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Bucharest, Romania

Bocker C.,Friedrich - Schiller University of Jena | Avramov I.,Institute of Physical Chemistry | Russel C.,Friedrich - Schiller University of Jena
Scripta Materialia | Year: 2010

Three different glasses in the system Na2O/K2O/Al2O3/KF/BaF2/SiO2 were crystallized. After cooling, in one sample only cubic BaF2 was detected, in the second sample predominantly the orthorhombic modification was found, and in the third sample, both phases were present. The orthorhombic phase according to the literature is thermodynamically stable only at high pressures. During crystallization, compressive stresses cannot relax and the orthorhombic phase is formed. This is the first experimental evidence of the formation of high pressure during crystallization of glass. © 2010 Acta Materialia Inc. Source

Avramov I.,Institute of Physical Chemistry
Journal of Non-Crystalline Solids | Year: 2011

The temperature dependence of viscosity of silicate melts is discussed in the framework of the Avramov-Milchev (AM) equation. The composition is described by means of two parameters: the molar fraction, x, and the "lubricant fraction", l. The molar fraction is the sum of the molar parts x i of all oxides dissolved in SiO2, the molar fraction of the latter being 1 - x. It is shown that, with sufficient precision, two of the parameters of the AM equation can be presented as unique functions of the molar fraction. On the other hand, x is not sufficient to determine properly the reference temperature Tr, at which viscosity is ηr = 1013 [dPa.s]. Therefore, additional parameter, "lubricant fraction" l, is introduced. For each of the components, li is a product of molar part xi and a specific dimensionless coefficient 0 ≤ ki ≤ 1 accounting for the specific contribution of this component to the increased mobility of the system. It is demonstrated that, for l > 0, the reference temperature is related to the "lubricant fraction" l through the reference temperature Tr,SiO2 of pure SiO2. © 2011 Elsevier B.V. All rights reserved. Source

Avramov I.,Institute of Physical Chemistry
Journal of Non-Crystalline Solids | Year: 2011

The number of free parameters of viscosity equations are reduced to two by assuming that the glass transition temperature Tg is the temperature at which viscosity is 1013.5 [dPa s]. In this way the Avramov and Milchev AM equation is becoming lgη=lgηA+13.5-lgηATgTα while the VFT equation is transformed to the form: lgη=lgηV+13.5-lgηVTg- ToT-To. It is demonstrated that the dimensionless fragility parameter a of the AM equation depends on composition, x, as α = αο + 6x, where αο = 1 - 2 for silicates and αο = 2.75 for borates. We show that the fitting parameters of AM and of VFT equations are correlated. Thus the relationship between the pre-exponential constants is lg ηV ≈ lg ηA - 2. There is also relationship ToTg=1-1.2α between the fragility parameter a and the divergence temperature To of the VFT equation. We develop an alternative approach to consider the vibration frequency of building units of the system. The superposition of the independent vibrations of the atoms constituting the building unit causes a "beat" of the latter. © 2010 Elsevier B.V.All rights reserved. Source

Lazzara T.D.,Institute of Organic and Biomolecular Chemistry | Mey I.,Institute of Organic and Biomolecular Chemistry | Steinem C.,Institute of Organic and Biomolecular Chemistry | Janshoff A.,Institute of Physical Chemistry
Analytical Chemistry | Year: 2011

Porous substrates have gained widespread interest for biosensor applications based on molecular recognition. Thus, there is a great demand to systematically investigate the parameters that limit the transport of molecules toward and within the porous matrix as a function of pore geometry. Finite element simulations (FES) and time-resolved optical waveguide spectroscopy (OWS) experiments were used to systematically study the transport of molecules and their binding on the inner surface of a porous material. OWS allowed us to measure the kinetics of protein adsorption within porous anodic aluminum oxide membranes composed of parallel-aligned, cylindrical pores with pore radii of 10-40 nm and pore depths of 0.8-9.6 μm. FES showed that protein adsorption on the inner surface of a porous matrix is almost exclusively governed by the flux into the pores. The pore-interior surface nearly acts as a perfect sink for the macromolecules. Neither diffusion within the pores nor adsorption on the surface are rate limiting steps, except for very low rate constants of adsorption. While adsorption on the pore walls is mainly governed by the stationary flux into the pores, desorption from the inner pore walls involves the rate constants of desorption and adsorption, essentially representing the protein-surface interaction potential. FES captured the essential features of the OWS experiments such as the initial linear slopes of the adsorption kinetics, which are inversely proportional to the pore depth and linearly proportional to protein concentration. We show that protein adsorption kinetics allows for an accurate determination of protein concentration, while desorption kinetics could be used to capture the interaction potential of the macromolecules with the pore walls. © 2011 American Chemical Society. Source

Saridakis E.,Institute of Physical Chemistry
Crystal Growth and Design | Year: 2011

A novel concept is presented for optimizing crystallization conditions, based on the principles of Genetic Algorithm stochastic optimization methods. This concept was tested with three model proteins as well as in a real problem situation involving crystallization of a DNA oligonucleotide and showed that initial microcrystalline suspensions or clusters of microcrystals could be optimized rapidly and with a minimal number of experiments. Macromolecular crystallization conditions are ususally discovered by screening against more or less standard sets of candidate conditions, known as "crystallization screens". One or more of these conditions may show some promise, most often in the form of microcrystals, clusters, or microcrystalline suspension: such conditions are referred to as "hits". The following, optimization step, generally consists in fine-tuning these promising conditions by changing the values of the various parameters, such as concentrations and pH, in small increments, until useful crystals are obtained. Very often, this obvious approach fails. In such cases, recombination and mutation of the initial "hits" using a simple Genetic Algorithm-inspired approach can be an effective alternative route. © 2011 American Chemical Society. Source

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