Research in our group aims at the understanding of complex interfacial phenomena, particularly  biomineralization and organic photovoltaics, at the molecular scale using computer simulation. We aim at understanding the chemistry and physics—including a variety of experimental data—to guide in the design of composite materials and devices such as for bone replacement, sensing, efficient energy generation, and treatment of diseases. Simulation with atomistic and coarse-grain models and the development of computational tools (force fields, methods) goes hand in hand with collaborative experimental efforts, for example peptide binding and dynamics at metallic and oxidic nanoparticle surfaces, phase transitions and switching of light-responsive structures with azobenzene, the prediction of conductivity in organic semiconductors, and the behavior of polyelectrolytes as dispersants or biological scaffold materials in solution and at surfaces.

Force fields for the simulation of inorganic-bio/organic interfaces

In computer simulations of hybrid materials, reliable energy models are needed. This is particularly challenging for inorganic components such as silicates and biologically important minerals. Improvements in the assignment of atomic charges and van-der-Waals parameters have been made, which lead to the reproduction of surface energies of various silicates and aluminates in very good agreement with experimental data (±5%), down from 50% to 500% deviations in existing force fields (Heinz, H.; Koerner, H.; Vaia, R. A.; Anderson, K. L.; Farmer, B. L., Chem. Mater. 2005, 17, 5658).

Snapshot of a peptide binding to mont-morillonite through a lysine side chain.

For the determination of atomic charges in molecular simulations, covalent and ionic contributions to chemical bonding must be taken into account (Heinz, H.; Suter, U. W. J. Phys. Chem. B 2004, 108, 18341). This can be achieved with an extended Born model, which provides reliable estimates for compounds across the periodic table. When available, experimental X-Ray measurements of the electron deformation density yield atomic charges (±0.1e) that are well suited for classical simulations. Together with a physical interpretation of the van-der-Waals (Lennard-Jones) parameters, these concepts are applied to develop energy models for a variety of solid state structures and molecules to carry out quantitative simulations of interfaces in composite materials, coatings, nanoelectronics, and biomedical materials. The energy models are compatible with chemically and biologically oriented force fields such as CHARMM, PCFF, CVFF, GROMACS.

Recently, improvements of the parameters for fcc metals (Cu, Ag, Au, Ni, Pd, Pt, Al, Pb) have been made to simulate more accurately interfacial processes, for example the interaction of metals with peptides.

The extended Born model (left) assumes ionization to partial charges and distinguishes covalent and ionic contributions to chemical bonding. Covalent bonding contributions are represented by atomization energies of the elements (right) and often dominate over ionic contributions represented by ionization potentials/electron affinities.

Snapshot of a gold-binding peptide on the (111) surface of gold.

Calculation of local and average pressure tensors in molecular simulations

A method to compute local and average stress tensors in systems with many-body interactions (angle, torsion, and out-of-plane potentials etc) has been proposed, including the contributions according to every term in the total energy expression. The approach is consistent with previous methods for systems with two-body interactions (e.g., Irving and Kirkwood) and calculations of the total pressure (e.g., the virial formula). The new method permits the three-dimensional analysis of pressure tensors at any length scale and for freely chosen volumes in molecular simulations. The method can help solve current challenges in nano-mechanics and interfacial thermodynamics, e.g., in composite materials and transport processes through cell membranes (Heinz, H.; Paul, W.; Binder, K. Phys. Rev. E, 2005, 72, 066704).

Illustration of the mechanical definition of the nanoscale pressure tensor as a force across a unit area. There are contributions from throughput of linear momentum and from dissected n-body forces. The dissection is determined by reducing n-body interactions to two-body interactions. If the vector r_n between the two geometric centers crosses A_α, a contribution is made, calculated from the forces on the n atoms due to the n-body potential.

Self-assembly of surfactants on mica and montmorillonite surfaces

The structure and phase transitions of various surfactants on mica and montmorillonite surfaces have been studied, leading to a consistent picture of the structure and chain dynamics as a function of CEC, head group structure, and chain length. On montmorillonite, the alkyl chains (one and two-armed surfactants) lie rather flat on the surface and have a low tilt angle. Primary ammonium head groups cause hydrogen bonds on the clay mineral surface, and the percentage of gauche torsions in the alkyl chains varies from 15% to 45% depending on the structure (Heinz, H.; Vaia, R. A.; Krishnamoorti, R.; Farmer, B. L. Chem. Mater., 2007, 19, 59).

                 C₂                                       C₆                                                C₁₄                                                     C₂₂

A series of alkyltrimethylammonium chains self-assembled on montmorillonite, CEC 145 mequiv/100g, viewed along the x axis.

In contrast, alkyl chains on mica have a substantial tilt angle and ion exchange can be incomplete. This leads to the formation of nuclei of alkyl chains, separated from alkali ions, or homogeneously mixed layers of potassium ions and alkyl chains (Heinz, H.; Suter, U. W. Angew. Chem. Int. Ed. 2004, 43, 2239).

The 2-phase system can be distinguished from a 1 phase-system by the gallery spacing, which is somewhat higher for a 2-phase system.

The peak in DSC that can be observed on heating corresponds to a reversible order-disorder phase transition (similar to melting), and the second transition observed in some cases corresponds to “jumps” of peralkylated ammonium head groups across the cavities in the surface leading to a structure even closer to liquid-like (Heinz, H.; Castelijns, H. J.; Suter, U. W., J. Am. Chem. Soc. 2003, 125, 9500). The second transition does not appear for primary ammonium surfactants (less flexibility due to hydrogen bonds to the surface) and for closely packed surfactants. Simulation results have explained/are in agreement with virtually all available experimental data, including XRD; IR, NMR, dielectric spectroscopy; DSC, TEM, AFM, and NEXAFS measurements.

UV spectral shifts in perylene due to solvation

Using a simple model of the electron density in the HOMO and in the LUMO of perylene, accurate computations of spectral shifts in a variety of low-temperature solid as well as liquid n-alkane environments have been obtained (Heinz, H.; Suter, U. W.; Leontidis, E. J. Am. Chem. Soc. 2001, 123, 11229). This approach demonstrates that electronic properties can be decribed on the basis of semiempirical models, which is tested further in the development of models for hole transport in organic semiconductors.

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Department of Polymer Engineering