Probing collective terahertz vibrations of a hydrogen-bonded water network at buried electrochemical interfaces.

The exceptional properties of liquid water such as thermodynamic, physical, and dielectric anomalies originate mostly from the hydrogen-bond networks of water molecules. The structural and dynamic properties of the hydrogen-bond networks have a significant impact on many biological and chemical processes in aqueous systems. In particular, the properties of interfacial water molecules with termination of the network at a solid surface are crucial to understanding the role of water in heterogeneous reactions. However, direct monitoring of the dynamics of hydrogen-bonded interfacial water molecules has been limited because of the lack of a suitable surface-selective spectroscopic means in the terahertz (THz) frequency range where collective vibrations of water exist. Here we show that hydrogen-bond vibrations below 9 THz can be measured in situ at an electrochemical interface, which is buried between two THz-opaque media, by using a density of states format of surface-enhanced inelastic light scattering spectra. The interpretation of the obtained spectra over the range 0.3-6 THz indicates that the negatively charged surface accelerates collective translational motions of water molecules in the lateral direction with the increase of hydrogen-bond defects. Alternatively, the positively charged surface results in suppression of lateral mobility. This work gives a new perspective on in situ spectroscopic investigations in heterogeneous reactions.

Protonation of Strained Epoxy Resin under Wet Conditions via First-Principles Calculations Using the H+-Shift Method.

A significant challenge in adhesive bonding is the accelerated breaking of stretched adhesives under wet conditions, which is known as cohesive failure. One group of commonly used adhesives consists of the amine-cured epoxy resins. Based on deprotonation free-energy calculations of the unstrained resin in water, it has recently been proposed that these adhesives can undergo failure through breakage originating at the protonated amine group under neutral or acidic conditions [J. Phys. Chem. B 2021, 125, 8989-8996]. In this study, we comprehensively investigated the degree of protonation of the amine group under both stretched and compressed conditions by devising a robust first-principles protonation calculation method applicable to strained materials. It was found that the amine group was partially protonated in neutral water at 298 K and that the amine group was protonated when the epoxy resin was stretched to a greater extent in water, and vice versa. These findings support the physicochemical cause of cohesive failure due to protonation of the amine group in the stretched amine-cured epoxy resins.

Pressure-assisted decomposition of tricresyl phosphate on amorphous FeO using hybrid quantum-classical simulations.

The moving components of combustion engines are operated under harsh conditions of high pressures and temperatures. Extreme-pressure anti-wear additives, such as tricresyl phosphate (TCP), are mixed with base oil to prevent wear through the formation of a lubricant film on the substrate. We studied the effect of liquid pressure on the decomposition pathway of TCP in base oil molecules (2,5-dimethylhexane) using hybrid quantum-classical simulations with density functional theory for electrons. At a temperature of 300 K, we found that: (i) bond-breaking barrier energies of both the OC and PO bonds of TCP decrease monotonically as the liquid pressure increases; (ii) the bond-breaking barrier energy of PO is lower than that of OC at pressures of 0 and 2.0 GPa, but is higher at a pressure of 5.0 GPa; and (iii) the applied pressure significantly lowers the bond-breaking barrier energies of both OC and PO when the PO bond of TCP is directed upward from the substrate. These findings are explained by the inhomogeneous distribution of base oil molecules around TCP and the steric repulsion of the PO bond of TCP. These results indicate that the internal structures of the lubricant films are pressure-dependent.

First-Principles Calculations of the Protonation and Weakening of Epoxy Resin under Wet Conditions.

In this study, we investigated the protonation of the amine group in epoxy resins prepared using amine-based curing agents by theoretical methods. Density functional theory (DFT)-based free-energy calculations of the corresponding deprotonation subreactions showed that the amine group of the epoxy resin is protonated at equilibrium depending on the location of the amine group when the epoxy resin is embedded in water under standard conditions. Additional DFT calculations demonstrate that the energetic barrier for breaking the ether bond of the epoxy resin is lowered by about 0.6 eV as a result of the cooperative effect of H2O dissociation and that the transition-state energy for breaking the amine group bond is lowered by about 0.4 eV after the protonation of the amine group. Comparing the transition-state energies, we predict that the bond breakage of the protonated amine groups is the principal process causing the weakening of epoxy resins under wet conditions.

Nonequilibrium molecular dynamics method based on coarse-graining formalism: Application to a nonuniform temperature field system.

A nonequilibrium molecular dynamics method is proposed to produce nonequilibrium states flexibly. In this method, virtual points are set in a simulation box, and coarse-grained physical quantities at these points are constrained using Gauss's principle of least constraint. The coarse-grained physical quantities are evaluated by averaging microscopic quantities with an appropriate weight. To obtain the weight to evaluate the coarse-grained physical quantities, a shape function matrix is initially constructed from the particle configuration. This matrix expresses an interpolation of the physical quantities at particle positions from the coarse-grained quantities at the virtual points. Then, a matrix form of the weight is calculated as the Moore-Penrose pseudoinverse matrix for the shape function matrix. This method is applied to constrain the coarse-grained kinetic energy and produce a nonuniform temperature field in the system. The temperature profile at a nonequilibrium steady state depends on the method for constructing the shape function matrix. In particular, a local temperature coincides with the coarse-grained temperature when the shape function matrix is constructed based on a higher-order interpolation.

Efficient scheme for calculating work of adhesion between a liquid and polymer-grafted substrate.

We propose a method for calculating the work of adhesion between a liquid and solid surface by using molecular simulations. Two ideas are introduced for efficient calculation when the proposed method is applied at the interface between a liquid and a polymer-grafted substrate. First, the liquid molecules are separated from the solid surface based on its shape by placing spherically symmetric potentials around the atoms selected from the substrate and the polymers grafted onto it. Second, to avoid deterioration of accuracy during numerical integration of the work, the parameters that appear in the potential are updated so that variations in the gradient of the work are suppressed. This method is applied to the interface between water and a gold substrate modified by poly(ethylene oxide) (PEO), and it is found that the work of adhesion is greater at intermediate PEO densities.

Self-interaction and slow dynamics.

A simple spin system with self-interaction is proposed and studied where the interaction among spins is taken into account within a mean field model. The self-interaction, which turns out to be a kind of activation energy, gives rise to slow relaxation due to disorder introduced to the self-interaction. In a thermodynamic limit (N-->infinity), a phase diagram is obtained and a spin-spin time correlation function (TCF) is calculated exactly to show that relaxation time diverges at low temperature. Dynamical property of a finite system, such as relaxation of the TCF, is studied by numerically solving a master equation.

Piston dynamics from a microcanonical ensemble.

The dynamical behavior of a system consisting of a (heavy) piston and two rectangular boxes, each containing two hard disks and in contact with the piston, is studied based on a projection operator method for a microcanonical ensemble. We derive a coupled set of nonlinear equations for slow variables of the system and solve it to confirm that our theory with no adjustable parameters reproduces experimental results fairly well. Some limitations of the theory are discussed from the viewpoint of the separation of slow and fast time scales and ergodicity.

Effects of conservation of total angular momentum on two-hard-particle systems.

In this paper we investigate some statistical mechanical properties of systems consisting of two hard disks in a circular cavity and two hard spheres in a cylindrical pore. Both systems conserve not only the total energy but the total angular momentum, and this conservation affects thermodynamic quantities such as the temperature and pressure of these systems. We show the dependence of the thermodynamic quantities of these systems on conserved quantities with a statistical mechanical treatment and molecular dynamics simulations.

Statistical mechanics of two hard spheres in a box.

We investigate some statistical mechanical properties of a system consisting of two hard spheres in a D-dimensional rectangular box (D=1, 2, ...). We give a theoretical method for computing a configurational partition function Zc,D of this system and compare the equation of state obtained from Zc,D with molecular dynamics simulations. Especially in D=3, we give a fully analytic expression for the pressure which turns out to have one or more negative compressibility regions when the box size is small.