ReaxFF Molecular Dynamics Study on the Influence of Temperature on Adsorption, Desorption, and Decomposition at the Acetic Acid/Water/ZnO(101̅0) Interface Enabling Cold Sintering

The reaction dynamics
of a liquid–solid interface with the
example of an acetic acid/water solution interacting with a ZnO(101̅0)
surface was investigated using ReaxFF reactive force field-based molecular
dynamics. The interactions were studied over a broad temperature range
to assess the kinetics and reaction pathways. Two different acetic
acid dissociation mechanisms are observed in the simulations: (1)
deprotonation to surface cation, which produces a terminal hydroxyl
and (2) deprotonation to a bridging hydroxyl, which results in water
production. An increase in temperature promotes the dissociation of
acetic acids and its adsorption to surface at first, but as the temperature
increase continues, the surface coverage by acetates decreases due
to evaporation from the surface or decomposition. The acetate decomposition
starts with a nucleophilic attack of oxygen to methyl carbon and results
in the production of carbon dioxide, which is consistent with experimental
findings in the literature. The coverage of the surface by water molecules
decreases as the system is heated up, which is also observed in other
molecular dynamics studies. At elevated temperatures, acetate molecules
are more stable than water molecules or bridging hydroxyls on the
surface. These simulations validate the ReaxFF method for the water/organic
mixture and metal oxide surface interactions and provide insights
into structure and reactivity of aqueous solvents on metal oxide surfaces
at elevated temperatures. Adsorption trends that are observed in this
study are consistent with phenomenological Langmuir models. The reaction
of acetic acid decomposition to smaller molecules such as CO2 and CH2O agrees with experimental observations. Understanding
the details of these dynamic surface reactions are critical to understand
important new cold sintering processes that utilize transient liquid
and solid reactions, and the latter could be used to predict solvent
selection for cold sintering.