Gold/Benzenedithiolate/Gold Molecular Junction: A Driven Dynamics Simulation on Structural Evolution and Breaking Force under Pulling

Dynamic evolutions of molecular binding structures and breaking forces of gold/thiolate molecular junctions under pulling are still not well understood. We perform driven dynamics simulations to show that there are essentially two distinct breaking force traces corresponding to the Au–Au and Au–S bond ruptures. The latter is attributed to the formation and breaking of an additional “–Au–SR–Au–” unit in the molecular junction. The force histogram shows two force quanta at 1.5 and 2.0 nN, corresponding to the Au–Au and Au–S bond breaking. Our findings provide new molecular insights into the gold–thiolate interactions. The intermediate metal–molecule–metal binding structures could be used for further molecular transport calculations

Methane Aqueous Fluids in Montmorillonite Clay Interlayer under Near-Surface Geological Conditions: A Grand Canonical Monte Carlo and Molecular Dynamics Simulation Study

The grand-canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations are performed to investigate the methane aqueous fluids in Na-montmorillonite clay interlayer under near-surface geological temperature and pressure conditions (<i>T</i> = 300 K and <i>P</i> = 20–50 bar). The chemical potentials of water and methane under these <i>T</i>/<i>P</i> conditions are calculated using the Widom’s insertion method. These chemical potentials are used in the GCMC simulations to determine the contents of different species in the clay interlayer, especially in those that correspond to the equilibrium stable spacing distances. Simulation results show that initial clay swelling is dominated by water adsorption into the clay interlayer, followed by the intercalation of methane as the basal spacing increases. However, it is found that this methane intercalation process is strongly influenced by the relative humidity and the total gas pressure of the system. High relative humidity may facilitate water molecules entering the clay interlayer region and inhibit the intercalation of methane molecules. MD simulations show that sodium ions are fully hydrated by water molecules and clay surface oxygen atoms, while methane molecules are not fully coordinated. This situation is attributed to the less water content in clay interlayer and the subsequent formation of methane dimer or trimer clusters due to the hydrophobic nature of small hydrocarbon molecules

Gold/Benzenedithiolate/Gold Molecular Junction: A Driven Dynamics Simulation on Structural Evolution and Breaking Force under Pulling

Dynamic evolutions of molecular binding structures and breaking forces of gold/thiolate molecular junctions under pulling are still not well understood. We perform driven dynamics simulations to show that there are essentially two distinct breaking force traces corresponding to the Au–Au and Au–S bond ruptures. The latter is attributed to the formation and breaking of an additional “–Au–SR–Au–” unit in the molecular junction. The force histogram shows two force quanta at 1.5 and 2.0 nN, corresponding to the Au–Au and Au–S bond breaking. Our findings provide new molecular insights into the gold–thiolate interactions. The intermediate metal–molecule–metal binding structures could be used for further molecular transport calculations

Gold/Benzenedithiolate/Gold Molecular Junction: A Driven Dynamics Simulation on Structural Evolution and Breaking Force under Pulling

Dynamic evolutions of molecular binding structures and breaking forces of gold/thiolate molecular junctions under pulling are still not well understood. We perform driven dynamics simulations to show that there are essentially two distinct breaking force traces corresponding to the Au–Au and Au–S bond ruptures. The latter is attributed to the formation and breaking of an additional “–Au–SR–Au–” unit in the molecular junction. The force histogram shows two force quanta at 1.5 and 2.0 nN, corresponding to the Au–Au and Au–S bond breaking. Our findings provide new molecular insights into the gold–thiolate interactions. The intermediate metal–molecule–metal binding structures could be used for further molecular transport calculations

Molecular Understanding of CO₂ and H₂O in a Montmorillonite Clay Interlayer under CO₂ Geological Sequestration Conditions

Grand canonical Monte Carlo (GCMC) simulations are carried out to investigate a supercritical carbon dioxide (scCO₂)-water mixture in the Na-montmorillonite clay interlayer under typical CO₂ geological sequestration conditions (<i>T</i> = 323 K, <i>P</i> = 90 bar and <i>T</i> = 348 K, <i>P</i> = 130 bar). The stable clay interlayer distances at different relative humidity (RH) are determined based on the normal pressure and free energy curves of the CO₂–H₂O–Na⁺ complex in the montmorillonite clay interlayer. Simulation results show that stable monolayer hydrates (1W) with a basal spacing around 12 Å are formed at RH = 30–60%. As RH is increased to 70% and above, bilayer CO₂–H₂O mixtures with a basal spacing around 15–16 Å (2W) are more stable. In general, the CO₂ intercalation process is strongly influenced by RH. While a high relative humidity facilitates water molecules entering the clay interlayer, it nonetheless decreases CO₂ intercalations. The sorbed H₂O concentrations from our simulations compare remarkably well with the in situ infrared (IR) spectroscopy experimental data by Loring et al. [<i>Langmuir</i>, <b>2014</b>, <i>30</i>, 6120–6128], if the continuous experimental curve is considered as the “smear-out” of the stepwise curve from our simulations. However, the overall sorbed CO₂ concentrations from our simulations are higher than the IR experimental results. We attribute these discrepancies in both sorbed H₂O and CO₂ concentrations (measured from experiments and simulations) to the complexity of hydrated clay particles in the IR spectroscopy experiment, to which the hydration-heterogeneity model could provide a reasonable interpretation. Molecular dynamics (MD) simulations show that the hydration state of CO₂ molecules is changed from the partial hydration in 1W to the full hydration in 2W with the increase in RH, and CO₂ dimers are frequently seen in both 1W and 2W hydration states. CO₂ dimers largely take the slipped parallel configurations, while the remaining dimers take the perpendicular T-shaped geometry. Further, sodium ions in the interlayer tend to be fully hydrated by water molecules due to their relatively large hydration energy. Moreover, we find that CO₂ molecules hardly migrate into the first hydration shell of sodium ions. The overall diffusion coefficients of CO₂ molecules are larger than those of water molecules and sodium ions. This comparably high mobility of CO₂ molecules in the clay interlayer, together with the low probability of CO₂ participation in the first hydration shell of Na⁺ ions, essentially prevents CO₂ and Na⁺ from direct interactions in clay interlayers

Gold/Benzenedithiolate/Gold Molecular Junction: A Driven Dynamics Simulation on Structural Evolution and Breaking Force under Pulling

Dynamic evolutions of molecular binding structures and breaking forces of gold/thiolate molecular junctions under pulling are still not well understood. We perform driven dynamics simulations to show that there are essentially two distinct breaking force traces corresponding to the Au–Au and Au–S bond ruptures. The latter is attributed to the formation and breaking of an additional “–Au–SR–Au–” unit in the molecular junction. The force histogram shows two force quanta at 1.5 and 2.0 nN, corresponding to the Au–Au and Au–S bond breaking. Our findings provide new molecular insights into the gold–thiolate interactions. The intermediate metal–molecule–metal binding structures could be used for further molecular transport calculations