ReaxFF molecular dynamics simulation of intermolecular structure formation in acetic acid-water mixtures at elevated temperatures and pressures
Summary¶
Acetic acid and water mix in technologically relevant ways—from acid catalysis to solution processing—and their hydrogen-bonded aggregates depend on temperature, pressure, and composition. This Journal of Chemical Physics article presents ReaxFF molecular dynamics of acetic acid–water mixtures across acid mole fractions down to \(x_{\mathrm{HAc}} \ge 0.2\), spanning ambient and near-critical conditions. The authors use metadynamics to refine the free-energy profile for acid dissociation and adjust the ReaxFF dissociation energy accordingly, seeking internal consistency between enhanced-sampling estimates and the reactive force field’s treatment of proton transfer. Randall and van Duin co-authorship ties the work to dielectric and ceramic processing communities interested in aqueous organic chemistry at high temperature. Near-critical water–organic mixtures also appear in green chemistry solvent discussions; the paper’s structural diagnostics (RDFs, cluster lifetimes) support interpreting those environments at atomistic resolution.
Methods¶
ReaxFF-based molecular dynamics (bulk liquid and supercritical regimes) samples radial distribution functions, hydrogen-bonded clusters, and bonding patterns for acetic acid–water mixtures with \(1.0 \ge x_{\mathrm{HAc}} \ge 0.2\) across ambient and near-critical states described in the abstract (papers/Sengul_aceticacid_water_JCP_2018.pdf). Metadynamics on the acetic acid dissociation coordinate supplies a dissociation free energy used to reoptimize the ReaxFF dissociation energy (abstract). Engine / code: N/A — MD package name not on the indexed excerpt (normalized/extracts/2018mert-y-sengul-the-journal-reaxff-molecular_p1-2.txt). System & PBC: periodic supercells of the mixture compositions in Methods (atom totals there). Ensemble / thermostat / barostat / timestep: N/A — not transcribed here; the article spans both ambient and critical conditions where pressure control matters—confirm NPT targets and barostat choices in Methods. Duration / stages: equilibration and production segments with lengths in ps/ns are tabulated in Methods (not duplicated on this excerpt-based page). Temperature: ambient vs critical-region temperature sweeps are central to the abstract’s structural claims. Electric field: N/A — not used. Enhanced sampling: metadynamics is used for the dissociation free-energy refinement noted above (not umbrella/replica exchange in the indexed abstract framing). Duration / stages: bulk mixture equilibration followed by production MD at each (T, P, composition) state point; cumulative trajectory lengths and the integration timestep (fs) are tabulated in J. Chem. Phys. §2 (papers/Sengul_aceticacid_water_JCP_2018.pdf)—N/A — exact ns not restated from the indexed excerpt alone.
Findings¶
At ambient conditions, dominant aggregates shift between acetic-acid-rich and water-rich motifs as composition changes; cyclic dimers and chain-like hydrogen-bonded clusters appear in acid-rich regimes and diminish as water content rises. Under near-critical conditions, longer-range correlations weaken and acid–acid versus water–water ordering becomes more disordered compared with ambient RDF fingerprints, as summarized in the abstract and figures. These trends are interpreted as signatures of how supercritical-like environments disrupt molecular clustering relative to room-temperature hydrogen-bond networks. The metadynamics refinement of the acid dissociation free-energy profile is the methodological centerpiece: it ties the reactive force field’s proton-transfer energetics to a statistically sampled reaction coordinate rather than relying on a single-point gas-phase estimate.
Limitations¶
Near-critical sampling is demanding; finite-size effects and simulation length can bias cluster statistics. ReaxFF proton-transfer barriers should be checked against experiment or higher-level electronic structure where quantitative rates matter. Metadynamics parameters influence the inferred dissociation free energy; sensitivity checks belong in the original article. Barostat choices near the critical point affect density fluctuations and cluster statistics.
Relevance to group¶
Group-linked ReaxFF application to aqueous organic mixtures with metadynamics-informed parameter consistency.