Hitoshi Washizu

Vapor-liquid equilibrium (VLE) data of fragrance components are crucial for product development and separation processes. However, experimentally obtaining these data can often be a high-cost and challenging task. In order to address this issue, simulations of VLE data using molecular dynamics (MD) methods have gained attention, though there are still relatively few studies about the vapor-liquid equilibrium calculations of fragrance components using MD. In this study, we focused on a mixture of d-limonene and 1-pentanol as representative components and conducted MD simulations. The VLE data obtained by varying the molar fraction of d-limonene, including x-y phase diagrams and activity coefficients, showed a high degree of agreement with the experimental data. Additionally, an analysis of the density profiles on a molecular level revealed a slight increase in the concentration of 1-pentanol at the vapor-liquid interface.

Molecular dynamics simulations were conducted using the generalized replica exchange method (gREM) on the 4-cyano-4′-n-alkyl biphenyl (nCB) system with n = 5, 6, 7, and 8, which exhibits a nematic–isotropic (NI) phase transition. Sampling near the phase transition temperature in systems undergoing first-order phase transitions, such as the NI phase transition, is demanding due to the substantial energy gap between the two phases. To address this, gREM, specifically designed for first-order phase transitions, was utilized to enhance sampling near the NI phase transition temperature. Free-energy calculations based on the energy representation (ER) theory were employed to characterize the NI phase transition. ER evaluates the insertion free energy of the nCB molecule for both nematic and isotropic phases, revealing a change in the temperature dependence across the NI phase transition. Further decomposition into energetic and entropic terms quantitatively shows the balance between these contributions at the NI phase transition temperature.

A reactive force field (ReaxFF) molecular dynamics simulation is performed for the sliding of diamond-like carbon (DLC) and yttria-stabilized zirconia (YSZ) under an ethanol gas environment, motivated by the previous experiment of ultralow friction phenomenon (friction fade-out). We observe (i) dissociation of ethanol into ethoxy and hydrogen, both of which simultaneously adsorb on the YSZ surface, and (ii) dissociation of ethanol into ethyl and hydroxy, the former of which forms a bond with another ethanol molecule and the latter of which adsorbs on the DLC surface. Reaction (i) is enhanced by the sliding motion, but occurs even without it, while reaction (ii) only occurs during sliding with a sufficiently high load pressure. The potentials of mean force for the two reactions are also calculated combining the steered MD and Jarzynski equality. It is shown that the activation energies of reactions (i) and (ii) are significantly lowered by the YSZ and DLC surfaces, respectively, as compared to those in a vacuum. The resultant activation energy is higher for reaction (ii) than for reaction (i).

In many physical or biological systems, diffusion can be described by Brownian motions with stochastic diffusion coefficients (DCs). In the present study, we investigate properties of the diffusion with a broad class of stochastic DCs with a novel approach. We show that for a finite time, the propagator is non-Gaussian and heavy-tailed. This means that when the mean square displacements are the same, for a finite time, some of the diffusing particles with stochastic DCs diffuse farther than the particles with deterministic DCs or exhibiting a fractional Brownian motion. We also show that when a stochastic DC is ergodic, the propagator converges to a Gaussian distribution in the long time limit. The speed of convergence is determined by the autocovariance function of the DC.

Organophosphates are well-known as the canonical additives for lubricants. Thus, understanding of the additive behaviour is a key aspect in the design of films on metal surfaces. Different types of phosphates are added to improve their antiwear properties, but the contributions of individual esters to these properties has not been studied using a combination of practical and theoretical approaches. In this study, organophosphates were isolated with high purity and their tribological characteristics were evaluated by using a Bowden-type reciprocating friction tester and a four-ball wear tester. Mono-oleylphosphate had a lower friction than di-oleylphosphate and exhibited excellent antiwear characteristics. Analysis of the sliding surfaces using desorption electrospray ionization-mass spectrometry (DESI-MS) and X-ray photoelectron spectroscopy (XPS) indicated that the film structure could predict the occurrence factor of the tribological characteristics of the oleylphosphates. Then the adsorption energies of the monoester on iron and iron oxide surfaces were higher than those of the diester, as assessed using density functional theory (DFT) calculations, owing to the difference in their chemisorption processes, as confirmed by further DFT analysis. Studies on the reactivity of additives and their interactions with surfaces are important for understanding the tribochemistry of additives.