Quantitative Simulation of Interface Properties in Inorganic-Organic Hybrid Materials

In computer simulation of complex structured materials, simple energy models for the quantitative simulation of inorganic-organic interfaces are an important challenge. Solutions for two major problems are outlined [1,2] and have been applied to the understanding of self-assembly on silicate surfaces and azobenzene-based actuators.

Atomic charges can be precisely determined on the basis of modern X-Ray methods with an error of about ±0.1e and these values are well suited for classical simulations. A theoretical model is presented to assign fast and accurately atomic charges for compounds across the periodic table, taking into account covalent and ionic contributions to chemical bonding.

Guidelines for the physical interpretation of van-der-Waals (Lenard-Jones) parameters in polar systems are given to facilitate more accurate assignments. The concept for atomic charges and van-der-Waals parameters has been applied in the derivation of force field parameters for various silicates and aluminates, which reproduce surface energies in very good agreement with experimental data (±5%), down from 50% to 500% deviations in earlier force fields. Energy models to date are integrated in chemically and biologically oriented force fields (CHARMM, PCFF, CVFF, GROMACS). The method can be employed to develop reliable models for a wide range of other solid-state structures and molecules, with broad applications to composite materials, coatings, nanoelectronics, and biomedical materials.

Quantitative simulations of nanoscale morphologies and thermodynamic properties in hybrid systems are demonstrated for two examples: (1) Chemically modified azobenzene units confined between silicate surfaces show different responses upon photoexcitation (trans-cis), guiding in the experimental design of novel actuators. (2) Surface reconstruction processes and interaction energies between sheets of layered silicates have been analyzed by molecular dynamics simulation to better understand exfoliation processes in nanocomposites. Results are currently employed as a guide in the laboratory synthesis of nanocomposites and actuators.

My research interests for the future are broadly aimed at promising challenges in nanotechnology and bioengineering. With emphasis on atomistic and coarse-grained models and computer simulation methods, I want to aid in the understanding of structure-property relationships and in the development of nanocomposite materials, energy generation and storage devices, and advanced biomaterials for biomedical applications. As an important part, effective computational tools for these purposes shall be developed, such as methods to simulate charge transport and responses to electric fields (combination of ab-inito calculations with classical simulation techniques) and development of reliable energy models for conjugated polymers and biominerals. To advance specific applications, collaboration with experimental (as well as theoretical) groups will be important.

[1] H. Heinz, H. J. Castelijns, and U. W. Suter, J. Am. Chem. Soc. 2003, 125, 9500-9510. [2] H. Heinz, K. L. Anderson, H. Koerner, R. A. Vaia, and B. L. Farmer, Chem. Mater. 2005 (to appear).