to 5.7 kcal/mol, and the corresponding percentage from 107.4 to 22.6% over hematite, while on γ-alumina the RMSD decreases from 21.5 to 7.6 kcal/mol, despite using a single parameter for all five chemically inequivalent surface aluminum atoms. Applying GLJ to the simulation of n-octadecanamine and N-tetradecyldiethanolamine adsorbed films on hematite and alumina respectively demonstrates that mobility of the surfactants is overestimated by the common LJ potential, while GLJ shows a strong structuration and slow dynamics of the surface films, as could be expected from the first-principles adsorption energies for model head-groups.

Bulk supercooled water versus adsorbed films on silica surfaces: specific heat by Monte Carlo simulation

Between 150 and 230.6 K, bulk supercooled water freezes upon cooling, and amorphous ice crystallizes upon heating: bulk water thus exists only in its stable ice form. To circumvent this...

Organic monolayers disrupt plastic flow in metals

Adsorbed films often influence mechanical behavior of surfaces, leading to well-known mechanochemical phenomena such as liquid metal embrittlement and environment-assisted cracking. Here, we demonstrate a mechanochemical phenomenon wherein adsorbed long-chain organic monolayers disrupt large-strain plastic deformation in metals. Using high-speed in situ imaging and post facto analysis, we show that the monolayers induce a ductile-to-brittle transition. Sinuous flow, characteristic of ductile metals, gives way to quasi-periodic fracture, typical of brittle materials, with 85% reduction in deformation forces. By independently varying surface energy and molecule chain length via molecular self-assembly, we argue that this “embrittlement” is driven by adsorbate-induced surface stress, as against surface energy reduction. Our observations, backed by modeling and molecular simulations, could provide a basis for explaining diverse mechanochemical phenomena in solids. The results also have implications for manufacturing processes such as machining and comminution, and wear.

When Thermodynamic Properties of Adsorbed Films Depend on Size: Fundamental Theory and Case Study

Small system properties are known to depend on geometric variables in ways that are insignificant for macroscopic systems. Small system considerations are therefore usually added to the conventional description as needed. This paper presents a thermodynamic analysis of adsorbed films of any size in a systematic and general way within the framework of Hill’s nanothermodynamics. Hill showed how to deal with size and shape as variables in a systematic manner. By doing this, the common thermodynamic equations for adsorption are changed. We derived the governing thermodynamic relations characteristic of adsorption in small systems, and point out the important distinctions between these and the corresponding conventional relations for macroscopic systems. We present operational versions of the relations specialized for adsorption of gas on colloid particles, and we applied them to analyze molecular simulation data. As an illustration of their use, we report results for CO2 adsorbed on graphite spheres. We focus on the spreading pressure, and the entropy and enthalpy of adsorption, and show how the intensive properties are affected by the size of the surface, a feature specific to small systems. The subdivision potential of the film is presented for the first time, as a measure of the film’s smallness. For the system chosen, it contributes with a substantial part to the film enthalpy. This work can be considered an extension and application of the nanothermodynamic theory developed by Hill. It provides a foundation for future thermodynamic analyses of size- and shape-dependent adsorbed film systems, alternative to that presented by Gibbs.