Nonequilibrium molecular dynamics simulations of acetonitrile/methanol mixtures in contact with a hydroxylated silica surface are used to elucidate the mechanism of molecular exchange at a hydrophilic liquid/solid interface. The different hydrogen-bonding ability of the two solvents provides a driving force for the adsorption/desorption process, which is followed by examining several structural and energetic properties of the system. Two different reaction coordinates for the hydrogen bonding exchange are defined and are used to identify transition states in which the methanol attains a well-defined orientation. The reaction coordinates are used to examine the mechanism and dynamics of the exchange. We find that the exchange process involves multiple recrossing of the transition state and can progress via two different mechanisms, depending whether the methanol first acts as a hydrogen bond donor or acceptor at the silica surface.

Light-driven and photocurable polymer-based additive manufacturing (AM) has enormous potential due to its excellent resolution and precision. Acrylated resins that undergo radical chain-growth polymerization are widely used in photopolymer AM due to their fast kinetics and often serve as a departure point for developing other resin materials for photopolymer-based AM technologies. For successful control of the photopolymer resins, the molecular basis of the acrylate free-radical polymerization has to be understood in detail. We present an optimized reactive force field (ReaxFF) for molecular dynamics (MD) simulations of acrylate polymer resins that captures radical polymerization thermodynamics and kinetics. The force field is trained against an extensive training set including density functional theory (DFT) calculations of reaction pathways along the radical polymerization from methyl acrylate to methyl butyrate, bond dissociation energies, and structures and partial charges of several molecules and radicals. We also found that it was critical to train the force field against an incorrect, nonphysical reaction pathway observed in simulations that used parameters not optimized for acrylate polymerization. The parameterization process utilizes a parallelized search algorithm, and the resulting model can describe polymer resin formation, crosslinking density, conversion rate, and residual monomers of the complex acrylate mixtures.

Vibrational sum frequency (VSF) spectroscopy and molecular dynamics simulations are used to investigate ethanol-silica and methanol-silica interfaces. We describe the subtle differences in molecular organization that result in the observed differences in the VSF spectra for methanol and ethanol at the alcohol-silica interface. Alcohol molecules hydrogen-bonded to the silica surface induce orientational opposition in an adjacent low-population region, which implies VSF signal reduction. This low population region is essentially of zero density in the ethanol system, implying less signal cancelation. Simulated silica defect sites increase the population of this region in both systems. Interestingly, the induced orientation in this region influences subsequent molecular orientation only in the ethanol-silica system, preserving the interfacial anisotropy. These effects suggest a stronger VSF response from the ethanol-silica system versus the methanol-silica system, where more methanol molecules reside in the low-population region, and this region does not induce order in subsequent solvent layers.

We present a general approach to isolate chemical reaction mechanism as an independently controllable variable across chemically distinct systems. Modern approaches to reduce the computational expense of molecular dynamics simulations often group multiple atoms into a single “coarse-grained” interaction site, which leads to a loss of chemical resolution. In this work we convert this shortcoming into a feature and use identical coarse-grained models to represent molecules that share nonreactive characteristics but react by different mechanisms. As a proof of concept, we use this approach to simulate and investigate distinct, yet similar, trifunctional isocyanurate resin formulations that polymerize by either chain- or step-growth. Because the underlying molecular mechanics of these models are identical, all emergent differences are a function of the reaction mechanism only. We find that the microscopic morphologies resemble related all-atom simulations and that simulated mechanical testing reasonably agrees with experiment.