Atomistic insight into the plasma-catalyzed nucleation of ice grains*

Reactive Molecular Dynamics (MD) and Quantum Mechanics (QM) computations are performed to investigate the nucleation and agglomeration processes of ice grains in a weakly-ionized plasma environment such as in the Caltech ice dust experiment [1] and in protoplanetary disks. The MD simulations use the state-of-the-art eReaxFF force field [2] which can describe the particle nature of both valence and free electrons. QM computations of the ground and ionized states of hydrate clusters provide electronic structure and potential energy that benchmark the much larger MD simulations. Findings indicate that solvated free electrons occupy a large volume, either localized on the surface or forming cavities within the water clusters, and attract the dipole moment of OH bonds of nearby water molecules. A methodology has been developed for the QM calibration of the eReaxFF description of solvated electrons in water clusters. Using this method, MD simulations of large spherical and ellipsoidal-shaped grains with solvated electrons are performed. These simulations are then analyzed to determine the impact of electrostatic interactions, surface tension, and binding energy on the shape and chemical composition of amorphous dust grains. The analysis shows that the distribution of free electrons in ice grains is not controlled by electron-electron repulsion because the Coulomb interaction energy between free electrons within the grain is much smaller than the binding energy of a solvated free electron to a cluster of water molecules. This strong binding of solvated free electrons to water molecules suggests that ice grains cannot be considered as being electrically conductive. QM computations on reactants, products, and transition states of hydrate systems reveal that O-, and especially OH-, are the most energetically favorable nucleation sites in a dusty plasma environment. MD simulations of dusty plasmas have been started to investigate the kinetics and nucleation process of ice grains.

[1] A. Nicolov, M. S. Gudipati, P. M. Bellan, The Astrophysical Journal 966 (1) (2024) 66.

[2] M. M. Islam, G. Kolesov, T. Verstraelen, E. Kaxiras, A. C. Van Duin, Journal of chemical theory and computation 12 (8) (2016) 3463–3472.