Center for Composite Materials - University of Delaware

Research Summary

Molecular Simulations of Silica Surface in Presence of Water

Authors: Sanjib C. Chowdhury, Bazle Z. (Gama) Haque, and John W. Gillespie Jr.

MOTIVATION

• When glass fiber comes out from the forehearth, water is sprayed on it for cooling purpose
• The reactivity of silica along with other elements of the glass fiber with water and silane controls surface properties and ultimately the adhesion of the glass fiber to the polymer matrix in the composite

OBJECTIVES

• Investigate structural properties of water and silica, silica surface reactivity, diffusion of water molecule into silica

APPROACH

• Molecular dynamics (MD) simulation using ReaxFF in LAMMPS

MODEL DEVELOPMENT

Silica Model
• Crystalline β-cristobalite silica. Total atoms = 5184. Model dimensions: 3.7×4.9×3.9 nm. Density = 2.2 gm/cc
• 1st Stage Annealing - Heated at 8000 K for 50 ps. Then gradually cooled to 300 K. NVT ensemble
• 2nd Stage Annealing - Heated at 4000 K for 50 ps. Then gradually cooled to 300 K. NPT ensemble

Water Model
• 1345 water molecules are randomly packed into a 3.7×4.9×2.0 nm box. Model is then relaxed for 10 ps with NVT ensemble

Silica-Water Model
• Amorphous silica model is sandwiched by the relaxed water model keeping 0.2 nm inter-surface gap

RESULTS & DISCUSSION

• Silica and water model are validated comparing bond length, angle, RDF profile, coordination number with the available literature values
• To study silica-water interface, density profile and snapshot of the model after 500 ps are checked
• Density distribution indicates water is diffusing in the silica
• Reactions:

CONCLUSIONS

• Silica-water interface is studied using MD simulation
• Simulation shows water diffuses into silica
• H and OH of dissociated water create bond with one-coordinated O and three-coordinated Si of silica and form silanol (SiOH)

ACKNOWLEDGEMENTS

This work is support by the ARL CRA-MEDE Program

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