Free energy decompositions illuminate synergistic effects in interfacial binding thermodynamics of mixed surfactant systems

Recent vibrational sum frequency generation spectroscopic experiments [Sengupta et al., J. Phys. Chem. Lett. 13, 11391-11397 (2022)] demonstrated synergistic interfacial adsorption effects between the anionic dodecyl sulfate (DS-) and the polar, but charge-neutral hexaethylene glycol monododecyl ether (C12E6), surfactants. In this study, the interfacial adsorption thermodynamics underlying these synergistic effects are analyzed through free energy decompositions. A general decomposition method utilizing alchemical intermediate states is outlined. Combining free energy decompositions with the potential distribution theorem illuminates the statistical interpretations of correlated effects between different system components. This approach allows for the identification of the physical effects leading to synergistic adsorption thermodynamics of DS- binding to the air-C12E6-water interface. The binding properties are found to result from a combination of effects predominantly including energetic van der Waals stabilization between DS- and C12E6, as well as competing energetic and entropic effects due to changes in the interfacial water structure as a result of introducing a C12E6 monolayer into the bare air-water interface.

The behavior of methane-water mixtures under elevated pressures from simulations using many-body potentials

Non-polarizable empirical potentials have been proven to be incapable of capturing the mixing of methane-water mixtures at elevated pressures. Although density functional theory-based ab initio simulations may circumvent this discrepancy, they are limited in terms of the relevant time and length scales associated with mixing phenomena. Here, we show that the many-body MB-nrg potential, designed to reproduce methane-water interactions with coupled cluster accuracy, successfully captures this phenomenon up to 3 GPa and 500 K with varying methane concentrations. Two-phase simulations and long time scales that are required to fully capture the mixing, affordable due to the speed and accuracy of the MBX software, are assessed. Constructing the methane-water equation of state across the phase diagram shows that the stable mixtures are denser than the sum of their parts at a given pressure and temperature. We find that many-body polarization plays a central role, enhancing the induced dipole moments of methane by 0.20 D during mixing under pressure. Overall, the mixed system adopts a denser state, which involves a significant enthalpic driving force as elucidated by a systematic many-body energy decomposition analysis.

Real time chemical imaging of a working catalytic membrane reactor during oxidative coupling of methane

We report the results from an operando XRD-CT study of a working catalytic membrane reactor for the oxidative coupling of methane. These results reveal the importance of the evolving solid state chemistry during catalytic reaction, particularly the chemical interaction between the catalyst and the oxygen transport membrane.