Effects of Adsorbed H2O amorphous ice on Surface Structure of Forsterite Glass
A. Kubo, J. Nishizawa, and T. Ikeda-Fukazawa
Department of Applied Chemistry, Meiji University, Japan

Forsterite (Mg2SiO4) glass exists as dust grains in interstellar molecular clouds and youngstellar objects [1]. In interstellar molecular clouds, elements such as hydrogen, oxygen, carbon,and nitrogen deposit on dust grains, and form various molecules (e.g., H2O, CO, CO2, NH3,CH4, H2CO, CH3OH, and so on) [2]. These molecules undergo chemical evolutions to organic
molecules through various processes on the surface of dust grains [2]. The structure of forsterite
glass is one of the important factors governing the chemical evolutions in interstellar molecularclouds. However, there are few studies for effects of molecular adsorption on the structure andproperties of forsterite glass [3]. To investigate the adsorption process of H2O and the effect ofadsorption on surface structure of forsterite glass, molecular dynamics (MD) calculations wereperformed.
The MD calculations were performed using an atom-atom potential model with MXDORTO
program [4]. The potential parameters for forsterite were empirically determined byconstraining the structure to reproduce the experimental results of density, thermal expansion
coefficient, and bulk modulus [5]. For H2O, KAWAMURA potential model was used [6]. A
fundamental orthorhombic cell consisting of 2400 Mg2SiO4 with two-dimensional periodic
boundary conditions was used. The glass structure was prepared by quenching the liquid phasefrom 3000 to 10 K with 2 K/fs in rate. The quenched glass was warmed to 300 K with the samerate, and 2682 H2O were adsorbed on the glass. Then, the system was quenched to 10 K with 2K/fs, and structures were analyzed at 10 K. The MD code was run with NTV ensemble.
The result shows that the atomic displacement parameter (ADP) of each atomic species inforsterite (i.e., Mg, Si, and O) in the interface decreases with adsorption of H2O amorphous ice.
For the forsterite glass without water adsorption, the ADP values of the surface layer werehigher than those of internal part due to existence of dangling bonds in the surface layer. To
investigate the mechanisms of ADP decrease observed in the interface with amorphous ice, thecoordination number of the magnesium atoms was analyzed. The result shows that thecoordination number in the interface layer increases with adsorption of H2O amorphous ice,
because new covalent bonds are formed with the adsorbed H2O molecules. The decrease in theADP values observed in the interface is resulted from the variation of the coordination number.
The mean Mg–O length of the newly formed covalent bonds is 0.256 nm, and is slightly longer
than that in the MgOx units in the forsterite glass without water adsorption (i.e., 0.238 nm). Dueto the formation of covalent bonds with H2O molecules, MgOxH2 units exist as an intermediate
state in the interface with forsterite glass and amorphous ice. The present results have important
implications for the role of forsterite glass in chemical evolutions in the universe.
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