Photonic crystals in biology

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Poster Session, Tuesday, June 15 Theme A1 - B702 Computational studies for free energies of solvation of additives in polymeric nanostructures Tuba Arzu Özal* Kadir Has University Abstract- Computational studies for free energies of solvation of additives in polymeric nanostructures are not trivial when there is no naturally occuring free volumes available in the system of a material matrix. In such system matrices, a new methodolgy should be applied. Solvation free energy calculations for several additive molecules (such as DMSO, propane and chloroform) based on a softcore reference state in a Bisphenol-A- Polycarbonate (BPA-PC) polimer matrix was performed. The applicability and usefulness of the methodology was discussed via comparison with some other basic and recent free-energy calculation methodologies. The numerical results obtained via these various methodologies which makes use of Molecular Dynamic simulation procedures at the atomic scale were compared and they are found to be in agreement quite well. Solubility is a crucial property in material science and in many other fields, and its thermodynamic correspondence solvation free energy and its individual thermodynamic terms had been our research interest thinking in terms of computational efforts at atomic scales. We had performed studies initially on solvation of small gas molecules such as methane in binary mixtures of liquids, and the thermodynamic terms, enthalpy and entropy was investigated deep into details leading to some critical questions about the terminology. [1] However, further studies on the solvation free energy computations based on molecular dynamic simulation techniques enhance the fact that there must be an already available free volumes for the methods such as Widom’s test particle insertion (TPI) to be applied. [2] When there is no such available volume, then some other methods should be applied by using the thermodynamic cycle properties. For example, solvation of additive molecules in polymer matrices this is generally the case. The methodology that we make use of the thermodynamic cycle in this work was introducing an intermediate soft-core spherical cavity and then performing the perturbation. By this means, our aim was first creating the volume to make the insertion possible and then performing the TPI. [3] In Figure 1, the soft-core reference state used as an intermediate state for the insertion in the thermodynamic cycle was depicted and the results compared with the ones obtained by Fast-Growth Thermodynamic Integration method was given in the table 1. [4] Table 1. Free energy changes computed for BPA-PC by FEP and TI, with long runs of 10ns and 1ns at each , respectively for these two methods. Data for various reference states are given for comparison and the criteria for a good choice of a reference state is discussed in the chapter. Errors (obtained by block averaging) are given in parentheses. [3] This study was supported by Max Planck Institute for Polymer Research Center. *Corresponding author: tugba.ozal@khas.edu.tr [1] Özal,T.A.; van der Vegt, N.F.A.,2006, “Confusing cause and effect: Energy-entropy compensation in preferential solvation of a nonpolar solute in DMSO/water mixtures”, Journal of Physical Chemistry B 110, 12104 [2] Peter, C.; van der Vegt, N.F.A., 2007, “Solvent reorganization contributions in solute transfer thermodynamics: Inferences from the solvent equation of state.”, Journal of Physical Chemistry B. 111(27), 7836-7842 [3] Özal, T.A.; Peter, C.; Hess, B.; van der Vegt, N.F.A.,2008, “Modeling Solubilities of Additives in Polymer Microstructures: Single Step Perturbation Method based on a Soft Cavity Reference State”, Macromolecules 41, 5055 [4] Hess, B.; Peter, C.; Ozal, T.; van der Vegt, N.F.A., 2008, “Fast growth thermodynamic integration: solubilities of additive molecules in polymer microstructures”, Macromolecules 41, 2283. Figure 1. Snap-shot presenting a Soft-core Reference state (yellow transparent) in BPA_PC polymer matrix at 480 K used for the free energy calculation based on a new combinatory method of Thermodynamic Integration (TI) and Free Energy Perturbation (FEP). [3] 6th Nanoscience and Nanotechnology Conference, zmir, 2010 343
Poster Session, Tuesday, June 15 Theme A1 - B702 Size effect on melting of ternary CuTiZr nanoparticles : a molecular dynamics study Serap Senturk Dalgic 1 * and Oguz Gulseren 2 1 Departmen of Physics, Trakya University, Edirne, 22030, Turkey 2 Department of Physics, Bilkent University, Ankara, 06800, Turkey Abstract-In the present study, the melting process of spherical CuTiZr ternary nanoparticles with the diameters around 3–9nm has been simulated by the tight binding second moment approximation (TB-SMA) model potential. Size depending melting properties of CuTiZr nanop articles are investigated with the composition Cu 50 Ti 25 Zr 25 . We find that the melting temperatures of CuTiZr nanoparticles are lower than that of the bulk. The size dependent melting temperature, surface energy and cohesive energy for corresponding ternary nanop articles have been predicted. The melting of nanomaterials is under considerable investigation because of the broad scientific and technological interest for possible applications. It is important and necessary to understand and predict the thermodynamics of nanomaterials for fabricating the materials for practical applications. In recent years, the ternary Cu-Ti-Zr system has attracted increasing interest because of its good glass forming ability [1]. Even though many atomistic simulation studies have been performed in order to investigate the atomic scale materials phenomena of bulk metallic glassy alloys [2], such as CuTiZr [3, 4], none has been done for nanoscale of this material. This is mostly because of the difficulty in developing the interatomic potentials for ternary systems which can be used successfully in molecular dynamics (MD) calculations. The objective of the present work is to determine the size dependent melting properties of ternary spherical nanoparticles CuTiZr for the selected alloy composition. For this purpose, we have studied the melting evolution of Cu 50 Ti 25 Zr 25 nanoparticles using molecular dynamics without the periodic boundary conditions. The temperature is controlled by Nose-Hoover thermostat. Newtonian equations of motion are integrated using the Leapfrog Verlet method with a time step 2 fs. For bulk and nanoparticles, system was subjected to heating process consisting of a series of MD simulations with temperature increments T=50 °K and relaxing time 50 ps. However, for a temperature near the melting region, we used smaller temperature increment, T=10 °K while keeping the relaxation time as 50 ps. calculations with the TB-SMA many body potential [5]. The potential parameters can deal with all the different elements, Cu, Ti, Zr proposed by Cleri and Rosato and alloys systems composed of those elements using common mathematical formalism developed in Ref. [4]. In order to validate the TB potential, we first simulate the melting process of bulk Cu 50Ti 25 Zr 25 alloys and check our results with those presented in the literatures. The simulation method was as follows: first of all, all spherical nanoparticles started with geometries constructed from a large cubic FCC crystal structure using a series of spherical cutoff centered at a core of cubes. The stable structure at 0 K is obtained through the initial configurations annealed fully at T=300 °K and then cooled to T=0 °K at a cooling rate 1 K/ps. In order to get an energy-optimized structure during heating at a given temperature for bulk systems, molecular dynamics under constant temperature and constant pressure conditions (NPT) with periodic boundary conditions has been performed. For the nanoparticles, we used the constant volume and constant temperature (NVT) molecular dynamics Cu 50 Ti 25 Zr 25 . The structural properties of nanoparticles were also studied via radial distribution function, mean atomic distances and coordination number. The size dependence diffusion have been established, the diffusion mechanism for ternary nanoparticles is dominated at surface diffusion. Surface energies for different nanoparticles have also been calculated. In summary, the presented study has been established to describe the size dependences of melting properties of ternary nanoparticles. The presented results will be verified by the experimental and other theoretical models developed for ternary nanoparticles in future studies. *Corresponding author: dserap@yahoo.com T=0 °K T =700 °K T=1100 °K Figure 1. Snapshot views of the MD sample with N=32085 (D=9nm) at a series of temperatures during heating. It is established experimentally that the melting begins preferentially at the surface in the melting process of nanoparticles and nanorods [6-8]. It has been shown in Figure 1 that the surface atoms much more active than the other atoms. The relations between the mean atomic energy and temperature for different size of nanoparticles and bulk have been obtained. The melting temperatures of ternary Cu50Ti25Zr25 nanoparticles show clear dependence of particle diameters. The heat of melting for the nanoparticles is lower than that of the bulk [1] S. Pauly, J. Das, N. Mattern, D.H. Kim, J. Eckert, Intermetallics, 17, 453 (2009). [2] A. Inoue, W. Zhang, T. Zhang, K. Kurosaka, Acta. Mater. 49, 2645 (2001). [3] X. J. Han and H. Teichler Physical Review E 75, 061501 (2007) [4] S. S. Dalgic and M. Celtek (unpublished results). [5] F. Cleri and V. Rosato Phys. Rev. B48, 22 (1993). [6] O Gulseren, F. Ercolessi ve E. Tosatti ; Phys. Rev B51,7377 (1995). [7] Z.L. Wang et al. Phys. Rev. B 67, 193403 (2003). [8] W. Hu, S. Xiao, J. Yang and Z. Zhang, Eur. Phys. J. B 45, 547 (2005). 6th Nanoscience and Nanotechnology Conference, zmir, 2010 344
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