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[PCG20-P02] Structure and diffusion dynamics of interstitial water of forsterite
Keywords:forsterite glass, water, molecular dynamics calculation
Structure and diffusion dynamics of interstitial water of forsterite
Yudai Kobayashi, Tomoko Ikeda-Fukazawa
Department of Applied Chemistry, Meiji University, Japan
In interstellar molecular clouds, various elements such as oxygen, hydrogen, nitrogen, and carbon adsorb on the surface of mineral dust grains and form amorphous ice and various molecules such as carbon monoxide and ammonia. These molecules undergo various reactions to produce complex organic substances, which is called chemical evolution. Since the interface between mineral and water is a dominant reaction field [1], it is essential to understand the structures and properties of the coexisting mineral and water.
Forsterite (Mg2SiO4) in glassy state is a major component of the mineral dust grains [2]. Kubo et al. [3] revealed that water molecules decompose to form hydroxides at the interface between forsterite glass and water using molecular dynamics (MD) calculations. The pressurization of water can be a cause of the decomposition because the forsterite glass acts as a pressure cell. However, the structures and dynamics of water molecules coexisting with forsterite have not yet been understood in detail. In this study, MD calculations of forsterite-water system were performed to investigate the structure and diffusion dynamics of water coexisting with forsterite in glassy and crystal states.
MD calculations were performed using the MXDORTO program developed by Kawamura [4]. Fundamental orthorhombic cells consisting of 3311 H2O and 1176 Mg2SiO4 (forsterite in crystal or glassy states) were used as the initial structures of forsterite-water systems. The MD code was run with an NTP ensemble in a temperature range of 10–690 K. The pressure was kept at 30 MPa. The structure and diffusion properties were analyzed using the equilibrated structures at each temperature. To compare the results, the calculations were performed using a pure water consisting of 360 H2O.
The result shows that the densities of the interstitial water coexisting with forsterites are larger than that of pure liquid water at temperatures above 470 K. This effect is remarkable for the interstitial water with forsterite glass. The result indicates that the pressure of the interstitial water becomes higher than its environment because the surrounding forsterite acts as a pressure cell. The pressurization causes a restrain of diffusive motions of water molecules. We analyzed self-diffusion coefficients of the interstitial water and pure liquid water using the Einstein relation [5]. The result shows that the self-diffusion coefficients of interstitial water with forsterite glass are larger than those of the interstitial water with forsterite crystal and pure liquid water. The difference between those with forsterites in crystal and glassy states is due to (i) an uneven structure of glass surface and (ii) a dissolution of forsterite glass toward coexisting liquid water phase. Water molecules form a covalent bond with dissolved Mg2+ in liquid water phase and are partially decomposed. The mean coordination number of the O–H covalent bonds in the interstitial water decreases with the decomposition. We discuss the changing mechanisms of the structure and diffusive properties of interstitial water.
[1] J.M. Greenberg, J.I. Hage, Astrophys. J. 361 (1990) 260.
[2] T. Henning, Annu. Rev. Astron. Astrophys. 48(2010) ,1
[3] A. Kubo, J. Nishizawa, and T. Ikeda-Fukazawa, Chemical Physics Letters 760 (2020) 810.
[4] K. Kawamura, MXDORTO, Japan Chemistry Program Exchange 029 (1996).
[5] A. Einstein, Investigations on the Theory of the Brownian Movement (Dover: New York, 1956).
Yudai Kobayashi, Tomoko Ikeda-Fukazawa
Department of Applied Chemistry, Meiji University, Japan
In interstellar molecular clouds, various elements such as oxygen, hydrogen, nitrogen, and carbon adsorb on the surface of mineral dust grains and form amorphous ice and various molecules such as carbon monoxide and ammonia. These molecules undergo various reactions to produce complex organic substances, which is called chemical evolution. Since the interface between mineral and water is a dominant reaction field [1], it is essential to understand the structures and properties of the coexisting mineral and water.
Forsterite (Mg2SiO4) in glassy state is a major component of the mineral dust grains [2]. Kubo et al. [3] revealed that water molecules decompose to form hydroxides at the interface between forsterite glass and water using molecular dynamics (MD) calculations. The pressurization of water can be a cause of the decomposition because the forsterite glass acts as a pressure cell. However, the structures and dynamics of water molecules coexisting with forsterite have not yet been understood in detail. In this study, MD calculations of forsterite-water system were performed to investigate the structure and diffusion dynamics of water coexisting with forsterite in glassy and crystal states.
MD calculations were performed using the MXDORTO program developed by Kawamura [4]. Fundamental orthorhombic cells consisting of 3311 H2O and 1176 Mg2SiO4 (forsterite in crystal or glassy states) were used as the initial structures of forsterite-water systems. The MD code was run with an NTP ensemble in a temperature range of 10–690 K. The pressure was kept at 30 MPa. The structure and diffusion properties were analyzed using the equilibrated structures at each temperature. To compare the results, the calculations were performed using a pure water consisting of 360 H2O.
The result shows that the densities of the interstitial water coexisting with forsterites are larger than that of pure liquid water at temperatures above 470 K. This effect is remarkable for the interstitial water with forsterite glass. The result indicates that the pressure of the interstitial water becomes higher than its environment because the surrounding forsterite acts as a pressure cell. The pressurization causes a restrain of diffusive motions of water molecules. We analyzed self-diffusion coefficients of the interstitial water and pure liquid water using the Einstein relation [5]. The result shows that the self-diffusion coefficients of interstitial water with forsterite glass are larger than those of the interstitial water with forsterite crystal and pure liquid water. The difference between those with forsterites in crystal and glassy states is due to (i) an uneven structure of glass surface and (ii) a dissolution of forsterite glass toward coexisting liquid water phase. Water molecules form a covalent bond with dissolved Mg2+ in liquid water phase and are partially decomposed. The mean coordination number of the O–H covalent bonds in the interstitial water decreases with the decomposition. We discuss the changing mechanisms of the structure and diffusive properties of interstitial water.
[1] J.M. Greenberg, J.I. Hage, Astrophys. J. 361 (1990) 260.
[2] T. Henning, Annu. Rev. Astron. Astrophys. 48(2010) ,1
[3] A. Kubo, J. Nishizawa, and T. Ikeda-Fukazawa, Chemical Physics Letters 760 (2020) 810.
[4] K. Kawamura, MXDORTO, Japan Chemistry Program Exchange 029 (1996).
[5] A. Einstein, Investigations on the Theory of the Brownian Movement (Dover: New York, 1956).