Entropic Stabilization of Water at Graphitic Interfaces

Metal-water interface formation: Thermodynamics from ab initio molecular dynamics simulations

Metal-water interfaces are central to many electrochemical, (electro)catalytic, and materials science processes and systems. However, our current understanding of their thermodynamic properties is limited by the scarcity of accurate experimental and computational data and procedures. In this work, thermodynamic quantities for metal-water interface formation are computed for a range of FCC(111) surfaces (Pd, Pt, Au, Ag, Rh, and PdAu) through extensive density functional theory based molecular dynamics and the two-phase entropy model. We show that metal-water interface formation is thermodynamically favorable and that most metal surfaces studied in this work are completely wettable, i.e., have contact angles of zero. Interfacial water has higher entropy than bulk water due to the increased population of low-frequency translational modes. The entropic contributions also correlate with the orientational water density, and the highest solvation entropies are observed for interfaces with a moderately ordered first water layer; the entropic contributions account for up to ∼25% of the formation free energy. Water adsorption energy correlates with the water orientation and structure and is found to be a good descriptor of the internal energy part of the interface formation free energy, but it alone cannot satisfactorily explain the interfacial thermodynamics; the interface formation is driven by the competition between energetic and entropic contributions. The obtained results and insight can be used to develop, parameterize, and benchmark theoretical and computational methods for studying metal-water interfaces. Overall, our study yields benchmark-quality data and fundamental insight into the thermodynamic forces driving metal-water interface formation.

Proton transfer and regulation across chemical interfaces by small-molecule assemblies

We report on measurements and control of proton gradient across interfaces of water and dichloroethane. Such interfaces are interesting as mimics of biological membranes. We use impedance spectroscopy to quantify interfacial proton gradient and identify proton transfer modes. We quantify proton movement using reciprocal of time constant (τ-1 ) acquired from electrochemical impedance modeling. We show that proton gradient across interfaces of water/dichloroethane and τ-1 correlate with the aqueous phase pH, changing from ca. 1 s-1 at pH 1 to 0.2 s-1 at pH 7. τ-1 changes in the presence of proton shuttling fat-soluble molecules. Dinitrophenol acts as a pH activated proton coupler which is active at around neutral pH and inert at pH <4. However, quinone type cofactors change the interfacial proton transport when activated by redox reactions with ferrocene type molecules, such as decamethyl ferrocence (DMFc). Quinone type cofactors show distinct features in their impedance response assigned to a proton coupled electron transfer (PCET) process, different from the uncoupled proton transfer activity of dinitrophenol. The observed PCET reaction significantly changes τ-1 . We use τ-1 as a proton transport descriptor. In particular, CoQ10 -DMFc shows a τ-1 of 3.5 s-1 at pH 7, indicating how small-molecule assemblies change proton availability.

Computational Strategies for Entropy Modeling in Chemical Processes

Computational simulations of entropy are important in understanding the thermodynamic forces that drive chemical reactions on a molecular scale. In recent years, various algorithms have been developed and applied in conjunction with molecular modeling techniques to evaluate the change of entropy in solvation, hydrophobic interactions, and chemical reactions. The aim of this review is to highlight four specific computational entropy calculation methods: normal mode analysis, free volume theory, two-phase thermodynamics, and configurational entropy modeling. The technical aspects, applications, and limitations of each method will be discussed in detail.

Nature of the Electrical Double Layer on Suspended Graphene Electrodes

The structure of interfacial water near suspended graphene electrodes in contact with aqueous solutions of Na₂SO₄, NH₄Cl, and (NH₄)₂SO₄ has been studied using confocal Raman spectroscopy, sum frequency vibrational spectroscopy, and Kelvin probe force microscopy. SO₄^2– anions were found to preferentially accumulate near the interface at an open circuit potential (OCP), creating an electrical field that orients water molecules below the interface, as revealed by the increased intensity of the O–H stretching peak of H-bonded water. No such increase is observed with NH₄Cl at the OCP. The intensity of the dangling O–H bond stretching peak however remains largely unchanged. The degree of orientation of the water molecules as well as the electrical double layer strength increased further when positive voltages are applied. Negative voltages on the other hand produced only small changes in the intensity of the H-bonded water peaks but affected the intensity and frequency of dangling O–H bond peaks. The TOC figure is an oversimplified representation of the system in this work.