ISSN 0018-1439, High Energy Chemistry, 2018, Vol. 52, No. 3, pp. 199–205. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © N.A. Sirotkin, V.A. Titov, 2018, published in Khimiya Vysokikh Energii, 2018, Vol. 52, No. 3, pp. 183–188.
Molecular Dynamics Simulation of Ion Sputtering
of a Sodium Chloride Solution
N. A. Sirotkin and V. A. Titov*
Krestov Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo, 153045 Russia
Received September 20, 2017
Abstract⎯The sputtering of an aqueous solution of sodium chloride with a concentration of 0.5 mol/L under
bombardment by 1–20 positive ions with initial energies of 50–500 eV has been simulated by a molecular
dynamics method. It has been found that the transfer of solute cations and anions to a gas phase requires a
threshold energy of bombardment. It has been shown that the solute components occurred in the gas phase
both as hydrated ions and in the form of ion pairs in the composition of water clusters. As the energy input
into the cell reached ~0.33 eV/particle, the clusters of five or more water molecules were predominantly sput-
Keywords: water, ion sputtering, liquid cathode, transfer processes, simulation, molecular dynamics, clusters
A study of gas discharges excited in liquids and in
contact with them is currently a priority line in the
physics of plasma and plasma chemistry [1, 2]. In par-
ticular, this is related to prospects for the practical
applications of these discharges; among them are the
decomposition of organic pollutants in water and the
disinfection of water [3–5], the modification of poly-
meric materials [6–8], the application of coatings ,
the formation of micro- and nanostructures [10–12],
and the quantitative spectral analysis of solutions for
metal ion contents [13–16].
The action of gas-discharge plasma on water and
aqueous solutions is accompanied by the transfer of
components from a liquid phase into plasma [17, 18].
A change in the composition of plasma leads to
changes in its properties and in the rates of processes
occurring in it.
Several hypotheses were proposed to explain the
mechanisms of the transfer of liquid cathode compo-
nents into the gas phase. It is assumed that this process
is similar to cathode sputtering in reduced-pressure
discharges with metal electrodes . An analogy
between the transfer of solute components into the gas
phase under the action of a discharge a matrix assisted
laser desorption/ionization (MALDI) process was
hypothesized [20, 21]. It was assumed [22, 23] that a
substance is transferred as microdroplets, which evap-
orate in the plasma, and the ions of a solute undergo
desolvation and neutralization. There is also a hypoth-
esis on the contribution of solution electrosputtering
under the action of a strong electric field .
The use of a transfer coefficient, a quantity analo-
gous to a cathode sputtering coefficient, was proposed
for the quantitative characterization of the process.
The transfer coefficient is equal to the number of par-
ticles passed from the solution into the gas phase per
ion incident on the solution surface [17, 18]. The
transfer coefficients (s) for solvent (water) molecules
and solute components were experimentally deter-
mined in studies of discharges with solutions of acids,
alkalis, and alkali or alkaline earth metal salts as a
cathode [17, 18, 25, 26]. The values of s = 300–
500 molecule/ion and s = 0.001–0.1 particle/ion were
obtained for water and solute components, respec-
tively. It was established that the transfer of solute
components comes into play at a threshold discharge
current (a threshold intensity of the ion bombardment
of a liquid cathode) . It was found that the thresh-
old intensity values depend on the masses of hydrated
cations in solution, and the transfer coefficients
increase with the intensity of ion bombardment.
The use of computer simulation in a study of the
interaction of plasma with solutions was reported .
The processes of the interaction of ions with water
were simulated with the use of a classical molecular
dynamics method [27, 28]. It was shown that the
transfer coefficients of water varied from 100 to
500 molecule/ion depending on the bombarding ion
energy (50–500 eV), and the energy consumed for the
transfer of a molecule was ~1.5 eV/molecule .
Minagawa et al.  determined the penetration depth
of an ion with an initial energy from 10 to 100 eV into
GENERAL ASPECTS OF HIGH ENERGY CHEMISTRY